Maintaining accurate weight data is critical for business of all sizes, whether you are small business which relies on accurate weight to sell products or a large cooperation which keeps all its employees and products to track, weighbridge calibration is essential.
In this blog post we’ll discuss about legal requirement, what is weighbridge calibration, process and requirements of weighbridge calibration.
Legal Requirement:
If you’re using your weighbridge for trade purposes, there is a legal requirement to ensure your weighbridge meets the stated accuracy (within certain tolerances) at all times. Anyone operating a weighbridge would be strongly advised to have it calibrated every 12 months to ensure compliance.
If a weighbridge is not calibrated at regular interval, it will be inaccurate by a percentage. For example a weighbridge may be low by 50kg when a 1 tone weight is placed on the scales, but when 10 tones are placed on the scales it will not read 9950kg. Instead, it will read 9500kg because the scale is reading 5% too low.
What is weighbridge calibration:
Calibration is the process of ensuring that a measuring instrument’s performance matches the standard of known or traceable accuracy; the measurement error is the difference between the measured value (the reading) and the correct value (the reference weight).
A set of weights ranging from weight equal to maximum capacity of the weighing machine to weight equal to the minimum reading indicated by the weighing machine is placed on the weighing machine one by one and the reading is noted simultaneously. The change in indication of reading is noted simultaneously after placing each weight, from maximum to minimum.
Weights are again placed on the weighing machine, this time from minimum to maximum capacity and change in indication is noted. Indication error is calculated. Following the series of calculations uncertainty of the weighing machine is calculated.
Benefits of weighbridge calibration:
Weighbridge calibration is critical to achieve accurate weighing results. the life of your weighbridge and its weighing accuracy is greatly prolonged when combined with periodic calibration and frequent routine testing.
Calibration performed by an authorized calibration provider using accurate reference standards ensures that the weight measurements will be accurate thus improving industrial process and therefore increases profitability.
Metrological requirements for reference standards used to calibrate the weighbridges:
The maximum permissible error of the Standard weight used to calibrate the weighing machine must not exceed one third of the maximum permissible error for the corresponding load considered on the weighing balance under verification. This is minimum requirement for calibration of the weighing balance.
The standard weight must be coated with materials suitable for providing protection against the corrosion.
Why chose our reference standards for calibration of weighbridges:
We here in Shanker Wire Products Industries (S.W.P.I.) produce 100 kg to 1000 kg high denominational weights as per OIML R 47, in all OIML accuracy class. Our Laboratory is NABL accredited in accordance with ISO/IEC 17025. We deliver NABL certificates of Calibration covering the basic reporting of conventional mass correction, uncertainty and traceability of standards.
Calibration Measurement Capability (CMC) of our 1000 kg weight is 6.102 g in M1 accuracy which is one of the best in India.
This standard weights are used for testing (and adjusting) of high capacity weighing machines in medium or ordinary accuracy class.
Calibration Weights and test weights are standardized masses certified by international, national, or industrial laboratories, such as NIST, ANSI, ASTM, or ISO. Each weight has a precise mass that makes it suitable for calibrating scales to ensure subsequent weight measurements are accurate. Documents often accompany calibration and test weights to guarantee they meet the standard body’s specifications for properties such as tolerance, material, surface finishing, design, cavities, and adjustments. Units of measurement are typically based on the kilogram, but other units are designated.
Calibration and test weights calibrate scales, balances, weight cells and other masses or weights. Weights are often accredited for certain applications, with those used in scientific settings held to higher benchmarks than those used for commercial or industrial uses, such as materials testing, material handling equipment load evaluation, pressure generation on deadweight testers, and cable testing.
ISO/IEC 17025: The International Organization for Standardization (ISO) is an independent non-governmental organization comprising a membership of 164 national standards bodies. The International Electrochemical Commission (IEC) is an international standards organization for electrotechnical fields. Together with other liaison organizations, they created the ISO/IEC 17025 standards. This is the international reference to be used by testing and calibration laboratories that want to display their ability to provide reliable results.
SWPI‘s world-leading expertise in metrology extends to Calibration Weights or Test weights, weight sets. Our weight portfolio covers weights according to OIML from fifty micrograms to one ton in all accuracy classes. Our test weights are used all over the world, not only for testing balances but also as primary standards in mass laboratories.
Calibrating Scale: Premium-quality weights to satisfy stringent testing requirements
Calibrating scale testing requirements have become more complex, requiring that the scales survive years of use in rough industrial environments. Ensuring scale calibration is key to extremely accurate and efficient production using a fully calibrated scale. Test weights for scales are important tools for weighing scale calibration.
If a scale is not calibrated, it can significantly cost a company financially, and even worse, it can damage its reputation. SWPI’s calibration Weights are perfectly designed to support testing and calibration of industrial scales. With a strong engineering focus on safe and productive testing, cast-iron weights up to 1 ton is perfect for this application, satisfying even the most stringent testing requirements. These test weights are available in different shapes and accuracy classes to ensure proper scale calibration and scale recalibration.
Weights are predominantly needed for performance tests and routine testing of balances and scales. In metrological terminology, a distinction is made between reference weights or “mass standards” (to calibrate other weights) and certified weights. National regulations and international recommendations define the error limits of certified weights. Weights are classified into tolerance limits which are defined either by OIML or ASTM. The conventional weight value (and not the mass) is used as the nominal value of the weight. For a high level of accuracy, certified weights are calibrated and traceable back to primary standards, which are usually national standards maintained by a National Metrology Institute (NMI).
2. What are OIML and ASTM calibration weight classes?
Weight classes are separated according to the error limits that are classified either according to OIML (International Organization of Legal Metrology) or ASTM (American Society for Testing and Materials) declarations as follows.
The OIML weight
Class E1 weights are intended to ensure traceability between national mass standards and weights of class E2 and lower (i.e. F1 and F2). Class E1 weights or weight sets shall be accompanied by a calibration certificate.
Class E2 weights are intended for use in the initial verification of class F1 weights and for use with weighing instruments of accuracy class I. Class E2 weights or weight sets shall always be accompanied by a calibration certificate. They may be used as class E1 weights if they comply with the requirements for surface roughness and magnetic susceptibility and magnetization for class E1 weights (and their calibration certificate gives the appropriate data).
Class F1 weights are intended for use in the initial verification of class F2 weights and for use with weighing instruments of accuracy class I and class II.
Class F2 weights are intended for use in the initial verification of class M1 and possibly class M2 weights. They are also intended for use in important commercial transactions (e.g. precious metals and stones) on weighing instruments of accuracy class II.
Class M1 weights are intended for use in the initial verification of class M2 weights and for use with weighing instruments of accuracy class III.
Class M2 weights are intended for use in the initial verification of class M3 weights and for use in general commercial transactions and with weighing instruments of accuracy class III.
Class M3 weights are intended for use with weighing instruments of accuracy class IIII.
Classes M3 and M2-3 are lower accuracy weights of 50 kg to 5 000 kg and are intended for use with weighing instruments of accuracy class III.*
*The error in a weight used for the verification of a weighing instrument shall not exceed one third of the maximum permissible error (MPE) for an instrument. These values are listed in section 3.7.1 of OIML International Recommendation 76 Non-automatic Weighing Instruments (1992).
ASTM Weight
ASTM Class 0: Used as primary reference standards for calibrating other reference standards and weights.
ASTM Class 1: Can be used as a reference standard in calibrating other weights and is appropriate for calibrating high-precision analytical balances with a readability as low as 0.1 mg to 0.01 mg.
ASTM Class 2: Appropriate for calibrating high-precision top loading balances with a readability as low as 0.01 g to 0.001 g.
ASTM Class 3: Appropriate for calibrating balances with moderate precision with a readability as low as 0.1 g to 0.01 g.
ASTM Class 4: For calibration of semi-analytical balances and for student use.
NIST Class F: Primarily used to test commercial weighing devices by state and local weights-and-measures officials, device installers and service technicians.
3. Why should I use certified calibration weights?
ASTM class 0 and ultra-class as well as OIML class “E0” and E1 should be used for the highest level of precision i.e. mass standards (calibrating other weights), micro-balance testing and calibration, and critical weighing applications. ASTM classes 1 & 2 and OIML classes E2 & F1 should be used for precision applications i.e. analytical balance testing and calibration. ASTM classes 3 & 4 and OIML classes F1 & F2 are best suited to top-loading balance calibrations and testing and moderate precision applications (laboratory non-critical).
Note: If a balance or scale is calibrated, the weight set used and the class must be documented.
4. Why / how often do I need to recalibrate my test weights?
Accurately calibrated test weights are the basis of accurate weighing results. The accuracy of test weights becomes less reliable over time. This is the result of normal wear and tear caused by regular use, dirt and dust. Periodic recalibration of test weights at an accredited mass-calibration laboratory is essential to ensure ongoing traceability. At our accredited mass-calibration laboratories, we clean, calibrate, and adjust each weight and then document the results in a calibration certificate. Our calibration services cover the basic reporting of conventional mass correction, uncertainty and traceability information in accordance with ISO/IEC 17025 requirements.
The frequency with which to recalibrate your test weights depends on the criticality of the weighing process. Selecting the correct test weight and weight class and also provides recommendations on how often to recalibrate your test weights. All of this information is determined based on your specific processes and risks.
