Anyone who wants to check a scale soon faces the question of which weight to use. A weight that is too coarse makes the check pointless, because you mainly measure the error of the weight itself. An unnecessarily fine weight is expensive and demands more care in use. The right choice follows from a simple calculation rule.
In this article we explain that rule, give examples for common scales and, at the bottom, you will find a selection tool that does the calculation for you. If you would like to have the scale calibrated with weights from our laboratory, read about scale calibration.
The calculation rule: one third of the error of the scale
OIML R76, the recommendation for non-automatic weighing instruments, puts it clearly: the error of the weights used may not exceed one third of the error the scale is allowed to make at that load. This keeps the influence of the weight small compared with what you want to assess.
For this you need two pieces of information:
- The permissible error of the scale at the load you want to check. This depends on the accuracy class of the scale (I, II, III or IIII) and on the verification scale interval e.
- The maximum permissible error of the weight according to OIML R111, which you can find per class and nominal value in the tolerance table of the weight classes.
The permissible error of a scale
R76 expresses the permissible error in verification scale intervals. At a low load a scale may deviate by half a scale interval, at a higher load by a whole one and at the highest loads by one and a half. Where those limits lie depends on the class of the scale:
| Scale class | ±0.5 e | ±1 e | ±1.5 e |
|---|---|---|---|
| I (special) | up to 50,000 e | up to 200,000 e | above that |
| II (high) | up to 5,000 e | up to 20,000 e | above that |
| III (medium) | up to 500 e | up to 2,000 e | above that |
| IIII (ordinary) | up to 50 e | up to 200 e | above that |
These values apply at the initial verification of a scale. They are also a good starting point for non-verified scales. If your scale has no verification scale interval on the type plate, use the readability d.
Worked example: a floor scale
Take a class III floor scale with a verification scale interval e = 500 g, which you want to check with 500 kg. That load is 1,000 scale intervals. It falls between 500 and 2,000 e, so the scale may deviate by 1 e: 500 g. The weight may then deviate by at most one third of that, about 167 g.
According to R111, a 500 kg weight may deviate by 80 g in class M2 and by 250 g in M3. So M2 complies, M3 does not. For this check you choose M2 or finer. Many companies nevertheless opt for M1, so that the same weights can also be used for a more accurate scale.
Rules of thumb per type of scale
The selection tool below calculates it precisely. As a first orientation, these rules of thumb apply:
- Analytical balance with a readability of 0.1 mg or finer: E2, for the highest requirements E1.
- Precision balance in class II: F1, with a coarse readability F2.
- Bench, retail and floor scales in class III: M1 is the usual choice.
- Weighbridge: M1 or M2, depending on the verification scale interval.
- Coarse scale in class IIII: M2 or M3.
If you check several loads, recalculate each measuring point separately. At low loads the scale is allowed to deviate less, so there the requirements on the weight are relatively stricter.
Class is not everything: certificate and handling
A weight in the right class is only usable if you can demonstrate that it still falls within that class. For this you need a valid calibration certificate, traceable to the national standard. Have your weights recalibrated periodically; how often is explained in how often to recalibrate weights. And handle them with care, because fingerprints and damage count immediately in the fine classes. Tips can be found in storing and handling calibration weights.
If you carry out a calibration according to EURAMET cg-18, not only the tolerance of the weight counts, but also the measurement uncertainty of the weight from the certificate. Working with the measured value and a small uncertainty can then be more favourable than simply choosing a finer class.
Advice from our laboratory
MASSLAB calibrates scales and weights every day. We are happy to help you choose the right class and supply weights in classes F1 to M3, if desired with a calibration certificate from our own laboratory. View the range in our webshop or use the selection tool below.