Page 307 - Introduction to Marine Engineering

Instrumentation and control 281

Spring
Evacuated corrugated cylinder Pointer
Linkage
Scale

Figure 15.3 Aneroid barometer

pressure acting on the mercury in the container. An absolute reading of
atmospheric pressure is thus given.
The aneroid barometer uses an evacuated corrugated cylinder to detect
changes in atmospheric pressure (Figure 15.3). The cylinder centre
tends to collapse as atmospheric pressure increases or is lifted by the
spring as atmospheric pressure falls. A series of linkages transfers the
movement to a pointer moving over a scale.

Bourdon tube
This is probably the most commonly used gauge pressure measuring
instrument and is shown in Figure 15.4. It is made up of an elliptical

Scale

Bourdon tube

Tube section

Adjustable
inkage

System
pressure
Figure 15.4 Bourdon tube pressure gauge