Pressure Sensors: The Design Engineers' Guide

Pneumatic and hydraulic pressure sensors

Applications

Examples of systems built around pneumatic technology include vehicle tyres, air brakes (on buses, trucks and trains), air compressors, compressed-air engines, vacuum pumps and more.

Examples of hydraulic applications include vehicle braking systems, power steering systems, shock absorbers, utility vehicles such as excavators and aerial platforms, lifts and industrial machinery such as hydraulic presses.

The overarching application of pressure sensors in pneumatics and hydraulics is to ensure the pressure within the system is at the correct level, or within an optimum range.

This is particularly important for hydraulics, where the liquid in the system may be volatile or flammable (for example, mineral oil) and reach very high pressures and temperatures, making leaks and accidents potentially dangerous.

Pressure sensors feature as part of pressure regulators, or automatic valves designed to control the pressure in the system (as shown below). Pressure regulators match the demand for gas or liquid to the demands of the system, while maintaining a constant output pressure. As the system demands more power, so the load flow increases, and the regulator flow must increase, or the controlled pressure will fall.


A load-sensing hydraulic system featuring three pressure sensors

Measurement options

Like other types of pressure sensor, sensors used in pneumatics and hydraulics can measure differential pressure (the difference between two pressures) or absolute pressure (measured against zero or another absolute value).

In pressure regulators, differential pressure sensors compare the pressure on either side of a valve, to determine whether the inlet flow is equal to the outlet flow.

Technology

Pneumatic and hydraulic pressure sensors are transducers, generating an electrical signal in proportion to the pressure they measure. This allows pressure to be monitored by a range of electronic devices.

The technology used most often in pneumatic and hydraulic pressure sensors uses a physical diaphragm, often made of silicon, which bends as pressure is applied to it. The diaphragm is a strain gauge, which varies its electrical resistance when force is applied – in this case, from pressure exerted by air, gas or hydraulic liquid on the sensor. This resistance is used to modify the output voltage of the sensor.


A cross-section linear variable differential transformer pressure sensor

Some pressure sensors for power-steering applications use a linear variable differential transformer. This includes a core that moves within a hollow tube to monitor the movement of a directional control valve with high precision, allowing hydraulic fluid to flow into different areas of the system.

Many pressure sensors are now standalone, incorporating all the electronics and temperature compensation technology they need into the unit itself.

However, as the pressure used in hydraulics systems increases to drive efficiency, and systems overall become smaller and more compact, this isn’t always possible. An alternative is embedded sensors, where the electronic components are located away from the sensor itself. This allows the sensor to work in environments characterised by high temperature, vibration and radiation.

To withstand harsh environments, pressure-sensing chips have been designed such that the medium (gas or liquid) only comes into contact with silicon, helping to protect electronic components.

Some pressure sensors for pneumatics and hydraulics function by measuring the expansion of a flexible tube, rather than the pressure in the gas or liquid directly. This can help to detect blockages within the tube and monitor pump performance.

Options and specifications

Pneumatic and hydraulic pressure sensors will usually have a range of pressures they can measure – for example, 0 to 200 bar. They may also specify a safe limit of pressure, above which the unit may malfunction, and a temperature range within which they will provide accurate readings (for example, -40ºC to 85ºC).

Most pneumatic and hydraulic pressure sensors will also specify an error band – for example, ±0.05% – indicating the level of accuracy of the sensor.

Other options may include input and output connection type and output signal, as well as physical specifications such as material (including for parts in contact with liquid), dimensions and thread sizes. 

Limitations

Hydraulics, in particular, are used in harsh and challenging environments involving extreme heat, water, dust and even radiation. Hydraulics on heavy vehicles may be subject to physical shocks and vibration, and may also be subject to sudden pressure spikes. Pressure sensors therefore need to be able to withstand these conditions and still function correctly.

If you want to learn more about the different types of media that pressure sensors can measure, the applications of each type, and the different sensor options for your design, click the links below to jump to the section you're interested in.


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.

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Pressure Sensors Chapter 1 GBL

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Chapter 1

How pressure sensors work

An introduction to pressure sensors covering the different types, how they work, their function, construction, and what to consider in your design choices.

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Pressure sensors chapter 5 GBL

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Chapter 5

Types of pressure measurement

What’s the difference between absolute, gauge and differential pressure sensors? And how do you know which one to use?

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Pressure Sensors Chapter 2 GBL

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Chapter 2

Pressure sensor applications

Discover the recent innovations in pressure sensor technology that are enabling smarter, safer, and more environmentally friendly electronics for businesses and consumers alike.

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Pressure sensors chapter 6 GBL

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Chapter 6

The core pressure sensor technologies

What’s the difference between the different pressure sensor technologies? And how do you know which one to use?

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Pressure Sensors Chapter 3 GBL

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Chapter 3

The different types of pressure sensors

Discover how pressure sensors vary according to the type of pressure measurement, sensing principles, output signal, media, MEMS technology, mounting and more.

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Chapter 8

Pressure sensing in harsh environments

An in-depth guide to pressure sensors for harsh environments - designing for extreme temperatures, high pressure, and corrosive and dynamic environments.

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Chapter 4

Pressure sensor output signals

Sensors, transducers, or transmitters? The right selection is important for your application. So what's the difference and how do you choose between them?

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Pressure sensors chapter 9 GBL

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Chapter 9

Understanding specifications

Explore the datasheet and the different factors affecting the accuracy of pressure sensor readings. Discover how to make the right choice for your application.

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