Principles of Measurement
About the measurement principles
Today, the techniques of measurement are of
immense importance in most facets of human civilization. Present-day
applications of measuring instruments can be classified into three major
areas. The first of these is their use in regulating trade, and includes
instruments which measure physical quantities such as length, volume and
mass in terms of standard units.
The second area for the application of measuring instruments is in
monitoring functions. These provide information which enables human beings
to take some prescribed action accordingly. Whilst there are thus
many uses of instrumentation in our normal domestic lives, the majority of
monitoring functions exist to provide the information necessary to allow a
human being to control some industrial operation or process. In a chemical
process for instance, the progress of chemical reactions is indicated by
the measurement of temperatures and pressures at various points, and
such measurements allow the operator to take correct decisions regarding
the electrical supply to heaters, cooling water flows, valve positions,
etc. One other important use of monitoring instruments is in calibrating
the instruments used in the automatic process control systems.
Use as part of automatic control systems forms the third area for the
application of measurement systems. The characteristics of measuring
instruments in such feedback control systems are of fundamental importance
to the quality of control achieved. The accuracy and resolution with which
an output variable of a process is controlled can never be better than the
accuracy and resolution of the measuring instruments used. This is a very
important principle, but one which is often inadequately discussed in many
texts on automatic control systems. Such texts explore the theoretical
aspects of control system design in considerable depth, but fail to give
sufficient emphasis to the fact that all gain and phase margin performance
calculations, etc., are entirely dependent on the quality of the process
measurements obtained.
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