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Pressure Gauges | Up
Fluid Power - Intro to Hydraulics, Pneumatics, and how it all works | Next
Spiral and Helical Bourdon Tubes |
C-shaped Bourdon Tube
The C-shaped Bourdon tube has a hollow,
elliptical cross section. It is closed at one end and
is connected to the fluid pressure at the other end.
When pressure is applied, its cross section
becomes more circular, causing the tube to
straighten out, like a garden hose when the water
is first turned on, until the force of the fluid
pressure is balanced by the elastic resistance of
the tube material. Since the open end of the tube
is anchored in a fixed position, changes in pressure
move the closed end. A pointer is attached to the
closed end of the tube through a linkage arm and
a gear and pinion assembly, which rotates the
pointer around a graduated scale.
Bourdon-tube pressure gauges are often
classified as simplex or duplex, depending upon
whether they measure one pressure or two
pressures. A simplex gauge has only one Bourdon
tube and measures only one pressure. The pressure
gauge shown in figure 8-1 is a simplex gauge. A
red hand is available on some gauges. This hand
is manually positioned at the maximum operating
pressure of the system or portion of the system
in which the gauge is installed.
When two Bourdon tubes are mounted in
a single case, with each mechanism acting
independently but with the two pointers mounted
on a common dial, the assembly is called a duplex
gauge. Figure 8-2 shows a duplex gauge with views
of the dial and the operating mechanism. Note
that each Bourdon tube has its own pressure
connection and its own pointer. Duplex gauges
are used to give a simultaneous indication of the
pressure from two different locations. For
example, it may be used to measure the inlet and
outlet pressures of a strainer to obtain the
differential pressure across it.
Differential pressure may also be measured
with Bourdon-tube gauges. One kind of Bourdon-
tube differential pressure gauge is shown in
figure 8-3. This gauge has two Bourdon tubes
but only one pointer. The Bourdon tubes are
connected in such a way that they indicate the
pressure difference, rather than either of two
actual pressures.
As mentioned earlier, Bourdon-tube pressure
gauges are used in many hydraulic systems. In this
application they are usually referred to as
hydraulic gauges. Bourdon-tube hydraulic gauges
are not particularly different from other types of
Bourdon-tube gauges in how they operate;
however, they do sometimes have special design
features because of the extremely high system
pressures to which they may be exposed. For
Figure 8-2.—Duplex Bourdon-tube pressure gauge.
8-2
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