Pressure sensor

Last updated February 15, 2023

Pressure transmitter Druckmessumformer.jpg — Pressure transmitter

A pressure sensor is a device for pressure measurement of gases or liquids. Pressure is an expression of the force required to stop a fluid from expanding, and is usually stated in terms of force per unit area. A pressure sensor usually acts as a transducer; it generates a signal as a function of the pressure imposed. For the purposes of this article, such a signal is electrical.

Pressure sensors are used for control and monitoring in thousands of everyday applications. Pressure sensors can also be used to indirectly measure other variables such as fluid/gas flow, speed, water level, and altitude. Pressure sensors can alternatively be called pressure transducers, pressure transmitters, pressure senders, pressure indicators, piezometers and manometers, among other names.

Pressure sensors can vary drastically in technology, design, performance, application suitability and cost. A conservative estimate would be that there may be over 50 technologies and at least 300 companies making pressure sensors worldwide.

There is also a category of pressure sensors that are designed to measure in a dynamic mode for capturing very high speed changes in pressure. Example applications for this type of sensor would be in the measuring of combustion pressure in an engine cylinder or in a gas turbine. These sensors are commonly manufactured out of piezoelectric materials such as quartz.

Some pressure sensors are pressure switches, which turn on or off at a particular pressure. For example, a water pump can be controlled by a pressure switch so that it starts when water is released from the system, reducing the pressure in a reservoir.

Types of pressure measurements

Pressure sensors can be classified in terms of pressure ranges they measure, temperature ranges of operation, and most importantly the type of pressure they measure. Pressure sensors are variously named according to their purpose, but the same technology may be used under different names.

Absolute pressure sensor

This sensor measures the pressure relative to perfect vacuum. Absolute pressure sensors are used in applications where a constant reference is required, like for example, high-performance industrial applications such as monitoring vacuum pumps, liquid pressure measurement, industrial packaging, industrial process control and aviation inspection. ^[1]

Gauge pressure sensor

This sensor measures the pressure relative to atmospheric pressure. A tire pressure gauge is an example of gauge pressure measurement; when it indicates zero, then the pressure it is measuring is the same as the ambient pressure. Most sensors for measuring up to 50 bar are manufactured in this way, since otherwise the atmospheric pressure fluctuation (weather) is reflected as an error in the measurement result.

Vacuum pressure sensor

This term can cause confusion. It may be used to describe a sensor that measures pressures below atmospheric pressure, showing the difference between that low pressure and atmospheric pressure, but it may also be used to describe a sensor that measures absolute pressure relative to a vacuum.

Differential pressure sensor

This sensor measures the difference between two pressures, one connected to each side of the sensor. Differential pressure sensors are used to measure many properties, such as pressure drops across oil filters or air filters, fluid levels (by comparing the pressure above and below the liquid) or flow rates (by measuring the change in pressure across a restriction). Technically speaking, most pressure sensors are really differential pressure sensors; for example a gauge pressure sensor is merely a differential pressure sensor in which one side is open to the ambient atmosphere.

Sealed pressure sensor

This sensor is similar to a gauge pressure sensor except that it measures pressure relative to some fixed pressure rather than the ambient atmospheric pressure (which varies according to the location and the weather).

Pressure-sensing technology

There are two basic categories of analog pressure sensors,

Force collector types These types of electronic pressure sensors generally use a force collector (such a diaphragm, piston, bourdon tube, or bellows) to measure strain (or deflection) due to applied force over an area (pressure).

Piezoresistive strain gauge

Uses the piezoresistive effect of bonded or formed strain gauges to detect strain due to an applied pressure, electrical resistance increasing as pressure deforms the material. Common technology types are silicon (monocrystalline), polysilicon thin film, bonded metal foil, thick film, silicon-on-sapphire and sputtered thin film. Generally, the strain gauges are connected to form a Wheatstone bridge circuit to maximize the output of the sensor and to reduce sensitivity to errors. This is the most commonly employed sensing technology for general purpose pressure measurement.

