Sensitivity of Measurement to Disturbance

How disturbances could affect the sensitivity of measurement

All calibrations and specifications of an instrument are only valid under controlled conditions of temperature, pressure, etc. These standard ambient conditions are usually defined in the instrument specification. As variations occur in the ambient temperature, etc., certain static instrument characteristics change, and the sensitivity to disturbance is a measure of the magnitude of this change. Such environmental changes affect instruments in two main ways, known as zero drift and sensitivity drift.

Zero drift describes the effect where the zero reading of an instrument is modified by a change in ambient conditions. Typical units by which zero drift is measured are volts/C, in the case of a voltmeter affected by ambient temperature changes. This is often called the zero drift coefficient related to temperature changes. If the characteristic of an instrument is sensitive to several environmental parameters, then it will have several zero drift coefficients, one for each environmental parameter. The effect of zero drift is to impose a bias in the instrument output readings; this is normally removable by recalibration in the usual way.

Sensitivity drift (also known as scale factor drift) defines the amount by which an instrument's sensitivity of measurement varies as ambient conditions change. It is quantified by sensitivity drift coefficients which define how much drift there is for a unit change in each environmental parameter that the instrument characteristics are sensitive to.

Many components within an instrument are affected by environmental fluctuations, such as temperature changes: for instance, the modulus of elasticity of a spring is temperature dependent. Sensitivity drift is measured in units of the form (angular degree/bar)/C.

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