Figure 11-1.C-shaped Bourdon tube.
There are many types of Bourdon-tube gauges used
in the Navy. The most common ones are the simplex,
duplex, vacuum, compound, and differential pressure
gauges. They operate on the principle that pressure in a
curved tube has a tendency to straighten out the tube.
This curved tube is made of bronze for pressure under
200 psi and of steel for pressures over 200 psi.
Simplex Bourdon-tube Gauge
Figure 11-2 shows a simplex Bourdon tube installed
in a gauge case. Notice that the Bourdon tube is in the
shape of the letter C and is welded or silver-brazed to
the stationary base. The free end of the tube is connected
to the indicating mechanism by a linkage assembly. The
threaded socket, welded to the stationary base, is the
pressure connection. When pressure enters the Bourdon
tube, the tube tends to straighten out. The tube
movement through linkage causes the pointer to move
proptionally to the pressure applied to the tube. The
simplex gauge is used for measuring the pressure of
steam, air, water, oil, and similar fluids or gases.
Duplex Bourdon-tube Gauge
The duplex Bourdon-tube gauge (fig. 11-3) has two
tubes and two separate gear mechanisms within the
same case. As shown in view B, a pointer is connected
to the gear mechanism of each tube. Each pointer
operates independently. Duplex gauges are normally
used to show pressure drops between the inlet and outlet
sides of lube oil strainers. If the pressure reading for the
inlet side of a strainer is much greater than the pressure
reading for the outlet side, you may assume that the
strainer is likely to be dirty and is restricting the flow of
lube oil through the strainer.
Bourdon-tube Vacuum Gauge, Compound
Gauge, and Differential Pressure Gauge
Bourdon-tube vacuum gauges are marked off in
inches of mercury (fig. 11-4). When a gauge is designed
to measure both vacuum and pressure, it is called a
compound gauge. Compound gauges are marked off
both in inches of mercury (in.Hg) and in psig (fig. 11-5).
Figure 11-2.Simplex Bourdon-tube pressure gauge.