What is calibration, and what is it not

Calibration is the comparison of an instrument against a reference of known accuracy, and the recording of the difference. That is the whole definition. Notice what it does not say: it does not say the instrument gets fixed, and it does not say the instrument passes.

Four words that get used interchangeably and should not be

The distinction that costs people money

Calibration: Measuring how far the instrument is from the reference, and recording it. The instrument is not changed.
Verification: Checking whether that difference falls inside a stated tolerance. A pass or fail decision, made after calibration.
Adjustment: Changing the instrument so it reads closer to the reference. This invalidates the calibration that preceded it, so a new calibration must follow.
Repair: Replacing or fixing a faulty component. Always invalidates the previous certificate, without exception.

The practical consequence is the one people miss. If an instrument is adjusted or repaired and then handed back without a new calibration, the certificate in the file describes an instrument that no longer exists. Every measurement made on the strength of it is unsupported — and that only becomes visible when someone challenges a result, by which time the measurements are already in a report.

Traceability — the part that makes it mean anything

Comparing your instrument against another instrument proves nothing on its own, because the question immediately becomes: what was that one compared against? Traceability is the answer. It is an unbroken documented chain from your instrument, through a laboratory reference, through a national measurement institute, to the SI definition of the unit — with a stated measurement uncertainty at every link.

That chain is what separates a calibration certificate from a sticker. A laboratory that cannot show the chain has not told you anything about your instrument; it has told you that two unknowns agreed with each other.

Accredited and traceable are not the same word

Any competent laboratory can calibrate traceably. Accreditation is a separate thing: an independent accreditation body has assessed the laboratory against ISO/IEC 17025 and published exactly which measurements it is competent to perform, with what uncertainty. That published list is the scope, and it is narrower than what most laboratories can technically do.

So the question to ask a supplier is not "are you accredited" — almost everyone will say yes. The question is "is this specific measurement inside your accredited scope, and can I see the scope document". A laboratory that answers plainly is telling you something useful. One that changes the subject is telling you something too.

How ALT answers that question

Three services are inside our accredited scope under certificate 24-LB0054: sound level meters (IEC 61672-3), sound calibrators (IEC 60942) and measurement microphones (IEC 61094-2 and -6). Dosimeters, vibration transducers and preamplifiers are calibrated traceably but outside that scope. Our scope document is published in full on this site — you do not have to ask for it.

How to read a calibration certificate

As-found results. What the instrument read before anything was done to it. This is the most important number on the document and the one most often missing — without it you cannot judge whether the work you did since the last calibration is still defensible.
As-left results, if the instrument was adjusted. What it reads now.
Measurement uncertainty, stated per result rather than as a single figure for the certificate. A result without its uncertainty is incomplete.
The scope statement. Whether this particular measurement was made under accreditation, or traceably outside it.
The date, and nothing about a due date. A calibration certificate records a condition on a date. The interval until the next one is your decision, not the laboratory's.

How often must an instrument be calibrated?

The interval is set by the user of the instrument, not by the laboratory and in most cases not by statute. What is required is that you can justify the interval you chose. Twelve months is the accepted default because it is short enough that drift usually stays inside tolerance. A specific regulation, client specification or management system may name something different, and that takes precedence.

My instrument failed. Are all my past measurements now invalid?

Not automatically, and this is exactly why as-found results matter. If the instrument failed by a small margin in one band, and your measurements were nowhere near a compliance limit, the impact may be nil. If it failed across the range, it is a different conversation. You cannot make that judgement at all without the as-found data, which is why a certificate that omits it is worth very little.

Is a calibration sticker enough?

No. A sticker records that something happened; it does not record what was measured, how far off the instrument was, or with what uncertainty. If a result is ever challenged, the certificate is the evidence and the sticker is decoration.

Who calibrates the laboratory's own reference equipment?

A laboratory higher up the traceability chain, and the certificates for those references are part of what an accreditation assessment examines. You are entitled to ask any laboratory how its references are traceable. A straight answer is a good sign.

Last updated: 2026-08-13