Capacitive

Uses a diaphragm and pressure cavity to create a variable capacitor to detect strain due to applied pressure, capacitance decreasing as pressure deforms the diaphragm. Common technologies use metal, ceramic, and silicon diaphragms. Capacitive pressure sensors are being integrated into CMOS technology^[2] and it is being explored if thin 2D materials can be used as diaphragm material.^[3]

Electromagnetic

Measures the displacement of a diaphragm by means of changes in inductance (reluctance), linear variable differential transformer (LVDT), Hall effect, or by eddy current principle.

Piezoelectric

Uses the piezoelectric effect in certain materials such as quartz to measure the strain upon the sensing mechanism due to pressure. This technology is commonly employed for the measurement of highly dynamic pressures. As the basic principle is dynamic no static pressures can be measured with piezoelectric sensors.

Strain-Gauge

Strain gauge based pressure sensors also use a pressure sensitive element where metal strain gauges are glued on or thin-film gauges are applied on by sputtering. This measuring element can either be a diaphragm or for metal foil gauges measuring bodies in can-type can also be used. The big advantages of this monolithic can-type design are an improved rigidity and the capability to measure highest pressures of up to 15,000 bar. The electrical connection is normally done via a Wheatstone bridge, which allows for a good amplification of the signal and precise and constant measuring results.^[4]

Optical

Techniques include the use of the physical change of an optical fiber to detect strain due to applied pressure. A common example of this type utilizes Fiber Bragg Gratings. This technology is employed in challenging applications where the measurement may be highly remote, under high temperature, or may benefit from technologies inherently immune to electromagnetic interference. Another analogous technique utilizes an elastic film constructed in layers that can change reflected wavelengths according to the applied pressure (strain).^[5]

Potentiometric

Uses the motion of a wiper along a resistive mechanism to detect the strain caused by applied pressure.

Force balancing

Force-balanced fused quartz bourdon tubes use a spiral bourdon tube to exert force on a pivoting armature containing a mirror, the reflection of a beam of light from the mirror senses the angular displacement and current is applied to electromagnets on the armature to balance the force from the tube and bring the angular displacement to zero, the current that is applied to the coils is used as the measurement. Due to the extremely stable and repeatable mechanical and thermal properties of fused quartz and the force balancing which eliminates most non-linear effects these sensors can be accurate to around 1PPM of full scale.^[6] Due to the extremely fine fused quartz structures which are made by hand and require expert skill to construct these sensors are generally limited to scientific and calibration purposes. Non force-balancing sensors have lower accuracy and reading the angular displacement cannot be done with the same precision as a force-balancing measurement, although easier to construct due to the larger size these are no longer used.

Other types

These types of electronic pressure sensors use other properties (such as density) to infer pressure of a gas, or liquid.

Resonant

Uses the changes in resonant frequency in a sensing mechanism to measure stress, or changes in gas density, caused by applied pressure. This technology may be used in conjunction with a force collector, such as those in the category above. Alternatively, resonant technology may be employed by exposing the resonating element itself to the media, whereby the resonant frequency is dependent upon the density of the media. Sensors have been made out of vibrating wire, vibrating cylinders, quartz, and silicon MEMS. Generally, this technology is considered to provide very stable readings over time. The squeeze-film pressure sensor is a type of MEMS resonant pressure sensor that operates by a thin membrane that compresses a thin film of gas at high frequency. Since the compressibility and stiffness of the gas film are pressure dependent, the resonance frequency of the squeeze-film pressure sensor is used as a measure of the gas pressure. ^[7]^[8]

A pressure sensor, a resonant quartz crystal strain gauge with a bourdon tube force collector, is the critical sensor of DART.^[9] DART detects tsunami waves from the bottom of the open ocean. It has a pressure resolution of approximately 1mm of water when measuring pressure at a depth of several kilometers.^[10]

Thermal

Uses the changes in thermal conductivity of a gas due to density changes to measure pressure. A common example of this type is the Pirani gauge.

