Pressure Transducers

A pressure transducer is a device which converts an applied pressure into a measurable electrical signal.

A pressure transducer consists of two main parts, an elastic material which will deform when exposed to a pressurized medium and a electrical device which detects the deformation.

The elastic material can be formed into many different shapes and sizes depending on the sensing principle and range of pressures to be measured. The most common method of utilising the elastic material is to form it into a thin flexible membrane called a diaphragm. The electrical device which is combined with the diaphragm to create a pressure transducer can be based on a resistive, capacitive or inductive principle of operation.

Product Selection Guides

Choose a pressure transducer for your application with one of these product selection guides:

Millivolt output – mV/V, 4 wire strain gauge signal
Voltage output – 0-10Vdc, 0-5Vdc, 1-5Vdc or 0.5 to 4.5Vdc, 3 wire amplified voltage signal
Current output – 2 wire 4-20mA or 3 wire 0-20mA
Digital output – RS485, USB, RS232, I²C, IO-Link or Modbus RTU

If you would like us to select a pressure transducer for you, then please use this form to specify your requirements.

Transducer Types

Strain Gauge

A resistive pressure transducer has strain gauges bonded or embedded into the surface of a diaphragm so that any change in pressure will cause a change in the electrical resistance of each strain gauge.

Variable Capacitance

A variable capacitance pressure transducer has a capacitive plate bonded to one side of the diaphragm and another capacitive plate bonded to an unpressurized surface in close proximty to the diaphragm. A change in pressure will widen or narrow the gap between the two plates which varies the capacitance.

Variable Inductance

An inductive pressure transducer uses the principle of inductance to convert the flexing of a diaphragm into the linear movement of a ferromagnetic core. The movement of the core is used to vary the induced current generated by an AC powered primary coil on another secondary pick-up coil.

Resonating Frequency

Other types of pressure transducer utilise the principle of the piezo-electric effect, or material structures which vary in resonant frequency with strain.

OEM Sensing Element Module

The pressure transducer sensing elements are mainly used by original equipment manufacturers (OEM) who incorporate them into their own products (e.g. gas analysers, leakage monitoring equipment) or pressure measurement manufacturers who incorporate them into more complex pressure sensors with compensated millivolt, amplified voltage, 4-20mA current loop or digital interface outputs, which are then sold to end users and equipment integrators.

Signal Conditioned

More complex pressure transducers include an electronic circuit for rationalising the transducer output signal so that there is very little difference from one transducer to another of the same type. A temperature compensation circuit is also added to reduce the errors associated with changes in media temperature. Often the rationalised and compensated output is then converted to a standardised output signal to make it universally compatible with end users instrumentation such as readouts, analog to digital converter cards, programmable logic controllers and data acquisition cards.

Connections & Fittings

Pressure transducers are fitted with a mechanical process connector (e.g. 1/4 BSP male, 1/2 NPT male) to the front of the diaphragm, and an electrical interface connection (e.g. DIN plug, M12) on the back of the electronics housing to enable the user to easily install the pressure transducer. In the case of pressure transducers sensing elements used by pressure measurement equipment manufacturers and OEMs, the pressure transducers may only have a prepared surface for clamping, bonding or welding and a PCB with pads or pin-holes suitable for soldering.

Basic Selection Considerations

If you need a transducer to measure gas or liquid pressure in a factory or laboratory environment there are a few parameters that should be considered in order to determine which pressure transducer is going to be suitable.

Installation & Location

The pressure of a fluid or gas is transmitted in all directions equally and over large distances, it is possible to install a pressure transducer some distance away from the process being measured via some suitable pipework.

A pressure transducer installed at a distance from the process greatly simplifies the selection process. Unfortunately it’s not always convenient to install remotely, for example you may require a fast response or the process media is hazardous and should be contained within a safe area.

Environmental Protection

If the pressure transducer is mounted indoors or inside an instrument enclosure outdoors, the temperature is unlikely to exceed ambient conditions due to the distance from the main process and the need to position instrument cabinets away from extremes of temperature to protect the electronics inside.

If the installation is outdoors an enclosure will protect the transducer from exposure to wet weather conditions which would otherwise require a high level of weatherproofing. If mounted indoors the transducer will be naturally protected as long as the ambient environment is relatively dry and clean, especially if the building is air conditioned. Electrical termination in these conditions does not require a high level of dirt or moisture ingress protection. The level of environmental protection is classified by IP Ratings, a DIN plug is more than adequate for these type of installations which has an IP65 rating and is very easy to install using the screw terminals without any soldering.

Process Connection

Fixing the transducer on to the end of a long piece of pipe directly or via a manifold block requires a very simple mechanical connection such as a G1/4 (1/4 BSP) or 1/4 NPT male/female thread which are both very popular fittings used in most countries.

Media Compatibility

The pressure media may vary enormously and it is impossible to specify a material that is compatible with every known substance. Stainless steel offers the most widespread protection whilst also providing the necessary elasticity to be used in the manufacture of sensing diaphragms.

