Technical explanations for pressure transmitters

A pressure transmitter (also called pressure transducer or pressure converter) is a component used to convert a pneumatic or hydraulic pressure to an electric (usually analogue and linear) output signal, such as a current or voltage.

The pressure measuring cell fitted has a membrane that is exposed to the pressure to be measured. Affixed on this membrane is a bridge circuit consisting of four ohmic resistors in the form of a Wheatstone bridge. The values of these resistors change proportionally to the pressure load present at the measuring cell or membrane. The bridge voltage of the measuring cell is amplified in the evaluation electronics and processed digitally by a microcontroller. The downstream output stage converts this signal to the output signal required (such as 4 - 20 mA or 0 - 10 V).

Transmitters with an output signal of 0 to 10 V are a commonly used variant due to their simple initial operation and straightforward scaling of the signal (0 V for 0 bar). The output load must be selected as highly resistive (with typical minimum value 4.7 k). SUCO transmitters with voltage output have a 3-wire design. The maximum connection length should not exceed 30 m to prevent signicant voltage drops in the signal line.

SUCO transmitters with ratiometric output are operated with a 5 V supply voltage as 3-wire conguration. The output signal is directly proportional/ dependent to/on the supply voltage; this is known as a ratiometric dependency. 0.5 – 4.5 V is established as an output voltage because many A/D converters work with reference voltage Uv+ of 5 V. The output voltage 0.5 V equals to 10% and 4.5 V corresponds to 90% of the supply voltage. The span is therefore 80% of the supply voltage. This variant is used for example when a trans mitter and a downstream A/D converter as an evaluation unit are to be powered with the same reference / operating voltage.

The most common analogue output signal of sensors is 4 – 20 mA current output (as 2-wire conguration). The advantage of a 4 – 20 mA output signal is the 4 mA oset which allows the monitoring of potential wire break and short-circuit (life zero signal). The signal can also be transmitted over long distances with no loss in accuracy. This variant is also the least sensitive to EMC factors. 2-wire technology also means wiring overhead is reduced.