Absolute Pressure

Let’s go friends today in this post we will go about Absolute Pressure. Absolute Pressure is the analysis of the force exerted by a fluid ( liquid or gas ) on a surface . Pressure is usually measured in units of force per unit of surface area . Several techniques have been developed to measure pressure and vacuum . Instruments used to measure and display pressure in an integral unit are called pressure meters or pressure gauges or vacuum gauges . a manometer A good example, as it uses the surface area and weight of a column of liquid to measure and indicate pressure. Similarly the widely used Bourdain gauge is a mechanical instrument, which measures and indicates and is probably the best known type of gauge.

Absolute, gauge and differential pressure — zero reference

The null reference in use is usually implied by context, and these words are only added when clarification is needed. Tire pressure and blood pressure are by convention gauge pressure , while atmospheric pressure , deep vacuum pressure and altitude pressure must be absolute.

For most working fluids where the fluid is present in a closed system , gauge pressure measurement prevails. Pressure devices attached to the system will indicate pressures relative to the current atmospheric pressure. The situation changes when extreme vacuum pressures are measured, so absolute pressure is usually used instead.

Differential pressures are commonly used in industrial process systems. Differential pressure gauges have two inlet ports, each connected to one of the volumes whose pressure is to be monitored. In effect, such a gauge performs the mathematical operation of subtraction by means of mechanical means, eliminating the need for an operator or control system to look at two different gauges and determine the difference in readings.

Medium vacuum pressure readings can be ambiguous without proper reference, as they may represent absolute pressure or gauge pressure without a negative sign. Thus a vacuum of 26 inHg gauge is equal to an absolute pressure of 4 inHg, calculated as 30 inHg (normal atmospheric pressure) – 26 inHg (gauge pressure).

Atmospheric pressure is usually about 100 kPa at sea level , but is variable with altitude and season. If the absolute pressure of a fluid remains constant, the gauge pressure of the same fluid will vary with changes in atmospheric pressure. For example, when a car goes up a mountain, the (gauge) tire pressure increases as atmospheric pressure decreases. The absolute pressure in the tire remains essentially unchanged.

Atmospheric pressure as a reference is usually followed by a pressure unit followed by a “g” for the gauge, for example 70 psig, meaning that the measured pressure is the total pressure minus atmospheric pressure . There are two types of gauge reference pressure: vented gauge (VG) and sealed gauge (SG).

A vented-gauge pressure transmitter , for example, allows outside air pressure to come into contact with the negative side of the pressure-sensing diaphragm, through a vented cable or a hole on the side of the device, so that it is always under pressure. It is called ambient barometric pressure . Thus a vented-gauge reference pressure sensor should always read zero when the process pressure connection is open to air.

A sealed gauge reference is very similar, except that the atmospheric pressure is sealed on the negative side of the diaphragm. This is usually adopted at higher pressure ranges, such as hydraulics , where changes in atmospheric pressure will have negligible effect on the accuracy of the readings, so venting is not necessary. This allows some manufacturers to provide secondary pressure control as an added precaution for pressure equipment safety if the burst pressure of the primary pressure sensing diaphragm is exceeded .

There is another way to make a sealed gauge reference, and that is to seal a high vacuum on the reverse side of the sensing diaphragm. Then the output signal is offset, so the pressure sensor reads close to zero when measuring atmospheric pressure.

A sealed gauge reference pressure transducer will never read zero because atmospheric pressure is always changing and in this case the reference is fixed at 1 bar.

To produce an absolute pressure sensor , the manufacturer seals a high vacuum behind the sensing diaphragm. If the process-pressure connection of the absolute-pressure transmitter is open to air, it will read the actual barometric pressure .

History

For much of human history, the pressure of gases like air has been ignored, denied, or assumed, but since the early 6th century BC. As in, the Greek philosopher Miletus of Anaximenes claimed that all things are made of air that change simply by changing the level of pressure. He could see the water evaporating, turning into a gas, and thought this applied to solids as well. More condensed air created colder, heavier objects and expanded air created lighter, warmer objects. This was similar to how gases actually become less dense when heated, becoming more dense when cooler.

In the 17th century, Evangelista Torricelli conducted experiments with mercury that allowed her to measure the presence of air. He dipped the closed glass tube at one end into a bowl of mercury and raised the closed end keeping the open end submerged. The weight of the mercury will pull it down, leaving a partial vacuum at the far end. This confirmed his belief that air/gas has mass, which exerts pressure on the things around it. The earlier, even more popular conclusion for Galileo, was that air was weightless and that it was the vacuum that provided the force, as in a siphon. The discovery helped Torricelli to conclude:

We live submerged in the bottom of an ocean of the element air, which is known to have weight from undisputed experiments.

