Page 305 - Introduction to Marine Engineering

_ Chapter 15

Instrumentation and control

All machinery must operate within certain desired parameters.
Instrumentation enables the parameters—pressure, temperature, and
so on—to be measured or displayed against a scale. A means of control is
also required in order to change or alter the displayed readings to meet
particular requirements. Control must be manual, the opening or
closing of a valve, or automatic, where a change in the system parameter
results in actions which return the value to that desired without human
involvement. The various display devices used for measurement of
system parameters will first be examined and then the theory and
application of automatic control.

Pressure measurement

The measurement of pressure may take place from one of two possible
datums, depending upon the type of instrument used. Absolute pressure is
a total measurement using zero pressure as datum. Gauge pressure is a
measurement above the atmospheric pressure which is used as a datum.
To express gauge pressure as an absolute value it is therefore necessary
to add the atmospheric pressure.

Manometer
A U-tube manometer is shown in Figure 15.1. One end is connected to
the pressure source; the other is open to atmosphere. The liquid in the
tube may be water or mercury and it will be positioned as shown. The
excess of pressure above atmospheric wil be shown as the difference in
liquid levels; this instrument therefore measures gauge pressure. It is
usually used for low value pressure readings such as air pressures.
Where two different system pressures are applied, this instrument will
measure differential pressure.

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