Selecting sensors: Specsmanship vs. reality

Accuracy and precision are of supreme importance when specifying sensors. The two terms are often used interchangeably, but there are fundamental differences between them. Accuracy, a qualitative concept, indicates the proximity of measurement results to the true value; precision reflects the repeatability or reproducibility of the measurement.

ISO 3534-1:2006 defines precision to mean the closeness of agreement between independent test results obtained under stipulated conditions, and views the concept of precision as encompassing both repeatability and reproducibility. The standard defines repeatability as precision under repeatable conditions, and reproducibility as precision under reproducible conditions.

Precision, accuracy, repeatability, reproducibility, variability and uncertainty represent qualitative concepts and thus should be applied with care. The precision of an instrument reflects the number of significant digits in a reading; the accuracy of an instrument reflects how close the reading is to the true value being measured. An accurate instrument is not necessarily precise, and instruments are often precise but far from accurate.

Figure 1 illustrates the difference between accuracy and precision, and shows that the precision of the measurement may instead vary in proportion to the signal level.

Concepts of accuracy
Sensor manufacturers and users employ one of two basic methods to specify sensor performance: parameter specification and the total error band envelope.

Parameter specification quantifies individual sensor characteristics without any attempt to combine them.

The total error band envelope yields a solution much nearer to that expected in practice, whereby sensor errors are expressed in the form of a total error band, or error envelope, into which all data points must fit regardless of their origin. As long as the sensor operates within the parameters specified in the data sheet, the sensor data can be relied on, giving the user confidence that all sensor data acquired will be accurate within the stated error band and thereby avoiding the need for lengthy and error-prone data analysis. Figure 2 illustrates the concept.

Many manufacturers, however, specify individual error parameters, unless there are legislative pressures compelling them to state the total error band of their sensors. For instance, if products or services are sold by weight, the weighing equipment is subject to legal metrology legislation and comes under the scrutiny of weights and measures authorities around the world.