Calibration, Verification and Adjustment
Calibration
How is it ensured that the specified uncertainty of a measurement system is not exceeded? This is done by calibration. During calibration, a measurement system is compared with a reference system whose uncertainty is known. This checks whether the required maximum uncertainty of the measurement system is not exceeded.
Let us look at an example: a folding-rule factory produces 100 folding rules. It must prove that all of them have a maximum length uncertainty of ±1 mm. For this, it obtains a reference system. In this case, it buys a length standard derived from the SI unit “metre” with an uncertainty of ±100 µm. To check the value at 2 m, it buys a length standard of length l = 2 m ± 100 µm.
An employee measures the length of the reference system with all folding rules of the production. If the measured folding-rule lengths ± the measurement uncertainty of the reference system lie in the range [1.999 m … 2.001 m], it is ensured that the folding rule measures sufficiently accurately within its specification of ±1 mm. A folding rule with a length of exactly 2.001 m must be rejected, because it could be 2.0011 m long and we would not notice this because of the measurement uncertainty of the reference system.
So when a measurement system is calibrated, an incorrect measured value is not corrected; a reference system is only used to determine whether the system lies within its specification with regard to its measurement uncertainty.
To simplify the example, it is assumed here that a folding rule is an ideal measuring instrument that only becomes inaccurate because its length deviates from the desired length of 2 m. Other causes of measurement uncertainty, such as reading errors, are not discussed for simplicity.
In practice, the measurement uncertainty is influenced by several further factors: the length of the folding rule depends on its ambient conditions. In humid air or with temperature variation, it changes its length. That is why reference conditions are defined for calibration (e.g. 20 °C, 50 % relative humidity, standard pressure etc.). For high-quality measurement technology, it is also stated how the measurement uncertainty changes when the ambient conditions vary.
As the folding rule gets older, its length also changes. That is why measurement systems are recalibrated at regular intervals. Again, the measured value is not corrected; it is only proven that the system still works within its specification.
The reference system also changes its value (here its length) over time. It is therefore compared at fixed intervals with a higher-grade standard, which has, for example, a measurement uncertainty of only 1 µm. And this higher-grade standard is in turn repeatedly compared with the next more accurate reference system, until eventually it is compared with the primary standard. In this way, a system of “traceably calibrated references” is created, which always have a regularly checked maximum deviation from the primary standard.
Traceably calibrated references must be used for calibration. The folding-rule factory must regularly recalibrate its reference system for length measurement against a higher-grade standard so that it is allowed to calibrate its folding rules with it at all.
Successful calibration is documented in a certificate that is enclosed with the system when it is sold. Among other things, the certificate contains the uncertainty of the reference system, the time and ambient conditions of the calibration and the period of validity of the calibration.
Legal verification (“Eichen”)
Calibration denotes ensuring a maximum uncertainty of a measurement system by the manufacturer. In some areas, this is too risky for the state. It carries out the check itself (through designated bodies). In German, this process is called “Eichen” (legal verification). Legal verification – just like calibration – only ensures a maximum measurement uncertainty. For example, flow meters at petrol stations, scales in supermarkets or water meters in households are verified. Legal verification is mostly carried out in cases where companies could achieve very large profit increases even with small deviations of the measured value.
Adjustment
Calibration and legal verification only determine a measurement deviation. It is not corrected. Of course, it is allowed to correct an incorrect measured value during production. We call this process “adjustment”. With a folding rule, adjustment is difficult, because once the scale has been printed, we cannot change it any more.
If it is found during production that a scale always shows 5 g too much, for example, we can subtract 5 g from the result in the scale's software after each measurement and only then display it.
Calibration or legal verification takes place at the very end of production, after adjustment. Many products only achieve the required measurement uncertainty because the measured value has been adjusted beforehand. Unfortunately, the measurement deviation has not completely disappeared after an adjustment. This is explained in the coming chapters.