The dynamic modelling of differential pressure transmitters

Abstract

The paper describes the dynamic modelling of two force-balance differential-pressure transmitters, one pneumatic and one electronic. In both cases an experimental transmitter transfer function is found from frequency response tests. This is then compared with a theoretical transfer function calculated from the differential equations describing the principles of transmitter operation. The theoretical transfer function for the pneumatic transmitter is fourth order; there is reasonable agreement between theoretical and experimental transfer-function coefficients. A more detailed model of the adjustable fluid damping mechanism is necessary for closer agreement to be obtained. The theoretical transfer function describing the electronic transmitter is third order, with close agreement between experimental and theoretical transfer-function coefficients. The bandwidth of the electronic differential-pressure transmitter tested was between 0 and 0.3 Hz compared with a bandwidth of between 0 and 3 Hz for the pneumatic transmitter. This large difference is mainly attributable to the electronic transmitter having a larger damping constant and a demodulator low-pass filter with a long time constant.