Anyone ordering weights or reading through a quality manual keeps coming across different words: test weight, test load, check weight, calibration weight, reference weight and mass standard. Sometimes exactly the same object is meant, sometimes there is a real difference in function. The confusion is understandable, because the words come from different eras and fields.
In this article we set out the terms. More important is what comes after that: what you need to look at when you check scales with a weight. That is not the name on the packaging, but the accuracy class and a current calibration certificate. How a laboratory determines those data is explained on our page about calibrating weights.
Six words side by side
The table shows where you usually encounter each word and what is meant by it in practice.
| Term | Where you encounter it | What is usually meant |
|---|---|---|
| Test weight | Everyday language, online shops, older documents | An accurate weight for checking scales |
| Test load | Verifications, calibration reports for scales | A weight with which a scale is loaded during a test |
| Check weight | Work instructions, HACCP plans | A weight for the periodic check by the user |
| Calibration weight | Quality systems, balance manuals | A weight with a known and documented mass |
| Reference weight | Laboratories, measuring equipment management | The most accurate weight within an organisation, against which other weights or instruments are compared |
| Mass standard | Metrology, calibration laboratories | A weight that serves as a reference in the chain of traceability |
A single 1 kg stainless steel weight can therefore carry all six names. The difference lies not in the weight, but in the role it plays in your process.
Where the Dutch word ijkgewicht comes from
In Dutch, a test weight is commonly called an ijkgewicht, from ijken: legal verification. In the past, not only scales but also the loose weights used in trade were officially checked and given a verification mark. Anyone weighing out grain, cheese or meat on a balance with loose weights had to be able to trust that those weights were correct. The word dates from that time, and it has stuck in everyday language.
Nowadays companies weigh with electronic scales. Weights mainly serve to check, calibrate and adjust those scales. For the scale, legal verification still exists, see the Dutch Metrology Act and scales. The weight itself is, in practice, calibrated. An old verification mark on a cast iron weight therefore says something about the history of that weight, but nothing about its mass today.
Reference weight and mass standard: the place in the chain
The words reference weight and mass standard say something about the place of a weight in the traceability chain. Since 2019 the kilogram has been defined via the Planck constant. National metrology institutes, VSL in the Netherlands, realise that definition and pass it on to calibration laboratories. Those laboratories use their mass standards to calibrate the weights of companies, and companies use those weights to check their scales.
Each step in that chain adds a little measurement uncertainty. That is why the reference weight is always more accurate than the weight or instrument that is checked with it. Within a company you see this reflected when, for example, you keep one set of F1 weights as a reference and use it to compare simpler working weights in the interim. More about this chain in our article on traceability and quality standards.
What really matters: the class
Whether you call it a test weight or a test load, its reliability follows from the accuracy class according to OIML R111. That recommendation has seven classes: E1, E2, F1, F2, M1, M2 and M3. For each class it is fixed how much a weight may deviate from its nominal value at most. Each class is about three times wider than the previous one.
An example makes the difference concrete. A 1 kg weight may deviate by at most 5 mg in class F1, at most 50 mg in class M1 and at most 500 mg in class M3. Anyone who buys a 1 kg test weight without a stated class therefore does not know whether the deviation is in milligrams or in tenths of a gram.
Which class you need depends on the scale. The rule of thumb from OIML R76 is that the error of the weight may be at most one third of the error the scale is permitted at that load. For most bench, floor and pallet scales that leads you to M1, for precision balances to F1 or F2. A detailed selection guide can be found in Which calibration weight do I need?
And a current certificate
A class on the packaging is a promise by the manufacturer. Only a calibration certificate shows how the weight actually stood at a given moment. That certificate states, among other things, the measured conventional mass, the deviation from the nominal value, the measurement uncertainty and the assessment against the class. It also states which references were used for the measurement and how they are traceable to national standards.
A certificate has no legal expiry date. Your quality system determines how long you rely on it, usually with a fixed calibration interval. How to choose that interval is explained in How often should you recalibrate weights?
What we encounter in practice
In our laboratory we regularly see that the name of a weight promises more than the weight delivers. A few examples:
- Weights without a class. Cheap test weights from an online shop sometimes have no OIML class and no certificate. Only at the first calibration does it become clear whether they meet a class.
- Old verification marks. Cast iron weights with a stamped verification mark are sometimes still regarded as reliable, while rust and dirt have been changing the mass for years.
- The internal calibration weight of a balance. Many balances have a built-in weight. The balance uses it to adjust itself, but that does not replace a check with an external weight with a certificate.
- Class and material that do not match. A cast iron weight can have a small deviation, but because of its material and surface it still does not meet the requirements of an F class.
In all these cases it helps not to rely on the name, but to assess the weight on class, condition and certificate.
How MASSLAB helps you
MASSLAB calibrates weights in-house in classes F1, F2, M1, M2 and M3, from 1 mg to 2,000 kg. We call your weights by the names you know them by, but assess them against the requirements of OIML R111. If a weight does not conform, we discuss whether cleaning, adjustment or use in a lower class is possible. If you want to know which class your test weights fall into, take a look at our working method under weight calibration or choose new weights in the webshop.