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Product category: Pressure sensors
News Release from: SensorsOne | Subject: Pressure transducers and transmitters
Edited by the Engineeringtalk Editorial Team on 10 October 2006

Pressure transducer or pressure
transmitter?

When does a pressure transducer become a transmitter? Wayne Bishop maintains there is no straightforward answer.

Distinguishing between a pressure transducer and a pressure transmitter can depend on which country you are working in and with which manufacturer you are dealing As with a lot of engineering terminology the original definitions have become a little blurred and have not always been used in the way that was originally intended

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 and semiconductor strain gauge to name just a few.

All of these sensing technologies are pressure transducers and they provide an electrical signal which varies characteristically with changes in pressure when connected to an appropriate power supply.

Typically a pressure transducers output signal is the one generated by the primary sensing element.

As 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 and 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 type of pressure sensor.

It is possible to source pressure transducers without compensation electronics which are also known as pressure capsules, modules or headers, but these tend to only be used by instrument manufacturers who design their own 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 has a lot of benefits over other types of pressure sensors, since they are smaller because there are fewer electronic components inside them and they have very few active components which 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 stations where they wanted to replace pressure gauges with pressure sensors.

Often 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 a system was developed to transmit the transducer signal over large distances around the plant without attenuation.

This system has seen some changes over the years as electronics have advanced and power considerations have changed but 4-20mA has been accepted worldwide as the main method for transmitting pressure transducer signals over long distances.

So a pressure transmitter is simply a pressure transducer with some extra electronics to transmit a 4-20mA output signal.

At first pressure transmitters would only be found in large sites and the sensors were bulky and expensive.

In recent years other industries have adopted the 4-20mA output signal pressure transmitter because they only need to use two wires 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 or a transmitter by just looking at its size and shape.

This is when the definitions started to get blurred between pressure transducers and transmitters, when they started to look the same.

Even more so when you throw in amplified voltage output pressure transducers which are a sort of halfway house between transducers and transmitters.

They have the signal strength to transmit over greater distances but use less power than the 4-20mA output signals.

One of the other benefits of pressure transmitters and amplified transducers is that they incorporate zero and 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 (eg 30 or 100mV) or nonamplified output it is a pressure transducer.

If the pressure sensor has a voltage (eg 0-5, 0-10 or 1-5V DC) output it is an amplified transducer.

If a pressure sensor has a current output (eg 4-20mA) it is a pressure transmitter.

But beware: you will find plenty of contradictions to this rule of thumb between manufacturers so always check the actual signal output on the datasheet where there should be no ambiguity.

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