Common Types of Pressure Sensors

Pressure sensors are instruments or devices that translate the magnitude of the physical pressure that is being exerted on the sensor into an output signal that can be used to establish a quantitative value for the pressure. There are many different types of pressure sensors available, which function similarly but rely on different underlying technologies to make the translation between pressure and an output signal. This article will discuss the most common types of pressure sensors, describe the working principals of pressure sensors, review the common specifications associated with pressure sensors, and present examples of applications.

One difference to note is that pressure sensors are different from pressure gauges. Pressure gauges by their design provide a direct output reading of a pressure value referred to as gauge pressure. This can be in the form of an analog (mechanical) display using a needle and graduated scale, or via a direct digital display of the pressure reading. Pressure sensors, on the other hand, do not directly provide a readable output of pressure, but instead generate an output signal value which is proportional to the pressure reading, but which first needs to be conditioned and processed to convert the output signal level to a calibrated pressure reading.

To learn more about other types of sensors, see our related guides that cover the different types of sensors or the use of sensors to empower the Internet of Things (IoT). To learn more about other pressure measurement devices, see our related guides on manometers and digital pressure gauges.

Pressure sensors, pressure transducers, and pressure transmitters

There are several common terms associated with pressure measurement devices that are often used interchangeably. Those terms are pressure sensors, pressure transducers, and pressure transmitters. Manufacturers and suppliers of these devices may use one or more of these terms to describe their product offerings. Generally, the primary difference between these terms has to do with the electrical output signal that is generated and the output interface of the device. Be aware that there is variation among suppliers with respect to how their devices are classified.

One way to think about the distinction between pressure sensors vs. pressure transducers and pressure transmitters is that pressure sensors do not have electronics built into them to provide signal conditioning and an amplified output, unlike the other two.

Pressure sensors, while used as an umbrella term for all these three types of devices, typically produce a millivolt output signal. The relatively low voltage output coupled with the resistance losses that occur with wiring implies that wire lengths must be kept short, which limits the utilization of the devices to around 10-20 feet from the electronics before too great of a signal loss is experienced. The output signal will be proportional to the supply voltage used with the sensor. So, for example, a sensor that generates a 10mV/V output used with a 5VDC supply will produce an output signal that ranges from 0-50mV in magnitude. Millivolt outputs allow the engineer to design signal conditioning as needed for the application and helps to reduce both the cost and the package size of the sensor. The limitations of these devices are that regulated power supplies must be used as the full-scale output is proportional to the supply voltage. Also, the low output signal means that these devices are less suitable for use in electrically noisy environments. An illustration of a half-bridge circuit with millivolt output is shown in Figure 1 below.

Electrical diagram of a strain gauge pressure sensor using a Wheatstone bridge — Figure 1: A strain gauge pressure sensor using a Wheatstone bridge

Image credit: https://www.avnet.com/wps/portal/abacus/solutions/technologies/sensors/pressure-sensors/output-signals

Pressure transducers generate a higher level of voltage or frequency output by having additional signal amplification capabilities built-in to boost the magnitude of the output signal to say 5V or 10V, and the frequency output to 1-6kHz. The increased signal strength allows for the use of pressure transducers at a greater range from the electronics, say 20 feet. These devices use a higher supply voltage level such as 8-28VDC. Higher output voltages reduce current consumption, making pressure transducers useable in applications where the equipment is battery powered.

While pressure sensors and pressure transducers generate a voltage output, pressure transmitters produce a low impedance current output, typically used as analog 4-20mA signals in a 2-wire or 4-wire configuration. Pressure transmitters feature good electrical noise immunity (EMI/RFI) and are therefore suitable for applications where it is necessary to transmit signals over longer distances. These devices do not require regulated power supplies, but the higher current output and power consumption make them unsuitable for applications with battery-powered equipment when the devices are operated at or near full pressure.

For simplicity in the balance of this article, we shall use the umbrella term pressure sensors rather than making distinct representations of pressure transducers and pressure transmitters.

