Many techniques have been developed for the measurement of pressure and vacuum. Brass is any Alloy of Copper and Zinc; the proportions of zinc and copper can be varied to create a range of brasses with varying properties Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface This vacuum means "absence of matter" or "an empty area or space" for the cleaning appliance see Vacuum cleaner. Instruments used to measure pressure are called pressure gauges or vacuum gauges
A manometer is a pressure measuring instrument, usually limited to measuring pressures near to atmospheric. Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface In the Physical sciences Quality assurance, and Engineering, Measurement is the activity of obtaining and comparing physical quantities The term manometer is often used to refer specifically to liquid column hydrostatic instruments.
A vacuum gauge is used to measure the pressure in a vacuum --- which is further divided into two subcategories: high and low vacuum (and sometimes ultra-high vacuum). Pressure (symbol 'p' is the force per unit Area applied to an object in a direction perpendicular to the surface This vacuum means "absence of matter" or "an empty area or space" for the cleaning appliance see Vacuum cleaner. Ultra high vacuum (UHV is the Vacuum regime characterised by Pressures lower than about 10−7 pascal or 100 nanopascals (~10−9 The applicable pressure range of many of the techniques used to measure vacuums have an overlap. Hence, by combining several different types of gauge, it is possible to measure system pressure continuously from 10 mbar down to 10-11 mbar. The bar (symbol bar) decibar (symbol dbar) and the millibar (symbol mbar, also mb are units of Pressure. 
Although pressure is an absolute quantity, everyday pressure measurements, such as for tire pressure, are usually made relative to ambient air pressure. In other cases measurements are made relative to a vacuum or to some other ad hoc reference. When distinguishing between these zero references, the following terms are used:
The zero reference in use is usually implied by context, and these words are only added when clarification is needed. Tire pressure and blood pressure are gauge pressures by convention, while atmospheric pressures, deep vacuum pressures, and altimeter pressures must be absolute. A tire-pressure gauge is a pressure gauge used to measure the Pressure of Tires on a vehicle A sphygmomanometer ( or blood pressure meter is a device used to measure Blood pressure, comprising an inflatable Cuff to restrict blood flow and a An altimeter is an instrument used to measure the Altitude of an object above a fixed level 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 through mechanical means, obviating the need for an operator or control system to watch two separate gauges and determine the difference in readings. Moderate vacuum pressures are often ambiguous, as they may represent absolute pressure or gauge pressure without a negative sign. Thus a vacuum of 26 inHg gauge is equivalent to an absolute pressure of 30 inHg (typical atmospheric pressure) − 26 inHg = 4 inHg.
Atmospheric pressure is typically about 100 kPa at sea level, but is variable with altitude and weather. If the absolute pressure of a fluid stays constant, the gauge pressure of the same fluid will vary as atmospheric pressure changes. For example, when a car drives up a mountain, the tire pressure goes up. Some standard values of atmospheric pressure such as 101. In Physical sciences standard conditions for temperature and pressure are Standard sets of conditions for experimental measurements to allow comparisons to be made 325 kPa or 100 kPa have been defined, and some instruments use one of these standard values as a constant zero reference instead of the actual variable ambient air pressure. This impairs the accuracy of these instruments, especially when used at high altitudes.