Ionization

Measures the flow of charged gas particles (ions) which varies due to density changes to measure pressure. Common examples are the Hot and Cold Cathode gauges.

Applications

There are many applications for pressure sensors:

Pressure sensing

This is where the measurement of interest is pressure, expressed as a force per unit area. This is useful in weather instrumentation, aircraft, automobiles, and any other machinery that has pressure functionality implemented.

Altitude sensing

This is useful in aircraft, rockets, satellites, weather balloons, and many other applications. All these applications make use of the relationship between changes in pressure relative to the altitude. This relationship is governed by the following equation:^[11]

h=(1-(P/P_{\mathrm {ref} })^{0.190284})\times 145366.45\mathrm {ft}

This equation is calibrated for an altimeter, up to 36,090 feet (11,000 m). Outside that range, an error will be introduced which can be calculated differently for each different pressure sensor. These error calculations will factor in the error introduced by the change in temperature as we go up.

Barometric pressure sensors can have an altitude resolution of less than 1 meter, which is significantly better than GPS systems (about 20 meters altitude resolution). In navigation applications altimeters are used to distinguish between stacked road levels for car navigation and floor levels in buildings for pedestrian navigation.

Flow sensing

This is the use of pressure sensors in conjunction with the venturi effect to measure flow. Differential pressure is measured between two segments of a venturi tube that have a different aperture. The pressure difference between the two segments is directly proportional to the flow rate through the venturi tube. A low pressure sensor is almost always required as the pressure difference is relatively small.

Level / depth sensing

A pressure sensor may also be used to calculate the level of a fluid. This technique is commonly employed to measure the depth of a submerged body (such as a diver or submarine), or level of contents in a tank (such as in a water tower). For most practical purposes, fluid level is directly proportional to pressure. In the case of fresh water where the contents are under atmospheric pressure, 1psi = 27.7 inH₂O / 1Pa = 9.81 mmH₂O. The basic equation for such a measurement is

P=\rho gh

where P = pressure, ρ = density of the fluid, g = standard gravity, h = height of fluid column above pressure sensor

Leak testing

A pressure sensor may be used to sense the decay of pressure due to a system leak. This is commonly done by either comparison to a known leak using differential pressure, or by means of utilizing the pressure sensor to measure pressure change over time.

Ratiometric correction of transducer output

Piezoresistive transducers configured as Wheatstone bridges often exhibit ratiometric behavior with respect not only to the measured pressure, but also the transducer supply voltage.

V_{\mathrm {out} }={P\times K\times Vs_{\mathrm {actual} } \over Vs_{\mathrm {ideal} }}

where:

$V_{\mathrm {out} }$ is the output voltage of the transducer.

$P$ is the actual measured pressure.

$K$ is the nominal transducer scale factor (given an ideal transducer supply voltage) in units of voltage per pressure.

$Vs_{\mathrm {actual} }$ is the actual transducer supply voltage.

$Vs_{\mathrm {ideal} }$ is the ideal transducer supply voltage.

Correcting measurements from transducers exhibiting this behavior requires measuring the actual transducer supply voltage as well as the output voltage and applying the inverse transform of this behavior to the output signal:

P={V_{\mathrm {out} }\times Vs_{\mathrm {ideal} } \over K\times Vs_{\mathrm {actual} }}

Note: Common mode signals often present in transducers configured as Wheatstone bridges are not considered in this analysis.

Related Research Articles

<span class="mw-page-title-main">Pressure measurement</span> Analysis of force applied by a fluid on a surface

Pressure measurement is the measurement of an applied force by a fluid on a surface. Pressure is typically measured in units of force per unit of surface area. Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure and display pressure mechanically are called pressure gauges,vacuum gauges or compound gauges. The widely used Bourdon gauge is a mechanical device, which both measures and indicates and is probably the best known type of gauge.

Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure is the pressure relative to the ambient pressure.

Piezoelectricity is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure and latent heat. It is derived from the Greek word πιέζειν; piezein, which means to squeeze or press, and ἤλεκτρον ēlektron, which means amber, an ancient source of electric charge.

