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, 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.