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.