# Pressure Transmitter

A Pressure Transmitter is a device for measuring the pressure 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 applied pressure . 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 Transmitter can also be used to indirectly measure other variables such as fluid/gas flow, speed, water level and altitude . Pressure Transmitter may alternatively be called pressure transducer , pressure transmitter , pressure sender , pressure indicator , piezometer and manometer , among other names.

Pressure sensors can vary greatly in technology, design, performance, application suitability and cost. A conservative estimate would be that there may be more than 50 technologies around the world and that there are at least 300 companies making Pressure Transmitter.

There is also a class of Pressure Transmitter designed to measure in dynamic mode to capture very rapid speed changes in pressure. Example applications for this type of sensor would be in measuring combustion pressure in engine cylinders or gas turbines. These sensors are typically fabricated from a piezoelectric material such as quartz.

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

## Type of pressure measurement

Pressure sensors can be classified according to the pressure range they measure, the temperature range of operation and most importantly the pressure they measure. Pressure sensors are given different names according to their purpose, but the same technology may be used under different names.

• absolute pressure sensor

This sensor accurately measures the pressure relative to the vacuum . Absolute pressure sensors are used in applications where a constant reference is required, for example, in high-performance industrial applications such as vacuum pump monitoring, liquid pressure measurement, industrial packaging, industrial process control, and aviation . Inspection

• gauge pressure sensor

This sensor measures pressure relative to atmospheric pressure . Tire pressure gauge is an example of gauge pressure measurement; When it indicates zero, the pressure it is measuring is the same as the ambient pressure. Most sensors are built in this way to measure up to 50 bar, otherwise fluctuations in atmospheric pressure (weather) are reflected as an error in the measurement result.

• vacuum pressure sensor

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

• differential pressure sensor

This sensor measures the difference between two pressures, one attached to each side of the sensor. Differential pressure sensors are used to measure a number of properties, such as in an oil filter or air filter , fluid level (by comparing the pressure at the top and bottom of the liquid) or flow rate (by measuring changes in the pressure of a restriction). The pressure drops. Technically speaking, most pressure sensors are actually differential Pressure Transmitter; For example the gauge Pressure Transmitter is simply a differential pressure sensor with one side open to the ambient environment.

• seal pressure sensor

This sensor is similar to a gauge pressure sensor, except that it measures pressure relative to a certain amount of pressure, rather than ambient atmospheric pressure (which varies by location and season).

## Pressure-sensing technology

There are two basic categories of analog pressure sensors,

Force modulated types These types of electronic Pressure Transmitter typically use a force modulator (such as a diaphragm, piston, Borden tube, or bellows) to measure the tension (or deflection) caused by a force applied to an area (pressure). .

• piezoresistive strain gauge

Uses the piezoresistive effect of a bonded or formed strain gauge to detect the stress caused by the applied pressure , the resistance increases as the 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. Typically, strain gauges are connected to form a Wheatstone bridge circuit to maximize the output of the sensor and reduce susceptibility to errors. It is the most commonly employed sensing technique for general purpose pressure measurement.

• capacitor

Uses a diaphragm and pressure cavity to form a variable capacitor to detect stress due to applied pressure, capacitance decreases as pressure deforms the diaphragm. Common technologies use metal, ceramic and silicon diaphragms.

• Electromagnetic

One measures the displacement of a diaphragm through a change in inductance (reluctance), LVDT , the Hall effect , or by the eddy current principle.

• piezoelectric

Uses the piezoelectric effect in some materials, such as quartz, to measure the stress on the sensing system due to pressure . This technique is commonly employed for the measurement of highly dynamic pressures. Since the basic principle is dynamic, no static pressure can be measured with a piezoelectric sensor.

• strain gauge

Strain gauge based pressure sensors also use a pressure sensitive element where a metal strain gauge is affixed or a thin film gauge is applied by sputtering. This measuring element can be either a diaphragm or can also be used for metal foil gauges to measure bodies in a can-type. The big advantages of this monolithic can-type design are an improved rigidity and the ability to measure the highest pressure of up to 15,000 bar. The electrical connection is typically made through a Wheatstone bridge which allows good amplification of the signal and accurate and consistent measurement results.