<span class="mw-page-title-main">Thermocouple</span> Electrical device for measuring temperature

A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the Seebeck effect, and this voltage can be interpreted to measure temperature. Thermocouples are widely used as temperature sensors.

Relative density, or specific gravity, is the ratio of the density of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ; for gases, the reference is air at room temperature. The term "relative density" is often preferred in scientific usage, whereas the term "specific gravity" is deprecated.

A viscometer is an instrument used to measure the viscosity of a fluid. For liquids with viscosities which vary with flow conditions, an instrument called a rheometer is used. Thus, a rheometer can be considered as a special type of viscometer. Viscometers only measure under one flow condition.

Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways. The common types of flowmeters with industrial applications are listed below:

A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another. Transducers are often employed at the boundaries of automation, measurement, and control systems, where electrical signals are converted to and from other physical quantities. The process of converting one form of energy to another is known as transduction.

A strain gauge is a device used to measure strain on an object. Invented by Edward E. Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the object by a suitable adhesive, such as cyanoacrylate. As the object is deformed, the foil is deformed, causing its electrical resistance to change. This resistance change, usually measured using a Wheatstone bridge, is related to the strain by the quantity known as the gauge factor.

A quartz crystal microbalance (QCM) measures a mass variation per unit area by measuring the change in frequency of a quartz crystal resonator. The resonance is disturbed by the addition or removal of a small mass due to oxide growth/decay or film deposition at the surface of the acoustic resonator. The QCM can be used under vacuum, in gas phase and more recently in liquid environments. It is useful for monitoring the rate of deposition in thin film deposition systems under vacuum. In liquid, it is highly effective at determining the affinity of molecules to surfaces functionalized with recognition sites. Larger entities such as viruses or polymers are investigated as well. QCM has also been used to investigate interactions between biomolecules. Frequency measurements are easily made to high precision ; hence, it is easy to measure mass densities down to a level of below 1 μg/cm². In addition to measuring the frequency, the dissipation factor is often measured to help analysis. The dissipation factor is the inverse quality factor of the resonance, Q⁻¹ = w/f_r ; it quantifies the damping in the system and is related to the sample's viscoelastic properties.

A gas meter is a specialized flow meter, used to measure the volume of fuel gases such as natural gas and liquefied petroleum gas. Gas meters are used at residential, commercial, and industrial buildings that consume fuel gas supplied by a gas utility. Gases are more difficult to measure than liquids, because measured volumes are highly affected by temperature and pressure. Gas meters measure a defined volume, regardless of the pressurized quantity or quality of the gas flowing through the meter. Temperature, pressure, and heating value compensation must be made to measure actual amount and value of gas moving through a meter.

A load cell converts a force such as tension, compression, pressure, or torque into an electrical signal that can be measured and standardized. It is a force transducer. As the force applied to the load cell increases, the electrical signal changes proportionally. The most common types of load cell are pneumatic, hydraulic, and strain gauges.

<span class="mw-page-title-main">Piezoelectric sensor</span>

A piezoelectric sensor is a device that uses the piezoelectric effect to measure changes in pressure, acceleration, temperature, strain, or force by converting them to an electrical charge. The prefix piezo- is Greek for 'press' or 'squeeze'.

Level sensors detect the level of liquids and other fluids and fluidized solids, including slurries, granular materials, and powders that exhibit an upper free surface. Substances that flow become essentially horizontal in their containers because of gravity whereas most bulk solids pile at an angle of repose to a peak. The substance to be measured can be inside a container or can be in its natural form. The level measurement can be either continuous or point values. Continuous level sensors measure level within a specified range and determine the exact amount of substance in a certain place, while point-level sensors only indicate whether the substance is above or below the sensing point. Generally the latter detect levels that are excessively high or low.

A pressure switch is a form of switch that operates an electrical contact when a certain set fluid pressure has been reached on its input. The switch may be designed to make contact either on pressure rise or on pressure fall. Pressure switches are widely used in industry to automatically supervise and control systems that use pressurized fluids.

