Differential Pressure Transmitter Construction

The most common industrial pressure measuring instrument is the differential pressure transmitter. This equipment detects the pressure difference between two ports and produces an output signal with reference to a calibrated pressure range.

Industrial differential pressure transmitters are made of two boxes. The pressure sensing element is housed in the lower half and the electronics are housed in the upper half. It has two pressure ports marked "High" and "Low". It is not mandatory that the high port is always under high pressure and the low port always under low pressure. This labeling is related to the effect of the port on the output signal.

A differential pressure transmitter has three functional parts.

1) Direct pressure sensing element (located in the lower compartment).

Most industrial differential pressure transmitters are equipped with diaphragm as a pressure sensing element. This diaphragm is a mechanical device. It is placed between the two pressure inlet ports. The diaphragm will be deflected by the applied pressure.

This deflection is converted into an electrical signal. This is usually done by the sensors. Commonly used sensors are (a) Voltage meter (b) Differential capacitance (c) Vibrating wire. The sensor output is proportional to the applied pressure.

2) Electronic Unit

The electrical signal generated in the lower chamber by the sensor is only in the milli-volt range.

This signal must be amplified to the 0-5V or 0-10V range or converted to 4-20mA for later transmission to a remote instrument. This upper enclosure is the transmitter portion of the differential pressure transmitter that houses the electronics unit.

3) 4-20mA 2-wire current transmitter

A DC output current is generated that is directly proportional to the pressure range of the differential pressure transmitter. The lower range is 4mA and the upper range is 20mA. This controlled current output is unaffected by load impedance variation and supply voltage fluctuations. This 4-20 mA output overlaps with BRAIN or HART FSK protocol digital communications.