Pressure sensor vs transducer vs transmitter

You may hear electronic pressure detectors referred to as sensors, transducers, or transmitters. And understanding the difference in what these three terms mean is important to ensure the chosen device is right for the end application; particularly with regards to cost, power consumption, susceptibility to noise, and constraints around wiring and installation.

So how do you differentiate between these terms?

The term ‘pressure sensor’ can be regarded as a generic description for any device that measures pressure and provides an appropriate output in response.
 

Properties of the output interface

The properties of the output interface define the type of sensor.

To differentiate between the different types, it can be helpful to consider transducers as devices that have a voltage output, which may have a magnitude of a few millivolts or several volts. Transmitters, on the other hand, have a current output, usually designed for connecting to the standard 4-20mA current loop widely used in industrial sensing and control. The distinctions we’ll discuss here are:

• Millivolt-output pressure transducers
• Voltage-output pressure transducers
• Transmitters

The diagrams below show how a voltage-output transducer or current-output transmitter can be connected to a programmable logic controller (PLC) or meter to monitor pressure in a typical industrial equipment or process control application.

How to connect a pressure transducer or transmitter to industrial instrumentation

Millivolt-output pressure transducers

As is in the name, these transducers output in millivolts (mV).  The output signal is proportional to the power supply, for example, a 5VDC supply with a 10mV/V output signal produces a 0-50mV output on the sensor. Older foil-type strain-gauge sensors can produce an output of about 2-3mV/V, whereas today’s MEMS sensors can provide about 20mV/V with good linearity. Any variation in pressure is determined by measuring small changes in this voltage, which is a result of tiny changes in resistance (about 0.1%) in the strain gauges themselves.

A half-bridge strain gauge with millivolt output

The diagram below shows a half-bridge strain-gauge pressure sensor, illustrating the excitation voltage and output voltage. A larger excitation voltage, say 10V as opposed to 3V, produces a larger output voltage.

The simple interface circuitry of a millivolt output transducer helps ensure low cost and small package size, and gives designers the flexibility to design interface circuitry to suit their own application. However, there are several limitations to consider.

Because the full-scale output is directly proportional to the excitation, the excitation voltage must usually be generated using a regulated power supply.

In addition, owing to the low amplitude of the output, millivolt-output transducers are not usually suitable for use in electrically noisy environments.

And because the output voltage is attenuated by the resistance in connecting wires, these wires must be kept short, implying that the sensor must be close to the monitoring instrumentation. About three to six meters is usually the maximum practicable distance.

Voltage-output pressure transducers

A voltage-output transducer contains additional signal amplification to increase the output voltage of the bridge to a larger value such as 5V or 10V.

Having a larger output, voltage-output transducers are less susceptible to noise, allowing for use in harsher electrical environments. Longer connecting wires can be used, allowing the sensor to be further from the panel.

Supply voltages are typically from 8-28VDC. This allows the use of a lower-cost unregulated power supply, except where the output is 0.5-4.5V, which requires a 5VDC regulated supply. Lower current consumption means they are also suitable for battery operated equipment.

Older voltage output transducers do not have a ‘live zero’, meaning they do not output a signal when at zero pressure. The risk with these is that the system can’t recognise the difference between a failed sensor with no output and zero pressure.

Pressure transmitters

In contrast to a voltage-output transducer, a pressure transmitter has a low-impedance current output, most commonly designed to transmit analogue 4-20mA signals. The output may be designed for use with either a 2-wire or 4-wire current loop, as both types are widely used throughout industry.

4-20mA pressure transmitters provide good electrical noise immunity (EMI/RFI), making them ideal when the signal must be transmitted long distances. Transmitters can be powered by an unregulated supply, but current output is generally unsuitable for battery powered equipment when operating at full pressure.
 

Transducers v transmitters: Choosing the right one

Pressure transducers and transmitters are categories of pressure sensors widely used in industrial equipment and process control applications and differ in their output characteristics. To help select the right sensor for a given task, their strengths and weaknesses can be summarised as follows:

Millivolt transducer:

• Lowest cost
• Suitable where connection distances can be short and noise is not a problem
• Needs stable bridge-excitation voltage

Voltage transducer:

• Less susceptible to noise
• Shorter connection distances than pressure transmitter
• Lower power consumption than pressure transmitter
• Can work with unregulated bridge-excitation voltage
• Low power consumption

Transmitter:

• Easy to use in ubiquitous 4-20mA industrial sensing
• Long communication distance
• Low susceptibility to noise
• Typically higher power consumption than transducer types

Looking for more on pressure sensor technology? Check out the further chapters of this guide below, or if you're pressed for time you can download it in a PDF format here.