Metal bellows are elastic vessels that can be compressed when pressure is applied to the outside of the vessel, or extended under vacuum. When the pressure or vacuum is released, the bellows will return to its original shape, provided the material has not been stressed past its yield strength. They are used both for their ability to deform under pressure and to provide a hermetic seal that allows movement.

<span class="mw-page-title-main">Ultrasonic transducer</span> Acoustic sensor

Ultrasonic transducers and ultrasonic sensors are devices that generate or sense ultrasound energy. They can be divided into three broad categories: transmitters, receivers and transceivers. Transmitters convert electrical signals into ultrasound, receivers convert ultrasound into electrical signals, and transceivers can both transmit and receive ultrasound.

A heat flux sensor is a transducer that generates an electrical signal proportional to the total heat rate applied to the surface of the sensor. The measured heat rate is divided by the surface area of the sensor to determine the heat flux.

A measuring instrument is a device to measure a physical quantity. In the physical sciences, quality assurance, and engineering, measurement is the activity of obtaining and comparing physical quantities of real-world objects and events. Established standard objects and events are used as units, and the process of measurement gives a number relating the item under study and the referenced unit of measurement. Measuring instruments, and formal test methods which define the instrument's use, are the means by which these relations of numbers are obtained. All measuring instruments are subject to varying degrees of instrument error and measurement uncertainty. These instruments may range from simple objects such as rulers and stopwatches to electron microscopes and particle accelerators. Virtual instrumentation is widely used in the development of modern measuring instruments.

Surface acoustic wave sensors are a class of microelectromechanical systems (MEMS) which rely on the modulation of surface acoustic waves to sense a physical phenomenon. The sensor transduces an input electrical signal into a mechanical wave which, unlike an electrical signal, can be easily influenced by physical phenomena. The device then transduces this wave back into an electrical signal. Changes in amplitude, phase, frequency, or time-delay between the input and output electrical signals can be used to measure the presence of the desired phenomenon.

References

↑ Taskos, Nikolaos (2020-09-16). "Pressure Sensing 101 – Absolute, Gauge, Differential & Sealed pressure". ES Systems. Retrieved 2020-09-16.
↑ Nagata, Tomio; Terabe, Hiroaki; Kuwahara, Sirou; Sakurai, Shizuki; Tabata, Osamu; Sugiyama, Susumu; Esashi, Masayoshi (1992-08-01). "Digital compensated capacitive pressure sensor using CMOS technology for low-pressure measurements". Sensors and Actuators A: Physical. 34 (2): 173–177. doi:10.1016/0924-4247(92)80189-A. ISSN 0924-4247.
↑ Lemme, Max C.; Wagner, Stefan; Lee, Kangho; Fan, Xuge; Verbiest, Gerard J.; Wittmann, Sebastian; Lukas, Sebastian; Dolleman, Robin J.; Niklaus, Frank; van der Zant, Herre S. J.; Duesberg, Georg S.; Steeneken, Peter G. (2020-07-20). "Nanoelectromechanical Sensors Based on Suspended 2D Materials". Research. 2020: 1–25. doi:10.34133/2020/8748602. PMC 7388062 . PMID 32766550.
↑ "What is a Pressure Sensor?". HBM. Retrieved 2018-05-09.
↑ Elastic hologram' pages 113-117, Proc. of the IGC 2010, ISBN 978-0-9566139-1-2 here: http://www.dspace.cam.ac.uk/handle/1810/225960
↑ [https://www.researchgate.net/publication/230966593_Characterization_of_quartz_Bourdon-type_high-pressure_transducers ^{[ bare URL ]}
↑ Andrews, M. K.; Turner, G. C.; Harris, P. D.; Harris, I. M. (1993-05-01). "A resonant pressure sensor based on a squeezed film of gas". Sensors and Actuators A: Physical. 36 (3): 219–226. doi:10.1016/0924-4247(93)80196-N. ISSN 0924-4247.
↑ Dolleman, Robin J.; Davidovikj, Dejan; Cartamil-Bueno, Santiago J.; van der Zant, Herre S. J.; Steeneken, Peter G. (2016-01-13). "Graphene Squeeze-Film Pressure Sensors". Nano Letters. 16 (1): 568–571. arXiv: 1510.06919 . doi:10.1021/acs.nanolett.5b04251. ISSN 1530-6984. PMID 26695136. S2CID 23331693.
↑ Milburn, Hugh. "The NOAA DART II Description and Disclosure" (PDF). noaa.gov. NOAA, U.S. Government. Retrieved 4 April 2020.
↑ Eble, M. C.; Gonzalez, F. I. "Deep-Ocean Bottom Pressure Measurements in the Northeast Pacific" (PDF). noaa.gov. NOAA, U.S. Government. Retrieved 4 April 2020.
↑ http://www.wrh.noaa.gov/slc/projects/wxcalc/formulas/pressureAltitude.pdf Archived 2017-07-03 at the Wayback Machine National Oceanic and Atmospheric Administration

