How often does this scale need to be calibrated? It is one of the questions we are asked most often. The honest answer is that there is no fixed period. No law and no quality standard prescribes that a scale must be calibrated every year or every six months. You must, however, be able to explain why you choose a particular period.
In this article you will read who determines the interval, which factors play a part, when it is better not to wait for the scheduled date and how calibration relates to the legal re-verification of verified scales. How the calibration itself is carried out is described under scale calibration.
Your quality system determines the interval
Calibration is not a legal requirement. The requirement comes from the standards and systems you work with, such as ISO 9001, HACCP, BRC, IFS or GMP. Each in its own words, they require measuring equipment to be calibrated or verified at defined intervals, traceable to national standards.
Defined intervals means that you choose and record the period yourself. An auditor mainly looks at whether that choice is justified and whether you stick to it. A calibration certificate also has no legal validity period; it describes the condition on the calibration date. It is your own management procedure that determines when a scale is overdue. What else an auditor wants to see is set out in our measuring equipment management checklist for the audit.
Factors that determine the interval
When choosing an interval you look at the risk: what happens if the scale deviates unnoticed, and how likely is that?
How critical is the weighing?
A scale with which you weigh out active substances, release products or settle transactions deserves a shorter interval than a scale for a rough indication in the warehouse. Also look at the margin. If you weigh close to the limit of what the scale can do, a small shift will fall outside your tolerance sooner.
How intensively is it used?
A floor scale that weighs pallets in three shifts has more to endure than a precision balance that is used a few times a week. Many weighings mean more mechanical load, more risk of overload and more risk of damage.
In which environment is it located?
Temperature changes, moisture, dust, vibrations and cleaning agents all have an influence. In practice we see that scales in wet production areas, in cold stores and freezers, or outdoors, such as weighbridges, start to deviate sooner than scales in a climate-controlled room.
What does the history say?
Your own results say the most here. How much did the scale deviate at previous calibrations? Did it need to be adjusted? What did the interim checks show? A scale that stays well within tolerance for years justifies a longer period than a scale that has to be corrected every time.
Annually as a starting point, then adjust
For a new scale or a scale without history, one year is a common starting interval. It fits in with the annual audit cycle and is easy to defend for most applications. For critical applications or heavy use, companies often opt for six months or less.
After a few calibrations you can adjust the interval on a sound basis. A simple approach:
| Result at calibration | Possible adjustment |
|---|---|
| Several times in a row well within tolerance, no adjustment needed | Extend the interval, for example from 12 to 18 or 24 months, possibly with additional interim checks |
| Within tolerance, but close to the limit | Maintain the interval and tighten the interim checks |
| Found outside tolerance | Shorten the interval, investigate the cause and assess the consequences for earlier weighings |
Record such a decision, together with the reason. Anyone looking for a more detailed method can consult the international guide of ILAC and OIML on determining calibration intervals (ILAC-G24, also published as OIML D 10), available via the OIML website.
Do not wait: when to calibrate earlier
An interval assumes normal use. Some events can cause a scale to deviate all at once. In those cases, have the scale checked or calibrated before you rely on it again:
- after moving it to another room or location;
- after a fall, a knock or a heavy overload;
- after a repair, such as replacing a load cell, circuit board or indicator;
- after water damage, lightning strike or serious contamination;
- if an interim check shows a deviation beyond your action limit;
- in case of doubt about results, for example after complaints or unexplained stock differences.
Many users underestimate relocation. A scale in fact measures a force, and that depends on the floor, the levelling and even the local gravity. With fine balances those differences are clearly visible. A relocated scale without damage does not always need to be fully recalibrated; a check with a calibrated weight often gives sufficient assurance. How to go about that is explained in checking your scale between calibrations.
Calibration and re-verification for legally verified scales
A legally verified scale falls under the Dutch Metrology Act (Metrologiewet). In the Netherlands there is no fixed period for re-verification for most scales. That does not make the calibration interval any less important. As the user you are responsible for ensuring that the scale always weighs within the legal tolerances, and a periodic calibration or check is the way to demonstrate that.
After a repair to the weighing system or a broken seal, re-verification is mandatory, regardless of your calibration interval. If a calibration shows that a legally verified scale needs to be adjusted, re-verification also follows after adjustment. It is practical to schedule calibration, adjustment and re-verification in the same visit. In Belgium, a periodic re-verification does apply to legally verified scales; in that case, align your calibration schedule with it. More about the legal side in the Dutch Metrology Act and scales.
How MASSLAB helps you
MASSLAB calibrates scales of all makes according to EURAMET cg-18 and records the results in a traceable certificate. In our customer portal you can see your certificates and the schedule for the next calibrations. That way you have the history of each scale at hand when you want to justify or adjust your interval. As a Recognised Verifier (Erkend Keurder) under the Dutch Metrology Act, we can include the re-verification after adjustment or repair for legally verified scales. More information under scale calibration and the customer portal.