Some transducers incorporate a ceramic diaphragm sealed onto a pressure housing via a Viton or FKM seal. This is also compatible with many types of media including caustic chemicals which can be highly corrosive to thin stainless steel membranes.

Vibration & Shock

Vibration and shock can be detrimental to the service life of a pressure transducer if it’s mechanical construction is not designed to a robust enough standard. The majority of standard commercially off the shelf type pressure transducers, are not designed for extreme or special conditions such as those specified by aerospace and military requirements, but should be more than adequate for most industrial environments where the transducer is installed in isolation from excessive shock & vibration via shock mounts or length of pipework.

Electrical Supply & Output

For the measurement signal, most interfaces still use an analog one for test and control purposes. There are mainly two types of analog output types which are compatible with most signal conditioning instrumentation, which are 4 to 20mA series current loop and 0 to 5 or 10 Volts. The required power supply for both of these output types varies slightly from transducer to transducer but are generally within the range of 15 to 30 Vdc. A common supply voltage available from most panel displays, plcs or separate power supplies is 24Vdc and this is suitable for powering most pressure transducers.

Pressure Range

The pressure range to specify will depend on the process but it should at the least reflect the maximum possible pressure of the process being measured. The other parameter to consider is the pressure reference which is one of 3 types: “gauge” is the pressure above or below atmospheric pressure, “absolute” is the pressure from 0 vacuum and “differential” is the pressure difference measured between two points.

Accuracy

The accuracy of the transducer is the main contributor to the measurement uncertainly in a system. Unless the transducer, along with its signal conditioning, are in a closely monitored and controlled environment, high levels of measurement accuracy are difficult to achieve in most applications with a general purpose pressure transducer. Although the linearity and thermal errors have improved in recent years with digital electronics, factors such as hysteresis, repeatability and stability are still limited by the transducer technology. An accuracy of 0.5% full scale is achievable for most transducers at room temperature. The temperature errors can vary as much as a few percent over a wide temperature range, but for ambient temperatures especially indoors the room temperature accuracy is a reasonable approximation of accuracy.

Checklist for pressure transducer requirements

Define your pressure requirements requirements using this checklist:

Pressure range?
Accuracy?
Electrical supply and output?
Media compatibility?
Process connection?
Installation and location?
Environmental protection?
Vibration and shock?

Pressure Transducer Help

Compound range -0.5…2 bar with 1 millibar accuracy

I need pressure transducers with the following characteristics; pressure range of -0.5 to 2 bar, accuracy of 1 mbar, output of 4 to 20 mA or 0 to 10VDC?

We would suggest the DMP 331i for your application to meet your technical requirements. The DMP331i high accuracy pressure transducer has the level of accuracy you need of 0.05% and is very stable and repeatable.

True analogue pressure transducers

Why is it so difficult to find a true analogue pressure transducer, that has no internal A/D and D/A conversion?

The majority of pressure transducers utilise ASIC chips to convert the low voltage output of the pressure sensing device to a signal conditioned amplified analogue output signal. For example a silicon strain gauge sensing element fitted internally to a transducer provides a millivolt output signal of around 5mV/V, this is then sent into a ZMDi ASIC which does the signal conditioning, calibration, compensation and provides a 0.5 to 4.5V output. This output is then fed into a further ASIC which converts the 0.5 to 4.5V output to a 4-20mA or 0-10V depending on customer requirements. This is therefore not a bare analogue device and unless you go for a straight millivolt bridge output this will be the case for majority of products.

Main cause of failure

What is the most likely cause of a pressure transducer failure?

Accidentally exceeding overpressure is probably the most common cause of pressure sensor failure. For more information see symptons and causes of pressure transducer diaphragm damage.

Measuring low pressures with miniature transducers

We are looking for a small pressure transducer a few millimeters in size which will be capable of measuring small water movements of a few Pa, is this possible?

Miniature diaphragm based pressure transducers are very poor at detecting very low pressures because there is insufficient surface area to generate enough movement in the sensing diaphragm to produce a strong output signal. Any signal that is achieved will have a low signal to noise ratio and will be largely susceptible to errors caused by changes in temperature.

2mV/V output for measuring hydraulic pressure to 700 bar

I am interested in a pressure transducer that has a 2mV/V output and can measure hydraulic pressure up to 700 bar. It will be supplied with an excitation voltage of 5V dc?

The IMP which has a 2mV/V output, it is low cost but has a low overpressure & burst pressure rating of 880 bar.

The TPS has a 3mV/V output, it is more robust and better suited to hydraulics, with a much higher overpressure rating of 2000 bar.

Variable capacitance vs strain gauge

How does variable capacitance compare to strain gauge sensor technology?

Variable capacitance sensors offer better performance than strain gauge devices at very low pressure ranges, although they are larger in diameter and are more expensive to manufacture.