Annual calibration of test and measurement equipment is recommended by the vast majority of manufacturers and it may also be appropriate to carry out more frequent calibration should test results start to fluctuate. However, the processes behind loss of accuracy are often little understood and they are influenced by a wide array of factors. This article aims to explain the influences behind variations in accuracy and clarify the actual calibration process your tester will undergo when it comes to our laboratory.
How do instruments lose accuracy over time?
Physical mechanical wear and tear is the biggest factor affecting instrument accuracy. In terms of test lead sockets, this can take the form of oxidised connections, contamination from fine particles and physical wear caused by the constant insertion and removal of test leads. All of these issues can cause the leads to sit loosely in the socket which in turn will prevent the tester from holding its null value and lead to variations in readings.
As far as internal components are concerned, relays and rotary switch assemblies are key. The components of high actuation relays are prone to burning and contamination which leads to reading fluctuation whilst impacts sustained during day-to-day use can weaken and damage mechanical relays or small surface mount components which are damaged by larger components banging against them.
What are the most important factors that influence accuracy?
So many factors influence accuracy from one moment to the next that it would be impossible to list them here but environmental conditions are particularly important. Manufacturers define an optimal temperature at which to store testers and, although this is impossible in practice, when instruments are left in extreme conditions - for example in cold vans overnight - they will read less accurately. There are also specific tests that can affect accuracy. For example, the heat produced by repetitive high current loop testing will condense the air inside the tester which also affects readings. It is therefore a good idea to know the instrument’s specification.
Why should calibration be annual?
Annual calibration is recommended in order to see how a unit has drifted from its factory-adjusted settings over the year. In the meantime, it is recommended to use a checkbox in order to verify a unit’s performance, just as a driver checks oil, water and tyre pressure before setting out on a long car journey. Experienced electricians can often spot a suspect reading but there is always a small degree of doubt about whether the tester is reading correctly so a quality checkbox constitutes a useful method of reaffirming the testers’ accuracy.
The calibration certificate will show the difference between the value applied to the unit and the actual reading the tester displays. It is quite uncommon for the unit to read the exact value applied by the calibrator, hence why manufacturers state a tolerance for each of the tester’s functions.
How does the calibration process work?
Before commencing calibration, instruments are stored in a temperature-controlled environment so that the air inside the unit is at the same temperature as the ambient air. Any visible contamination to sockets and plugs is also removed. The tester will then be hooked up to calibration equipment, preferably using test leads supplied by the end user. Otherwise, laboratory test leads will be used and this will be indicated on the final certificate.
For each of the tester’s functional settings, a series of values will be applied going from the lowest point to the highest point in the measuring range. Computer software compares the variation in the supplied and displayed values against permitted tolerances and thereby calculates a pass or fail result. Should the unit be given a fail result, it will be adjusted back into specification according to the manufacturer’s procedures and tolerances.
Once the reading for all applied values achieve a pass result and the unit meets the criteria set out in the manufacturer’s specification, a calibration certificate will be issued and anti-tamper seals will be fitted to the unit.