Pressure sensors: The design engineer's guide
The advent of smaller, lower-cost and lower-powered pressure sensors has increased efficiency and performance, and generated a new wave of innovation, in both the sensors themselves and their applications. As demand for these technologies is increasing, the worldwide pressure sensor market is estimated to grow to $11.4 billion by 2024. With a variety of different sensor technologies now available on the market, it can be difficult to know where to start.
The Design Engineer’s Guide to Pressure Sensors was created to help you understand the types of sensors in common use, their operating principles, and their modes of use (absolute, gauge, or differential). Whether you’ve designed with pressure sensors before or not, this field guide is intended to provide a comprehensive overview of the technologies, insight into the various applications, and assistance in finding the sensor that’s right for your design.
Further down this page, you’ll find an introduction to how pressure sensors work, or you can jump ahead to the most relevant chapters for you:
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Since there are many different types of applications for pressure sensors, there are many types of sensors available with a wide variety of characteristics, whether it’s for harsh or corrosive environments, medical equipment or mobile devices. Selecting a pressure sensor means choosing from a vast array of technologies, packages, performance levels and features in order to meet multiple demands for accurate pressure measurement, such as:
- Gas pressure inside a tank, such as an industrial-compressor reservoir
- Measuring level or volume of liquid contained by sensing the pressure at the bottom of a vessel
- Measuring pressure differences between two points in a system, as a means of monitoring or quantifying the flow of liquids or gases
- Barometric pressure: change in atmospheric pressure with weather conditions or with altitude. Useful in weather stations, environmental monitoring, or to assist navigation dead reckoning alongside GPS or cell triangulation
What is pressure?
Pressure = Force / Area
In SI (MKS) units, a force of one Newton, applied to an area of one square meter, exerts a pressure of one Newton per square meter, or one Pascal.
Any kind of pressure sensor contains a mechanism or structure that reacts proportionately to a force applied. The area over which the force is applied is constant, for a given sensor structure.
There are three different types of pressures that can be measured: gauge, absolute, and differential.
Gauge pressure is the pressure measured relative to the ambient atmospheric pressure. It can be positive for pressures higher than atmospheric, or negative for lower pressures. A gauge pressure sensor will have two ports, allowing the media at the reference pressure, and at the pressure. A typical application for a gauge pressure sensor is to measure liquid levels in a vented tank using the difference in hydrostatic pressure and ambient atmospheric pressure.
You can read more on gauge pressure in chapter five: types of pressure measurement.
Absolute pressure sensors will give the result relative to zero (a perfect vacuum). Sensors will have one port for the media to enter and exert pressure on the sensing element, producing a positive change in output, of magnitude proportional to the pressure applied.
This is useful in applications that are measuring atmospheric pressure, perhaps to determine altitude. Absolute pressure sensors are also used in pressure measurement applications that will be used at different altitudes, since atmospheric pressure varies with altitude, gauge pressure wouldn’t give an accurate reading. This type of sensor is used in tyre pressure monitoring systems to optimise tyre performance.
For more on absolute pressure sensors, head to chapter five.
Differential pressure sensors measure the difference in pressure between two points, similar to how a gauge sensor works. But in this case the reference pressure is one of the points in the system, as determined by the system designer. The change in differential output is positive or negative, depending on which is greater. The magnitude of the change is proportional to the pressure difference between the two domains.
As an example, differential sensors are sometimes used to detect the pressure difference either side of an object. Differential pressure sensors are often used to monitor airflow in HVAC applications.
In chapter five we explore differential pressure sensors in more detail.
To an extent, the operating principle – absolute, gauge or differential – determines the sensor’s construction. An absolute pressure sensor may be designed to respond to pressure applied at the top side or the back side, when mounted on a circuit board or a panel, for example. Creating a port for the measured media to enter through the top side may leave the sensor vulnerable to hazards such as physical damage or contamination with dirt or moisture. A bottom-side entry sensor may be chosen to overcome this. The diagrams below compare the layout of both types.
A diagram of top-side or bottom entry absolute pressure sensors
A gauge sensor is typically designed to allow atmospheric pressure to apply to one port, while permitting the measured pressure to be applied to the other.
Similarly, a differential sensor will feature two ports, through which each of the measured media is designed to come into contact with the sensing element. The diagram below compares the construction of gauge and differential sensors.
Examples of gauge (left) and two-port differential (right) pressure sensor packages
Pressure sensor, transducer, or transmitter?
It is worth noting that “pressure sensor” is a generic term to describe a pressure-sensing device that may be a sensors, a transducer or a transmitter, depending on the design of associated electrical circuitry.
The sensing element responsible for detecting and quantifying the effects of applied pressure produces an output that cannot be used directly in an electronic circuit – like a microcontroller-based system. The physical response needs to be translated into an electrical signal, and then signal conditioning is required to create a suitable, usable signal.
Pressure sensors produce an output voltage that varies with the pressure it experiences, usually referring to the sensor element that is physically detecting the pressure. Packaged board-mount pressure sensors are available which will require the designer to consider calibration, temperature compensation and amplification separately. Confusingly, the phrase “pressure sensor” is also sometimes used to describe transducers and transmitters in general.
Pressure transducers, like pressure sensors, produce an output voltage that varies with pressure. A transducer in this context is a sensing element combined with signal conditioning circuitry, perhaps to compensate for temperature fluctuations, and most likely an amplifier to allow transmission of signals further from the source. Note that for most applications there is an advantage to specifying pressure transducers that are temperature compensated rather than trying to implement custom temperature compensation on a pressure sensing element, as the testing required can be complicated and difficult.
