Types of pressure sensors: A comprehensive overview
There’s a vast array of pressure sensors available on the market today - and wrapping your head around all the differences can take a bit of time.
That said, the sensors can largely be categorised according to the type of pressure measurement they make, the sensing principle employed, the output signal and the media they’re measuring.
Beyond that, there are a few other distinguishing factors, like whether or not they’re MEMS sensors, or whether they’re medically-approved.
Below we’ll take you through a brief explanation of the different types of pressure sensors to help you understand your options.
Type of Pressure Measurement
Pressure sensors can be categorised in one of three main measurement modes:
In an absolute pressure sensor (see diagram to the right), the reference point is zero, or a vacuum. One side of the sensor is exposed to the medium to be measured, and the other side is sealed to effect a vacuum.
Click the button below to learn more about absolute pressure sensors in chapter five.LEARN MORE
A gauge sensor (see diagram to the right) measures pressure relative to atmospheric pressure. One side is connected to the system, which may be a pump such as a suction pump, while the other side is vented to the atmosphere. It’s important to ensure the vent hole won’t become obstructed.
Click the button below for more on gauge pressure measurement in chapter five.Learn More
A differential pressure sensor (see diagram to the right) measures the difference between pressure experienced at two exposed ports. Typical uses include measuring liquid or gas flow in pipes or ducts, or detecting a blockage or seized valve.
For more on differential pressure measurement, click the button below to jump to chapter five.Learn More
The sensing principle employed by a sensor, can influence accuracy, reliability, measurement range, and compatibility with the target environment. Below we’ll look at 5 different ways the mechanical displacement taking place inside a sensor is turned into an electrical output:
Resistive pressure sensors utilise the change in electrical resistance of a strain gauge bonded to the diaphragm that’s exposed to the pressure medium.
The strain gauges often comprise of a metal resistive element on a flexible backing bonded to the diaphragm, or deposited directly using thin-film processes. The metal diaphragm gives high over-pressure and burst-pressure capability.
Otherwise, strain gauges can be deposited on a ceramic diaphragm using a thick-film deposition process. Over-pressure and burst-pressure tolerance are typically much lower than for metal-diaphragm devices.
Piezoresistive sensors take advantage of the change in resistivity of semiconductor materials, when subjected to strain due to diaphragm deflection. The magnitude of the change can be 100 times greater than the resistance change produced in a metal strain gauge. Hence piezoresistive sensors can measure smaller pressure changes than metal or ceramic sensors.
The table below compares the relative strengths of metal, ceramic, and piezoresistive sensors. Click here to learn more about piezoresistive pressure sensors in chapter six.
Capacitive sensors, which display a capacitance change as one plate deflects under applied pressure, can be highly sensitive, can measure pressures below 10mbar, and withstand large overloads. Constraints on materials, and joining and sealing requirements, however, can restrict applications.
Piezoelectric pressure sensors utilise the property of piezoelectric materials like quartz, to generate a charge on the surface when pressure is applied. The charge magnitude is proportional to the force applied, and the polarity expresses its direction. The charge accumulates and dissipates quickly as pressure changes, allowing measurement of fast-changing dynamic pressures.
Optical sensors, which utilise interferometry to measure pressure-induced changes in optical fibre, are undisturbed by electromagnetic interference, allowing use in noisy environments or near sources such as radiography equipment. They can be created using tiny components or MEMS technology, can be medically safe for implantation or topical use, and can measure the pressure at multiple points along the fibre.
MEMS (Micro Electro-Mechanical System) sensors contain a piezo or capacitive pressure-sensing mechanism fabricated on silicon at micron-level resolution. Co-packaged signal-conditioning electronics convert the small-magnitude MEMS electrical output to an analogue or digital signal. They are tiny surface-mount devices typically only about 2-3mm per side.
Output signal: Transducer or transmitter?
The terms sensors, transducers and transmitters often appear to be used interchangeably. To clarify things, a ‘sensor’ can be seen as an umbrella term for devices that perform as a transducer or a transmitter.[MJ1] [MJ(2]
In simple terms transducers produce an output voltage that varies with the pressure experienced, while transmitters produce an output current. The most common distinctions here are the following:
In practice, the excitation voltage for a resistive bridge can be as low as 3V or 5V, or 10V-30V, or higher. Sensitivity is typically only a few millivolts per volt which means the raw output signal has low magnitude.
If the connection distance is short, and noise is not a problem, a millivolt-output sensor can be easy to design-in but requires a regulated power supply to prevent fluctuations in the excitation voltage affecting the output.
A voltage-output transducer [MJ3] [MJ(4] amplifies the bridge signal, making it a good choice where longer cable lengths are required. Lower noise susceptibility, and a lower-cost unregulated power supply are additional advantages.
A pressure transmitter converts the voltage output to a current signal, typically 4-20mA. Noise susceptibility is extremely low and cable lengths can be several hundred metres, although power consumption is greater.
When searching for the right pressure sensor, you’ll want to consider the media they’re designed to measure, i.e. the different types of gases and liquids:
- Atmospheric / barometric
- Hydraulic / pneumatic
- Corrosive media
Although many of the above can be adapted for use with corrosive substances, you can also find sensors specifically designed to measure corrosive media.
Sensors used in chemical processes may need to withstand exposure to corrosive media such as acids or alkalis. Many can be specified with a stainless steel case and/or diaphragm for increased corrosion resistance. These can also withstand corrosion due to atmospheric humidity or water splashes, or withstand permanent immersion in untreated water, or in water containing chemicals such as in treatment plants or swimming pools.
Sea water, salt spray or coastal environments can present corrosion hazards beyond the resistance of ordinary low-grade stainless steels. Case and diaphragm materials such as super-nickel alloys or titanium are often recommended.
Sensors may also be specified with parts such as o-rings made from viton, instead of rubber, for increased resistance to ageing and corrosion. Alternatively, the diaphragm may be welded to the sensor body to enhance corrosion resistance.
Other factors to consider
Among general industrial pressure sensors, the measurement range can be 0-25 bar or 0-50 bar for light hydraulics applications or similar, while higher-range sensors can be designed for measuring up to 1000 bar or 5000 bar, or more. Sensors designed for general industrial applications can be used in a wide variety of hydraulic or pneumatic systems.
Medically safe sensors
Medical sensors in contact with the body must be safe for the patient. This impacts not only the choice of sensor materials, but also hygiene. Some manufacturers’ medical ranges include disposable sensors that are discarded after use.
Surface-mount packages or ready-to-use modules
Pressure sensors are available in a variety of forms, such as individual sensing elements in surface-mount packages, or ready-to-use sensor modules complete with process connection and electrical interface.
Screw-mount process connections in general industrial sensors may conform to a standard size, such as G ¼” or G ½”, or UNF or NPT sizes. Specifications such as DIN 3852 or EN 837 define various types of seals. High-pressure sensors may utilise a larger thread size, such as M16 x 1.5, and metal-to-metal sealing.
Small board-mount sensors can be specified with a moulded manifold, a standard-size barbed port for push-on tube connection, or port-less.
Overall, the variety of individual sensor types now available in the marketplace provides flexibility for design engineers to identify a suitable sensor for almost any given application.
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.
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
Need a more digestible introduction to pressure sensors? Download the white paper, 'Pressure sensors: Design considerations and technology options'.Download
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