Davidson Measurement Tech Tips

Interpreting Error Statements

This article is dedicated to clarifying the difference between error statements of Percent of reading and Percent of full scale. Both measurement and calibration procedures are affected by the type of error specified so it is important to realise the implications of those errors.

We will take the example of a sensor with a full scale of 1000 kPa, however, the information can obviously be applied to other types of measurement devices such as displacement and vibration sensors etc.

Let's say the error is +/-0.1% FS (full scale), giving an error band of +/-1kPa. The error of +/-1 kPa (+/-0.1%FS) is applied across the full range of the device, then as you reduce the pressure, the actual error for the reading becomes worse as shown below:

Thus you can see that as your pressure value moves to the lower end of the scale the actual error as a percent of the reading increases quite considerably.

Using the same error, this time as a percent of reading rather than full scale, the values become significantly different. This time we have a constant percentage error of reading of +/-0.1%. See how the error band changes to a smaller value:-

The above is vital when deciding what calibration device to specify; you need to ensure it provides you with the best possible accuracy statement to meet the demands of your measuring parameters.