Pressure Terminology

There is some key terminology that relates to pressure sensors which is presented in this section.

Gauge pressure is a measurement of pressure made relative to the ambient pressure. A common example of this is when a tire pressure gauge is used to measure the air pressure in an automobile tire. If the gauge reads 35 psi, that indicates that the tire’s pressure is 35 psi above the local ambient pressure.
Absolute pressure is a measurement made relative to a pure vacuum condition, such as the vacuum of space. This type of pressure measurement is important in aerospace engineering applications as the air pressure changes with altitude.
Differential pressure is a measurement of the pressure difference between two pressure values, therefore measuring by how much the two differ from each other, not their magnitude relative to atmospheric pressure or to another reference pressure.
Vacuum pressure is the pressure measurement of values that are in a negative direction with respect to atmospheric pressure.

Figure 2 below illustrates these terms on a diagram showing the relative relationships between each of them.

Graphical representation of the relationships between pressure measurements — Figure 2: Relationships of the different pressure measurements

Image credit: https://www.engineeringtoolbox.com

Pressure measurement technologies

There are six primary pressure sensor technologies used to sense pressure. These are:

Potentiometric pressure sensors
Inductive pressure sensors
Capacitive pressure sensors
Piezoelectric pressure sensors
Strain gauge pressure sensors
Variable reluctance pressure sensors

Potentiometric pressure sensors use a Bourdon tube, capsule, or bellows which drives a wiper arm, providing relatively course pressure measurements.

Inductive pressure sensors use a linear variable differential transformer (LVDT) to vary the degree of inductive coupling that occurs between the primary and secondary coils of the transformer.

Capacitive pressure sensors use a diaphragm that is deflected by the applied pressure which results in a change in the capacitance value, which can then be calibrated to provide a pressure reading.

Piezoelectric pressure sensors rely on the ability of materials such as ceramic or metalized quartz to generate an electrical potential when the material is subjected to mechanical stress.

Strain gauge pressure sensors rely on a measurement of the change in resistance that occurs in a material such as silicon when it is subjected to mechanical stress, known as the piezoresistive effect.

Variable reluctance pressure sensors make use of a diaphragm that is contained in a magnetic circuit. When pressure is applied to the sensor, the diaphragm deflection causes a change in the reluctance of the circuit, and that change can be measured and used as an indicator of the applied pressure.

Types of Pressure Sensors

Using a pressure sensor, pressure measurements can be taken to determine a range of different values and different types of pressure depending on whether the pressure measurement is performed relative to atmosphere, vacuum conditions, or other pressure reference levels. Pressure sensors are instruments that can be designed and configured to detect pressure across these variables. Absolute pressure sensors are intended to measure pressure relative to a vacuum and they are designed with a reference vacuum enclosed within the sensor itself. These sensors can also measure atmospheric pressure. Similarly, a gauge pressure sensor detects values relative to atmospheric pressure, and part of the device is usually exposed to ambient conditions. This device may be employed for blood pressure measurements.

An important aspect of industrial pressure detection processes involves comparisons between multiple pressure levels. Differential pressure sensors are used for these applications, which can be challenging due to the presence of at least two different pressures on a single mechanical structure. Differential pressure sensors are relatively complex in design because they are often needed to measure minute pressure differentials across larger static pressures. The principles of transduction and mechanical pressure sensing are common to most standard pressure sensing units, regardless of their categorization as differential, absolute, or gauge pressure instruments. Below we look at the most common type of pressure sensors.

Aneroid Barometer Sensors

An aneroid barometer device is composed of a hollow metal casing that has flexible surfaces on its top and bottom. What is the barometric pressure sensor working principle? Atmospheric pressure changes cause this metal casing to change shape, with mechanical levers augmenting the deformation in order to provide more noticeable results. The level of deformation can also be enhanced by manufacturing the sensor in a bellows design. The levers are usually attached to a pointer dial that translates pressurized deformation into scaled measurements or to a barograph that records pressure change over time. Aneroid barometer sensors are compact and durable, employing no liquid in their operations. However, the mass of the pressure sensing elements limits the device’s response rate, making it less effective for dynamic pressure sensing projects.