This test, known as Torricelli’s experiment , was essentially the first documented pressure gauge.

Blaise Pascal went ahead, having his brother-in-law try experimenting on a mountain at different altitudes, and indeed found that the lower in the ocean of the atmosphere, the higher the pressure.

vte	Pascal	Bar	Technical atmosphere	Standard atmosphere	Torr	Pound per square inch
(Pa)	(bar)	(at)	(atm)	(Torr)	(lbf/in2)
1 Pa	1 Pa ≡ 1 Pa	1 Pa = 10−5 bar	1 Pa = 1.0197×10−5 at	1 Pa = 9.8692×10−6 atm	1 Pa = 7.5006×10−3 Torr	1 Pa = 0.000 145 037 737 730 lbf/in2
1 bar	105	≡ 100 kPa≡ 106 dyn/cm2	= 1.0197	= 0.98692	= 750.06	= 14.503 773 773 022
1 at	98066.5	0.980665	≡ 1 kgf/cm2	0.967 841 105 354 1	735.559 240 1	14.223 343 307 120 3
1 atm	≡ 101325	≡ 1.01325	1.0332	1	760	14.695 948 775 514 2
1 Torr	133.322 368 421	0.001 333 224	0.001 359 51	1/760 ≈ 0.001 315 789	1 Torr≈ 1 mmHg	0.019 336 775
1 lbf/in2	6894.757 293 168	0.068 947 573	0.070 306 958	0.068 045 964	51.714 932 572	≡ 1 lbf/in2

The SI unit for pressure is the pascal (Pa), equal to one newton per square meter (N m ^-2 or kg m ^-1 s ^-2 ). This special name for the unit was added in 1971; Prior to this, pressure in SI was expressed in units such as N m ^{−2 .} When indicated, the zero in parentheses after the unit is called a reference, for example 101 kPa (abs). Pounds per square inch (psi) is still in widespread use in the US and Canada to measure, for example, tire pressure. Often a letter is added to the psi unit to indicate the zero reference of the measurement; psia for absolute, psig for gauge, psid for difference, although this practice is discouraged by NIST. ^[1]

Since pressure was usually measured by the manometer’s ability to displace a column of liquid, pressure is often expressed as the depth of a particular fluid ( eg, inches of water). Manometric measurement is the subject of pressure head calculation. The most common substitutes for the fluid of a manometer are mercury (Hg) and water; Water is nontoxic and readily available, while the density of mercury allows a smaller column (and so smaller manometer) to measure a given pressure. The abbreviation “WC” or the words “water column” are often printed on gauges and measurements that use water for manometers.

Fluid density and local gravity can vary from one reading to another depending on local factors, so the height of the fluid column does not accurately define pressure. Therefore measurements in “millimeters of mercury” or “inches of mercury” can be converted to SI units as long as the local factors of fluid density and gravity are taken into account. Fluctuations in temperature change the value of fluid density, while location can affect gravity.

Although no longer preferred, these manometric units still come to the fore in many fields. Blood pressure is measured in millimeters of mercury (see tor) in most parts of the world, with central venous pressure and lung pressure in centimeters of water still normal, as in the setting of CPAP machines. Natural gas pipeline pressure is measured in inches of water, expressed as “inches WC”

Underwater divers use manometric units: the ambient pressure meter is measured in units of seawater (MSW) defined as equal to one-tenth of a bar. ^{[2] [3] The unit used in the US is the} foot of seawater ( FSW ) , based on standard gravity and a seawater density of ⁶⁴ lb/ft . According to the US Navy Diving Manual, one fsw is equal to 0.30643 msw,0.030 643  times , or0.444 44  psi , ^[2] [3] although elsewhere it states 33 fsw14.7 psi (one atmosphere), which gives an fsw roughly equivalent to 0.445 psi. ^[4] msw and fsw are the traditional units for measurement of diver pressure exposure used in decompression tables and units of calibration for pneumophathometers and hyperbaric chamber pressure gauges. ^[5] Both msw and fsw are measured relative to normal atmospheric pressure.

In vacuum systems, the tor (millimeter of mercury), the micron (micrometer of mercury), ^[6] and the inch of mercury (inHg) are most commonly used. Torr and µm usually indicate an absolute pressure, while inHg usually indicates a gauge pressure.

Atmospheric pressure is usually stated using the hectopascal (hPa), kilopascal (kPa), millibar (mbar) or atmosphere (atm). In American and Canadian engineering, stress is often measured in kip. Note that stress is not a real pressure because it is not scalar. In cgs system the unit of pressure was barye (Ba), equal to 1 dyn cm ^-2 . In the meter system, the unit of pressure was the pieze, equal to 1 sthene per square meter.

