An introduction to pressure sensors
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 different features. Here is our guide to pressure sensors, how they work and what to look out for. Use the links below to jump to a specific section.
- What is a pressure sensor?
- Types of pressure sensing element
- Choosing a pressure sensor
- Products and suppliers
What is a pressure sensor?
Broadly speaking, pressure sensors convert the pressure of the atmosphere, gas or liquid they are exposed to into an electrical signal.
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. Ambient atmospheric pressure is usually sensed via a hole in the packaging. 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.
Absolute pressure sensors will give the result relative to zero (a perfect vacuum). 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.
Differential pressure sensors measure the difference in pressure between two samples, similar to how a gauge sensor works, but differential sensors are sometimes used to detect the pressure difference either side of an object, for example. Differential pressure sensors are often used to monitor airflow in HVAC applications.
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.
Pressure sensors come in several different types. You will see pressure sensors described as sensors, transducers and transmitters, and while these terms are sometimes used interchangeably, the devices they describe aren’t technically the same.
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 their output current varies with pressure, rather than the voltage. 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.
Types of pressure sensing element
The most common types of pressure sensing technology around today are strain gauges. These sensors use some kind of diaphragm, which deflects due to the pressure it experiences. A strain gauge is attached to the diaphragm, which changes its resistance as the diaphragm deflects, that is, as the pressure changes. This change in resistance is usually measured by a circuit, called a Wheatstone bridge.
Strain gauge technologies include bonded foil, in which a metal foil gauge is glued or bonded to the metal diaphragm, and then two or four diaphragms are arranged into a Wheatstone bridge. These sensors can resist high pressures over a wide temperature range and they respond quickly to changes in pressure.
Also available are sputtered glass strain gauges, in which a layer of glass is sputtered onto the diaphragm, then a foil strain gauge is sputtered onto the glass – that is, there is a molecular bond between the strain gauge, the insulating layer and the diaphragm, rather than it being simply glued on. These sensors are very robust, suitable for long-term use and harsh environments.
Silicon strain gauges are very common today using a silicon MEMS diaphragm with a strain gauge device and temperature sensor grown onto it. These devices can be integrated at chip level with signal conditioning electronics to make pressure transducers or transmitters.
Aside from traditional strain gauges, there are also capacitive and piezoelectric pressure sensors. Capacitive pressure sensors use a MEMS diaphragm over a metal surface, which deflects as the pressure changes, changing the system’s capacitance. These sensing elements are very stable and linear, but they can be sensitive to high temperatures.
Piezoelectric sensors use an element made of a material which generates electrical energy when they are under strain, such as quartz or tourmaline. Crucially, they only produce energy when the pressure changes, and are therefore suitable only for dynamic pressure measurements (not static pressure). They are also susceptible to shock and vibration.
Choosing a Pressure Sensor
When comparing pressure sensors, there are a number of physical and performance attributes to be considered.
Firstly, you’ll want to consider the pressure range each sensor is capable of measuring and how that compares to the pressures you want to measure. You may also wish to consider proof pressure, the maximum pressure the device can withstand and then retain functionality when the pressure returns to the operating range, and burst pressure, the pressure that breaks the component such that fluids can leak (which may be dangerous in some applications).
Pressure sensors’ accuracy is an important performance attribute, which is typically given as a percentage of full-scale pressure over a certain temperature range. Some sensors also exhibit hysteresis, non-repeatability and non-linearity, which should be described on the data sheet, if they apply. Linearity is generally expressed as a percentage of full scale pressure, but there are two methods of measurement (best fit straight line and terminal point) which are not equivalent, so be sure to compare like with like. Long-term stability of devices is also desirable – look for low drift over time as well as good stability over a wide temperature and humidity range – while short-term stability after soldering can also be an issue if the device needs to be used straight away (some sensor types can take hours, or even weeks to stabilise).
You should also consider how long you have to spend on integrating the sensor into your system. If time is short, a transducer with integrated signal conditioning electronics, temperature compensation, self-calibration, internal diagnostic functions and a digital output may be the best choice. However, if your system has specialist needs and you are working with appropriate design resources, your own custom implementation of the electronics could be the right choice, especially if you are prepared to calibrate the sensor after assembly.
