In practice, calibrating, adjusting and legal verification are often used interchangeably. A buyer asks for a verified scale while the auditor wants to see a calibration certificate. Or a technician reports that he has calibrated a scale when in fact he has adjusted it. That may seem like a matter of words, but the consequences are concrete: the wrong document at an audit, a legal verification that has lapsed unnoticed, or a scale that may not be used for legally regulated applications.
In this article we set the three terms side by side. You will read what happens in each operation, when you need which one, and which misunderstandings we as a calibration laboratory encounter most often. If you want to know straight away how a calibration of your weighing equipment is carried out, see scale calibration.
Calibrating: measuring and recording the error
In a calibration you compare the indication of the scale with weights of known mass. The difference between what the display shows and what is actually on the platform is the indication error. That error is recorded for each measuring point, together with the measurement uncertainty: the margin within which the true value lies with a given probability.
Important to remember: a calibration does not change anything about the scale. Nothing is adjusted. The result is information, recorded in a certificate. With that information you, or your quality system, assess whether the scale is suitable for the work you do with it.
For scales, EURAMET cg-18 is the usual method. It describes tests for repeatability, eccentricity and indication error, and explains how the measurement uncertainty is calculated. Read more in EURAMET cg-18 explained. What exactly should appear on a certificate is covered in the calibration certificate explained.
Adjusting: setting the scale right again
Adjusting is an intervention. The technician sets the scale so that the indication is once again as close as possible to the true mass. This is usually done via an adjustment menu or a switch in the scale, with a weight of known mass as the reference. Many laboratory balances have a built-in adjustment weight, with which the balance adjusts itself at the press of a button or automatically when the temperature changes.
Manufacturers often call that function internal calibration. Technically it is adjustment: the balance corrects itself but does not record an error with measurement uncertainty against a traceable reference. It is a useful function, but not a substitute for a calibration with external weights.
Adjusting only makes sense in combination with measuring. Without a measurement beforehand, you do not know how large the error was. Without a measurement afterwards, you do not know whether the intervention succeeded. That is why, during a calibration, we first record the condition in which we find the scale, adjust only after consultation, and then measure again. That first measurement is not a formality. If the scale was weighing outside your tolerance, most quality systems require you to assess what that means for the weighings made since the previous calibration.
Legal verification: the statutory inspection
Legal verification (in Dutch: ijken) is of a different order. It is a statutory inspection establishing that a weighing instrument meets the requirements for its accuracy class. In the Netherlands this is regulated by the Dutch Metrology Act (Metrologiewet) and the Measuring Instruments Decree (Meetinstrumentenbesluit), which transpose European Directive 2014/31/EU on non-automatic weighing instruments.
You can recognise a verified scale by the metrology marking next to the CE mark: a capital M in a rectangle, the last two digits of the year and the number of the notified body. Seals prevent anyone from gaining unnoticed access to the adjustment. The initial verification of a new instrument is carried out by the manufacturer or a notified body. After a repair to the weighing mechanism or a broken seal, a re-verification follows, for example by a Recognised Verifier (Erkend Keurder) under the Dutch Metrology Act. Read more on the page scale verification and re-verification.
A legal verification says that the scale weighs within the statutory tolerances. It does not come with a table of errors per measuring point or a measurement uncertainty. If your auditor expects a calibration certificate, a verification marking is therefore usually not sufficient.
When do you need what?
Which operation is required depends on what you use the scale for. The table gives an overview of situations we often encounter.
| Application | What is required |
|---|---|
| Selling by weight in a shop, at a market or in wholesale | Legal verification mandatory; calibration if your quality system requires it |
| Charging for waste, scrap or freight on a weighbridge | Legal verification mandatory; calibration often added for your own substantiation |
| Determining the quantity in prepackages | Legal verification mandatory; calibration and interim checks for the production process |
| Weighing patients, pharmacy, medical or pharmaceutical analyses | Legal verification mandatory; calibration for the quality system |
| Recipes and production control under ISO 9001, HACCP, BRC or IFS | Calibration with certificate; legal verification not mandatory |
| Other laboratory analyses and research | Calibration, possibly with uncertainty in use; legal verification not mandatory |
| Scale deviates too much during calibration or a check | Adjustment with measurement before and after; for a verified scale, followed by re-verification |
Adjustment as such is not a requirement in any law or standard. It is a means of making a scale that deviates too much usable again.
Combinations in practice
Many companies deal with more than one of these terms. Three combinations are common.
- Calibrating without adjusting. The scale weighs within your tolerance. Nothing is changed and the certificate contains a single series of results. For stable scales this is the normal situation.
- Calibrating, adjusting and calibrating again. The first measurement shows an error that is too large. After adjustment, the scale is measured again. The certificate shows the results before and after adjustment.
- Calibrating, adjusting and re-verifying. On a verified scale, the adjustment access is behind a seal. If that seal is broken, the verification lapses and a re-verification is required before the scale is used again for legally regulated applications. It is practical to have this done during the same visit.
For that last combination, it is wise to provide the make, type and serial number in advance. A verifier must check for each instrument whether he is permitted to perform that verification.
Common misunderstandings
- The CE mark means the scale is legally verified. No. Only the supplementary metrology marking with the M indicates that the scale has been verified. Many scales carry only a CE mark and are not intended for legally regulated applications.
- A verified scale is also calibrated. A legal verification does not provide measurement results per point or a measurement uncertainty. For your quality system you generally need a separate calibration certificate.
- The balance calibrates itself. An internal adjustment function adjusts the balance with a built-in weight. Whether the balance and the built-in weight are still correct only becomes apparent from a measurement with traceable external weights.
- A calibration certificate is valid for one year. A certificate has no statutory period of validity. It describes the condition on the calibration date. You determine the interval yourself; read about this in how often to have a scale calibrated.
- A verified scale in the Netherlands must be re-verified every year. For most scales there is no fixed re-verification period in the Netherlands. You are, however, responsible for ensuring that the scale remains within the tolerances. In Belgium, periodic re-verification is mandatory.
- A quick adjustment can do no harm. On a verified scale, a broken seal invalidates the verification. And without a measurement beforehand, the information about the old error is lost.
How MASSLAB helps you
MASSLAB calibrates scales of all makes according to EURAMET cg-18, on site or in our laboratory in Geldermalsen. We first measure the condition in which we find the scale, adjust only after consultation and record the results in a certificate that is traceable to national standards. As a Recognised Verifier, we can carry out the re-verification of verified scales after adjustment or repair during the same visit, once we have checked that we are permitted to verify that instrument. Between two visits you can monitor the scale yourself; see checking your scale between calibrations. More information can be found at scale calibration.