Many other hybrid units are used such as mmHg/cm2 ^or gram-force/cm2 ⁽ sometimes without properly identifying [[kg/cm2]] force units) ^. The use of the names kilogram, gram, kilogram-force, or gram-force (or their symbols) as a unit of force in SI is prohibited; The unit of force in SI is Newton (N).

Static and dynamic pressure

Static pressure is the same in all directions, so pressure measurements are independent of direction in a stationary fluid. Flow, however, applies additional pressure to surfaces perpendicular to the flow direction, while surfaces parallel to the flow direction have little effect. This directional component of pressure in a moving (moving) fluid is called dynamic pressure. An instrument facing the direction of flow measures the sum of the static and dynamic pressures; This measurement is called total pressure or stagnation pressure. Since dynamic pressure is referred to as static pressure, it is neither a gauge nor an absolute; This is a differential pressure.

While static gauge pressure is of primary importance for determining net load on pipe walls, dynamic pressure is used to measure flow rate and airspeed. Dynamic pressure can be measured by taking the differential pressure between devices parallel to and perpendicular to the flow. For example, pitot-static tubes make this measurement on airplanes to determine airspeed. The presence of a measuring device essentially serves to divert flow and create turbulence, so its shape is critical to accuracy and calibration curves are often non-linear.

Application

• altimeter
• barometer
• depth gauge
• MAP sensor
• pitot tube
• sphygmomanometer

Device

A number of instruments have been invented for measuring pressure with various advantages and disadvantages. Pressure range, sensitivity, dynamic response and cost all vary by several orders of magnitude from one device design to another. The oldest type is the liquid column (a vertical tube filled with mercury) manometer invented by Evangelista Torricelli in 1643. YouTube was invented by Christian Huygens in 1661.

Hydrostatic

Hydrostatic gauges (such as mercury column manometers) compare the pressure per unit area of ​​a column of fluid to the hydrostatic force. Hydrostatic gauge measurements are independent of the type of gas being measured, and can be designed for very linear calibration. They have poor dynamic response.

Piston

Piston-type gauges balance the pressure of a fluid with a spring (e.g. tire-pressure gauges of comparatively low accuracy) or a solid weight, in which case it is known as a deadweight tester. and can be used for calibration of other gauges.

Liquid column (manometer)

A liquid-column gauge consists of a column of liquid in a tube whose ends are exposed to different pressures. The column will rise or fall until its weight (the force exerted due to gravity) is in equilibrium with the pressure difference between the two ends of the tube (the force exerted due to fluid pressure). A very simple version is a U-shaped tube half-filled with liquid, with one side attached to the region of interest while a reference pressure (which can be atmospheric pressure or vacuum) is applied to the other. The difference in liquid levels represents the applied pressure. The pressure exerted by a column of fluid of height h and density is given by the hydrostatic pressure equation, p = hgρ . _{Therefore, the} pressure P applied in the U-tube manometer isThe pressure difference between P and the reference pressure P ₀ can be found by solving P _a – P ₀ = Hgρ . In other words, the pressure at either end of the liquid (shown in blue in the figure) must be balanced (since the liquid is stationary), and therefore P _a = P ₀ + hgρ .

In most liquid-column measurements, the result of the measurement is a height h , usually expressed in mm, cm, or inches. Also known as h pressure head. When expressed as a pressure head, the pressure is specified in units of length and the measurement fluid must be specified. When accuracy is important, the temperature of the measurement fluid must also be specified, as the density of the liquid is a function of temperature. So, for example, the pressure head for measurements taken with mercury or water as the manometric fluid, respectively, is referred to as “742.2 mm _Hg ” or “4.2 _{H 2 O “.}59 degrees Fahrenheit”. The term “gauge” or “vacuum” may be added to such measurements to differentiate between pressures above or below atmospheric pressure. Both mm of mercury and inches of water There are normal pressure heads, which can be converted to SI units of pressure using unit conversions and the above formulas.

If the fluid being measured is dense enough, in addition to measuring the differential pressure of the fluid, hydrostatic correction may have to be made for the height between the moving surface of the pressure measuring fluid and the location where the pressure measurement is desired ( For example, across an orifice plate or venturi), in which case the density must be corrected by subtracting the density of the fluid being measured. ^[7]

Although any liquid can be used, mercury is preferred for its high density (13.534 g/cm3 ⁾ and low vapor pressure. Its convex meniscus is advantageous because it means that wetting the glass will result in no pressure error, although in exceptionally clean conditions, mercury will stick to the glass and the barometer may become trapped (mercury may maintain a negative absolute pressure). Even under a strong vacuum. ^[8] For low pressure differences, light oil or water is usually used (the latter gives rise to units of measurement such as the inch water gauge and the millimeter H2O). Liquid-column pressure gauges have highly linear calibration. They have poor dynamic response because the fluid in the column can react slowly to pressure changes.

