Pressure sensor configurations
2 wire, 3 wire or 4 wire interfaces
The most typical interface configurations for industrial analogue outputs include 2 wire and 3 wire (current or voltage). The 2 wire system shares power supply and signal on the same 2 wires as it generally operates as a current “loop” (e.g. 4-20mA). The 3 wire system typically uses separate cores for supply and signal with a common 0V. This also facilitates low power ratiometric interfaces. The ratiometric configuration enables an output which is proportional to the supply voltage of typically 5V within a 10-90% range, eg 0.5V to 4.5V. This provides a low power solution for remote system applications such as environmental data logging. 4 wire systems are also used in some test and research applications, for example isolated input/output voltage or raw millivolt output transducers and other configurations.
The full range output of a pressure transducer or transmitter is usually calibrated for the full pressure range of the device (usually a standardized / DIN range eg 0 to10 bar, with the zero point being atmospheric pressure for gauge (g) units or a sealed vacuum reference for absolute (a) units. Gauge (g) atmospheric reference applications are the most common type of measurement, where the ambient barometric pressure is compensated by using a breather “vent” tube within the cable assembly or by a fine porous element. The ambient air pressure is directed through a vent hole or a vent tube to the back of the sensing element. A vented gauge pressure transducer / transmitter enables the local air pressure to be exposed to the negative side of the pressure sensing diaphragm so that it always measures with reference to ambient barometric pressure. Therefore a vented gauge (g or vg) pressure sensor always reads zero pressure when the process pressure connection is held open to atmospheric air, whereas an absolute pressure sensor will actually measure the atmospheric pressure.
A sealed gauge reference (sg) or sealed reference (sr) is similar except that atmospheric pressure is sealed on the negative side of the diaphragm at the time of manufacture. This is generally for high pressure applications (60 bar+ hydraulic) where atmospheric pressure changes will have a negligible effect on sensor accuracy. The zero set point is simply the ambient pressure value sealed into the reference chamber when the sensor was originally made. This means that the sealed gauge / sealed reference unit does not require a local atmospheric reference or vented cable when installed for operation.
For standard type sensors, to provide the most cost effective and readily available solution, popular fixed (DIN) ranges are manufactured and typically specified by the user allowing for sufficient overpressure whilst considering the working range and accuracy required (accuracy is usually expressed as a maximum error related to the full scale calibrated pressure range). They can be supplied calibrated to the user preferred pressure units (for example bar, PSI, KPa, mH2O, InHg etc). For higher quantity / volume requirements the pressure ranges can also be calibrated to intermediate / bespoke values as this will not compromise manufacturing costs. Overall full scale operating ranges can be supplied from just a few mbar to several thousand bars.