Calibration certificate versus an ordinary thermometer: what you are actually buying
Temperature loggers often ship with a document called a certificate. Sometimes it is a calibration certificate from an accredited laboratory; sometimes it is a printout from the production line. The difference only becomes visible during an inspection — and by then it is too late.
EnviSensor Team
In short
- Metrological traceability is the property of a result whereby it can be related to a reference through a documented unbroken chain of calibrations, each contributing to measurement uncertainty.
- Evidence of traceability is a certificate from an accredited laboratory bearing the accreditation symbol of an ILAC MRA signatory, or calibration by a national metrology institute.
- Calibration laboratories are accredited against ISO/IEC 17025.
- A calibration certificate states the measurement uncertainty — a document without it says nothing about how far the reading can be trusted.
- Calibration is not adjustment: it documents the deviation but does not remove it.
What metrological traceability means
A thermometer reads 4.0 °C. How do you know that is true? The only answer metrology accepts is: because the reading can be related to a recognised reference through a chain of comparisons in which every link is documented.
Formally, metrological traceability is the property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty.
Two elements of that definition decide everything. An unbroken chain — if a link is missing in the middle, there is no relation. And a contribution to uncertainty — every link adds its own uncertainty, so a document that does not state the final uncertainty does not close the chain.
What counts as evidence, and what does not
There are two accepted routes to demonstrating traceability.
- A calibration certificate from an accredited calibration laboratory — sufficient provided it bears the accreditation symbol of an accreditation body that is a signatory to the ILAC Mutual Recognition Arrangement or to regional arrangements recognised by ILAC.
- Calibration by a national metrology institute covered by the CIPM Mutual Recognition Arrangement.
This yields a simple practical test. A document without an accreditation symbol — however it is titled and however impressive it looks — does not demonstrate traceability. It may be perfectly honest, but it is not the evidence regulations require and auditors look for.
| Element | Why it matters |
|---|---|
| Accreditation symbol | Without it the document does not demonstrate traceability |
| Laboratory accreditation number | Lets you verify the laboratory in the national register |
| Measurement uncertainty | Without it there is no way to know how wide the confidence interval is |
| Calibration points | Calibration at 20 °C says nothing about behaviour at 4 °C |
| Date of calibration | The reference point for planning the next one |
The fourth row is often underestimated. If a logger will work in a refrigerator across 2–8 °C, the certificate should cover a point in that range. Calibration performed only at room temperature says little about accuracy under working conditions.
Calibration is not a repair
The most common misunderstanding: calibration does not set the instrument right. Calibration compares the instrument reading against a reference and documents the deviation. If the instrument reads 0.3 °C high, the certificate will say so — but it will not stop it reading high.
Removing the deviation is adjustment, a separate operation. In practice this means that once you hold a certificate you must decide: apply a correction to readings, order an adjustment and recalibration, or accept the deviation as within the permissible error for the application.
That decision should be written down. An inspection asks not only for the certificate but for what followed from it.
How often to calibrate
There is no single answer, and it is worth being wary of anyone who offers one. EU Good Distribution Practice guidelines put it this way: equipment is calibrated at intervals determined on the basis of a risk and reliability assessment, and calibration should be traceable to a national or international measurement standard.
The exception where a concrete figure appears is transport: temperature monitoring equipment in vehicles and containers is maintained and calibrated at least once a year.
When setting your own interval, consider the stability declared by the manufacturer, the consequences of a wrong reading in your application, operating conditions (work at extreme temperatures accelerates drift), and the results of previous calibrations — if the deviation grows year on year, the interval needs shortening.
When the rules require it
The requirement for calibrated equipment appears wherever a measurement carries legal or health consequences. In pharmaceutical distribution it follows from Good Distribution Practice, which covers all equipment monitoring storage conditions and requires calibration traceable to a measurement standard.
National rules for pharmacies and healthcare add their own detail, typically focused on refrigeration equipment and on the ability to read minimum and maximum values reached between readings.
There are also areas where regulations mandate measurement and recording but do not explicitly require a calibration certificate — archive storage is a common example. In such cases calibration is not a legal duty, but it remains the only way to demonstrate that the recorded values mean anything at all.
Frequently asked questions
What is a calibration certificate?
It is a document reporting the comparison of an instrument reading against a reference, stating the measured deviations and the measurement uncertainty. Issued by an accredited calibration laboratory and bearing the accreditation symbol, it constitutes evidence of metrological traceability.
Is a manufacturer certificate enough instead of an accredited calibration certificate?
Evidence of traceability is a document from an accredited laboratory bearing the accreditation symbol of an ILAC MRA signatory, or calibration by a national metrology institute. A manufacturer document without an accreditation symbol does not provide that evidence.
How often must a temperature logger be calibrated?
EU Good Distribution Practice guidelines require calibration at intervals determined by a risk and reliability assessment, without specifying a universal period. The exception is monitoring equipment used in transport, which must be maintained and calibrated at least once a year.
Does calibration fix the instrument?
No. Calibration compares readings against a reference and documents the deviation. Removing the deviation is adjustment, a separate operation. After receiving a certificate you must decide whether to apply a correction, order an adjustment, or accept the deviation as permissible.
What should a good calibration certificate contain?
The accreditation symbol and laboratory accreditation number, the measurement uncertainty, calibration points matching the instrument working range, and the date of calibration. Calibration only at room temperature says little about the accuracy of an instrument operating in a refrigerator.
Need monitoring at your site?
We will help you choose sensors and the way they run — locally on your LAN, in your own monitoring system or in the CE MonitorApp cloud.