When measuring a vacuum, the working liquid may evaporate and contaminate the vacuum if its vapor pressure is too high. When measuring liquid pressure, a loop filled with a gas or lighter fluid may separate the fluid to prevent mixing, but this may be unnecessary, for example, when mercury is used to measure the differential pressure of a fluid. The manometer is used in the form of a liquid such as water. Simple hydrostatic gauges can measure pressures ranging from a few tors (some 100 Pa) to a few atmospheres (approximately)1 000 000  Pa ).

Single-limb liquid-column manometers have a rather large reservoir on one side of the U-tube and a scale beside the narrow column. The column may be inclined to further enhance the fluid movement. Depending on the use and composition, the following types of manometers are used ^[9]

1. Ordinary pressure gauge
2. micromanometer
3. differential manometer
4. inverted differential pressure gauge

McLeod Gage

A McLeod gauge separates a sample of gas and compresses it in a modified mercury manometer until the pressure is a few millimeters of mercury. The technique is too slow and unsuitable for continuous monitoring, but capable of good accuracy. Unlike other manometer gauges, the McLeod gauge reading is dependent on the composition of the gas, as the interpretation depends on the sample compression as in an ideal gas. Due to the compression process, the McLeod gauge completely ignores partial pressures from non-ideal vapors that condense, such as pump oil, mercury, and even water if sufficiently compressed.Useful range : from about 10 ⁻⁴  Torr ^[10] (approximately 10 ⁻²  Pa) to vacuums up to 10 ⁻⁶  Torr (0.1 mPa),

0.1 MPa is the lowest direct measurement of pressure that is possible with current technology. Other vacuum gauges can measure low pressure, but only indirectly by measurement of other pressure-dependent properties. These indirect measurements must be calibrated to SI units by direct measurement, usually the McLeod gauge. 

Anhydrous

Aneroid gauges are based on a metal pressure-sensing element that elastically flexes under the influence of pressure differences across the element. “Aneroid” means “without fluid”, and the term originally distinguished these gauges from the hydrostatic gauges described above. However, aneroid gauges can be used to measure the pressure of a liquid as well as a gas, and they are not the only type of gauge that can operate without a fluid. For this reason, they are often called mechanical gauges in modern parlance. Unlike thermal and ionization gauges, aneroid gauges do not depend on the type of gas being measured, and are less likely to contaminate the system than hydrostatic gauges. Pressure Sensing Element A Borden Tube, may be a diaphragm, a capsule, or a set of bellows, which will change shape in response to the pressure of the area in question. The deflection of the pressure sensing element can be read by the linkage attached to the needle, or it can be read by the secondary transducer. The most common secondary transducers in modern vacuum gauges measure changes in capacitance due to mechanical deflection. Gauges that rely on changes in capacitance are often referred to as capacitance manometers.

Bourdon gauge

The Bourdon pressure gauge uses the principle that a flattened tube tends to straighten or regain its spherical form in cross-section when pressure is applied. (A party horn illustrates this principle.) This change in cross-section is hardly noticeable, including moderate stresses within the elastic range of an easily workable material. The tension of the tube’s material is enhanced by the formation of the tube in a C shape or even a helix, such that the entire tube is straightened or flexed as it is pressurized. Eugene Bourdain patented his gauge in France in 1849, and it was widely adopted because of its superior sensitivity, linearity, and accuracy; Edward Ashcroft purchased Borden’s US patent rights in 1852 and became a major manufacturer of gauges. Also in 1849,^[12] But after Borden’s patent expired in 1875, his company Schaefer & Budenberg also manufactured Borden tube gauges.

In practice, a flat thin-walled, closed-end tube is attached to a fixed pipe with the fluid pressure to be measured at the hollow end. As the pressure increases, the closed end moves in an arc, and this motion is converted by a connecting link (section of A) into the rotation of the gear which is usually adjustable. There is a small diameter pinion gear on the pointer shaft, so the speed is further increased by the gear ratio. The position of the pointer card on the back of the pointer, starting pointer shaft position, linkage length and starting position, all provide a means of calibrating the pointer to indicate the desired range of pressure for a change in the behavior of the Bourdon tube. Differential pressure can be measured by a gauge having two separate Borden tubes with connecting linkages.