Next, consider the environment the sensor will be operating in. Mechanical robustness may be an issue – the sensor’s specification may give an idea of its expected cycle life. The ability to withstand liquids or contaminants may be attained by selecting a stainless steel part (note that most gauge pressure sensors have a hole or vent in the packaging for reading atmospheric pressure which can get clogged with dirt). Sensitivity to shock and vibration is also particularly important to automotive, transportation and industrial applications.
Some other vital parameters are the sensor’s response time (vital if real time feedback is required), energy efficiency (check the current consumption figures, especially for transmitters), and physical size. For hard to reach areas or portable equipment, you’ll be looking for a compact solution. Modern sensors come in a variety of package sizes and options that also need to be investigated. For example, does the sensor need to be surface mounted onto a PCB or does it need to be mounted in a specific orientation? Both obviously have implications for packaging choice.
There are of course many other factors that determine pressure sensor choice in specialist applications, such as heavy-duty sensors for industrial and transportation systems, highly accurate sensors for instrumentation and medical equipment, and low cost sensors for consumer devices, but this summary is a good starting point. If you'd like to read about pressure sensor technology selection in more detail, why not download our pressure sensor white paper, or visit our sensors page to find out more about other sensor technologies.
Products and suppliers
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.
TE Connectivity offers a wide range of digital and analogue board level pressure sensors, media isolated pressure sensors and pressure transducers.
The company’s board level pressure sensors come in ceramic and plastic SMD, ceramic leaded and TO can packaging, with custom packaging also available. This product range includes small footprint digital altimeters with integrated signal conditioning and temperature compensation that operate from as little as 1.8V for battery powered applications. There is also a range of piezoresistive silicon MEMS pressure sensors that are suitable for measuring the pressure of non-corrosive dry gases – these are mounted on a reliable ceramic substrate and come fully calibrated with temperature compensation included. TE also provides signal conditioned and temperature compensated analogue pressure sensor modules, with a total error band of less than 1% over the compensated temperature range of -10 to 85°C, and disposable medical pressure sensors for low cost, high volume applications.
TE’s range also spans media isolated pressure sensors, which are intended for harsh environments. These devices isolate a silicon strain gauge chip in air-evacuated oil with a stainless steel, titanium, or nickel alloy diaphragm. And finally, a series of heavy-duty pressure transducers are available that are built to withstand corrosive liquids and gases.
TE Connectivity can also design custom and semi-custom products to meet the needs of the application.
Amphenol Advanced Sensors
Amphenol Advanced Sensors offers a range of MEMS pressure sensors as part of its NovaSensor brand. This includes surface mount, hybrid, and media isolated sensors that serve medical, industrial and transportation applications. Parts are available with all levels of calibration, from uncalibrated to fully calibrated, and in amplified analogue and digital output versions. Products can also be customised on request.
Amphenol’s range includes the cost-effective NPA series of gauge and differential pressure sensors, for board-mounted applications. This now includes the NPA 201 absolute pressure sensor, designed for barometric sensing in low power consumption, low cost applications such as mobile. Other surface mount series include the NPX1 series for remote tyre pressure monitoring, the NPP-301 series of absolute pressure sensors and the digitally compensated NPX-SPI range for low power operation.
Amphenol also has MEMS pressure sensor products designed for disposable medical applications; media isolated pressure sensors for exposure to harsh media such as oil and fuel; and pump blockage detectors. There are also several differential pressure transducers and transmitters for HVAC applications.
Omron offers several gauge and absolute pressure sensors for measuring air pressure as part of its MEMS portfolio. Omron’s devices offer small size, low power consumption and good temperature dependency.
Panasonic offers a wide portfolio of high precision and accuracy relative (gauge) pressure sensors. These sensors are available with or without an on-board amplifier. Including amplification on the device removes the need for external peripheral circuitry and these devices also include temperature compensation.
SMI develops and manufactures MEMS-based pressure sensors that serve challenging applications in the automotive, medical, and industrial markets. Its unique products are developed for applications requiring ultra-low pressure ranges, extraordinary robustness in harsh environments, and extremely small size.
Discover the keys to designing pressure sensor applications with this 30-minute technical presentation and Q&A with Nicholas Argyle, Applications Engineer EMEA, TE Connectivity.Watch On Demand
Take your pressure sensor knowledge a step further with our white paper 'Pressure sensors: Design considerations and technology options'.Download
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