Manometer Sensors

A manometer is a fluid pressure sensor that provides a relatively simple design structure and an accuracy level greater than that afforded by most aneroid barometers. It takes measurements by recording the effect of pressure on a column of liquid. The most common form of the manometer is the U-shaped model in which pressure is applied to one side of a tube, displacing liquid and causing a drop in fluid level at one end and a correlating rise at the other. The pressure level is indicated by the difference in height between the two ends of the tube, and measurement is taken according to a scale built into the device.

The precision of reading can be increased by tilting one of the manometer’s legs. A fluid reservoir can also be attached to render the height decreases in one of the legs insignificant. Manometers can be effective as gauge sensors if one leg of the U-shaped tube vents into the atmosphere and they can function as differential sensors when pressure is applied to both legs. However, they are only effective within a specific pressure range and, like aneroid barometers, have a slow response rate that is inadequate for dynamic pressure sensing.

Bourdon Tube Pressure Sensors

Although they function according to the same essential principles as aneroid barometers, bourdon tubes employ a helical or C-shaped sensing element instead of a hollow capsule. One end of the bourdon tube is fixed into connection with the pressure, while the other end is closed. Each tube has an elliptical cross-section that causes the tube to straighten as more pressure is applied. The instrument will continue to straighten until fluid pressure is matched by the elastic resistance of the tube. For this reason, different tube materials are associated with different pressure ranges. A gear assembly is attached to the closed end of the tube and moves a pointer along a graduated dial to provide readings. Bourdon tube devices are commonly used as gauge pressure sensors and as differential sensors when two tubes are connected to a single pointer. Generally, the helical tube is more compact and offers a more reliable performance than the C-shaped sensing element.

Vacuum Pressure Sensors

Vacuum pressure is below atmospheric pressure levels, and it can be challenging to detect through mechanical methods. Pirani sensors are commonly used for measurements in the low vacuum range. These sensors rely on a heated wire with electrical resistance correlating to temperature. When vacuum pressure increases, convection is reduced, and wire temperature rises. Electrical resistance rises proportionally and is calibrated against pressure in order to provide an effective measurement of the vacuum.

Ion or cold cathode sensors are commonly used for higher vacuum range applications. These instruments rely on a filament that generates electron emissions. The electrons pass onto a grid where they may collide with gas molecules, thereby causing them to be ionized. A charged collection device attracts the charged ions, and the number of ions it accumulates directly corresponds to the number of molecules within the vacuum, thus providing an accurate reading of the vacuum pressure.

Sealed Pressure Sensors

Sealed pressure sensors are used when it is desired to obtain a pressure measurement relative to a reference value (such as atmospheric pressure at sea level), but where it is not possible to have the sensor directly open to that reference pressure. For example, on submersible vehicles, a sealed pressure sensor may be used to establish the depth of the vehicle by measuring the ambient pressure and comparing it to atmospheric pressure that is available in the sealed device.

Pressure sensor specifications

Pressure sensors are typically sized and specified by several common parameters which are shown below. Note that the specifications for these devices may vary from manufacturer to manufacturer and note as well that the specifications can be different depending on the specific type of pressure sensor being sourced. Having a basic understanding of these specifications will make the process of sourcing or specifying one of these sensors easier to accomplish.

