At a calibration it turns out that a weight falls outside the tolerance of its class. What now? Adjustment is often the logical next step: the weight is modified so that the mass is once again close to the nominal value. But not every weight can be adjusted, and not every adjustment is advisable.
In this article you will read how adjustment works for different types of weight, when it is not possible and which alternatives exist. In practice, adjustment is most common for industrial weights; see also our page on calibrating and adjusting M1, M2 and M3 weights.
Calibration and adjustment are two different steps
Calibration is measuring. The laboratory determines the conventional mass of the weight, the deviation from the nominal value and the measurement uncertainty. Nothing changes to the weight itself.
Adjustment is intervening. Material is added or removed so that the deviation becomes smaller. After an adjustment the earlier measurement is no longer valid and the weight must be calibrated again. Adjustment without a new measurement therefore does not exist. The same distinction applies to scales, see Calibrating, adjusting and verifying scales.
Weights with an adjustment cavity
Many weights have a sealed cavity containing a small quantity of adjustment material. Through that cavity the mass can be adjusted in both directions.
Knob weights
In cylindrical knob weights the adjustment cavity is usually under the knob. The cavity is closed with the screwed-in knob or with a plug. The laboratory opens the cavity, adds or removes adjustment material, closes the weight again and weighs it again. Sometimes several rounds are needed.
Block weights
Rectangular block weights have the adjustment cavity in the handle or in the body of the weight, closed with a plug. For cast iron block weights the order matters. First, dirt and rust are removed and the weight is repainted. Only then is it adjusted, because the paint layer also has mass. Anyone who adjusts first and paints afterwards ends up with a weight that is too heavy again.
Heavy mass pieces
Large mass pieces often have a more spacious adjustment cavity. The principle is the same, except that it concerns grams instead of milligrams. A tricky point with older weights is a cavity that has rusted shut or been welded closed. Then it must first be assessed whether the cavity can be opened safely.
Solid weights: can only be made lighter
Weights made from a single piece have no adjustment cavity. E-class weights are always solid, and many F1 weights are also made from one piece, because a cavity is an additional risk to stability and cleanliness.
If a solid weight is too heavy, it can sometimes be carefully corrected by finely polishing the surface. That takes patience: only a very small amount of material may be removed, and the surface must afterwards still meet the requirements of the class. If a solid weight is too light, adjustment is not possible. Adding material without a cavity cannot be done in a way that complies with the standard.
What a laboratory pays attention to when adjusting
- Towards the nominal value, not towards the limit. A weight that is brought just within tolerance quickly falls outside it again at the next calibration. The aim is a deviation that lies well within the margin.
- Deviation and uncertainty together. A weight only conforms if the deviation plus the measurement uncertainty remains within the permissible error. More on this in Measurement uncertainty on the calibration certificate of weights.
- Stability after intervention. After opening, cleaning or polishing, a weight must be stable before the final measurement is meaningful.
- Closure. The cavity must be properly closed again, so that no dirt or moisture can get in and the adjustment material cannot shift or disappear.
When adjustment is not possible or not advisable
There are situations in which adjustment is technically not feasible, and situations in which another solution fits better.
| Situation | Why not adjust | Alternative |
|---|---|---|
| Solid weight is too light | No material can be added | Downgrade or use with correction value |
| Weight falls just outside F1, but well within F2 | F2 is sufficient for the use | Downgrade to F2 |
| Reference weight with a long, stable history | Adjustment interrupts the series with which you assess drift | Use with correction value |
| Sheet or wire weight is bent or damaged | Cannot be reliably restored | Replace |
| Severe corrosion or a cavity rusted shut | The weight will not remain stable | Replace, or first restore and then assess |
Downgrading to a lower class
A weight that no longer meets F1 often still meets F2. As far as the requirements for material and surface are concerned, a lower class is no problem. Do check whether the lower class suits the scale you check with it. The selection rule from OIML R76 still applies: the error of the weight is at most one third of the permissible error of the scale. Record the new class in your measuring equipment register and mark the weight or the case clearly.
Use with the measured value
If your procedure allows it, you can also continue to use a weight with the conventional mass from the certificate instead of the nominal value. With a deviation of +7 mg on a 1 kg weight, you then calculate with 1,000.007 g. That is mainly useful for reference weights in a laboratory. On the shop floor, where weights are used quickly and with their nominal value, it is prone to error.
What the certificate states
If a weight is adjusted, two results belong on the certificate: the value before adjustment and the value after adjustment, known as the as found and as left values. The value before adjustment is no minor detail. It shows the condition the weight was in while you were using it.
If a weight was outside tolerance before adjustment, assess whether the checks you have carried out since the previous calibration are still reliable. An auditor often asks about this. Without the value before adjustment, that assessment cannot be made. How to ensure that adjustment is needed less often is explained in Storing and handling calibration weights.
How MASSLAB helps you
MASSLAB cleans, derusts, paints and adjusts weights in-house, from small knob weights to mass pieces of 2,000 kg. If a weight falls outside tolerance, we first consult with you: adjust, downgrade, use with the measured value or replace. After adjustment we measure again, and the certificate states the values before and after. More about the working method under calibrating M1, M2 and M3 weights and calibrating F1 and F2 weights.