A weight has no moving parts and no electronics. Yet its mass slowly changes through wear, dirt, moisture and corrosion. The question is therefore not whether a weight needs to be recalibrated, but how often. There is no standard answer to that question: neither OIML R111 nor the law prescribes a period.
In this article you will read who determines the interval, which factors play a part and how you justify an interval with certificates. What happens during the calibration itself is described on our page about weight calibration. If it concerns the period for scales, read How often should a scale be calibrated? Different factors play a role for weights.
Who determines the interval?
The owner of the weight determines the interval. ISO 9001 requires measuring equipment to be calibrated or verified at defined intervals, but does not specify a period. The same applies to HACCP, BRC, IFS and GMP. Those systems do expect you to be able to explain your choice to an auditor.
Sometimes a customer, a trade association or an internal procedure imposes a maximum period. That is then the upper limit. Within that limit you are free to choose, as long as you justify it. A calibration laboratory can advise, but the decision remains yours. For those who want to approach it thoroughly, the guide OIML D 10, drawn up together with ILAC, describes various methods for establishing intervals.
For a first calibration, one year is a common starting point. That is not a rule, but a reasonable choice as long as you have no data yet on the behaviour of your weights.
Five factors that determine the interval
1. Class
The finer the class, the smaller the permitted deviation and the sooner a small change counts. A 1 kg F1 weight may deviate by 5 mg. A fingerprint or some dust is already noticeable there. An M1 weight of the same nominal value has 50 mg of leeway and tolerates more.
2. Frequency of use
A weight that is placed on a scale daily wears faster than a reference weight that comes out of its case a few times a year. Every contact with a scale, a workbench or another weight can remove a little material or add dirt.
3. Handling
Who uses the weight and how? Weights that are picked up with tweezers or gloves remain more stable than weights that are moved with bare hands. For heavy weights it matters whether they are lifted by a lifting eye or slid across a floor.
4. Environment
A dry, clean laboratory room is favourable. A wet production hall, a cold store or an outdoor set-up at a weighbridge is not. Moisture accelerates corrosion, certainly for cast iron with a damaged paint layer. In dusty environments, dirt accumulates in seams and adjustment cavities.
5. History
The most important factor is what previous certificates show. A weight that has virtually the same value year after year deserves a different interval from a weight that shifts a little each time.
From starting interval to justified interval
After three or more calibrations you can assess the behaviour of a weight. Look not only at whether the weight falls within its class, but above all at how much room is left. That room is the maximum permissible error (mpe), minus the measured deviation, minus the measurement uncertainty. Two examples of weights that were calibrated three years in a row:
| Weight | mpe | Calibration 1 | Calibration 2 | Calibration 3 | Assessment |
|---|---|---|---|---|---|
| 20 kg, class M1 | ±1,000 mg | +180 mg | +200 mg | +215 mg | Shifts about 20 mg per year, ample margin: extending is defensible |
| 1 kg, class F1 | ±5 mg | +1.2 mg | +2.3 mg | +3.1 mg | Shifts about 1 mg per year, margin almost used up: do not extend, rather clean or adjust |
Suppose the measurement uncertainty for the F1 weight is 1.0 mg. Then the last result is 3.1 mg plus 1.0 mg, 4.1 mg in total. That still fits within 5 mg, but with the same shift that room will be used up after another year. The M1 weight still has hundreds of milligrams of margin after three years. How to read deviation and uncertainty together is explained in Measurement uncertainty on the calibration certificate of weights.
Extending or shortening: how to justify it
You may change an interval, provided you record it. A practical approach:
- Collect the results of at least three calibrations per weight or per set.
- Determine the shift between successive calibrations. Watch out for adjustments: after an adjustment the series starts again.
- Compare with the margin. If the shift per interval is small relative to the remaining margin, you can extend, for example from one to two years.
- Shorten in case of unrest. Fluctuating or growing deviations are a reason to shorten the interval instead.
- Record the decision in your measuring equipment register, with the data on which you base it.
Do not extend everything at once. A set with many weights can in practice vary widely: the small sheet weights often change faster than the large ones. The interval of the set is then determined by the most sensitive weights. An overview of what an auditor wants to see can be found in the measuring equipment management checklist.
Signals to calibrate earlier
Apart from the fixed interval, there are situations in which you should not wait for the next calibration date:
- the weight has been dropped, knocked or struck forcefully against something else;
- scratches, dents or damage to the surface are visible;
- the paint layer of a cast iron weight is damaged or rust has formed;
- the closure of the adjustment cavity is loose, damaged or missing;
- the weight has come into contact with liquid, oil or chemicals;
- your own checks on a scale show an unexplained shift;
- the weight has been lent out, hired or used by unknown persons.
If in doubt, set the weight aside until it has been checked. A deviating weight can cause a good scale to be rejected, or a deviating scale to be passed.
Interim checks
Between two calibrations you can pick up early signals yourself. Place a working weight on a stable scale and note the reading. Or compare it with a reference weight that you keep carefully. Such a check does not replace a calibration, but it quickly reveals a sudden change. Good storage and handling makes the difference between a weight that remains stable for years and a weight that has to go back time and again.
How MASSLAB helps you
In our customer portal you can see, for each weight, the history of all calibrations and the date of the next one. That history is exactly what you need to justify an interval. If a weight shows a striking shift, we are happy to discuss with you whether cleaning, adjustment or a shorter period is advisable. With a maintenance contract we schedule the recalibration of your weights automatically. More about our working method under weight calibration.