After a calibration you receive a certificate. Often it disappears straight into a folder until the auditor asks for it. That is a pity, because the certificate tells you more than the fact that a calibration took place. It shows how much your scale or weight deviates and how certain that statement is.
In this article you will read which elements make up a calibration certificate, how to read the results and the measurement uncertainty, and how to assess them against your own requirements. The examples concern scales and weights, as we calibrate them under scale calibration and weight calibration.
What must a calibration certificate contain as a minimum?
A complete certificate contains at least the elements below. For each element you can see what to look for:
| Element | What to look for |
|---|---|
| Identification of the instrument | Make, type, serial number or your own asset number match your measuring equipment register. |
| Place and date | Where and when the measurement was made: on site or in the laboratory. |
| References used and traceability | Which weights or references were used and how they are traceable to national standards. |
| Environmental conditions | Temperature and, where relevant, humidity and air pressure during the measurement. |
| Measurement results per point | Load or nominal value, indication or measured value, and the error. |
| Measurement uncertainty | Per point or as a formula, stating the coverage factor. |
| Performed by | Who carried out the measurement. |
A document without measurement uncertainty is not a calibration certificate, however extensive it may otherwise be. At best it is an inspection report. That difference is exactly what an auditor spots quickly.
Reading the results for a scale
A scale calibration according to EURAMET cg-18 consists of three tests, which you will find on the certificate:
- Repeatability: the same load is weighed several times. The spread, usually given as a standard deviation, shows how stable the scale is.
- Eccentricity: a load is placed in the centre and at the corners of the platform. The differences show whether the position of the load matters.
- Indication error: increasing loads across the range. For each point you see the load, the indication and the error.
For the indication error, look not only at the largest figure but also at the pattern. An error that increases with the load often points to an adjustment issue. Large differences in eccentricity are more likely to have a mechanical cause, such as dirt under the platform. Read more about the tests in EURAMET cg-18 explained.
If the certificate shows results both before and after adjustment, both are relevant. The results before adjustment say something about the period since the previous calibration. The results afterwards are the starting point for the next period.
Reading the results for weights
For weights, you see for each piece the nominal value, the conventional mass or its deviation from nominal, and the expanded uncertainty. The OIML R111 class is often stated too, along with whether the weight falls within the maximum permissible error (mpe) of that class.
An example: according to OIML R111, a 1 kg weight in class M1 may deviate by no more than ±50 mg. The certificate shows an error of +18 mg with a measurement uncertainty of 5 mg. Even in the worst case, at 23 mg the weight remains well within the mpe. For class F1, however, the mpe at 1 kg is ±5 mg, so as an F1 weight the same piece would not conform.
The conventional mass is not a true mass but an agreed value under reference conditions: a density of 8,000 kg/m³, an air density of 1.2 kg/m³ and 20 °C. Read more about uncertainty for weights in measurement uncertainty on the calibration certificate of weights.
Measurement uncertainty and assessment against your tolerances
The measurement uncertainty indicates the margin within which the true error probably lies. Usually an expanded uncertainty U with coverage factor k = 2 is stated. That corresponds to a confidence level of approximately 95%.
A certificate does not in itself tell you whether your instrument is good enough. You assess that yourself, against the tolerance you have set for your process. A simple and cautious approach:
- Determine the permitted error for each measuring instrument, derived from your process or product.
- At each measuring point, add the absolute error and the measurement uncertainty together.
- If that sum is within your tolerance, the instrument conforms at that point.
- If the error itself is already outside the tolerance, it does not conform.
- If the error is within the tolerance but the sum is outside it, you are in a grey area. Define in your procedure beforehand how you deal with this.
An example: you require that a floor scale deviates by no more than 0.2 kg at 300 kg. The certificate shows +0.10 kg with U = 0.06 kg. The sum is 0.16 kg, so the scale conforms. Had the error been +0.17 kg, the sum would have been 0.23 kg and you would have been in the grey area.
Record the assessment on or with the certificate, with date, name and conclusion. In practice, it is precisely this step that is most often missing.
Validity, audit and retention
How long is a certificate valid?
A calibration certificate has no statutory period of validity. It describes the condition on the day of the measurement. How long you rely on it is something you define yourself in a calibration interval. Annually is a common starting point, which you adjust on the basis of the results. See how often should a scale be calibrated. If the certificate states a date for the next calibration, that is a recommendation or an agreement, not a statutory deadline.
What does an auditor check?
- whether the certificate belongs to the instrument in your register;
- whether the calibration was performed within the specified interval;
- whether traceability to national standards has been demonstrated;
- whether measurement results, measurement uncertainty and environmental conditions are stated;
- whether the result has been assessed and whether action was taken in the event of rejection.
Retention
Keep certificates for as long as the results may be relevant to your products, and keep the history of several calibrations per instrument. With that history you can later justify a longer or shorter interval. A fixed, searchable place per instrument saves searching during the audit.
How MASSLAB helps you
MASSLAB calibrates weights in classes F1, F2, M1, M2 and M3 and scales of all makes according to EURAMET cg-18. Our certificates are traceable to national standards and state the measurement uncertainty. All certificates are available in our customer portal, together with the schedule, so that you have them to hand immediately during an audit. If you need an accredited certificate, we can arrange this on request through an accredited partner.