5. What are buoyancy artifacts?
Air density is usually calculated from relevant air parameters such as air temperature, pressure, humidity and CO2 concentration. An alternative method of determining air density may be applied by utilizing two specially designed buoyancy artifacts. Both artifacts are compared in vacuum and in air. By comparing the two artifacts of identical nominal weight, the large volume difference reflects the air buoyancy and therefore results in a highly accurate determination of air density. The buoyancy artifacts are mainly used for the M_one vacuum mass comparator.
6. Why is a silicon sphere used for specialized volume measurement?
Spheres are used because the volume can be determined according to the definition of volume by a length measurement. Silicon (Si) spheres have the same homogenous atomic structure as a perfect diamond without voids or dislocations, so the density is more accurate than other materials. This is why a silicon sphere with a homogenous atomic structure serves as a reference for specialized volume measurement.
7. What are heavy-capacity weights used for?
Mass comparators go up to a capacity of six tons. Industrial scales go up to several hundred tons. Heavy-capacity weights—typically those in the range of 100 kg, 200 kg, 500 kg, 1 t and 2 t are used for sensitivity, eccentricity, linearity and repeatability testing of these higher-capacity devices. Weights are less than 2 t due to the maximum lifting capability of machines, typically forklifts and cranes. However, these weights can be combined to reach the desired weight. Check out our range of heavy-capacity weights, Click here
Heavy-capacity weights must be transported in heavy-duty trucks and it is important to ensure trucks do not exceed their rated load limit due to safety and government regulations. Heavy-capacity weights are generally constructed of cast iron not stainless steel due to the cost.
8. What are calibrated test weights used for? Are they used to calibrate weight scale systems? Do you offer test weights for scales?
Calibration Weights are used in scale calibration. This is a process that ensures scale accuracy. Test weights for scales or precision weights are used to calibrate weight scale systems of various levels of accuracy depending on the use and requirements. Certified test weights or precision weights should be used in these processes to calibrate weight scale systems. It is important to ensure the test weights are calibrated test weights and that they are accurate to provide accurate calibration results. Check out our range of scale calibration weights, click here.
9. What is the weighing scale tolerance limit of any scale? Can all scales offer precision weights?
This is the required accuracy of the scale, and specifically the tolerance of inaccuracy allowed before it is out of tolerance and in need of a weigh scale calibration by certified calibration weights. A calibrated scale will operate at a higher level of accuracy and maintain tolerance better. For this reason, weight scale calibration with certified weights for keeping the weighing scale tolerance limit is key for accurate, calibrated scales and weigh scale calibration. Learn more about keeping your weighing scale tolerance limit in your weighing processes.
10. What are scale weights? Are they calibration weights for scales? Must they be certified weights?
Scale weights are weights for scale calibration. These weights for scale calibration may be certified weights. Generally, weights for scale calibration are certified. When calibrating scale procedures are performed, it is necessary to have calibration weights for scales. Weighing scale calibration with scale weights or test weights should be performed on a regular basis depending on use. Learn more about scale calibration weights and weigh scale calibration.
SWPI‘s world leading expertise in metrology extends to certified test weights, weight sets as well as calibration weights for scales. The weight portfolio covers scale weights according to OIML or ASTM from fifty micrograms to one ton in all accuracy classes. Our test weights are used all over the world, not only for testing balances but also as primary standards in mass laboratories. We invite you to learn more about our certified test weights and consider using them in your weighing scale calibration and weigh-scale calibration processes.
11. What are the differences between OIML classes?
The exact difference is explained in the OIML guideline, but at a basic level, E1 has the narrowest and M1 the highest tolerance limit, i.e. E1 is the most accurate.
12. Plus Tolerance
Weights are calibrated according to OIML maximum permissible errors (+/- in mg). If the result of the calibration is in the plus range, it means that the weight is heavier than the specified nominal value, but still within the tolerance. Since most weights lose weight over time due to wear, it is more likely that this weight will take longer to fall out of tolerance (maximum permissible error). Through our production processes, most of our weights are calibrated in the plus range.
13. How often do you need to re-calibrate your weights?
Depending on how often the weights are in use, weights should be re-calibrated every 1-2 years.
Selection of standard weights for calibration of weighing instruments
TEUVOLAMMI
The Finnish Association of Technology for Weighing, Helsinki, Finland
Abstract
This paper deals with the selection of standard weights or test loads for the calibration of single-interval weighing instruments. Four tables are given for the selection of weights of at most 50 kg. The tables contain information about the accuracy of the weights and the instruments to be calibrated. According to the accuracy of the instrument a table is chosen; with its aid the weights are selected so that their accuracy is appropriate in relation to that of the instrument.
1 Introduction
The weights dealt with here are those given in OIML Recommendations:
R 111, “Weights of classes E1, E2, F1, F2, M1, M2, M3” (1994) [1] or
R 47, “Standard weights for testing of high capacity weighing machines” (1979–1978) [2]
R 111 covers weights of at most 50 kg and R 47 those from 50 kg to 5000 kg. Their errors are measured in connection with either the calibration or the verification of the weights. In both these cases the following conditions are supposed to be met:
The errors of the weights comply with the maximum permissible errors (mpe’s) given in the Recommenda- tions;
The measurement uncertainty of the error of each weight is a fractional part of the mpe of the weight (usually at most 1/3 ´ mpe). This uncertainty is the uncertainty of the weight.
A generally accepted principle for selecting the weights for calibrating an instrument is that the accuracy of the weights should be appropriate in relation to that of the instrument and the influence of the errors of the weights on the calibration results should be controlled.
One way to achieve this is to select the weights for each applied load so that the quotient of the error of the weights and a certain error of the instrument specified by its user (maximum tolerable error) is not greater than a chosen fraction.
Usually, the value of the fraction chosen is 1/3, but sometimes it is 1/6. The idea of using 1/6 is explained in 4.2.2.
The user can specify the maximum tolerable errors,
e.g. by giving maximum differences between the indications of the instrument and the corresponding true values, as determined by the weights. In other words, he gives limits for the errors of the instrument obtained by means of calibration, and his expectation is that the errors are within the limits, the maximum tolerable errors. This is dealt with in more detail in Section 2.
In Section 3 the general rules for selecting R 111 and R 47 weights for the calibration of instruments are given, though these have been dealt with previously in the author’s publication Calibration of Weighing Instruments and Uncertainty of Calibration [3]. However, the main subject of this paper is to select the weights of class E2 to M3 of R 111 (class E1 is not dealt with here) using the tables given at the end of Section 4.
2 Maximum tolerable errors (MTEs) of instruments
Suppose that the user of an instrument has selected an error f representing the accuracy of the instrument or the accuracy of weighing with it (compare f with e in OIML R 76-1, T.3.2.3, 2.2 and 3.5.1 [4]). f may equal the scale interval of the instrument or a multiple thereof (OIML R 76-1, T.3.2.2). With the aid of f the user can define the maximum tolerable errors (MTEs) of the instrument. The MTEs can be:
± f for all the loads, or
± 0.5 f or ± f for certain “small” loads but ± f or ± 2 f for the larger loads, or
± 0.5 f or ± f for “small” loads, ± f or ± 2 f for certain “medium” loads and ± 1.5 f or ± 3 f for larger loads.
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In the following the absolute values êMTEú of the MTEs are used. The cases:
1) êMTEú = 0.5 f, f & 1.5 f; 0.5 f & f, or only f and
2) êMTEú = f, 2 f & 3 f, or f & 2 f are dealt with separately.
The “small” and “medium” loads expressed in terms of f are defined in 4.3.
General rules for selecting the weights used for calibrating instruments
The quotient Max/f, where Max is the maximum weighing capacity of the instrument, plays an important role. It is used in the tables in Section 4 but also in one of the following rules based on the requirement of R 76-1, 3.7.1 concerning standard weights for the verifi- cation of instruments.
Verified weights
Weights of at most 50 kg (R 111)
The sum of the absolute values of the mpe’s (sum of
êmpe÷ ’s) of the weights shall not be greater than 1/3 or 1/6 of the êMTEú of the instrument for the applied load (1/3 is used in R 76-1).
Weights from 50 kg to 5000 kg (R 47)
For these weights, rule 3.1.1 with the fraction 1/3 can be considered to be met if Max/f of the instrument is equal to or less than the n marked on the weights.
Calibrated weights
Errors of the indications of the instrument are not corrected for the errors of the weights
The sum of the absolute values of the errors of the weights shall not be greater than 1/3 or 1/6 of the êMTEú of the instrument for the applied load. However, on the basis of condition 1) in “Introduction” this rule is replaced with rule 3.1.1 here.
Errors of the indications of the instrument are corrected for the errors of the weights
The sum of the absolute values of the uncertainties of the weights shall not be greater than 1/3 of the êMTEú of the instrument for the applied load. The fraction 1/6 is not used here for this case.
Rules 3.1.1 to 3.2.2 only approximately met
Sometimes it is reasonable to allow the previous rules to be met only approximately. For example, 3.1.1 with the fraction 1/3 is approximately met if the sum of the
êmpeú ’s of the weights exceeds the limit 1/3 ´ êMTEú and the quotient of the excess and the limit is less than or about 1/10 for the applied load. This is applied similarly to the other rules too.
