steel, beryllium-copper, or other metal suitable
but only one pointer. The Bourdon tubes are
for the intended service of the gauge.
connected in such a way that they are the pressure
difference, rather than either of the two actual
Most bellows gauges are spring-loaded--that
pressures indicated by the pointer.
is, a spring opposes the bellows and thus prevents
full expansion of the bellows. Limiting the
expansion of the bellows in this way protects the
bellows and prolongs its life. In a spring-loaded
bellows element, the deflection is the result of the
A bellows elastic element is a convoluted unit
force acting on the bellows and the opposing force
that expands and contracts axially with changes
of the spring.
in pressure. The pressure to be measured can be
applied to the outside or to the inside of the
Although some bellows instruments can be
bellows; in practice, most bellows measuring
designed for measuring pressures up to 800 psig,
devices have the pressure applied to the outside
their primary application aboard ship is in the
of the bellows, as shown in figure 5-17. Like
measurement of quite low pressures or small
Bourdon-tube elements, bellows elastic elements
are made of brass, phosphor bronze, stainless
Figure 5-17.--Bellows gauge.