Monitoring and Control Instruments
About the instruments used for monitoring
and controlling
An important distinction between
different instruments is whether they are suitable only for monitoring
functions or whether their output is in a form that can be directly
included as part of an automatic control system. Instruments which
only give an audio or visual indication of the magnitude of the
physical quantity measured, such as a liquid-in-glass thermometer, are
only suitable for monitoring purposes. This class normally includes
all null-type instruments and most passive transducers.
For an instrument to be suitable for inclusion in an automatic control
system, its output must be in a suitable form for direct input to the
controller. Usually, this means that an instrument with an electrical
output is required, although other forms of output such as optical or
pneumatic signals are used in some systems.

More Instrument Classification
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