Sensor type – reflects the pressure type for which the sensor is designed to operate. This may include absolute pressure, compound pressure, differential pressure, gauge pressure, or vacuum pressure.
Operating pressure range – provides the range of pressures over which the sensor can operate and generate a signal output.
Maximum pressure – the absolute maximum value of pressure in which the device can reliably function without damaging the sensor. Exceeding the maximum pressure can result in device failure or inaccurate signal output.
Full scale – is the difference between the maximum pressure that the sensor can measure and zero pressure.
Output type – describes the general nature of the output signal characteristics from the pressure sensor. Examples include analog current, analog voltage, frequency, or other formats.
Output level – the range of output, such as 0-25mV, associated with the pressure sensor over its range of operation. For electrical signal outputs, this will usually be a millivolt or Volt rage, or a current output range in milliamps.
Accuracy – a measure of the deviation in measurement between the pressure level as defined by the sensor output versus the true value of pressure. Accuracy is often expressed as a +/- range of pressure unit (such as psi or millibars) or as a +/- percentage error. Accuracy of pressure sensors is usually defined against a best fit straight line of datapoints for signal output values against various applied pressure readings.
Resolution – represents the smallest difference in the output signal that can be distinguished by the sensor.
Drift – a measure of the gradual change in the calibrated state of the sensor over time.
Supply voltage – the magnitude of the voltage source needed for powering the pressure sensor, measured in volts, most typically expressed as a range of input voltage that is acceptable.
Operating temperature range – the temperature extremes (high and low) over which the sensor is designed to operate reliably and provide an output signal.

Applications of Pressure Sensors

Pressure sensors find wide applications in a range of markets including medical, general industrial, automotive, HVAC, and energy, to name a few. It is important to realize that while these devices sense pressure, they can be used to perform other important measurements since there is a relationship between a recorded pressure and the value of these other parameters.

Some examples of pressure sensor use are summarized below:

In automotive brake systems, pressure sensors may be used to detect fault conditions in hydraulic brakes that could impact their ability to function.
Automobile engines use pressure sensors to optimize the fuel/air mixture as driving conditions change and to monitor the oil pressure level of the operating engine.
Pressure sensors in cars can be used to detect collisions and trigger the activation of safety devices such as airbags.
In medical ventilators, pressure sensors are used to monitor oxygen pressure and to help control the mix of air and oxygen supplied to a patient.
Hyperbaric chambers use pressure sensors to monitor and control the pressure applied during the treatment process.
Pressure sensors are used in spirometry devices that measure the lung capacity of patients.
Automated drug delivery systems that infuse medication into a patient in the form of IV fluids use pressure sensors to deliver the proper dosage at the correct time of day.
In HVAC systems, pressure sensors can be used to monitor the condition of air filters. As the filters clog with particulates, the differential pressure across the filter rises and can be detected.
Airflow speed can be monitored using pressure sensors as the rate of airflow is proportional to the pressure differential.
In industrial process applications, pressure sensors can detect when a filter has become clogged in a process flow by assessing the difference between the influent and effluent pressures.
Tank fluid levels can be effectively monitored using pressure sensors placed at the bottom of the tank. As the level of fluid in the tank decreases, the head pressure (caused by the weight of the volume of liquid above the sensor) also decreases. This measurement is a direct indicator of the amount of fluid in the tank and is independent of the shape of the tank, being solely a function of the fluid height. Here pressure sensors provide an alternative to other forms of liquid level sensors.
Improved GPS location is provided by pressure sensors. A measurement of altitude can be inferred by detecting the barometric pressure due to the relationship between barometric pressure and altitude in the atmosphere.
High-efficiency washing machines may use pressure sensors to determine the volume of water that should be added to clean a load of dirty clothes – thereby making the best use of natural resources.
Pressure sensors are used in wearable devices for monitoring patients and the elderly in assisted living environments, detecting when a fall may have occurred, and notifying staff or a family member. By measuring small changes in air pressure on the order of 2 millibars, these sensors can detect a change in altitude that is on the order of 10 cm in distance.

Summary

This article presented a summary of pressure sensors, including what they are, the types, key specifications, and examples of applications. For information on other topics, consult our additional guides or visit the Thomas Supplier Discovery Platform where you can locate potential sources of supply for over 70,000 different product and service categories.