Test equipment calibration

- the essential facts about test equipment calibration to enable the best to be gained from the test equipment.

In order that any item of electronics test equipment performs to its specification, it is necessary to ensure that it receives its periodic calibration. Like any other item, electronics test equipment tolerances change slightly with time. There are a variety of reasons for this, and while it is possible to minimise the levels of drift, they cannot be totally eliminated. As a result, it is necessary to ensure that the test equipment receives its calibration. Only when an item of test equipment is within its calibration period, will it be guaranteed to produce results that are within its stated specification. Once the calibration period has been exceeded, then it is not possible to guarantee the results.

When using an item of test equipment there are some circumstances when the calibration of the test equipment is required, under other circumstances it is not. It may be that a measurement is only required to provide a rough order of magnitude reading and for this a fully calibrated test instrument may not be needed. However for development test, production test and other areas such as qualification test, it is crucial that the results from the test equipment can be guaranteed. Under these circumstances it is necessary to ensure that the test instrument is within its calibration period.

In view of the fact that most test instruments and other pieces of test equipment are used both for ad hoc measurements as well as those for more formal test applications, it means that all test equipment available for use should be calibrated. While calibration adds an additional expense to be covered by the company, the consequences of not having all equipment calibrated is far more costly.


Test equipment calibration

Test equipment, or test instrument calibration is more than just comparing an instrument with a similar one to check whether the two readings are the same. It requires the full calibration procedure for the instrument to be followed. Normally a calibration procedure will be available for sophisticated instruments. Specific adjusters will be provided to enable the instrument to be calibrated. Additionally the required test set-up and suitable test equipment for performing the calibration procedure will be defined.


Test equipment calibration traceability

When any test equipment calibration is performed, it is obviously necessary to ensure that the equipment against which the item is being calibrated is itself accurate. In order for this to the calibration of the standard equipment must be performed against a high quality standard. To ensure this is of a high standard, many test houses offering test equipment calibration are able to prove or trace back the measurements to primary measurement standards held at national institutes such as NPL (National Physical Laboratory) in the UK, and NIST (National Institute of Standards and Technology) in the USA. Here highly accurate measurements of fundamental quantities of time, current, voltage and the like can be made against known standards.

In view of the fact that the measurements can be traced back to basic standards, the calibration of the final test equipment is know to be accurate to the basic known or absolute figures, rather than relative measurements that may not be accurate to the absolute standards.


Calibration policy

In any company or organisation using test equipment, it is necessary to set up a properly controlled calibration procedure. In this way, all the required equipment can be maintained in a calibrated state and it will be available for use. It is also necessary for the system to visibly flag or note when any equipment is due for calibration.

The most obvious method of controlling test equipment is to use adhesive labels attached to each piece of equipment. This provides information of the date when the equipment was calibrated and the date when the test equipment is next due for calibration. This shows any users instantly, the state of the equipment.

Visible calibration stickers should not be the only method of tracking the equipment state. It is also necessary to ensure that a central record of the test equipment is maintained. In this way it is possible to view the status of all the test equipment that is on site and plan the overall calibration strategy. Trying to achieve this by locating the equipment and noting its status is not viable. It is always difficult and time consuming to locate all the test equipment on any site or within a laboratory. Locating the exact location of the test equipment is often left to the time when it is needed for calibration.

It is also necessary to record each piece of test equipment that is used when making measurements. For example all test results sheets for a production test will record the serial numbers of the test equipment used. In this way if any problems or discrepancies are noticed later, these can be investigated and all the relevant test equipment identified in case the discrepancy has been caused by a test equipment failure.


Summary

For any organisation using electronics test equipment, it is necessary to ensure that the equipment is operating to its specification. This is achieved by rigorously maintaining a process whereby all the equipment in use is calibrated. This will ensure that all the measurements are accurately made and with sufficient records, any test equipment problems can be traced and rectified.

By Ian Poole

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