Calibrating M1, M2 and M3 weights

The M classes have wider tolerances than the F classes, but here too every gram counts. A 20 kg block weight in class M1 may deviate by no more than 1 gram.

The weight must be more accurate than the scale you test with it. The common rule from OIML R76 is that the error of the test weight is at most one third of the error the scale is permitted at that load. In practice this means:

• M1 for most class III scales: retail, bench, floor and pallet scales and many weighbridges;
• M2 for coarser class III scales and class IIII scales;
• M3 for coarse industrial applications and as a loading weight, for example in proof load tests.

If in doubt, choose the finer class. You can also use an M1 weight later if you purchase a more accurate scale. For the calibration of scales according to EURAMET cg-18, the measurement uncertainty of the weight also determines the final result. Read more under scale calibration.

With us, a calibration of M-class weights is a complete overhaul of the test weight: inspection, cleaning, derusting and repainting where necessary, adjustment and recalibration. This way a cast iron weight lasts for years to come.

In practice, M-class weights are used heavily: they stand on floors, are moved with forklifts and come into contact with dust, moisture and oil. At intake we therefore first assess the condition of each weight.

• Cleaning: we remove dirt, oil and loose rust before measuring. Otherwise the dirt is calibrated along with the weight.
• Repainting: cast iron weights with a damaged paint coat are treated and given a new protective coat. This prevents corrosion and the resulting loss of mass.
• Adjusting: block and knob weights have an adjustment cavity. We add or remove adjustment material until the weight is within tolerance, close the cavity again and measure again.

Because all these steps take place in our own laboratory and our own workshop, weights do not have to go back and forth between different parties. That saves time and prevents damage in transit.

We compare M-class weights with reference weights of a higher class, using the OIML R111 weighing schemes. Our own LIMS software records every measurement, corrects for air density and calculates the conventional mass with its measurement uncertainty.

For each weight, the certificate states the measured value, the deviation, the measurement uncertainty and the assessment in its class. If a weight was adjusted, the values before and after adjustment are stated. The certificate is also available in your customer portal, together with the date of the next calibration.