Weight calibration in our own laboratory

We calibrate all common designs in classes F1 to M3, regardless of brand:

OIML R111 specifies for each class how much a weight may deviate from its nominal value. Each class is roughly three times wider than the class above it. According to R111, the measurement uncertainty of the calibration must not exceed one third of that maximum permissible error. A finer class therefore requires a more accurate reference and a better measuring set-up.

We have deliberately chosen not to outsource the process. From collection to certificate, everything stays in the hands of our own team and our own systems. You know exactly who is working on your weights, and we keep control of the turnaround time ourselves.

Calibrating means determining how much a weight deviates. Adjusting means bringing that deviation back within tolerance. How this is done depends on the design:

• Weights with an adjustment cavity, such as knob weights and many block weights, are adjusted by adding or removing adjustment material. We then close the cavity again.
• Solid precision weights made from a single piece have no adjustment cavity. If such a weight is too heavy, it can sometimes be carefully reworked. If it is too light, adjustment is usually not possible and we advise on replacement or on using the weight with its measured value.
• Cast iron weights are cleaned and derusted where necessary and given a new protective coat of paint before we adjust them.

Cleaning is also part of the calibration. Dirt and grease on a weight are mass too; if you do not clean, you calibrate the dirt along with the weight.

For each weight, the certificate states:

• the identification of the weight and the class in which it was assessed;
• the measured conventional mass and the deviation from the nominal value;
• the expanded measurement uncertainty, with the coverage factor;
• the assessment: within or outside the tolerance of the class;
• the reference weights used and their traceability to national standards;
• the environmental conditions during the measurement and the date of calibration.

If a weight is adjusted, both the value before and the value after adjustment are stated on the certificate. This matters to your auditor: it allows you to verify whether checks you carried out with that weight were still reliable.

MASSLAB is a Recognised Verifier (Erkend Keurder) under the Dutch Metrology Act and our calibrations are traceable to national standards. Calibrations are performed according to internally established procedures based on the relevant requirements of ISO/IEC 17025. Where applicable, measurement results are stated with measurement uncertainty and supported by traceable measurement references. If you require an accredited certificate, we can arrange this on request through an accredited partner.

A calibration certificate gives a measurement result: the measured conventional mass with its measurement uncertainty. A statement of conformity goes a step further: it states whether the weight falls within the maximum permissible error of its OIML class. Because every measurement has an uncertainty, such a statement requires a decision rule.

• Tolerance: the maximum permissible error (mpe) from OIML R111 for the class and the nominal value, see the table above.
• Measurement uncertainty: according to R111 at most one third of that tolerance, so that the uncertainty is small enough for a meaningful statement.
• Assessment: we assess in accordance with OIML R111: a weight conforms if the measured deviation, including the expanded measurement uncertainty, remains within the tolerance. If only part of the uncertainty interval crosses the limit, we state this on the certificate so that you can decide for yourself whether to apply a guard band or have the weight adjusted.

Have you laid down your own decision rule in your quality system, for example a stricter limit for a specific scale? Let us know when registering your weights and we will apply it in the assessment. That way the certificate matches what your auditor expects.

No law prescribes a fixed interval. The interval comes from your own quality system. Annual calibration is customary for F-class weights. M-class weights that are used gently and stored properly are sometimes given a longer interval. Have a weight calibrated sooner if it has been dropped, is visibly damaged or dirty, or if your own checks show a deviation.

If successive certificates show that a weight is stable, you can extend the interval on a documented basis. Our customer portal lets you track when the next calibration is due for each weight. For storage tips, see storing calibration weights.

In practice the terms are used interchangeably. A calibration weight, test weight or check weight is a weight with a known mass in an OIML class. The Dutch term 'ijkgewicht' dates from the time when weights themselves were subject to legal verification. Today it is mainly the class, together with a valid calibration certificate, that tells you something about reliability. Read more in the difference between test weights and calibration weights.

Which class you need depends on the scale you check with it. The rule of thumb from OIML R76: the error of the check weight is at most one third of the error the scale is permitted at that load. If you are torn between two classes, we are happy to advise. New weights can be bought in our webshop, if desired with a certificate from our laboratory straight away.

Our customers use weights to monitor their own weighing processes. This happens in the food industry, where HACCP, BRC and IFS demand demonstrable checks. In the pharmaceutical industry and laboratories, GMP and internal procedures call for traceable references. In chemicals and petrochemicals it is about recipes and dosing; in logistics and transport it is about weighbridges and floor scales. We also work for the agricultural sector, the metal and plastics industries and maintenance companies that check scales for their own customers.

Whether it concerns a single reference weight or the annual recalibration of hundreds of weights across several sites, you have one point of contact. We schedule the calibrations and take care of transport. With a maintenance contract you no longer have to think about it yourself.

Need force measuring instruments calibrated too? We calibrate force transducers, dynamometers and crane scales in our force laboratory; see force calibration in tension and compression.