Tables for selecting weights of class E2 to M3 (R 111) according to Max/f of the instrument
General
Scope
Tables 1, 2, 3 and 4 at the end of this section cover the selection of the weights of class E2 to M3 of R 111 according to Max/f of the instrument to be calibrated. The tables are compiled so that the weights selected with their aid meet rule 3.1.1 above without any further action, however, with the exception of the weights for instruments/balances with “very” high Max/f.
The weights dealt with here are normally verified weights, but under the practice of 3.2.1 calibrated weights may also be concerned. The weights for the balances with “very” high Max/f are calibrated weights of class E2 which meet rule 3.2.2, if applicable. This is one of the two procedures to be dealt with in the tables.
Differences between the tables
In Tables 1 and 2 the values of êMTEú are: 0.5 f, f & 1.5 f or 0.5 f & f, or only f, and in Tables 3 and 4: f, 2 f & 3 f or f & 2 f (if êMTEú only takes on the value f, Table 1 or 2 is referred to). The fraction is 1/3 in Tables 1 and 3 and 1/6 in Tables 2 and 4.
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Selectionofatable,itsuse,groups1), 2), 3) and 4) of the instruments and procedures
The table is selected according to êMTEú and the frac- tion 1/3 or 1/6. Then Max/f of the instrument/ balance is calculated and, following the instructions given in the tables, it is assigned to one of the following groups (compare the groups with the accuracy classes for instruments/balances in R 76-1, 3.1.1 and 3.2):
Group 1): Special balances (Max/f is unlimited, special accuracy)
Group 2): Laboratory or precision balances (Max/f
£ 100 000, high accuracy)
Group 3): Instruments for industrial weighing (Max/f
£ 10 000, medium accuracy)
Group 4): Instruments for industrial weighing (Max/f
£ 1 000, low accuracy)
On the basis of Max/f and the group of the instru- ment/balance the accuracy class of the weights, or the procedure to be applied (see 4.1.1), is obtained from the table chosen.
The procedures are:
“Apply 3.2.2” or “No calibration”. If Max/f is high enough, they are applied for some balances of Group 1).
“Apply 3.2.2” means that calibrated weights of class E2 are selected applying 3.2.2 and “No calibration” means that some balances are not calibrated with the weights dealt with here. The procedure “Apply 3.2.2” is used for Tables 1 and 3. It cannot be used for Tables 2 and 4 because the fraction is 1/6 for them. Due to this fraction rule 3.2.2 is excluded. Therefore, the procedure “No calibration” has to be used for Tables 2 and 4 instead of “Apply 3.2.2”. Note that the highest value of Max/f dealt with in the tables is 650 000. More information about the use of the tables is given in the text below each table.
If weights £ 50 g are selected, problems caused by these weights are explained in 4.4. The application of the tables to the selection of the weights for verification of instruments/balances is dealt with in 4.5.
The use of Tables 1, 2, 3 and 4 is illustrated in 4.2.1 to
4.2.4 by means of examples. In order to use the tables properly the “small” and “medium” loads for which the values of êMTEú are given in Section 2 should be defined. This is done in 4.3.
Table1
This table is for êMTEú = 0.5 f, f & 1.5 f; 0.5 f & f or only f and for the fraction 1/3. Table A in 4.3 shows in which cases the values of êMTEú are used. According to Max/f and the group of the instrument/balance the accuracy class E2 to M3 of the weights (3.1.1 or 3.2.1) or the procedure “Apply 3.2.2” is obtained from Table 1.
Example 1: Group 4): Instruments for industrial weighing (Max/f £ 1 000, low accuracy)
If Max/f £ 660, weights of class M3 are selected irres- pective of the possible values of êMTEú . (Consider an instrument with Max 6 600 g, f = 10 g and Max/f = 660. Let the weights for the Max load be 5 kg, 1 kg , 500 g and 100 g of class M3. Their êmpeï’s are 2.5 g, 0.5 g, 0.25 g and 0.05 g respectively.
Let êMTEïassume the value f = 10 g for all the loads. For the Max load the sum of the êmpeï’s of the weights is Sêmpeï = (2.5 + 0.5 + 0.25 + 0.05) g = 3.3 g » 1/3 ´ êMTEï » 3.3 g.
Let êMTEïassume the values 0.5 f = 5 g, f = 10 g & 1.5 f = 15 g so that êMTEï= 0.5 f is used for the loads a) from 0 to 50 f (the loads are expressed in terms of f), ïMTEï = f for the loads,
> 50 f but £ 200 f and ïMTEï = 1.5 f for the loads, c) over 200 f to Max. Let us investigate the sums S êmpeï of the weights (the test loads) which can be used at the greatest loads of the ranges a), b) and c) respectively. For the greatest load of the range a) Sïmpeï = 0.25 g < 1/3 ´ 0.5 f » 1.7 g, for that of
b) Sï mpe ï = 1 g < 1 / 3 ´ f » 3.3 g and for that of
c) Sïmpeï = 3.3 g < 1/3 ´ 1.5 f = 5 g).
b) If 660 < Max/f £ 1 000
and êMTEú takes on the values 0.5 f, f & 1.5 f, the class is M3
the class is M2 if êMTEú takes on the values 0.5 f & f or only f.
Example 2: Group 3): Instruments for industrial weighing (Max/f £ 10 000, medium accuracy)
If 2 200 < Max/f £ 3 300
and êMTEú takes on the values 0.5 f, f & 1.5 f, the class is M2
the class is M1 if êMTEú takes on the values 0.5 f & f or only f.
Example 3: Group 2): Laboratory or precision balances
(Max/f £ 100 000, high accuracy)
Max/f = 6 500 (also see 4.4)
Consider a balance with Max 650 g and f = 0.1 g
– let êMTEú be 0.5 f = 0.05 g for loads £ 500 g and f = 0.1 g for > 500 g to 650 g. The quotient L/(0.5 f)
= 500/0.05 has to be compared with Max/f = 6 500. Because 500/0.05 > 6 500 (L/(0.5 f) > Max/f), class
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F2 has to be used. Note that M1 would be suitable for the load 650 g but not for 500 g. (For 650 g the sum of theïmpeï’s for weights of class M1 is (25 + 5 + 3) mg = 33 mg
» 1/3 ´ ïMTEï= 1/3 ´ 0.1 g » 33 mg but for 500 g it is 25 mg > 1/3 ´ ïMTEï = 1/3 ´ 0.05 g » 16.7 mg).
– M1 would be suitable if the choice of the values of
êMTEú were made so that êMTEú = 0.5 f is used for loads £ 300 g and ïMTEï= f for > 300 g to 650 g (thus L/(0.5 f) = 300 g/(0.5 f) < Max/f ), or if êMTEú
= f for all loads.
Consider a balance with Max 65 g and f = 10 mg. Obviously, the weights used are £ 50 g and they should be of class F2 irrespective of the possible values of êMTEú .
Example 4: Group 1): Special balances
(Max/f unlimited, special accuracy)
a) If 65 000 < Max/f £ 200 000 (also see 4.4)
and êMTEú assumes the possible values 0.5 f & f or only f, weights of class E2 are selected
exceptionally, if weights of £ 50 g are used, 170 000
< Max/f < 200 000 and f = 1 mg, calibrated weights of class E2 are selected applying 3.2.2, i.e., the procedure “Apply 3.2.2” is used. Such a balance might have Max 190 g, f = 1 mg, Max/f = 190 000. However, if f > 1 mg (e.g., Max 380 g, f = 2 mg, Max/f = 190 000), weights (3.1.1 or 3.2.1) of class E2 are used.
b) If 200 000 < Max/f £ 300 000 (also see 4.4)
and êMTEú = 0.5 f, f & 1.5 f, the class of the weights is E2. (Consider a balance with Max 290g, f = 1mg, Max/f = 290 000 and ïMTEï = 0.5 f, f & 1.5 f. Let the weights for the Max load be 200 g, 50 g and two of 20 g of class E2. The sum of their ïmpeï’s is (0.30 + 0.10 + 2 ´ 0.080) mg = 0.56 mg which exceeds 1/3 ´ ïMTEï = 1/3 ´ 1.5 mg = 0.5 mg by 0.06 mg. This excess is neglected (3.3) because 0.06 mg/0.5 mg is near to 1/10).
if êMTEú = 0.5 f & f or only f, calibrated weights of class E2 are selected applying 3.2.2, i.e., the pro- cedure “Apply 3.2.2” is used.
Table 2 and the idea of using the fraction 1/6
In this table êMTEú = 0.5 f, f & 1.5 f; 0.5 f & f or only f as in Table 1 but the fraction is 1/6. Table A in 4.3 shows in which cases the values of êMTEú are used. According to Max/f and the group of the instrument/balance the accuracy class E2 to M3 of the weights (3.1.1 or 3.2.1) or the procedure “No calibration” is obtained from Table 2.
If the weights are within the mpe’s, as they should be, the sum of their êmpeú’s is £ 1/6 ´ êMTEú of the instrument/balance for the applied load. The sum reveals the influence of the errors of the weights on the calibration results.
Suppose that due to wear and tear the weights are not within the mpe’s. However, if their errors can be estimated to be within the mpe’s multiplied by 2, the weights can conditionally be used for the calibration of instruments/balances. The sum of the doubled
êmpe÷’s of the weights is £ 1/3 ´ êMTEú . So the
influence of the errors of the weights on the calibration results is twice that in 1) and thus at most 1/3 ´ êMTEú . If this is accepted, the calibration with these weights can be regarded as correct.