Bourdon tubes measure gauge pressure relative to ambient atmospheric pressure, as opposed to absolute pressure; Vacuum is felt as reverse motion. Some aneroid barometers use Bourdon tubes closed at both ends (but most use diaphragms or capsules, see below). When the measured pressure is rapidly pulsating, such as when the gauge is near a reciprocating pump, an orifice restriction in the connecting pipe is often used to avoid unnecessary wear on the gears and to provide an average reading; When the entire gauge is subjected to mechanical vibration, So the whole case including the indicator and the indicator card can be filled with oil or glycerin. Tapping on the face of the gauge is not recommended as this will falsify the actual reading initially presented by the gauge. The Bourdon tube is isolated from the face of the gauge and thus has no effect on the actual reading of the pressure. Typical high-quality modern gauges provide ±2% span accuracy, and a specialized high-precision gauge can be as accurate as 0.1% of the full scale.^[13]

Force-balanced Fused Quartz Borden Tube sensors work on the same principle, but use the reflection of a beam of light from a mirror to sense angular displacement and electricity to balance the force of the tube and return the angular displacement. Current is applied to the magnets. Zero, the current being applied to the coil, is used as a measurement. Because of quartz’s extremely stable and repeatable mechanical and thermal properties and a force balance that nearly eliminates all physical motion, these sensors can be accurate to about 1 ppm of full scale. ^[14] Due to the extremely fine fused quartz structures that must be made by hand, these sensors are usually limited to scientific and calibration purposes.

The transparent cover face of the combination pressure and vacuum gauges depicted in the following illustrations has been removed and the mechanism removed from the case. This particular gauge is a combination vacuum and pressure gauge used for automotive diagnostics:

• On the left side of the face, the manifold is used to measure vacuum, with its internal scale calibrated in centimeters of mercury and the outer scale calibrated in inches of mercury.
• The right side of the face is used to measure fuel pump pressure or turbo boost and is calibrated in fractions of 1 kg/cm on its inner scale and pounds per square inch on its outer scale.

Mechanical details

Fixed part:

• A: Receiver block. It connects the inlet pipe to the fixed end of the borden tube (1) and secures to the chassis plate (B). The two holes receive the screws that secure the case.
• B: Chassis plate. Face card is attached with it. It has bearing holes for the axle.
• C: Secondary chassis plate. It supports the outer ends of the axle.
• D: Post for connecting and holding the two chassis plates.

moving parts:

1. Fixed end of Bourdon tube. It communicates with the inlet pipe through the receiver block.
2. The moving end of the Bourdon tube. This end is sealed.
3. axle and spindle pin
4. Link the lever (5) with the pivot pin to the pin to allow joint rotation
5. Extending Lever, Sector Gear (7)
6. sector gear axle pin
7. sector gear
8. Indicator needle axle. It has a spur gear that engages the sector gear (7) and extends through the face to drive the indicator needle. Because of the short distance between the lever arm link boss and the pivot pin, and the difference between the effective radius of the sector gear and the spur gear, any motion of the Bourdon tube is greatly increased. A small movement of the tube results in a large movement of the pointer needle.
9. Hair spring to preload gear train to eliminate gear lash and hysteresis

Diaphragm

A second type of aneroid gauge uses the deflection of a flexible membrane that separates areas of different pressures. The amount of deflection can be repeated for known pressures so the pressure can be determined using calibration. The deformation of a thin diaphragm depends on the pressure difference between its two faces. The reference face gauge may be open to the atmosphere to measure pressure, may be open to another port to measure differential pressure, or may be sealed against vacuum or other fixed reference pressure to measure absolute pressure. Is. Distortion can be measured using mechanical, optical or capacitive techniques. Ceramic and metal diaphragms are used.Useful range : above 10 ⁻² Torr ^[15] (approximately 1 Pa)

For absolute measurement, welded pressure capsules with diaphragms on both sides are often used.

shape:

• even
• corrugated
• flat tube
• capsules

Bellows

In a gauge to sense small pressures or pressure differences, or require that an absolute pressure be measured, the gear train and needle may be driven by an enclosed and sealed bellows chamber, called an aneroid ., which means “without liquid”. (Early barometers used a column of liquid suspended by a vacuum, such as water or the liquid metal mercury.) This bellows configuration was used for aneroid barometers (barometers with an indication needle and dial card), altimeters, altitude recording barographs and altitudes. is done in. Telemetry equipment used in weather balloon radiosondes. These devices use a sealed chamber as the reference pressure and are driven by external pressure. Other sensitive aircraft instruments such as wind speed indicators and rate of climb indicators (variometers) are concerned with both the interior of the aneroid chamber and the external enclosing chamber.

Magnetic coupling

These gauges use the attraction of two magnets to translate differential pressure into the movement of the dial pointer. As the differential pressure increases, a magnet attached to the piston or rubber diaphragm moves. A rotary magnet that is attached to a pointer and then moves uniformly. To create different pressure ranges, the spring rate can be increased or decreased.

