Measurement accuracy

No instrument can be guaranteed to give readings that are absolutely accurate every time or exactly the same as another apparently identical instrument. Several factors influence the ability of an instrument to make and display a measurement.

Before we look at sources of error during a measurement, it is important to understand the limitations of an instrument and how they affect the performance. Part 1 explains instrument range, count, resolution and accuracy and how to interpret them. Part 2 looks at sources of error and where possible, how to avoid them.

For tests on electrical installations in the UK, BS 7671 references BS EN 61557. This European wide standard defines the performance of test instruments used in the verification of low voltage electrical installations under 1000 V AC.

BS EN 61557 defines an internationally agreed minimum performance for which all instruments must conform on any particular application. This includes the test range, the types of test necessary and the minimum accuracy required.

Frequently, engineers find they are working at the upper or lower limits of an instruments capabilities. It is under these conditions that the declared resolution and accuracy has the greatest effect. Awareness of these limitations is essential to ensure correct application of a test and interpretation of results.