In case 2), the increase of the influence of the errors of the weights from £ 1/6 ´ êMTEú to £ 1/3 ´ êMTEú has to be accepted. In principle this is not difficult because £ 1/3 ´ êMTEú is a generally accepted in- fluence. Because the errors of the weights may exceed the limits of the mpe’s even by 100 %, the period of readjustment of the weights can be extended. This is a considerable advantage. From this angle there are reasons to apply the fraction 1/6.
If the aim is to minimize the uncertainty of the calibration of instruments/balances, the influence of the errors of the weights should be kept as small as possible. £ 1/6 ´ êMTEú could be suitable. Therefore, the errors of the weights should strictly be within the mpe’s as in 1) and the fraction 1/6 should be applied.
Note 1: In R 111 mpe’s on initial verification (mpe’s in 1) above) and in service are given. The latter are twice the mpe’s on initial verification. The mpe’s in service can be used in situations similar to the following. Parties concerned by weighings with legally controlled instruments/balances (e.g., non-self- indicating instruments) in which balance (the position of equilibrium) is obtained with the aid of weights, want to check whether the weights used are “acceptable”. The weights were adjusted to be within the mpe’s on initial verification. Now the errors of the weights are acceptable if they are within the mpe’s in service. One could say that the mpe’s in service give the user of the instrument protection against complaints about the incorrectness of the results of the instrument as far as the weights are concerned.
Note 2: Notwithstanding 2) above the weights, the errors of which are within the mpe’s in service, are not for calibration, verification or testing of instruments/balances.
Example 5: Group 3): Instruments for industrial weighing (Max/f £ 10 000, medium accuracy)
If 1 100 < Max/f £ 3 300, the class is M irrespective of
When the weights of class E2 to M3 selected by
êMTEú .
1
(Consider an instrument with Max 6 000 g, f = 2 g,
means of Table 2 (with the fraction 1/6) are used for the calibration of instruments/balances, the consequences of their errors could be as follows.
Max/f = 3 000 and ïMTEï = f = 2 g for all the loads. Let the weights for the Max load be 5 kg and 1 kg of class M1. The sum of their
ïmpeï’s is (250 + 50) mg = 0.30 g < 1/6 ´ ïMTEï = 1/6 ´ 2 g » 0.33 g).
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Example 6: Group 2): Laboratory or precision balances (Max/f £ 100 000, high accuracy)
Max/f = 6 500 (6 000 < Max/f £ 11 000; also see 4.4)
Consider a balance with Max 650 g and f = 0.1 g. Let
êMTEú be 0.5 f = 0.05 g for loads £ 500g and f = 0.1 g for > 500 g to 650 g. Weights of class F2 are selected.
Consider a balance with Max 65 g and f = 10 mg. Obviously, the weights used are £ 50 g and they should be of class F1 irrespective of the possible values of êMTEú .
Example 7: Group 1): Special balances
(Max/f unlimited, special accuracy)
If Max/f £ 60 000 (also see 4.4)
and the weights are > 50 g, calibration is per- formed with the weights of class E2
if the weights are £ 50 g and êMTEú = 0.5 f & f, calibration is not performed with the weights dealt with here, i.e., the procedure “No calibration” is used. However, calibration is performed with the weights £ 50 g of class E2 if êMTEú = f for all the loads.
Table3
Table 3 is for êMTEú = f, 2 f & 3 f or f & 2 f and for the fraction 1/3. If êMTEú = f for all the loads, apply Table 1. Table B in 4.3 shows in which cases the values of êMTEú are used. According to Max/f and the group of the instrument/balance the accuracy class E2 to M3 of the weights (3.1.1 or 3.2.1) or the procedure “Apply 3.2.2” is obtained from Table 3.
Example 8: Group 2): Laboratory or precision balances (Max/f £ 100 000, high accuracy)
Max/f = 6 500 (also see 4.4)
Consider a balance with Max 650 g and f = 0.1 g. Let
êMTEú be f = 0.1 g for loads £ 500 g and 2 f =0.2 g for
> 500 g to 650 g. Weights of class M1 are selected.
Consider a balance with Max 65g and f = 10 mg. Let
êMTEú be f = 10 mg for loads £ 50 g and 2 f = 20 mg for > 50 g to 65 g. Weights of class M1 are selected.
Table4
Table 4 is for êMTEú = f, 2 f & 3 f or f & 2 f and for the fraction 1/6. If êMTEú = f for all the loads, apply Table 2. Table B in 4.3 shows in which cases the values of êMTEú are used. According to Max/f and the group of the
instrument/balance the accuracy class E2 to M3 of the weights (3.1.1 or 3.2.1) or the procedure “No calibra- tion” is obtained from Table 4.
The consequences of using the fraction 1/6 are the same as in 1) and 2) in 4.2.2.
Example 9: Group 2): Laboratory or precision balances (Max/f £ 100 000, high accuracy)
Max/f = 6 500 (also see 4.4)
Consider a balance with Max 650 g and f = 0.1 g
let êMTEú be f = 0.1 g for loads £ 500 g and 2 f =
0.2 g for > 500 g to 650 g. The class is F2 because L/f = 500 g/0.1 g = 5 000 > 3 000 (e.g. F2 is necessary for the load 500 g)
if êMTEú = f = 0.1 g for loads £ 300 g and 2 f = 0.2 g for >300 g to 650 g, then L/f = 300 g/0.1 g = 3 000. So weights of class M1 are selected.
Consider a balance with Max 65 g and f = 10 mg. Let
êMTEú be f = 10 mg for loads £ 50 g and 2 f = 20 mg for > 50 g to 65 g. Because the weights for this balance are £ 50 g their class is F2.
Values of êMTEú for Tables 1, 2, 3 and 4
The following auxiliary tables A and B give the values of
êMTEú which are to be used when selecting weights for the calibration of instruments/balances with the aid of Tables 1, 2, 3 and 4. Table A (for Tables 1 and 2) and B (for Tables 3 and 4) are patterned on the model of R 76-1, 3.5.1.
Definition 1: “Small” loads for an instrument/ balance (expressed in terms of f) are those less than or equal to some chosen load which is not greater than 50 000 f, 5 000 f, 500 f
or 50 f for groups 1), 2), 3) or 4) res- pectively. For example, for a balance of group 2) the “small” loads can be from 0 to 5 000 f or from 0 to a load less than 5 000 f, say, 3 000 f. 5 000 f or 3 000 f is the greatest “small” load L.
Example 10: If Max of a balance of group 2) equals 15 000 f, then Max/f = 15 000 and thus
< 20 000. If the greatest “small” load L is 3 000 f, then according to Table A êMTEú is 0.5 f for the loads from 0 to 3 000 f and f for the loads over 3 000 f to Max. êMTEú can also be chosen to be only f from 0 to Max.
Definition 2: “Medium” loads for an instrument/ balance (expressed in terms of f) are those
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Table A The values of êMTE ê = 0.5 f, f & 1.5 f, or 0.5 f & f, or only f in relation to Max/f and the group of an instrument/balance for Tables 1 and 2 (the groups are defined in 4.1.3)
Max/f of an instrument/balance in: Group 1) Group 2) Group 3) Group 4)
êMTE ê
£ 50 000
£ 5 000
£ 500
£ 50
only f 1)
£ 200 000 2)
£ 20 000 2)
£ 2 000 2)
£ 200 2)
0.5 f & f, or only f 3)
> 200 000
> 20 000
> 2 000
> 200
0.5 f, f &1.5 f, or 0.5 f & f, or only f 4)
1) from 0 to the greatest “small” load L (see Definition 1). In this case L = Max for the instrument/balance.
2) but greater than L/f in the same group.
3) 0.5 f for the “small” loads and f for larger loads, or only f for all the loads (see Example 10).
4) 0.5 f for the “small” loads, f for the “medium” loads (see Definition 2) and 1.5 f for the larger loads but êMTE ê can also be chosen to be 0.5 f for the “small” loads and f for larger loads, or only f for all the loads.
Table B The values of êMTE ê = f, 2 f & 3 f, or f & 2 f in relation to Max/f and the group of an instrument/balance for Tables 3 and 4 (the groups are defined in 4.1.3). (If for an instrument/ balance êMTE ê = f for all the loads, then according to 4.2.3 and 4.2.4 Table 1 or 2 is used instead of Table 3 or 4 respectively.)
Max/f of an instrument/balance in: Group 1) Group 2) Group 3) Group 4)
êMTE ê
£ 200 000 1)
£ 20 000 1)
£ 2 000 1)
£ 200 1)
f & 2 f 2)
> 200 000
> 20 000
> 2 000
> 200
f, 2 f & 3 f, or f & 2 f 3)
1) but greater than L/f in the same group (L = the greatest “small” load, see Definition 1).
2) f for the “small” loads and 2 f for larger loads.
3) f for the “small” loads, 2 f for the “medium” loads (Definition 2) and 3 f for the larger loads but êMTE ê can also be chosen to be f for the “small” loads and 2 f for larger loads (see Example 11 below).
greater than the greatest “small” load L but not greater than 200 000 f, 20 000 f,
2 000 f or 200 f for groups 1), 2), 3) or 4) respectively. For example, if the “small” loads for an instrument of group 3) are from 0 to 300 f, the “medium” loads are in the interval over 300 f to 2 000 f. Note: The lower limit of the “medium” loads is not predetermined because it depends on the choice of the greatest “small” load L. However, the corresponding upper limit is. It takes on the values 200 000 f to 200 f in the different groups respectively.
