A loss of differential pressure integrity of the secondary element, the DP transmitter, will
introduce an error into the indicated flow. This loss of integrity implies an impaired or degraded
pressure boundary between the high-pressure and low-pressure sides of the transmitter. A loss
of differential pressure boundary is caused by anything that results in the high- and low-pressure
sides of the DP transmitter being allowed to equalize pressure.
As previously discussed, flow rate is proportional to the square root of the differential pressure.
The extractor is used to electronically calculate the square root of the differential pressure and
provide an output proportional to system flow. The constants are determined by selection of the
appropriate electronic components.
The extractor output is amplified and sent to an indicator. The indicator provides either a local
or a remote indication of system flow.
Use of Flow Indication
The flow of liquids and gases carries energy through the piping system. In many situations, it
is very important to know whether there is flow and the rate at which the flow is occurring. An
example of flow that is important to a facility operator is equipment cooling flow. The flow of
coolant is essential in removing the heat generated by the system, thereby preventing damage to
the equipment. Typically, flow indication is used in protection systems and control systems that
help maintain system temperature.
Another method of determining system coolant flow is by using pump differential pressure. If
all means of flow indication are lost, flow can be approximated using pump differential pressure.
Pump differential pressure is proportional to the square of pump flow.
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
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