A self-balancing pneumatic potentiometer and Wheatstone bridge with electrical readout: applications to surface roughness measurement, pneumatic gauging and to measurement of pressure difference ratios

Abstract Earlier papers by the author have shown how an exact pneumatic analogue of the Wheatstone bridge can be used for surface roughness measurement. The further developments now described make the bridge self-balancing and give a direct electrical readout. This is achieved by using a new detector giving an electrical output, which is used to control an electrically variable fluid resistor. The result is a roughness measuring instrument which is robust and which requires minimal skill in operation. The bridge could equally well be used in pneumatic gauging. The same elements may also be used in a self-balancing potentiometer for measurement of pressure difference ratios such as aerodynamic pressure coefficients