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About Pressure Sensors

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Exerpts below are from Wikipedia, reference: http://en.wikipedia.org/wiki/Pressure_sensor

pressure sensor measures pressure, typically 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 typically generates an electrical signal as a function of the pressure imposed.

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.

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.
  • 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.
  • 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 (i.e. negative gauge pressure), but it may also be used to describe a sensor that measures low pressure relative to perfect vacuum (i.e. absolute pressure).
  • 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).

Transmitters vs. transducers vs. sensors

What is the difference between a pressure transmitter, pressure transducer and a pressure sensor? Many people (and some manufacturers) use the terms interchageably but there are differences between them technically. The discussion here will focus on how GE Druck refers to the terms, which is generally in line with how most people use them.

Pressure Sensor
It is a general term that refers to all types and classes of devices that measure pressure.

Pressure Transducers (PDCR)
A device that measures pressure with a Millivolt (mV) output signal which is ratiometric to the supply voltage (output signal is proportional to the supply). The millivolt output signal can typically be used ten (10) to (20) feet away from the electronics without significant signal loss.

Pressure Transducers (PMP)
Pressure sensors with Amplified Output in Volts with ranges that can be 0-10 Vdc, 0-5 Vdc, 1-5 Vdc etc. The outputs can be either 3 wire (excitation power, sensor output, and common ground) or 4 wire (excitation power +/-, sensor output +/-). 

Pressure Transmitters (PTX)
Current output signal, i.e. 4-20mA (4 to 20mA); the current is measured on the device, rather than the voltage; pressure transmitters are two wire devices (red for supply. black for the ground). 4-20mA pressure transmitters offer good electrical noise immunity (EMI/RFI). 

Pressure Switches 
A pressure sensing device that has contacts at set points to open or close a circuit. 

Digital Pressure Gauges
Pressure measurement with a display; digital pressure gauges originated as battery powered gauges, but now feature output signals as well.

PRESSURE SENSING TECHNOLOGIES

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 (pressure) over an area.

Piezoresistive strain gauge
Uses the piezoresistive effect of bonded or formed strain gauges to detect strain due to applied pressure, resistance increasing as pressure deforms the material. Common technology types are Silicon (Monocrystalline), Polysilicon Thin Film, Bonded Metal Foil, Thick Film, 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.

Electromagnetic
Measures the displacement of a diaphragm by means of changes in inductance (reluctance), 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.

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).[1]

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

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.

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.