Pressure transmitters are similar to transducers, but generate a current signal across a low-impedance load rather than a voltage signal. Typically the output will be a 4-20mA standard industrial output. Be aware that in portable applications, transmitters can wear the batteries down if they are consistently used at the top end of their pressure range.
Specifying absolute pressure sensors where they aren’t really required is a common mistake; the majority of industrial applications can use gauge pressure. It’s important to fully understand the application’s requirements before making a selection to ensure an accurate, efficient and economical choice.
Working principle of a pressure sensor
An electronic pressure sensor relies on a physical reaction to applied pressure, and then measuring the resulting proportional change electronically. Commonly used phenomena include changes in capacitance, or changes in ohmic resistance of a strain gauge or piezoelectric element, which are proportional to the magnitude of the deflection when pressure is applied.
Important criteria such as measurement range, environmental suitability, physical size, and power requirements, and the type of pressure measurement required will have a significant guiding influence on engineers looking for an application specific solution.
Capacitive pressure sensors
A capacitive pressure sensor contains a capacitor with one rigid plate and one flexible membrane as electrodes. The area of these electrodes being fixed, the capacitance is proportional to the distance between the electrodes. The pressure to be measured is applied to the flexible-membrane side, and the resulting deflection causes a change in capacitance that can be measured using an electrical circuit.
The diagram to the left (click to expand) illustrates the operating principle behind capacitive pressure sensing.
Strain-gauge pressure sensors
In a strain-gauge type pressure sensor, foil or silicon strain gauges are arranged as a Wheatstone bridge. The strain gauge is attached to some kind of diaphragm, which deflects when pressure is applied. The resulting signal is then measured, amplified and conditioned by the Wheatstone bridge circuit to provide a suitable transducer-voltage or transmitter-current output representative of the applied pressure, as shown in the diagram to the right.
Piezoresistive pressure sensors
Piezoresistive sensing elements can also be arranged in a similar bridge formation. The diagram below illustrates how the sensing elements of a bridge-type pressure sensor are attached to a flexible diaphragm, so that resistance changes according to the magnitude of the diaphragm deflection. The overall linearity of the sensor is dependent on the stability of the diaphragm, over the stated measurement range, as well as the linearity of the strain gauges or piezoresistive elements.
It’s easy to imagine a piezoresistive or capacitive pressure sensor as a large device like a through-hole electronic component or a module ready to screw into the side of a tank – but that’s not always the case.
A piezo or capacitive pressure-sensing mechanism can also be fabricated on silicon as a MEMS (Micro Electro Mechanical System) device and packaged as a compact surface-mount device typically measuring only about 2-3mm per side.
MEMS devices, which include not only pressure sensors but also motion or position sensors, and silicon microphones, are extremely small, stable, and cost-effective, bringing advanced functionality to space- and cost-constrained equipment like mobiles and IoT endpoints.
MEMS devices are fabricated in silicon using doping and etching processes. These processes are performed at chip scale, resulting in a tiny device that can be co-packaged with signal-conditioning electronics. The electronic circuitry may comprise simple amplification to produce an analogue output, and may also include analogue-to-digital conversion to generate a digital output.
An analogue output may be advantageous if the sensor signal is to be handled entirely in the analogue domain, or if the designer wants to use an ADC of particularly high resolution or accuracy, or if the system-host microcontroller contains a suitable integrated ADC on-chip. A digital sensor can be designed-in with no need for external conversion components, thereby saving overall component count.
Perhaps the easiest type of sensor to visualise is a barometric pressure sensor. These can be used for measuring ordinary atmospheric pressure, and are used in a range of applications including context sensing or indoor navigation in smartphones. Typically, this is a tiny MEMS sensor.
Detecting changes in atmospheric pressure enables the device to theoretically be able to calculate its height above sea level – for example on a road (to assist satellite navigation and aid dead reckoning in the event of loss of satellite signal), or to detect what level of building the user is situated, such as in a multi-storey car park, office block, apartment block, or shopping mall.
Understanding the types of sensors in common use, their operating principles, and modes of use (absolute, gauge, or differential) can help engineers make initial selection decisions when identifying the most suitable sensor to choose for a given application.
The materials used and type of construction can have an important influence over aspects such as the measurement range, limiting factors like the maximum survivable pressure to which a sensor can be exposed, its stabilising time after soldering, and long-term stability in the intended application.
An understanding of the electrical output properties, and the circuitry needed to interact properly with the host electronic system – typically a microcontroller- or microprocessor-based control system — can help assess how the choice of pressure sensor will influence the likely electronic integration challenges.
This introduction only scratches the surface of pressure sensor technology. Chapter 1.2 will explore the different types of sensing elements that are used in pressure sensors, how they work, and their advantages and disadvantages. If you're looking for more detail on anything discussed here, you can check out the later chapters of this guide below. Alternatively, if you're pressed for time, the full guide is available as a downloadable PDF here.
For more information on our other sensor technologies, visit our sensors page.
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Avnet Abacus works with some of the best suppliers of pressure sensors in the world. Our team of pan-European technical specialists work closely with them to offer you the highest level of engineering support for your designs. If you have a question on pressure sensors, or you would like some advice on sensor selection, visit the Ask an Expert page to get in touch.