Differential pressure flowmeter measurements are inherently affected by fluid density. Density changes in liquid applications are usually small because of their non-compressible nature and because (in many applications) process temperature has a relatively small affect on density. In gas and vapor applications, both temperature and pressure can affect density and significantly degrade the quality of the flow measurement. Notwithstanding the above, note that changes in fluid composition can affect the density of the fluid.

Flow computers can be used to compensate for density and other operating parameters in applications where degradation of the flow measurement produces unacceptable flow measurement performance. A flow computer can be implemented as a separate hardware device that calculates the compensated flow measurement from field devices, such as differential pressure, pressure and/or temperature instruments. These calculations can also be performed in the process control system. In addition to measuring flow, temperature and pressure, some multivariable differential pressure flow transmitters can perform these calculations internally.

Multivariable flowmeters, such as multivariable differential pressure flow transmitters and other multivariable flowmeter technologies, are outside the scope of this article because even though there is some overlap with information contained here, additional parameters are used to evaluate the relative performance of the different technologies.

Differential pressure flow transmitters generate an electrical signal that represents the differential pressure at its ports. Of importance is how well the differential pressure flow transmitter performs this function. Because performance is the prime concern in many applications, the technology used to effect the measurement is typically a secondary or tertiary matter.