As previously discussed, the density of the fluid whose flow is to be measured can have a large effect on flow sensing instrumentation. The effect of density is most important when the flow sensing instrumentation is measuring gas flows, such as steam. Since the density of a gas is directly affected by temperature and pressure, any changes in either of these parameters will have a direct effect on the measured flow. Therefore, any changes in fluid temperature or pressure must be compensated for to achieve an accurate measurement of flow.

Ambient temperature variations will affect the accuracy and reliability of flow sensing instrumentation. Variations in ambient temperature can directly affect the resistance of components in the instrumentation circuitry, and, therefore, affect the calibration of electric/electronic equipment. The effects of temperature variations are reduced by the design of the circuitry and by maintaining the flow sensing instrumentation in the proper environment.

The presence of humidity will also affect most electrical equipment, especially electronic equipment. High humidity causes moisture to collect on the equipment. This moisture can cause short circuits, grounds, and corrosion, which, in turn, may damage components. The effects due to humidity are controlled by maintaining the equipment in the proper environment.

Summary

The density of the fluid, ambient temperature, and humidity are the three factors which can affect the accuracy and reliability of flow sensing instrumentation. The purpose of each block of a typical differential pressure flow detection circuit is summarized below.

Flow Circuitry Summary

The differential pressure is used by the DP transmitter to provide an output proportional to the flow.

The extractor is used to electronically calculate the square root of the differential pressure and to provide an output proportional to system flow.

The indicator provides either a local or a remote indication of system flow.