How often should a scale be calibrated?
The question we get most often, and the honest answer is: it depends. This is what it depends on.
Who sets the interval
Not the law. Legal metrology rules require a verified instrument to remain compliant, but do not specify an interval; and for a non-verified scale there is no legal requirement at all. The interval comes from your own quality system: ISO 9001 requires measuring equipment to be calibrated at specified intervals, and HACCP, GMP, BRC and IFS ask the same. How long that interval is, you set yourself, and the auditor judges whether it is reasonable.
What a reasonable interval depends on
- How critical the weighing is. A dosage in a pharmacy or a recipe where a one percent difference rejects a batch calls for more frequent calibration than a check at goods receipt.
- How heavy the use is. A floor scale that forklifts drive over every day drifts faster than a balance in a quiet laboratory.
- The environment. Temperature fluctuations, vibration, moisture and dust speed up the drift.
- What the previous calibrations showed. If the deviation stayed well within the limit twice in a row, the interval can be extended; if it was close to the limit, shortened.
Once a year is the usual starting point. After that you adjust the interval to what you measure.
Checking it yourself in between
A calibration is a snapshot. In between, you check it yourself: every day, week or month, put a check weight in the right OIML class on the scale and note what it reads. If it deviates more than you have set as your limit, have it calibrated sooner and adjusted if necessary. With that log you also show the auditor that the scale was in order between two calibrations. Which weight to use is explained under OIML classes explained.
When sooner
After the scale has been moved or relocated, after a fall or knock, after a repair, when a new user notices something odd, and always when your own check shows a deviation. Is that deviation in the scale or in the set-up? Knowing that saves a service call: first go through why a scale deviates. For a verified instrument there is also the legal moment: after work on the weighing technology or a broken seal, re-verification is required.
How we do it
We calibrate with a certificate on site or in our workshop, and keep track of when the next visit is due for you. If you want several scales done in one visit, we plan them together. With a new scale, a certificate on delivery provides the starting point of your records.
Frequently asked questions
Is annual calibration compulsory?
Not by law. It is required by most quality systems, which ask for a defined interval; one year is the usual choice.
How often should an analytical balance be calibrated?
Usually once a year by us, with a daily or weekly check of your own using an E2 or F1 weight, and internal adjustment after every change in temperature.
Can I extend the interval?
Yes, if successive calibrations show that the scale is stable and your quality system allows it. Record the justification for the auditor.
What if the scale deviates too much during calibration?
Then we adjust it and measure again; both values appear on the certificate. For a verified scale, re-verification is then arranged through the manufacturer.