Example 11: If Max of an instrument of group 3) equals 2 500 f, then Max/f = 2 500 and thus
> 2 000. Let the greatest “small” load L be 400 f. According to Table B êMTEú is f for the loads from 0 to 400 f, 2 f for the “medium” loads over 400 f to 2 000 f and
3 f for the loads over 2 000 f to Max.
êMTEú can also be chosen to be f from 0 to 400 f and 2 f for the loads over 400 f to Max = 2 500 f.
Weights of nominal values £ 50 g
There are problems when selecting weights for balances in group 1) or 2), especially if weights of £ 50 g are to be used for the Max load.
In order to explain the nature of the problems consider Max 65 kg and Max 65 g balances both in group
1) with Max/f = 65 000. For the Max 65 kg balance the sum of the êmpeú ’s of class F1 weights of > 50 g is slightly below the limit 1/3 ´ êMTEú for the Max load (3.1.1), but for the Max 65 g balance the corresponding sum of the class F1 weights of £ 50 g exceeds the limit.
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In the tables the above problem is solved by giving two accuracy classes for some balances in group 1) or 2). One class is for weights > 50 g for balances with certain Max/f ’s and “large” Max loads (e.g: F1, Max/f = 65 000, Max 65 kg,), and the other for weights £ 50 g for balances with the same Max/f ’s as above and “small” Max loads respectively (e.g.: E2, Max/f = 65 000, Max 65 g).
Note: For a “large” Max load, e.g. 650 g there is no problem with a single weight of £ 50 g (i.e., weights of > 50 g are dominating) but for a “small” Max load, e.g. Max near to 100 g there may be.
In the column “Instruments/balances” of the tables several intervals of the values of Max/f are given. When using only weights > 50 g for balances of group 1) and 2) the upper limits of the intervals could be higher than those given in the tables. For example, in Table 1 the upper limits 20 000 (6 500 < Max/f £ 20 000) and 300 000
(200 000 < Max/f £ 300 000) could be raised to 22 000 and 330 000 respectively. But if weights £ 50 g were selected using the tables with the higher limits, their accuracy would not be suitable in all cases. Since weights £ 50 g are important for the calibration of the balances in question the limits have not been raised. As a result of this weights > 50 g selected using the tables may sometimes be more accurate than necessary.
Use of the tables to select weights for verification of instruments/balances
Table 1 or 2 ( êMTEú = 0.5 f, f &1.5 f; 0.5 f & f, or f) can be applied to select the weights for the verification of instruments/balances. Then “ f ” is replaced with “e”, “MTE” with “MPE”(maximum permissible error for instruments/balances), the “groups 1), 2), 3) and 4)” of the instruments/balances with the “accuracy classes I, II, III and IIII” respectively and “calibration” with “verifica- tion”. If in Table 1 or 2:
only one accuracy class of weights is given for instruments/balances with a certain n = Max/e, then the correct class is obtained from the tables without any further action.
two accuracy classes of weights are given for instruments/balances with a certain n = Max/e, then to choose the right class the OIML requirements in R 76-1, 3.2 and 3.5.1 have to be taken into account. This is elucidated in the following.
4.5.1 ÷ MPE÷ = 0.5 e, e & 1.5 e
For certain instruments/balances in Table 1 and 2 the accuracy classes of the weights are given in the form
e.g.: M2(M1if ï MTEï = 0.5 f & f or f) or F2 (F1 if ï MTEï = 0.5 f & f or f).
Use the replacements for êMTEú , f and groups 1) to 4) as given above. These accuracy classes are for instruments/balances with n = Max/e > 200 000 in class I, n > 20 000 in class II, n > 2 000 in class III or n > 200 in class IIII. Thus the values of the ÷MPE÷ ’s to be applied are 0.5 e, e & 1.5 e. According to the informa- tion on the use of the tables (given in the text below the tables) the accuracy class of the weights given first (M2 or F2 in the above examples) is used. The second accuracy class given in parentheses is to be ignored because the condition “if÷ MPE÷ = 0.5 e & e or e” is not in accordance with the OIML requirements for the instruments/balances in question.
4.5.2 ÷ MPE÷ = 0.5 e & e
For some balances in Table 1 and 2 there are accuracy classes of the weights in the form e.g.: M1 (F2 if 1). F2 if
ï MTEï = 0.5 f & f and L/(0.5 f) > Max/f …), F2(F1if 1))
or E2 (No calibration if 1) and ïMTEï= 0.5 f & f ). 1) refers to the use of weights of £ 50 g. Use the replacements for
êMTEú, f, groups 1) to 4) and calibration. This concerns class I balances with n = Max/e £ 200 000 but n > 50 000 and class II balances with n £ 20 000 but n > 5 000. The values of the êMPEú ’s to be applied are 0.5 e & e. Accuracy classes of the weights similar to those in the above examples, and in advice under the heading “Exception” in Table 1, can be used. However, one has to check that only those instructions in the tables are followed which are or lead to results which are compatible with the OIML requirements (also see 4.5.4).
4.5.3 ÷ MPE÷ = 0.5 e
In the case where êMPEú = 0.5 e is used for all the loads (e.g., n = Max/e = 50 000 and e ³ 1 mg for class I balances or n = 5 000 and e ³ 0.1 g for class II balances), Table 1 or 2 is exceptionally applied so that the weights are chosen according to Max/f where f = 0.5 e.
4.5.4 Restriction concerning balances of class II
The sections of Tables 1 and 2 which are intended for class II balances (originally intended for group 2) balances) can be used for the selection of weights only if for the balances e ³ 10 mg. So if 1 mg £ e £ 5 mg (R 76-1, 3.2) for class II balances with êMPEú ’s of 0.5 e & e, or only 0.5 e, the weights cannot be obtained correctly from the tables in all cases.
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Table 1 Max/f and accuracy classes E2 to M3 of weights or procedure to be applied
ïMTEï of the instrument/balance takes on the values: 1) 0.5 f, f & 1.5 f or 2) 0.5 f & f or 3) only f (the values are chosen following the instructions in Table A in 4.3)
The fraction is 1/3 (the error of the weights shall not be greater than 1/3 ´ ïMTE÷ for the applied load)
Instruments/balances Max/f
Weights Accuracy class or procedure
Group 1): Special balances (Max/f unlimited, special accuracy); f ³ 1 mg, e.g. f = 1 mg, 2 mg, 5 mg, 10 mg, 20 mg, etc.
Apply 3.2.2 E2 (Apply 3.2.2 if ïMTEï = 0.5 f & f or f) E2 Exception: Apply 3.2.2 if 1), 170 000 < Max/f < 200 000 and f = 1 mg (E2 if f >1 mg) F1 (E2 if 1). E2 if ïMTEï = 0.5 f & f and L/(0.5 f ) > Max/f 2); L is the greatest “small” load (4.3) for which ïMTEï = 0.5 f)
Group 2): Laboratory or precision balances (Max/f £ 100 000, high accuracy); f ³ 10 mg, e.g., f = 10 mg, 20 mg, 50 mg or ³ 0.1 g.
F1 (E2 if ïMTEï = 0.5 f & f or f) F1 F2 (F1 if ïMTEï = 0.5 f & f or f) F2 Exception: F1 if 1) , 17 000 < Max/f < 20 000 and f = 10 mg (F2 if f > 10 mg) M1 (F2 if 1). F2 if ïMTEï = 0.5 f & f and L/(0.5 f ) > Max/f 3); L is the greatest “small” load (4.3) for which ïMTEï= 0.5 f)
Group 3): Instruments for industrial weighing
(Max/f £ 10 000, medium accuracy);
f ³ 1 g, e.g., f = 2 g or 20 kg.
6 600 < Max/f £ 10 000
M1 (F2 if ïMTEï = 0.5 f & f or f)
3 300 < Max/f £ 6 600
M1
2 200 < Max/f £ 3 300
M2 (M1 if ïMTEï = 0.5 f & f or f)
Max/f £ 2 200
M2
Group 4): Instruments for industrial weighing (Max/f £ 1 000, low accuracy); f ³ 5 g, e.g., f = 50 g or 50 kg. 660 < Max/f £ 1 000 Max/f £ 660
M3 (M2 if ïMTEï = 0.5 f & f or f) M3
1) Weights of £ 50 g are used (4.4).
2) F1 if L/(0.5 f) £ Max/f, or if ïMTEï = f for all the loads. Weights of > 50 g are used/dominating (4.4).
3) M1 if L/(0.5 f) £ Max/f, or if ïMTEï = f for all the loads. Weights of > 50 g are used/dominating (4.4).
In the column “Weights” the accuracy classes of the weights (3.1.1 or 3.2.1) and the procedure “Apply 3.2.2” (4.1.3) are given for the instruments/balances to be calibrated.
If there is only one accuracy class corresponding to a Max/f, it can be used irrespective of the values of êMTEú given in 1), 2) or 3) above. Frequently, another accuracy class along with conditions for its use is given in parentheses. This class must be applied if the conditions are met, e.g., if êMTEú = 0.5 f & f or f . Otherwise if êMTEú = 0.5 f, f & 1.5 f, the class given first is used.