• Optical

Techniques include the use of physical transformation of the optical fiber to detect the strain due to applied pressure. A common example of this type is the use of fiber Bragg gratings . This technology is employed in challenging applications where measurements may be highly remote under high temperatures, or may benefit from technologies inherently immune to electromagnetic interference. Another similar technique uses an elastic film built into layers that can change the reflected wavelength according to the applied pressure (strain).

• potentiometric

Uses wiper motion with a resistive mechanism to detect stress due to applied pressure.

• force balance

Force-balanced fused quartz Bourdon tubes use a spiral Bourdon tube to exert force on a mirrored pivot armature, the reflection of a beam of light from the mirror senses angular displacement and electromagnets on the armature to balance the force. A current is applied. To bring the angular displacement to and from the tube to zero, the current applied to the coil is used as a measurement. Due to the extremely stable and repeatable mechanical and thermal properties of fused quartz and the force balance that eliminates most non-linear effects, these sensors can be accurate to approximately 1 ppm of full scale. [4]Due to the extremely fine fused quartz structures that are made by hand and the specialist skills required to manufacture these sensors, these sensors are usually limited to scientific and calibration purposes. Non-force-balancing sensors have low accuracy and reading angular displacement cannot be done with the same accuracy as force-balance measurements, although they are easier to manufacture due to their larger size, they are no longer used.

other types

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

• resonant

Uses changes in resonant frequency in a sensing system to measure stresses, or changes in gas density, due to applied pressure . This technique can be used in conjunction with a force collector, such as in the range above. Alternatively, the resonant technique can be employed by exposing the resonant element to the media, whereby the resonant frequency is dependent on the density of the media. The sensor is made from vibrating wire, vibrating cylinders, quartz and silicon MEMS. In general, this technique is considered to provide very stable readings over time.

A pressure sensor, a resonant quartz crystal strain gauge with a Bourdain tube force modulator is the key sensor of the DART . [5] DART detects tsunami waves from the bottom of the open ocean . It has about 1 mm of water pressure when measuring pressure at a depth of several kilometers. [6]

• Thermal

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

• ionization

Measures the flow of charged gas particles (ions) that change due to the density change to measure the pressure. Common examples are hot and cold cathode gauges.

## Application

There are many applications for pressure sensors:

• pressure sensing

This is where the measurement of interest is pressure, which is expressed as force per unit area. It is useful in weathering equipment, aircraft, automobiles, and any other machinery to which pressurization functionality is applied.

• height sensing

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

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 that can be calculated separately for each of the different pressure sensors. These error calculations will factor in the error introduced by changes in temperature as we go up.

Barometric pressure sensors can have an altitude resolution of less than 1 m, which is much better than GPS systems (about 20 m altitude resolution). In navigation applications altimeters are used to differentiate between steep street level for car navigation and floor level in buildings for pedestrian navigation.

• flow sensing

This is the use of pressure sensors with the Venturi effect to measure flow. The differential pressure is measured between two sections of a venturi tube with a separate orifice. The pressure difference between the two sections is directly proportional to the flow rate through the venturi tube. Low pressure sensors are almost always needed because the pressure difference is relatively small.

• Level / Depth Sensing

A pressure sensor can also be used to calculate the fluid level. This technique is commonly employed to measure the depth of a submerged body (such as a diver or submarine), or the level of material 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 material is at atmospheric pressure, 1psi = 27.7 inH20 / 1Pa = 9.81 mmH20. The basic equation for such a measurement is

P=\rho gh

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

• Leak test

A pressure sensor can be used to sense the loss of pressure due to system leakage. This is usually done using differential pressure to compare a known leak or by using a pressure sensor to measure the pressure change over time.

## Ratiometric correction of transducer output

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

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

Where from:

Vout is the output voltage of the transducer.

P is the actual measured pressure.

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

Vs actual The actual transducer is the supply voltage.

Vs ideal The ideal transducer is the supply voltage.

Correcting a measurement from a transducer exhibiting this behavior requires measuring the actual transducer supply voltage as well as the output voltage and applying the inverse transformation 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.