Spinning-rotor gauge

The spinning-rotor gauge works by measuring the amount of time a rotating ball is slowed by the viscosity of the gas being measured. The ball is made of steel and is magnetically levitated inside a steel tube closed at one end and exposed to the gas being measured at the other. The ball is brought up to speed (rad/s about 2500 or 3800), and the deceleration rate measured by the electromagnetic transducer after the drive is turned off. ^[16] Equipment ranges from 5 ⁻⁵ to 10 ²  Pa (with a low accuracy of 10 ³Pa) is. It is accurate and stable enough to be used as a secondary standard. During the last years this type of gauge became more user friendly and easier to operate. The tool in the past was famous for requiring certain skills and knowledge to be used correctly. Various corrections must be applied for high accuracy measurement and the ball must spin at a pressure well below the intended measurement pressure for five hours before use. It is most useful in calibration and research laboratories where high accuracy is required and qualified technicians are available. ^[17]Insulation vacuum monitoring of cryogenic liquids is also an ideally suited application for this system. Inexpensive and with a long-term stable, weldable sensor that can be separated from more expensive electronics/read it is a perfect fit for all static vacuums.

Electronic pressure device

metal strain gaugeThe strain gauge is typically affixed to a membrane (foil strain gauge) or deposited (thin-film strain gauge). The deflection of the membrane due to pressure causes a resistance change in the strain gauge that can be measured electronically.piezoresistive strain gaugeUses the piezoresistive effect of a bonded or formed strain gauge to detect stress due to applied pressure.Piezoresistive Silicon Pressure SensorThe sensor is typically a temperature compensated, piezoresistive silicon pressure sensor chosen for its excellent performance and long-term stability. Integral temperature compensation is provided over the range of 0–50 °C using laser-trimmed resistors. An additional laser-trimmed resistor is incorporated to normalize pressure sensitivity variations by programming the gain of an external differential amplifier. It offers good sensitivity and long-term stability. The two ports of the sensor exert pressure on the same single transducer, please refer to the pressure flow diagram below.

This is an overly simplified diagram, but you can see the fundamental design of the internal ports in the sensor. The important thing to note here is the “diaphragm” as it is the sensor itself. Please note whether it is slightly convex in shape (highly exaggerated in the drawing), this is important because it affects the accuracy of the sensor in use. Sensor size is important because it is calibrated to operate in the direction of air flow as shown by the red arrow. This is normal operation for the pressure sensor, which provides a positive reading on the digital pressure meter’s display. Applying pressure in the opposite direction can result in errors because the movement of air pressure is trying to force the diaphragm to move in the opposite direction. The errors induced by this are small, but it is always better to ensure that more positive pressure is always applied to the positive (+ve) port and lower pressure is applied to the negative (-ve) port. , for general ‘pressure gauge’ application. The same applies for measuring the difference between two vacuums, the larger vacuum should always be applied to the negative (-ve) port. The pressure measurement through the Wheatstone Bridge looks like this…

The effective electrical model of the transducer, along with a basic signal conditioning circuit, is shown in the application schematic. The pressure sensor is a fully actuated Wheatstone bridge with temperature compensation and offset adjusted through thick film, laser trimmed resistors. The excitation for the bridge is applied through a constant current. The low-level bridge output is at +O and -O, and the amplified span gain is determined by the programming resistor (r). The electrical design is microprocessor controlled, allowing for calibration, additional functions for the user, such as scale selection, data hold, zero and filter functions, record function that stores/displays MAX/MIN.capacitorUses a diaphragm and pressure cavity to form a variable capacitor to detect stress due to applied pressure.magneticOne measures the displacement of a diaphragm through a change in inductance (reluctance), LVDT, the Hall effect, or by the eddy current principle.piezoelectricUses the piezoelectric effect in some materials, such as quartz, to measure the stress on the sensing system due to pressure.OpticalUses the physical changes of the optical fiber to detect the strain caused by the applied pressure.potentiometricUses wiper motion with a resistive mechanism to detect stress due to applied pressure.resonantUses changes in resonant frequency in a sensing system to measure stresses, or changes in gas density, due to applied pressure.

Thermal conductivity

In general, as the density of a real gas increases – which may indicate an increase in pressure – its ability to conduct heat increases. In this type of gauge, the filament of a wire is heated by running current through it. A thermocouple or resistance thermometer (RTD) can then be used to measure the temperature of the filament. This temperature depends on the rate at which the filament loses heat to the surrounding gas, and hence on the thermal conductivity. A common type is the Pirani gauge, which uses a single platinum filament as both the hot element and the RTD. These gauges are accurate from 10 ⁻³  Torr to 10 Torr, but their calibration is sensitive to the chemical composition of the gases being measured.