This scheme is analogously applied to the case where the procedure “Apply 3.2.2” is used. For example, if only “Apply 3.2.2 ” is given, it is applied irrespective of the values of êMTEú .
Advice under the heading “Exception” is for certain special cases.
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Table 2 Max/f and accuracy classes E2 to M3 of weights or procedure to be applied
ïMTEï of the instrument/balance takes on the values: 1) 0.5 f, f & 1.5 f or 2) 0.5 f & f or 3) only f (the values are chosen following the instructions in Table A in 4.3)
The fraction is 1/6 (the error of the weights shall not be greater than 1/6 ´ ïMTE÷ for the applied load)
Instruments/balances Max/f
Weights Accuracy class or procedure
Group 1): Special balances (Max/f unlimited, special accuracy);
f ³ 1mg, e.g. f = 1 mg, 2 mg, 5 mg, 10 mg, 20 mg, etc.
Max/f > 110 000
No calibration
60 000 < Max/f £ 110 000
E2 (No calibration if 1) )
Max/f £ 60 000
E2 (No calibration if 1) and ïMTEï = 0.5 f & f 2) )
Group 2): Laboratory or precision balances
(Max/f £ 100 000, high accuracy);
f ³ 10 mg, e.g., f = 10 mg, 20 mg, 50 mg or ³ 0.1 g.
50 000 < Max/f £ 100 000
E2
30 000 < Max/f £ 50 000
F1 (E2 if ïMTEï= 0.5 f & f or f)
11 000 < Max/f £ 30 000
F1
6 000 < Max/f £ 11 000
F2 (F1 if 1))
Max/f £ 6 000
F2 (F1 if 1) and ïMTEï = 0.5 f & f 3) )
Group 3): Instruments for industrial weighing
(Max/f £ 10 000, medium accuracy);
f ³ 1 g, e.g., f = 2 g or 20 kg.
5 000 < Max/f £ 10 000
F2
3 300 < Max/f £ 5 000
M1 (F2 if ïMTEï = 0.5 f & f or f)
1 100 < Max/f £ 3 300
M1
Max/f £ 1 100
M2
Group 4): Instruments for industrial weighing
(Max/f £ 1 000, low accuracy);
f ³ 5 g, e.g., f =50 g or 50 kg.
500 < Max/f £ 1 000
M2
330 < Max/f £ 500
M3 (M2 if ïMTEï = 0.5 f & f or f)
Max/f £ 330
M3
1) weights of £ 50 g are used (4.4).
2) E2 if 1) and ïMTEï= f for all the loads or if weights of > 50 g are used/dominating (4.4).
3) F2 if 1) and ïMTEï= f for all the loads or if weights of > 50 g are used/dominating (4.4).
In the column “Weights” the accuracy classes of the weights (3.1.1 or 3.2.1) and the procedure “No calibration” (4.1.3) are given for the instruments/ balances to be calibrated.
If there is only one accuracy class corresponding to a Max/f, it can be used irrespective of the values of êMTEú given in 1), 2) or 3) above. Sometimes, another accuracy class along with conditions for its use is given in parentheses. This class must be applied if the conditions are met, e.g., if 1) (if weights of £ 50 g are used). Otherwise if the weights are > 50 g, the class given first is used.
This scheme is analogously applied to the case where the procedure “No calibration” is used. For example, consider “E2 (No calibration if 1) )”. If the weights are £ 50 g, calibration is not performed with the weights dealt with here. Otherwise, if the weights are > 50 g, calibration is performed with weights of class E2.
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Table 3 Max/f and accuracy classes E2 to M3 of weights or procedure to be applied
ïMTEï of the instrument/balance takes on the values: 1) f, 2 f & 3 f or 2) f & 2 f (the values are chosen following the instructions in Table B in 4.3). If ïMTEï = f for all the loads, apply Table 1
The fraction is 1/3 (the error of the weights shall not be greater than 1/3 ´ ïMTE÷ for the applied load)
Instruments/balances Max/f
Weights Accuracy class or procedure
Group 1): Special balances
(Max/f unlimited, special accuracy);
f ³ 1 mg, e.g., f = 1 mg, 2 mg, 5 mg, 10 mg, 20 mg etc.
400 000 < Max/f £ 650 000
E2 (Apply 3.2.2 if ïMTEï= f & 2 f)
130 000 < Max/f £ 400 000
E2
65 000 < Max/f £ 130 000
F1 (E2 if 1)) Exception: F1 if 1) , Max/f = 70 000 or 105 000
and L = 50 000 f 2)
Max/f £ 65 000
F1
Group 2): Laboratory or precision balances
(Max/f £ 100 000, high accuracy);
f ³ 10 mg, e.g., f = 10 mg, 20 mg, 50 mg or f ³ 0.1 g.
65 000 < Max/f £ 100 000
F1
40 000 < Max/f £ 65 000
F2 (F1 if ïMTEï= f & 2 f)
13 000 < Max/f £ 40 000
F2
6 500 < Max/f £ 13 000
M1 (F2 if 1)) Exception: M1 if 1) , Max/f = 7 000 or 10 500
and L = 5 000 f 3)
Max/f £ 6 500
M1
Group 3): Instruments for industrial weighing
(Max/f £ 10 000, medium accuracy);
f ³ 1 g, e.g., f = 2 g or 20 kg.
6 600 < Max/f £ 10 000
M1
4 400 < Max/f £ 6 600
M2 (M1 if ïMTEï= f & 2 f)
1 300 < Max/f £ 4 400
M2
Max/f £ 1 300
M3
Group 4): Instruments for industrial weighing (Max/f £ 1 000, low accuracy); f ³ 5 g, e.g., f = 50 g or 50 kg. Max/f £ 1 000
M3
1) weights of £ 50g are used (4.4).
2) L is the greatest “small” load for which ïMTE÷ = f (see Definition 1 in 4.3).
3) L is the greatest “small” load for which ïMTE÷ = f (see Definition 1 in 4.3).
In the column “Weights” the accuracy classes of the weights (3.1.1 or 3.2.1) and the procedure “Apply 3.2.2” (4.1.3) are given for the instruments/balances to be calibrated.
If there is only one accuracy class corresponding to a Max/f, it can be used irrespective of the values of êMTEú given in 1) or 2) above. Sometimes, another accuracy class along with conditions for its use is given in parentheses. This class must be applied if the conditions are met, e.g., if êMTEú = f & 2 f. Otherwise if êMTEú = f, 2 f & 3 f, the class given first is used.
This scheme is analogously applied to the case where the procedure “Apply 3.2.2” is used. For example, consider “E2 (Apply 3.2.2 if êMTEú = f &2 f )”. If êMTEú = f & 2 f , calibrated weights of class E2 are used applying 3.2.2. Otherwise, if êMTEú = f, 2 f & 3 f, weights (3.1.1 or 3.2.1) of class E2 are used.
Advice under the heading “Exception” is for certain special cases.
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Table 4 Max/f and accuracy classes E2 to M3 of weights or procedure to be applied
ïMTEïof the instrument/balance takes on the values: 1) f, 2 f & 3 f or 2) f & 2 f (the values are chosen following the instructions in Table B in 4.3). If ïMTEï = f for all the loads, apply Table 2
The fraction is 1/6 (the error of the weights shall not be greater than 1/6 ´ ïMTE÷ for the applied load)
Instruments/balances Max/f
Weights Accuracy class or procedure
Group 1): Special balances
(Max/f unlimited, special accuracy);
f ³ 1mg, e.g. f = 1 mg, 2 mg, 5 mg, 10 mg, 20 mg etc.
Max/f > 300 000
No calibration
200 000 < Max/f £ 300 000
E2 (No calibration if ôMTEô= f & 2 f )
65 000 < Max/f £ 200 000
E2 Exception: No calibration if 1), 170 000 < Max/f < 200 000 and
f = 1 mg (E2 if f > 1 mg)
Max/f £ 65 000
F1 (E2 if 1). E2 if L/f > 30 000 2); L is the greatest “small” load
(4.3) for whichôMTEô = f)
Group 2): Laboratory or precision balances (Max/f £ 100 000, high accuracy); f ³ 10 mg, e.g., f = 10 mg, 20 mg, 50 mg or ³ 0.1 g.
F1 (E2 ifôMTEô= f & 2 f) F1 F2 (F1 if ôMTEô= f & 2 f) F2 Exception: F1 if 1), 17 000< Max/f < 20 000 and f = 10 mg (F2 if f > 10 mg) M1 (F2 if 1). F2 if L/f > 3 000 3); L is the greatest “small” load (4.3) for whichôMTEô = f)
Group 3): Instruments for industrial weighing
(Max/f £ 10 000, medium accuracy);
f ³ 1 g, e.g., f = 2 g or 20 kg.
6 600 < Max/f £ 10 000
M1 (F2 if ôMTEô= f & 2 f)
3 300 < Max/f £ 6 600
M1
2 200 < Max/f £ 3 300
M2 (M1 if ôMTEô= f & 2 f)
Max/f £ 2 200
M2
Group 4): Instruments for industrial weighing (Max/f £ 1 000, low accuracy); f ³ 5 g, e.g., f = 50 g or 50 kg. 660 < Max/f £ 1 000 Max/f £ 660
M3 (M2 if ôMTEô= f & 2 f) M3
1) weights of £ 50g are used (4.4).
