What is Calibrations?

In practice, we receive many questions about various subjects. This shows that the matter is extremely complex. In this regular section, which we have called edutorial, we tackle a subject by the horns of every newsletter and try to provide our customers and other interested parties with insight and clarity.

The definitions

What is calibration? Isn't that the same as adjusting or gauging? These are frequently heard answers to this question. Firstly, it must be stated that the definitions used in the United States are different from the ones used in Europe, but more on that later. First what calibration is not!!

Calibration is neither gauging nor adjustment of equipment, although these terms are closely related. First, the activity of calibration falls under the metrology law (former calibration law) and mainly applies to quantities (a meter, a kilo, a liter, etc. are examples of quantities) intended for trade purposes, or for use by police or customs.

For example, the butcher's scales are calibrated because it is important that a kilo of steak is actually a kilo. Otherwise, the customer will receive too much, which is not good for the butcher, or too little, which is not good for the customer. For example, you would also like to receive the number of liters of petrol for which you have to pay. Measuring instruments that are not calibrated often also bear the statement: "not suitable for commercial purposes". You can find this, for example, on centimeters or on personal scales.

Adjust

It's clear that these measuring instruments must indicate the correct value and that is why these instruments are actually adjusted when this is no longer the case. We call the latter adjustment.

Calibrate

Before we can adjust or adjust an instrument, however, we must first know what the deviation is. This determination of the deviation in a way that compies with international standards is called calibration.

Practice

In practice, we mainly encounter two situations for electronic calibrations.

In one case the customer only wants to know whether the measuring instrument still meets the specifications, while in another case the customer is interested in the deviation of the instrument.

Testing specifications

In many cases, a company has selected a measurement tool for a specific purpose. The accuracy of the measuring instrument is tailored to this purpose. In other words, as long as the instrument meets these specifications, the instrument is usable. Many ISO/IEC 9000 certified companies are interested in such a calibration with the so-called testing against specifications linked to it. If the instrument still complies, a so-called calibration certificate is issued, which is proof that the device meets the specification. A situation directly related to this occurs when a measuring instrument does not have to meet the specifications but must meet a specific standard. It then becomes testing against the standard. This occurs, for example, when calibrating an ESD gun to generate static electricity. The generator must comply with an IEC61000-4-2 standard. If a measuring instrument does not comply, it must be adjusted and this ins't possible, repaired or else used for another purpose or iwritten off.

Correction tables

A completely different situation arises if the aim is to measure as accurately as possible. This is common at measurement institutions, whether or not they are accredited. Since most measurements are now performed automatically, it is possible to correct the measurement on the basis of so-called correction tables. In this case the correction table and not the calibration certificate are the main purpose of the calibratio. These tables are entered into the measurement software on the computer and the measurement values ​​are corrected as it were 'live'. An example of this is an antenna calibration. The antenna is being calibrated. The deviations per frequency point are measured and stored in a correction table. During the actual measurements, these corrections are directly included in the measurement.

Measurement uncertainty

In the above situationuation, it is not desirable for the instrument to be adjusted. The fact that an instrument deviates is actually not that interesting as long as the instrument is stable and the deviation is known. In addition, no measurement is 100% accurate. There are always deviations. We call this the so-called measurement uncertainty. Internationally, certain standards are maintained with the smallest possible measurement uncertainty. From this point on, all calibration should be traceable to these international standards. Measurement uncertainty increases with every step from international standards. In the correction table, not only the deviation but also the measurement uncertainty is stated per measured point.

Best Accuracy Limits

The accuracy of a calibration laboratory can be found in the so-called scope of the laboratory. A so-called best accuracy is specified for each area. It's considered the best accuracy because the instrument to be calibrated itself also contributes to the inaccuracy. When specifying a laboratory's best accuracy limits, this influence is disregarded.

Limits

From the above it immediately becomes clear that when testing against specifications or standards, a situation can arise in which the measured value is so close to the specification or standard that the deviation is smaller than the measurement uncertainty. In such a case we speak of a limit value.

Outsourcing calibrations

The vast majority of organizations are unable to perform their own calibrations and therefore have no other option than to outsource their calibrations. Then the question immediately arises: "What to keep in mind when outsourcing calibrations?" However, this is quite a lot of points.

What is being calibrated?

First of all, one has to ask the question what needs to be calibrated? If a measuring instrument is only used as an indication, there is really no reason to calibrate the instrument. It is then wise to make this clear by means of, for example, a sticker. If a multi-meter is only used to measure voltage, why do the other ranges need to be calibrated? If it is clear which areas should be applied for which purpose (accuracy). It's important to establish that this is carried out in the correct manner and with accuracy over the entire desired range.

Accreditation

The easiest (and often the only) way to ensure that the calibration is done correctly is to have the calibration performed under accreditation. As soon as a calibration is carried out under accreditation, you can assume that all relevant matters to achieve a traceable calibration have been arranged, such as for example:

• Is the calibration performed with the correct (suitable) equipment?
• Is this equipment traceably calibrated?
• Is the calibration performed under the correct climatic conditions?
• Are the employees sufficiently trained to carry out the calibration?

You will understand that a lot of effort is needed to arrange the above matters. It is therefore extremely doubtful that an institution that does not carry out calibrations under accreditation will meet all the requirements. If that is the case, then it would also be carried out under accreditation. Contrary to popular belief, calibrations under accreditation are not more expensive at all.

Calibration Period

Another important parameter is the determination of the calibration period. In general, a year is taken for this. However, this is not at all self-evident. The aim is to measure with the desired accuracy during the calibration period. If history shows that the instrument is stable, the calibration period can be extended. If the instrument is dropped or overloaded, it is important that the instrument is recalibrated quickly. A disadvantage of a calibration period that is too long is that there is a risk that the instrument will show a deviation within the calibration period, for whatever reason. It is then possible that you carry out incorrect measurements for a considerable time. A (partial) solution to this problem is to carry out interim verification measurements yourself.

In the United States

To make things a bit more difficult, in the United States calibration is indeed understood to mean adjustment.

Kiwa Dare

Kiwa Dare is an accredited calibration laboratory for all electrical quantities, specialized in EMC and high-frequency calibrations.
Click here for more information about our calibrations Kiwa Dare actually carries out all calibrations for which it is accredited by the Dutch Accreditation Council, under accreditation.

See RvA\Kiwa Dare for our scope and our best accuracy limits.

Kiwa Dare scope & certificates

More information: Kiwa Dare +31 348 200 951 or send an e-mail to: Calibrations Kiwa Dare NL  NL.Calibrations.Kiwa.Dare@kiwa.com