Pirani (one string)

A Pirani gauge consists of a metal coil that is open to the pressure being measured. The wire is heated by the current flowing through it and cooled by the gas around it. If the gas pressure is reduced, the cooling effect will decrease, so the equilibrium temperature of the wire will increase. The resistance of the wire is a function of its temperature: by measuring the voltage flowing through the wire and the current across, the resistance (and hence the gas pressure) can be determined. This type of gauge was invented by Marcelo Pirani.

Two wires

In a two-wire gauge, one wire is used as a coil heater, and the other is used to measure temperature due to convection. Thermocouple gauges and thermistor gauges work this way by using a thermocouple or thermistor, respectively, to measure the temperature of the hot wire.

Ionization gauge

Ionization gauges are the most sensitive gauges for very low pressures (also called hard or high vacuum). When a gas is bombarded with electrons, they sense pressure indirectly by measuring the electric ions produced. Low-density gases will produce fewer ions. Ion gauge calibration is unstable and depends on the nature of the gases being measured, which is not always known. They can be calibrated against a McLeod gauge which is more stable and independent of gas chemistry.

Thermionic emission produces electrons, which collide with gas atoms and produce positive ions. The ions are attracted to a suitably biased electrode known as a collector. The current in the collector is proportional to the rate of ionization, which is a function of the pressure in the system. Therefore, measuring the collector current gives the gas pressure. There are several sub-types of ionization gauges.Useful range : 10 ⁻¹⁰ – 10 ⁻³ Torr (approximately 10 ⁻⁸ – 10 ⁻¹ Pa)

Most ion gauges are of two types: hot cathode and cold cathode. In the hot cathode version, an electrically heated filament produces an electron beam. Electrons travel through the gauge and ionize the gas molecules around them. The resulting ions are collected at a negative electrode. The current depends on the number of ions, which depends on the pressure in the gauge. Hot cathode gauges are accurate from 10 ⁻³  Torr to 10 ⁻¹⁰  Torr. The principle behind the cold cathode version is the same, except that electrons are produced in a high voltage discharge. Cold Cathode Gauge 10 ⁻²  Torr to 10 ⁻⁹  Accurate up to Torr. Ionization gauge calibration is very sensitive to construction geometry, chemical composition of the gases being measured, corrosion and surface deposits. Their calibration can be invalidated by activation at atmospheric pressure or low vacuum. The composition of gases at high vacuum will usually be unpredictable, so a mass spectrometer must be used in conjunction with an ionization gauge for precise measurement. ^[18]

Hot cathode

A hot-cathode ionization gauge is primarily composed of three electrodes that work together as a triode, which contains the cathode filament. The three electrodes are a collector or plate, a filament and a grid. The collector current is measured in picoamperes by an electrometer. The filament voltage to ground is typically at a 30 volt potential, while the grid voltage at 180–210 volt DC, unless there is an optional electron bombardment feature by heating the grid, which can have a higher potential of around 565 volts.

The most common ion gauge is the hot-cathode Bayard–Alpert gauge , which has a small ion collector inside the grid. A glass envelope with vacuum openings may enclose the electrodes, but usually the naked gauge is inserted directly into the vacuum chamber, with pins fed through a ceramic plate in the wall of the chamber. Hot-cathode gauges can be damaged or lose their calibration if they are exposed to atmospheric pressure or low vacuum when heated. The measurements of a hot-cathode ionization gauge are always logarithmic.

The electrons released from the filament move back and forth around the grid several times before eventually entering the grid. During these movements, some electrons collide with a gaseous molecule to form a pair of an ion and an electron (electron ionization). The number of these ions is proportional to the gaseous molecule density multiplied by the electron current emitted from the filament, and these ions enter the collector to form the ion stream. Since gaseous molecule density is proportional to pressure, pressure is estimated by measuring the ion current.

The low pressure sensitivity of the hot-cathode gauge is limited by the photoelectric effect. Electrons hitting the grid generate X-rays that generate photoelectric noise in the ion collector. This limits the range of the older hot-cathode gauge to 10 ^-8  torr and the Bayard-Alpert to about 10 ^-10  torr. Additional wires at the cathode potential in the line of sight between the ion collector and the grid prevent this effect. In the extraction type the ions are attracted not by a wire, but by an open cone. Since the ions cannot decide which part of the cone to collide with, they pass through the hole and form an ion beam. This ion beam can be passed on a:

• faraday cup
• Microchannel Plate Detector with Faraday Cup
• Quadruple mass analyzer with faraday cup
• Quadruple mass analyzer with microchannel plate detector and Faraday cup
• Ion lens and acceleration voltage and directed at a target to create a sputter gun. In this case a valve lets the gas into the grid-cage.

