ALL ABOUT CALIBRATION
We clarify the concepts of calibration.
In the field of calibration there are many different concepts such as. self-checking, traceability, calibration and adjustment. If you don’t know what they really mean, it can be tricky to order a calibration of your measuring instruments.
But what is calibration?
Calibration is to find out the difference between the displayed and the true measurement value. In practice, this means that the instrument’s measurements are compared with reference values and the difference or measurement error is documented for each measurement point. After calibration, a certificate or proof is issued indicating the extent of the deviations.
Customers may take it for granted that a measuring instrument will “show correctly” after a calibration. But the fact is that many, perhaps most, instruments and sensors on the market cannot be adjusted in this way. Instead, you get a document in your hand that tells you the difference between the real “true value” and the value measured by the instrument.
What does adjustment mean?
In some instruments, the lab can change or “adjust” the value so that the display is as close to the true value as possible. After such an adjustment, we do another calibration to see how big (or rather small) the measurement error has become.
Who needs to calibrate?
Calibration is the quality assurance of your measurement results. Often laws and authorities require the documented measurement results to have a quality stamp, but the requirements can also come from your own business or your own customers.
For example, ISO 9001-certified industrial companies are required to use calibrated measuring instruments for all measurements important to their business.
Another example is companies in the pharmaceutical sector that have to comply with Good Manufacturing Practice (GMP) requirements.
To find out what criteria apply in a specific area, it may be a good start to contact your trade association. Businesses in the food sector can visit the website of the Finnish Food Authority and look under “Industry guidelines”. There are downloadable documents on what applies to everything from bakeries to breweries.
Accredited calibration
If a calibration is accredited , this means that Sweden’s national accreditation body Swedac has approved the method used in the calibration. Thus, there is also a traceability back to the international bodies that control Swedac’s activities and to international standards and quality norms. Furthermore, all accredited calibration laboratories in Sweden must regularly submit their reference instruments for calibration by other calibration providers, where the measurement uncertainty is better than in their own lab.
Self-monitoring
Comparing measured values against a known reference is called self-monitoring. For example, temperature and relative humidity can be easily controlled on their own.
In terms of temperature, a ‘known reference’ at 0°C can be achieved quite easily by taking an ordinary thermos and filling it with as much ice as possible. Then pour in water to fill the thermos completely and leave it in the fridge for 30 minutes. Then stir the ice bath and place the measuring tip of the sensor in the middle of the thermos.
Self-checks of moisture measuring instruments can be carried out using so-called moisture jars. They contain a salt solution and are available with different relative humidity levels. A common option is to buy a standard set consisting of two cans of 11.3 and 75.3 %RH. To check this, the moisture sensor is inserted into the jar through a clamping screw in the lid. It should then be left in place for a certain amount of time (see instructions attached to the jars).
How do I select calibration points?
The choice of calibration points is always the responsibility of the client. During an inspection, it can never be said that the calibration laboratory selected the points.
Thus, you need to identify which calibration points within the measurement area are to be checked. If you just want to check that grilled chicken meat has a core temperature of +72°C, it is enough to check just that point. But if, for example, a thermometer is used in a range between -18 °C and +120 °C, you need to select a number of measurement points where it is particularly important that the measurement results are accurate.
