The Bourdon tube, capsule, or bellows convert pressure into mechanical motion
which is well suited for conversion to direct visual indication; the Bourdon tube
for instance is normally an integral part of the indicator. Figure 12.5a shows a
mechanical linkage from a sensor to a direct reading indicator as is normally used
for pressure sensing. The Bourdon tube is normally located behind the dial. As
the pressure changes the Bourdon tube changes its radius and moves the toothed
slider to operate the pointer. The pointer moves over a scale which is graduated
in pounds per square inch and so on. These devices are cost effective and are in
wide scale use, but are not temperature compensated and the cheaper instruments
do not have a zero or span adjustment. More expensive devices may have
screw adjustments and a limited temperature range.