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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