Cold cathode

Penning Vacuum Gauge (Open)

There are two subtypes of cold-cathode ionization gauges: the Penning gauge (invented by France Michel Penning), and the inverse magnetron , also known as the redhead gauge . The main difference between the two is the position of the anode with respect to the cathode. Neither is a filament, and each may require a DC potential of about 4 kV for operation. Inverted magnetrons can measure up to 1 × 10 ^{-12 Torr.} 

Similarly, cold-cathode gauges may be reluctant to start at very low pressures, in which the near-absence of a gas makes it difficult to establish electrode current—particularly in Penning gauges, which axially form paths. using a symmetrical magnetic field. Lengths for electrons of the order of metres. In ambient air, suitable ion pairs are ubiquitously formed by cosmic radiation; In Penning gauges, design features are used to simplify the set-up of the discharge path. For example, the electrodes of a Penning gauge are usually finely taped to facilitate the field emission of electrons.

Maintenance cycles of cold cathode gauges are, in general, measured in years depending on the type of gas and the pressure in which they operate. Using a cold cathode gauge in gases with substantial organic components, such as pump oil fractions, can result in the development of fragile carbon films and shards within the gauge that eventually short-circuits the gauge’s electrodes or the generation of a discharge path. interrupts.

Physical phenomena	Instrument	Governing equation	Limiting factors	Practical pressure range	Ideal accuracy	Response time
Mechanical	Liquid column manometer	{\displaystyle \Delta P=\rho gh}		atm. to 1 mbar
Mechanical	Capsule dial gauge		Friction	1000 to 1 mbar	±5% of full scale	Slow
Mechanical	Strain gauge			1000 to 1 mbar		Fast
Mechanical	Capacitance manometer		Temperature fluctuations	atm to 10−6 mbar	±1% of reading	Slower when filter mounted
Mechanical	McLeod	Boyle’s law		10 to 10−3 mbar	±10% of reading between 10−4 and 5⋅10−2 mbar
Transport	Spinning rotor (drag)			10−1 to 10−7 mbar	±2.5% of reading between 10−7 and 10−2 mbar2.5 to 13.5% between 10−2 and 1 mbar
Transport	Pirani (Wheatstone bridge)		Thermal conductivity	1000 to 10−3 mbar (const. temperature)10 to 10−3 mbar (const. voltage)	±6% of reading between 10−2 and 10 mbar	Fast
Transport	Thermocouple (Seebeck effect)		Thermal conductivity	5 to 10−3 mbar	±10% of reading between 10−2 and 1 mbar
Ionization	Cold cathode (Penning)		Ionization yield	10−2 to 10−7 mbar	+100 to -50% of reading
Ionization	Hot cathode (ionization induced by thermionic emission)		Low current measurement; parasitic x-ray emission	10−3 to 10−10 mbar	±10% between 10−7 and 10−4 mbar±20% at 10−3 and 10−9 mbar ±100% at 10−10 mbar

Dynamic transients

When the fluid flow is not in equilibrium, the local pressure can be higher or lower than the average pressure in a medium. These disturbances spread from their source as a longitudinal pressure variation in their path of propagation. It is also called sound. The sound pressure is the instantaneous local pressure deviation from the average pressure caused by the sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water. The effective sound pressure is the root mean square of the instantaneous sound pressure over a given time interval. Sound pressures are normally small and are often expressed in units of microbars.

• Frequency response of pressure sensor
• echo

Calibration and Standards

The American Society of Mechanical Engineers (ASME) has developed two separate and distinct standards on pressure measurement, B40.100 and PTC 19.2. B40.100 provides directions on pressure indicated dial type and pressure digital indicator gauges, diaphragm seals, snubbers and pressure limiter valves. PTC 19.2 provides instructions and guidance for the accurate determination of pressure values ​​in support of the ASME Performance Test Code. The choice of method, equipment, calculations required and corrections to be applied depend on the purpose of the measurement, the allowable uncertainty and the characteristics of the equipment being tested.

The methods of pressure measurement and the protocol used for data transmission are also provided. Guidance is given for setting up the equipment and determining the uncertainty of measurement. Information about equipment type, design, applicable pressure range, accuracy, output and relative cost is provided. Information is also provided on pressure measuring instruments used in field environments, such as piston gauges, manometers, and low-absolute-pressure (vacuum) instruments.

These methods are designed to aid in the assessment of measurement uncertainty based on current technology and engineering knowledge, taking into account published instrumentation specifications and measurement and application techniques. This supplement provides guidance in the use of methods to establish pressure-measurement uncertainty.