2) F1 if L/f £ 30 000. Weights of > 50 g are used/dominating (4.4).
3) M1 if L/f £ 3 000. Weights of > 50 g are used/dominating (4.4).
In the column “Weights” the accuracy classes of the weights (3.1.1 or 3.2.1) and the procedure “No calibration” (4.1.3) are given for the instruments/ balances to be calibrated.
If there is only one accuracy class corresponding to a Max/f, it can be used irrespective of the values of êMTEú given in 1) or 2) above. Frequently, another accuracy class along with conditions for its use is given in parentheses. This class must be applied if the conditions are met, e.g., if L/f > 3 000 (a balance in group 2) with Max/f £ 6 500). Otherwise if L/f £ 3 000, the class M1 given first is used.
This scheme is analogously applied to the case where the procedure “No calibration” is used. For example consider “E2 (No calibration if
êMTEú = f & 2 f)”. If êMTEú = f & 2 f, calibration is not performed with the weights dealt with here. Otherwise, if êMTEú = f, 2 f & 3 f, calibration is performed with weights of class E2.
Advice under the heading “Exception” is for certain special cases.
OIML BULLETIN V OLUME XLIV • N UMBER 4 • O CTOBER 2003 15
technique
References
OIML Recommendation R 111, Weights of classes E1, E2, F1, F2, M1, M2, M3 (1994)
OIML Recommendation R 47 Standard weights for testing of high capacity weighing machines (1979-1978)
T. Lammi: Calibration of Weighing Instruments and Uncertainty of Calibration. OIML Bulletin Volume XLII,
Number 4, October 2001
OIML Recommendation R 76-1, Nonautomatic weighing instruments. Part 1: Metrological and technical requirements – Tests (1992)
16 OIML BULLETIN V OLUME XLIV • N UMBER 4 • O CTOBER 2003
Calibration weights or Calibrated test weights or scale calibration weights are used in scale calibration. This is a process that ensures scale accuracy. Test weights for scales or precision weights are used to calibrate weight scale systems of various levels of accuracy depending on the use and requirements. Certified test weights or precision weights should be used in these processes to calibrate weight scale systems. It is important to ensure the test weights are calibrated test weights and that they are accurate to provide accurate calibration results. Check out our range of scale calibration weights.
Generally, weights for scale calibration are certified. When calibrating scale procedures are performed, it is necessary to have calibration weights for scales. Weighing scale calibration with scale weights or test weights should be performed on a regular basis depending on use. Learn more about scale calibration weights and weigh scale calibration.
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KLAUS-DIETERSOMMER, Landesamt für Mess- und Eichwesen Thüringen (LMET), GermanySAMUELE. CHAPPELL, Consultant, Formerly of theNational Institute of Standards and Technology(NIST), USAMANFREDKOCHSIEK, Physikalisch-TechnischeBundesanstalt (PTB), Germany
Abstract
The most important actions required to ensure the correct indication of measuring instruments are: Kin industrial metrology, regular calibration of the measuring instruments according to the implemented quality systems; and Kin legal metrology, periodic verification or conformity testing of the instruments according to legal regulations. Both actions are strongly inter-related and are pre-dominantly based on the same measuring procedures. Historically, however, these actions have been established with separate rules, metrological infrastructures and activities. This paper, therefore, addresses the differences, common bases and the relationship between calibration and verification. In particular, the relationships between legally prescribed error limits and uncertainty and the uncertainty contribution of verified measuring instruments are discussed.
Introduction
The correctness of measurements and measuring instruments is one of the most important prerequisites for the assurance of the quality and quantity of products and services, and the accuracy of the instruments must be consistent with their intended use.
In compliance with the ISO 9000 standard series and the ISO/IEC 17025 standard, traceability of measuring and test equipment to the realization of SI units must be guaranteed by an unbroken chain of comparison measurements to allow the necessary statements about their metrological quality. The most important actions to ensure the correct indication of measuring instruments are: Kin industrial metrology: regular calibration of the measuring instruments according to the implemented quality systems; and Kin legal metrology: periodic verification or conformity testing of the measuring instruments according to legal regulations. Both actions are closely related and are mostly based on the same measuring procedures. Historically, however, these actions have been established with separate rules and metrological infra-structures and activities. Verification has become a principal part of legal metrology systems and calibration is widely used in quality assurance and industrial metrology – accreditation bodies prefer calibration as a primary action to provide proof of the correctness of the indication of measuring instruments. As a result, today it must be acknowledged that there is a lack of reciprocal understanding of the identical metrological nature of these activities between the different communities of users. In particular, their specific concerns are insufficiently understood, and there is widespread incomprehension concerning the relation-ship of error limits and uncertainty of measurement. For instance, the use of legally verified instruments within the framework of quality management some times presents problems since only the MPEs for the instruments are provided, without the measurement uncertainties being explicitly given.
1 Calibration
Usually, calibration is carried out in order to provide a quantitative statement about the correctness of the measurement results of a measuring instrument. For economic reasons, laboratories strive for broad recognition of their calibration and measurement results. Confidence in results, therefore, is achieved through both establishing the traceability and providing the un-certainty of the measurement results. According to the VIM [1], calibration may be defined as a “set of operations that establish, under specified conditions, the relationship between values of quantities indicated by a measuring instrument or measuring system, or values represented by a material measure or a reference material, and the corresponding values realized by standards”. This means that the calibration shows how the nominal value of a material or the indication of an instrument relates to the conventional true values of the measurand. The conventional true value is realized by a traceable reference standard [1]. According to this definition, calibration does not necessarily contain any actions of adjustment or maintenance of the instrument to be calibrated. Figures 1 and 2 show examples of calibration by means of the comparison method, i.e. by comparison of the indication of the instrument under test, and the corresponding indication of appropriate standards respectively. Calibration certificates for measuring instruments give the measurement deviation, or correction, and the uncertainty of measurement. Only this combination characterizes the quality of the relation of the measurement result to the appropriate (SI) unit. Figure 3illustrates the meaning of a (single) calibration result as it is typically presented. The uncertainty of measurement is a parameter, associated with the result of measurement, that characterizes the (possible) dispersion of the values that could reasonably be attributed to the measurand [1]. In other words, uncertainty is a measure of the in completeness of knowledge about the measurand. It is determined according to unified rules [2, 3] and is usually stated for a coverage probability of 95 %. Its value, together with the determined measurement error, is valid at the moment of calibration and under the relevant calibration conditions. If a recently calibrated measuring instrument is used under the same conditions as during the calibration, the measurand Y may be reduced to the following parts: Y = XS+ δX(1)where XS represents the corrected indication of the calibrated instrument. δX may be the combination of all other (unknown) measurement deviations due to imperfections in the measuring procedure. Thus, it follows that the associated standard uncertainty of the measurement carried out by means of a calibrated instrument is:u2(y) = u2(xs) + u2(δx)(2)
This means that the calibration uncertainty u(xs) of a newly calibrated instrument enters directly into the total uncertainty of the measurement u(y) as an (inde-pendent) contribution. When the calibrated instrument is used in a different environment, the measurement uncertainty determined by the calibration laboratory will often be exceeded if the instrument is susceptible to environmental influences. A problem can also arise if the instrument’s performance is degraded after prolonged use. Furthermore, the stated uncertainty of measurement can be considered as being related to national standards only for certificates issued by laboratories that have demonstrated their competence beyond reasonable doubt. Such laboratories are normally well recognized by their customers. In other cases, for example, when working standard calibration certificates are used, reference to the national standards cannot be taken for granted and the user must be satisfied as to the proper traceability – or take other actions. Sometimes, calibration certificates give a conformity statement, i.e. a statement of compliance with given specifications or requirements. In these cases, according to the EA document EA-3/02 [4], the obtained measurement result, extended by the associated uncertainty, must not exceed the specified tolerance or limit. Figure 4 illustrates this approach.
2 Verification and error limits in legal metrology
2.1 Verification
Verification of the conformity of measuring instruments is a method of testing covered by legal regulations. It is a part of a process of legal metrological control that in many economies requires type evaluation and approval
of some models of instruments subject to legal regulations as a first step. Figure 5 shows the typical test sequence over the lifetime of a measuring instrument subject to legal regulations. Type evaluation is usually more stringent than verification. It includes testing the instrument’s performance when subjected to environmental influence factors in order to determine whether the specified error limits for the instrument at rated or foreseeable in situoperating conditions are met [5].The basic elements of verification are [5]:K qualitative tests, e.g. for the state of the instrument(which is essentially an inspection); and K quantitative metrological tests. The aim of the quantitative metrological tests is to determine the errors with the associated uncertainty of measurement (cf. 1) at prescribed testing values. These tests are carried out according to well-established and harmonized testing procedures [5].
Following the definition of calibration, as given in 1,the quantitative metrological tests may be considered a calibration. This means that an instrument’s assurance of metrological conformity involves both verification and calibration, and the measuring equipment necessary to determine conformity during verification might be the same as that used for calibration, e.g. as shown in Figs. 1 and 2.The results of the verification tests are then evaluated to ensure that the legal requirements are being met(see 2.2). Provided that this assessment of conformity leads to the instrument being accepted, a verification mark should be fixed to it and a verification certificate may be issued. Figure 6 illustrates these elements of verification. According to the above definitions and explanations, Table 1 compares the primary goals and the actions of calibration and verification.
