A pressure transducer is fundamentally any device that converts an applied pressure into an electrical signal. There have been many different types of pressure transducer developed over the years such as bonded foil, thick film, thin film & semiconductor strain gauge to name just a few. All of these sensing technologies are pressure transducers and they provide an electrical signal typically a millivolt output signal which varies with changes in pressure when connected to an appropriate power supply.
Typically a pressure transducers output signal is one generated by the primary sensing element. Since it is difficult to achieve tight electrical tolerances with the sensing elements during manufacture it is often necessary to add a circuit to trim the zero & span offsets and compensate for errors over the operating temperature range. However the pressure transducer still retains the natural characteristics of the particular sensing technology employed such as linearity, hysteresis, repeatability, stability and frequency response. In fact these are the main reasons for using a pressure transducer which apart from the compensation circuitry are the purest form of pressure sensor.
It is possible to source pressure transducers without compensation electronics which are also known as pressure capsules, modules or headers, which are mostly used by instrument manufacturers who utilise their own micro-electronics to compensate the pressure transducer.
If the electrical connections are short, such as in the laboratory or inside an electronics enclosure the pressure transducer is more desirable, since it is smaller because it has virtually no pcb electronic components inside and there are none or very few active electronic components that can be upset by electromagnetic interference.
So what happens when you want to run the electrical output signal of a pressure transducer over long distances? In the past this was required when engineers started to automate process plants such as power generation plants where they wanted to replace pressure gauges with pressure sensors. The distance was too far between the control system and the measurement point to use a pressure transducer, because the signal losses and interference of the cables would be too great. Therefore the transducer output signal needed to be amplified to transmit the transducer signal over large distances around the plant without attenuation. Many types of amplified signals have been developed over the years as electronics have advanced and power considerations have changed but the 2 wire 4-20mA current loop output has long been accepted worldwide as the main method for transmitting pressure transducer signals over long distances and is still the most requested output signal particularly in the process control industry.
So a pressure transmitter is simply a pressure transducer with some extra electronics to transmit a 4 to 20 mA output signal. At first pressure transmitters would only be found in large process plants and the sensors were bulky and relatively expensive. In recent years other industries have adopted the 4-20mA output signal pressure transmitter because it only needs 2 wire connections and solid-state electronics has shrunk the size and cost of these pressure sensors so much, that it is now difficult to tell the difference between a pressure transducer and transmitter by just looking at its size and shape.
The shrinking of the size of a pressure transmitter is when the definitions started to get blurred between pressure transducers and transmitters. The divisions have become even more blurred with amplified voltage output pressure transducers. They have the signal strength to transmit over greater distances than a strain gauge output pressure transducer but use less power than the 4-20mA output signals.
One of the other benefits of pressure transmitters and high voltage output pressure transducers is that they incorporate zero & span adjustment potentiometers so that the pressure sensors can be easily calibrated.
So today the general rule of thumb is that if the pressure sensor has a millivolt (e.g. 30mV or 100mV) or non-amplified output it is a pressure transducer. If the pressure sensor has a voltage (e.g. 0-5Vdc , 0-10Vdc or 1-5Vdc ) output it is an amplified voltage output pressure transducer. If a pressure sensor has a current loop output (e.g. 2 wire 4-20mA or 3 wire 0 /4-20mA ) it is a pressure transmitter .
However you should be aware that you will find plenty of contradictions to this rule of thumb between manufacturers so always check the actual signal output on the data sheet where there should be no ambiguity and it is best not to rely on the descriptive name to define the output signal.
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