FAQs-METROLOGY

Why should I have my testing and measuring equipment calibrated and at what intervals?

The performance of measuring and test equipment may change with time, under the influence of the environment to which it is exposed, wear and tear, overload or because of improper use. The accuracy of the measurement given by the equipment needs therefore to be checked from time to time.
To do this, the value of a quantity measured by the equipment is compared with the value of the same quantity provided by a measurement standard. This procedure is called calibration. For example, a vernier calliper or a micrometer screw can be calibrated by a set of standard gauge blocks, and a set of standard weights is used to calibrate a weighing instrument. Comparison with the standard reveals whether the accuracy of the measuring equipment is within the tolerances specified by the manufacturer or within prescribed error limits.
You can use the results of a calibration, showing the deviation from the standard that represents the correct value, to correct your measurement readings or to design a correction curve. You should repeat the calibration of your equipment from time to time for the earlier-mentioned reasons. The time span between two calibrations is called a calibration interval; every calibration after the first is called a recalibration.
Calibration intervals are usually recommended by the manufacturer of the measuring or test equipment, and these should be observed. However, the performance of the instrument depends on its treatment and its use. In some situations, immediate recalibration is required, for instance, because the measuring result obtained is doubtful or unexpected. Recalibration is certainly necessary after overloading, either mechanical or electrical, or after the equipment has been exposed to shock, vibration, improper electrical supply or other instances of mishandling. The user of the instrument is responsible for requesting recalibration in such cases. Waiting until the date of a recommended reverification would imply the risk of incorrect measurements.
Metrological confirmation comprises the set of operations required to ensure that a measuring equipment complies with the requirements for its intended use. Calibration is part of the metrological confirmation process (maintenance, necessary adjustments and labelling being others).
Confirmation intervals should be fixed taking into account the most important factors relevant to the equipment in question, such as the type of equipment; manufacturer’s recommendations; a record of the history of maintenance, calibration and servicing; extent and severity of use; environmental conditions; and the consequences of incorrect measurements. Clearly, in the light of these factors, drawing up a list of confirmation intervals that can be universally applied is not feasible. 
The cost of confirmation does not allow periodic confirmations at overly short intervals. If no information on recalibration periods is available from the manufacturer and the equipment functions as expected, it is recommended that recalibrations be carried out once a year, and records of the results should be kept. Depending on the consistency of these results, you may decide to prolong or shorten periods between recalibrations.
When choosing measurement standards for the calibration exercise, it is essential to use standards that are traceable to national and/or international standards.