Figure 8-3.Bourdon-tube differential pressure gauge.
example, some hydraulic gauges have a special
type of spring-loaded linkage that is capable of
taking overpressure and underpressure without
damage to the movement and that keeps the
pointer from slamming back to zero when the
pressure is suddenly changed. A hydraulic gauge
that does not have such a device must be protected
by a suitable check valve. Some hydraulic gauges
may also have special dials that indicate both the
pressure (in psi) and the corresponding total force
being applied, for example tons of force produced
by a hydraulic press.
Spiral and Helical Bourdon Tubes
Spiral and helical Bourdon tubes (figs. 8-4 and
8-5) are made from tubing with a flattened cross
Figure 8-4.Spiral Bourdon tube.
section. Both were designed to provide more travel
of the tube tip, primarily for moving the recording
pen of pressure recorders.
BELLOWS ELASTIC ELEMENTS
A bellows elastic element is a convoluted unit
that expands and contracts axially with changes
in pressure. The pressure to be measured can be
applied to either the outside or the inside of the
bellows; in practice, most bellows measuring
Figure 8-5.Helical Bourdon tube.