2.2 Maximum permissible errors on verification and in service
In many economies with developed legal metrology systems, two kinds of error limits have been defined: K the maximum permissible errors (MPEs) on verification; and K the maximum permissible errors (MPEs) in service. The latter is normally twice the first. MPEs on verification equal “MPEs on testing” that are valid at the time of verification. For the measuring instrument user, the MPEs in service are the error limits that are legally relevant. This approach is explained and illustrated in detail in 4.3 of [5].
The values of the error limits are related to the intended use of the respective kind of instrument and determined by the state of the art of measurement technology.
3 Relationship between legally prescribed error limits and uncertainty
3.1 General
If a measuring instrument is tested for conformity with a given specification or with a requirement with regard to the error limits, this test consists of comparisons of measurements with those resulting from use of a physical standard or calibrated standard instrument. The uncertainty of measurement inherent in the measurement process then inevitably leads to an uncertainty of decision of conformity. Figure 7 (taken from the standard ISO 14253-1) [6] makes this problem quite clear: between the conformance zones and the upper and lower non-conformance zones there is in each case an uncertainty zone whose width corresponds approximately to twice the expanded uncertainty of measurement at the 95 % probability level. The uncertainty comprises contributions of the standard(s) used and the instrument under test as well as contributions that are related to the measuring procedure and to the in-complete knowledge about the existing environmental conditions (cf. 3).Because of the uncertainty of measurement, measurement results affected by measurement deviations lying within the range of the uncertainty zones cannot definitely be regarded as being, or not being, in conformity with the given tolerance requirement.
3.2 Relationship upon verification
In practice, measuring instruments are considered to comply with the legal requirements for error limits if: K the absolute value of the measurement deviations is smaller than or equal to the absolute value of the legally prescribed MPEs on verification when the testis performed under prescribed test conditions; and K the expanded uncertainty of measurement of the previous quantitative metrological test (cf. 2.1), for a coverage probability of 95 %, is small compared with the legally prescribed error limits. The expanded measurement uncertainty at the 95 %probability level, U0.95, is usually considered to be small enough if the following relationship is fulfilled:U0.95≤1–3⋅MPEV(3)
where MPEV is the absolute value of the MPE on verification. Umaxis, therefore, the maximum acceptable value of the expanded measurement uncertainty of the quantitative test. The criteria for the assessment of compliance are illustrated in Fig. 8 (cf.[5]): cases a, b, c and d comply with the requirements of the verification regulations, whereas cases e and f will be rejected. Values in all cases, including their uncertainty of measurement, lie within the tolerances fixed by the MPEs in service. Consequently, the MPE on verification of a newly verified measuring instrument will in the worst case be exceeded by 33 %. However, as the legally prescribed MPEs in service are valid for the instrument users, there is, therefore, negligible risk in the sense that no measured value under verification – even if the measurement uncertainty is taken into account – will be outside this tolerance band. So far, the MPEs on verification may be seen as supporting the conclusion that an instrument would be in conformity with required MPEs in service (MPES) taking into consideration the above-mentioned influences. The advantages of this verification system are that it is practical in terms of legal enforcement, and – due to the widened tolerance band in service [MPES–; MPES+]- it is potentially tolerant of external influences and of drifts in indication over the legally fixed validity periods. Verification validity only expires early in cases of un-authorized manipulations and damage that could reduce the accuracy of the instrument.
3.3 Relationship upon testing of working standards In legal metrology, working standards are the standards that are used routinely to verify measuring instruments. In several economies, some of the working standards used in legal metrology must be tested or verified according to special regulations. The MPEs of such working standards depend on their intended use. In general, they should be significantly lower than the expanded uncertainties that are required by equation(3).
Usually, a working standard, e.g. mass (weight) [7], is considered to comply with the respective requirements for legal error limits if the difference between its indication, or measured value, and the corresponding value realized by a reference standard is equal to or less than the difference between the prescribed error limits, MPEws, and the expanded uncertainty of measurement,U0.95:|Iws– xs| ≤MPEws– U0.95(4) where: Iws= the indication of the working standard under test;andxs= the value provided by a reference standard. In practice, this means that with respect to measurement deviations, a tolerance band is defined that is significantly reduced when compared with the range between the legally prescribed error limits[MPEws–;MPEws+] (see Fig. 4). The magnitude of this tolerance band may be described by the interval [MPEws–+ U; MPEws+– U].This approach is consistent with the prescribed procedures for statements of conformity on calibration certificates (cf. 1 and [4]).
4 Uncertainty contribution of verified instruments
In practice, it is often necessary or desirable to deter-mine the uncertainty of measurements that are carried out by means of legally verified measuring instruments. If only the positive statement of conformity with the legal requirements is known, for example in the case of verified instruments without a certificate, the uncertainty of measurements for such instruments can be derived only from the information available about the prescribed error limits (on verification and in service)and about the related uncertainty budgets according to the requirements established in 2.2 and 3.2.On the assumption that no further information is available, according to the principle of maximum entropy, the following treatment is justified: K The range of values between the MPEs on verification can be assumed to be equally probable. K Due to uncertainty in measurement, the probability that indications of verified instruments are actually beyond the acceptance limits of the respective verification declines in proportion to the increase in distance from these limits. A trapezoidal probability
distribution according to Fig. 9 can, therefore, reflect adequately the probable dispersion of the deviation of verified measuring instruments. K Immediately after verification, the indications of measuring instruments may exceed the MPEs on verification by the maximum value of the expanded uncertainty of measurements at most. K After prolonged use and under varying environmental conditions, it can be assumed that the expanded measurement uncertainty, compared with its initial value, may have increased significantly. In particular, the following evaluation of the un-certainty contribution of verified instruments seems to be appropriate: a) Immediately after verification, the trapezoidal probability distribution of the errors according to plot (a)of Fig. 9 can be taken as a basis for the determination of the uncertainty contribution of the instruments. The following may, therefore, be assumed for this standard uncertainty contribution uINSTR[2]:uINSTR= a⋅ (1 + β2) / 6 ≈0.7 ⋅MPEV(5)where a= 1.33–⋅MPEVand β= 3 / 4.MPEVis the absolute value of the MPEs on verification.
b) After prolonged use and under varying environ-mental conditions, it can be assumed that, in the worst case, the measurement error extended by the measurement uncertainty will reach the values of the MPEs in service. The resulting trapezoidal distribution could more or less be represented by plot (b)of Fig. 9. In this case, the following may be assumed for the standard uncertainty contribution [2]:uINSTR= a⋅ (1 + β2) / 6 ≈0.9 ⋅MPEV(6) where: a= 2 ⋅MPEVandβ= 1 / 2
5 System comparison
Table 2 shows a comparison between verification and calibration, which is partially based on Volkmann [8].In conclusion, verification offers assurance of correct measurements by a measuring instrument according to its intended use especially for those instruments that
require type evaluation and approval. It is based on technical procedures equivalent to those used in calibration and provides confidence in the correctness of indications of verified instruments although no expert knowledge by the instrument’s user is required. Verification, therefore, may be considered a strong tool in both legal metrology and quality assurance when large numbers of measuring instruments are involved. In particular, it excels as a simple means by which enforcement can be realized, and because the user is only affected by the MPEs in service, it provides a high degree of confidence over a long time period.
One disadvantage in verification is that the influence of uncertainty on a decision of conformity of a measuring instrument to specific requirements is not completely clear. In comparison, traditional calibration is considered an important basic procedure for legal metrology activities and also for fundamental measurement applications in scientific and industrial metrology. It is practically not limited as far as the measurement task is concerned, but does require sound expert knowledge on the part of the instrument’s user in carrying out and evaluating measurements.
References
[1]International Vocabulary of Basic and General Termsin Metrology: BIPM, IEC, IFCC, ISO, IUPAC, IUPAP,OIML, 1993[2]Guide to the Expression of Uncertainty in Measure-ment(Corrected and reprinted 1995): BIPM, IEC,IFCC, ISO, IUPAC, IUPAP, OIML, page 101[3] EA-4/02, Expression of the Uncertainty of Measure-ment in Calibration, Ed. 1: European Cooperationfor Accreditation (EA), April 1997 (previously EAL-R2)[4] EA-3/02, The Expression of Uncertainty in Quanti-tative Testing, Ed. 1: European Cooperation forAccreditation (EA), August 1996 (previously EAL-G23)[5] Schulz, W.; Sommer, K.-D.: Uncertainty of Measure-ment and Error Limits in Legal Metrology: OIMLBulletin, October 1999, pp. 5–15[6] Geometrical Product Specification (GPS) –Inspection by measurement of workpieces andmeasuring equipment, Part 1: Decision rules forproving conformance or nonconformance withspecification, ISO 14253–1: 1998, InternationalOrganization for Standardization (ISO), Geneva,1998[7] OIML R 111 (1994): Weights of classes E1, E2, F1, F2,M1, M2, M3[8] Volkmann, Chr.: Messgeräte in der Qualitäts-sicherung geeicht oder kalibriert. AWA-PTB-Gespräch 1997, Braunschweig 1997[9] Klaus Weise, Wolfgang Wöger: Messunsicherheitund Messdatenauswertung. Verlag Weinheim, NewYork, Chichester, Singapore, Toronto: Wiley-VCH,1999
