2.4 Necessity for calibration
The foregoing discussion has described the static and dynamic
characteristics of measur[1]ing instruments
in some detail. However, an important qualification that has been omitted from
this discussion is that an instrument only conforms to stated static and
dynamic patterns of behaviour after it has been calibrated. It can normally be
assumed that a new instrument will have been calibrated when it is obtained
from an instrument manufacturer, and will therefore initially behave according
to the characteristics stated in the specifications. During use, however, its
behaviour will gradually diverge from the stated specification for a variety of
reasons. Such reasons include mechanical wear, and the effects of dirt, dust,
fumes and chemicals in the operating environment. The rate of divergence from
standard specifications varies according to the type of instru[1]ment, the frequency
of usage and the severity of the operating conditions. However, there will come
a time, determined by practical knowledge, when the characteristics of the
instrument will have drifted from the standard specification by an unacceptable
amount. When this situation is reached, it is necessary to recalibrate the
instrument to the standard specifications. Such recalibration is performed by
adjusting the instrument at each point in its output range until its output
readings are the same as those of a second standard instrument to which the
same inputs are applied. This second instru[1]ment
is one kept solely for calibration purposes whose specifications are accurately
known. Calibration procedures are discussed more fully in Chapter 4.
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