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.
For more on capacitive pressure sensing technology, head to chapter six.
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.
If you would like to learn more about piezoelectric pressure sensing then click here to jump to chapter six.
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.
Want more information on optical pressure sensing? Click here to go to chapter six.
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.
We explore MEMS technology in more detail in chapter six. You can jump there now by clicking here.
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.
For a more in-depth look at sensors, transducers and transmitters, read chapter four.
Media Compatibility
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:
- Air
- Atmospheric / barometric
- Gas
- Water
- Liquid
- 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.
Chapter seven goes into detail on pressure sensors for different media types. Click here to jump straight there.
Other factors to consider
Industrial sensing
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.
