Pressure Sensors: The Design Engineers' Guide

Gauge pressure sensors

What are gauge pressure sensor?

In some applications the exact pressure or vacuum being generated is not of key importance. Instead, you just want to understand how much the pressure or vacuum differs in comparison to atmospheric pressure.

Cross-section of a typical gauge pressure sensor
A gauge pressure sensor measures pressure relative to the local atmospheric pressure

Atmospheric pressure varies across the globe depending on our altitude and even changes in the weather.

Consider, as an example, the vacuum pumps used during or after surgery. These are used to remove bodily fluids, gases and even tissue. Typically, only a small, finely controlled vacuum is required in order to avoid injury. This needs to be set in relation to the local atmospheric pressure. In a hospital at sea level, the atmospheric pressure will be higher than at a hospital high in the mountains.

A gauge pressure sensor measures the pressure at its port with respect to the local atmospheric pressure. This can be compared to using a multimeter’s DC measurement range, where the display shows the voltage at the positive probe with respect to the negative probe.

Gauge pressure sensors are typically packaged with a port, to which a pipe can be attached (as shown to the right), as well as a vent that is open to the atmosphere. The pipe can then be connected to the system where the measurement is to be made.

When positioning the sensor in your application, it is important that the vent is open to the atmosphere. This may require a hole in your printed circuit board and even the housing of your product.
 

How does a gauge pressure sensor work?




A piezoresistive Wheatstone bridge circuit with amplifier,
powered by a constant current circuit

There are a variety of strategies for measuring the pressure in a gauge sensor. Most of them use a membrane that is fitted with an electrical component, such as a resistor, whose value varies when flexed.

Nowadays, microelectromechanical systems, commonly known as MEMS, are utilised. Small and light structures are etched into silicon that can flex or vibrate. Since the base medium is silicon, further electronic circuitry can be integrated alongside the MEMS element.

The short electrical paths help to ensure low noise and high measurement accuracy, whilst the MEMS element can result in a sensor that is better isolated from temperature changes.

In all likelihood, your chosen commercial gauge sensor will probably be a piezoresistive type. Such sensors are constructed as a Wheatstone bridge (see diagram below) and require some analogue circuitry to amplify the signal for use with a microcontroller or other systems.

This can also include compensation for temperature and a calibration adjustment. A constant current circuit is used to power the bridge.
 

How do I integrate a gauge pressure sensor into my circuit?

With such a broad array of gauge pressure sensors available, it can be difficult to know where to start.

If you’re intending to interface your sensor with a microcontroller, you may find that a board-mounted sensor is the best option. Gauge pressure sensors tend to provide an analogue output, although some suppliers provide devices with digital interfaces.

Board mounted sensors come in a variety of packages, often with a short section of barbed manifold (see examples below) allowing the sensor to be connected by tube to the system to be measured.

A range of SMI pressure sensors
A range of pressure sensors with barbed manifolds

Many analogue sensors integrate the amplifier, temperature compensation and support for calibrating the device (see image below). This leaves an output signal in the range of 0 – 5 VDC which is suitable to connect to the analogue-to-digital converter of a microcontroller.

One thing to watch is the quality of the power supply, as many sensors are ratiometric. This means that the output signal varies with the input supply to the sensor.

 


Some analogue sensors provide a considerable amount
of analogue circuitry around the sensing element

When developing firmware to read the pressure provided, note should also be taken of the warm-up time of the sensor. After initial power on, it may take several milliseconds before the output can be relied upon.

If the sensor is part of a control loop, the response time for the sensor should also be reviewed. Gauge pressure sensors’ response times are often specified by the time taken for the output to change from 10% to 90% for a step change in pressure.

Sensors with a digital output tend to support the I2C or SPI protocol. However, there are exceptions. Some sensors use the single wire bi-directional ZACwire™ communication protocol (See diagram below). This is typically not native to microcontrollers, so supporting this interface in firmware will require some significant programming effort, as well as demanding a lot of processing time.

If you’re developing a low-power application, you will probably want to stick with protocols natively supported by the microcontroller’s peripherals.


Some manufacturers offer the single-wire ZACwire™ protocol on their gauge sensors, although this is uncommon in microcontrollers
 

Can I use an industrial gauge pressure sensor in my design?

Board level sensors are ideal for integrating onto a PCB, but they’re typically limited in the temperature range they support. You will also find that they’re not suited for monitoring most liquids and chemicals.

Industrial gauge sensors are an ideal alternative. They’re typically housed in a robust metal case, making them suitable for use in damp and corrosive environments. They also feature a screw thread, allowing them to be fixed to tanks and pipes with ease.

However, one of the challenges you may face is interfacing them to your system, especially a microcontroller.

Industrial complexes are often required to fulfil high levels of safety to protect their workers from the high pressures, corrosive liquids and dangerous equipment in their environment. Industrial gauge sensors are therefore designed with interfaces that are intended to guarantee that the measurements they deliver are always reliable.

Many ‘transmitter’ type sensors encode their output as an analogue signal in the form of a current between 4 and 20 mA. This can be in the form of a two-wire interface that doubles as the power supply to the sensor. This would need to be replicated in your circuitry as shown in the diagram below.


An example circuit showing how a gauge type transmitter may be implemented

Some industrial sensors have now gone digital, utilising protocols such as Fieldbus, standardised as IEC 61158, IO-Link, PROFIBUS and CANopen. Some of the electrical interfaces and associated signalling are compatible with microcontrollers, such as CANopen. All that is required is the matching software stack. Others, such as IO-Link and PROFIBUS, may require specialised microcontrollers or external circuitry to implement the interface.
 

What applications are gauge pressure sensors used in?

If the pressure measurement you intend to take needs to be relative to the local atmospheric pressure, you will need to use a gauge pressure sensor. For example, the level of a liquid in an open tank will change with variation in atmospheric pressure. A gauge pressure sensor allows this to be measured and compensates for those atmospheric pressure changes.

Medical applications also make regular use of gauge pressure sensors, for fluid extraction from wounds, in hyperbaric chambers, and ex-vivo blood pressure measurements. In such situations, the pressure, or vacuum, to be developed is often small, and requires fine control in order to avoid harm to the patient.

If you'd like to read more on other pressure measurement types then click the links below:

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 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 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 7 GBL

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Chapter 7

Pressure sensors for different media types

An in-depth guide to pressure sensors for different media types. Learn about the technology, applications, different options, their specifications and their limitations.

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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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Pressure sensors chapter 8 GBL

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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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