Скачать или смотреть Working of Differential pressure Transmitter

By viewing this video you can get an idea how the differential pressure transmitter works and how to measure the flow in a pipeline.
this is the function of secondary elements of flowmeter
Differential pressure flow meters have a primary and a secondary element. Generally speaking , the primary element is designed to produce a difference in pressure as the flow increases. There are many different types of primary element, the most common being the orifice plate, venturi, flow nozzle and pitot tube.

The secondary element of the flow meter is the differential pressure transmitter. It is designed to measure the differential pressure produced by the primary element as accurately as possible. In particular it is important that the differential pressure measurement is not affected by changes in the fluid pressure, temperature or other properties such as ambient temperature.

A good dp transmitter will ensure that the differential pressure is measured accurately independent of other changing parameters and will reliably transmit a signal to represent the differential pressure. In the case of a dp flow transmitter the output signal may also include square root extraction. Although it is common these days for this function to be carried our in a flow computer of Dcs system.
The output signal from and industrial DP transmitter is likely to be 4 to 20milliAmpheres, but it may also include digital communications such as HART, Profibusm Fieldbus, Modbus 485 RTU or one of many other communication protocols.

The majority of industrial DP Transmitters are fitted with diaphragm as the pressure sensing element. This diaphragm is a mechanical device. It is placed in between the two pressure inlet ports. The diaphragm will be deflected by the applied pressure. This deflection is converted into an electrical signal.