ABSTRACT
This chapter considers some aspects of instrumentation that do not appear to come under a specific category but nevertheless have to be considered when a system is being designed so that the final results are meaningful. For every installation where a differential-pressure instrument has been used, valve manifolds have to be included at both locations, primary device and transmitter; for without them it would be difficult to disconnect these from the process. The pressure difference across a distillation column, say, is best measured with two pressure instruments, and the difference being obtained by subtraction of the two signals, rather than a single differential-pressure instrument. Since pH is a highly nonlinear measurement, reagent addition in a pH control loop requires a controlled output that is nonlinear also. To obtain a pressure/temperature-compensated flow measurement using a differential-pressure transmitter, make the compensation calculation on the signal from the differential-pressure transmitter before extracting the square root to produce a measurement of flow.