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[1] Taskos, Nikolaos (2020-09-16). "Pressure Sensing 101 – Absolute, Gauge, Differential & Sealed pressure". ES Systems. Retrieved 2020-09-16.

[2] Nagata, Tomio; Terabe, Hiroaki; Kuwahara, Sirou; Sakurai, Shizuki; Tabata, Osamu; Sugiyama, Susumu; Esashi, Masayoshi (1992-08-01). "Digital compensated capacitive pressure sensor using CMOS technology for low-pressure measurements". Sensors and Actuators A: Physical. 34 (2): 173–177. doi:10.1016/0924-4247(92)80189-A. ISSN 0924-4247.

[3] Lemme, Max C.; Wagner, Stefan; Lee, Kangho; Fan, Xuge; Verbiest, Gerard J.; Wittmann, Sebastian; Lukas, Sebastian; Dolleman, Robin J.; Niklaus, Frank; van der Zant, Herre S. J.; Duesberg, Georg S.; Steeneken, Peter G. (2020-07-20). "Nanoelectromechanical Sensors Based on Suspended 2D Materials". Research. 2020: 1–25. doi:10.34133/2020/8748602. PMC 7388062 . PMID 32766550.

[4] "What is a Pressure Sensor?". HBM. Retrieved 2018-05-09.

[5] Elastic hologram' pages 113-117, Proc. of the IGC 2010, ISBN 978-0-9566139-1-2 here: http://www.dspace.cam.ac.uk/handle/1810/225960

[6] [https://www.researchgate.net/publication/230966593_Characterization_of_quartz_Bourdon-type_high-pressure_transducers ^{[ bare URL ]}

[7] Andrews, M. K.; Turner, G. C.; Harris, P. D.; Harris, I. M. (1993-05-01). "A resonant pressure sensor based on a squeezed film of gas". Sensors and Actuators A: Physical. 36 (3): 219–226. doi:10.1016/0924-4247(93)80196-N. ISSN 0924-4247.

[8] Dolleman, Robin J.; Davidovikj, Dejan; Cartamil-Bueno, Santiago J.; van der Zant, Herre S. J.; Steeneken, Peter G. (2016-01-13). "Graphene Squeeze-Film Pressure Sensors". Nano Letters. 16 (1): 568–571. arXiv: 1510.06919 . doi:10.1021/acs.nanolett.5b04251. ISSN 1530-6984. PMID 26695136. S2CID 23331693.

[9] Milburn, Hugh. "The NOAA DART II Description and Disclosure" (PDF). noaa.gov. NOAA, U.S. Government. Retrieved 4 April 2020.

[10] Eble, M. C.; Gonzalez, F. I. "Deep-Ocean Bottom Pressure Measurements in the Northeast Pacific" (PDF). noaa.gov. NOAA, U.S. Government. Retrieved 4 April 2020.

[11] ttp://www.wrh.noaa.gov/slc/projects/wxcalc/formulas/pressureAltitude.pdf Archived 2017-07-03 at the Wayback Machine National Oceanic and Atmospheric Administration

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