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The need for a pressure sensing element that
was extremely sensitive to low pressures and provided power
for activating recording and indicating mechanisms resulted
in the development of the metallic bellows pressure sensing
element. The metallic bellows is most accurate when measuring
pressures from 0.5 to 75 psig. However, when used in
conjunction with a heavy range spring, some bellows can be
used to measure pressures of over 1000 psig. Figure 1 shows a
basic metallic bellows pressure sensing element.
The bellows is a one-piece, collapsible,
seamless metallic unit that has deep folds formed from very
thin-walled tubing. The diameter of the bellows ranges from
0.5 to 12 in. and may have as many as 24 folds. System
pressure is applied to the internal volume of the bellows. As
the inlet pressure to the instrument varies, the bellows will
expand or contract. The moving end of the bellows is
connected to a mechanical linkage assembly. As the bellows
and linkage assembly moves, either an electrical signal is
generated or a direct pressure indication is provided. The
flexibility of a metallic bellows is similar in character to
that of a helical, coiled compression spring. Up to the
elastic limit of the bellows, the relation between increments
of load and deflection is linear. However, this relationship
exists only when the bellows is under compression. It is
necessary to construct the bellows such that all of the
travel occurs on the compression side of the point of
equilibrium. Therefore, in practice, the bellows must always
be opposed by a spring, and the deflection characteristics
will be the resulting force of the spring and bellows.
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