|1 Pa||≡ 1 N/m2||10−5||1. The bar (symbol bar) decibar (symbol dbar) and the millibar (symbol mbar, also mb are units of Pressure. A technical atmosphere (symbol at is a non- SI unit of Pressure equal to one Kilogram-force per square centimeter The Standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of Pressure (symbol atm The torr (symbol Torr) is a non- SI unit of Pressure defined as 1/760 of an atmosphere. The pound per square inch or more accurately pound-force per square inch (symbol psi or lbf/in² or lbf/in²) is a unit of The newton (symbol N) is the SI derived unit of Force, named after Isaac Newton in recognition of his work on Classical 0197×10−5||9. 8692×10−6||7. 5006×10−3||145. 04×10−6|
|1 bar||100,000||≡ 106 dyn/cm2||1. 0197||0. 98692||750. 06||14. 504|
|1 at||98,066. 5||0. 980665||≡ 1 kgf/cm2||0. The unit kilogram-force ( kgf, often incorrectly just kg) or kilopond ( kp) is defined as the Force exerted by Earth's gravity 96784||735. 56||14. 223|
|1 atm||101,325||1. 01325||1. 0332||≡ 1 atm||760||14. The Standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of Pressure (symbol atm 696|
|1 torr||133. 322||1. 3332×10−3||1. 3595×10−3||1. 3158×10−3||≡ 1 Torr; ≈ 1 mmHg||19. The torr (symbol Torr) is a non- SI unit of Pressure defined as 1/760 of an atmosphere. 337×10−3|
|1 psi||6,894. 76||68. 948×10−3||70. 307×10−3||68. 046×10−3||51. 715||≡ 1 lbf/in2|
Example reading: 1 Pa = 1 N/m2 = 10−5 bar = 10. This article deals with the unit of force For the unit of mass see Pound (mass. 197×10−6 at = 9. 8692×10−6 atm, etc.
Note: mmHg is an abbreviation for millimetres of mercury.
The SI unit for pressure is the pascal (Pa), equal to one newton per square metre (N·m-2 or kg·m-1·s-2). The newton (symbol N) is the SI derived unit of Force, named after Isaac Newton in recognition of his work on Classical M^2 redirects here For other uses see M². CM2 redirects here This special name for the unit was added in 1971; before that, pressure in SI was expressed in units such as N/m². When indicated, the zero reference is stated in parenthesis following the unit, for example 101 kPa (abs). The Pounds per square inch (psi) is still in widespread use in the US and Canada, notably for cars. The pound per square inch or more accurately pound-force per square inch (symbol psi or lbf/in² or lbf/in²) is a unit of A letter is often appended to the psi unit to indicate the measurement's zero reference; psia for absolute, psig for gauge, psid for differential, although this practice is discouraged by the NIST .
Because pressure was once commonly measured by its ability to displace a column of liquid in a manometer, pressures are often expressed as a depth of a particular fluid (e. g. inches of water). The most common choices are mercury (Hg) and water; water is nontoxic and readily available, while mercury's density allows for a shorter column (and so a smaller manometer) to measure a given pressure. Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum Water is a common Chemical substance that is essential for the survival of all known forms of Life.
Fluid density and local gravity can vary from one reading to another depending on local factors, so the height of a fluid column does not define pressure precisely. When 'millimetres of mercury' or 'inches of mercury' are quoted today, these units are not based on a physical column of mercury; rather, they have been given precise definitions that can be expressed in terms of SI units. The torr (symbol Torr) is a non- SI unit of Pressure defined as 1/760 of an atmosphere. Inches of mercury, inHg or "Hg is a measuring unit for Pressure. The water-based units usually assume one of the older definitions of the kilogram as the weight of a litre of water.
Although no longer favoured by measurement experts, these manometric units are still encountered in many fields. Blood pressure is measured in millimetres of mercury in most of the world, and lung pressures in centimeters of water are still common. Blood pressure is also the title of a short story by Damon Runyan in Guys and Dolls and Other Stories A centimeter (centimetre of water or cm H2O is a less commonly used unit of Pressure. Natural gas pipeline pressures are measured in inches of water, expressed as '"WC' ('Water Column'). Natural gas is a Gaseous Fossil fuel consisting primarily of Methane but including significant quantities of Ethane, Propane, Scuba divers often use a manometric rule of thumb: the pressure exerted by ten metres depth of water is approximately equal to one atmosphere. Scuba diving is swimming underwater, or taking part in another activity while using a Scuba set. A rule of thumb is a principle with broad application that is not intended to be strictly accurate or reliable for every situation In vacuum systems, the units torr, micrometre of mercury (micron), and inch of mercury (inHg) are most commonly used. The torr (symbol Torr) is a non- SI unit of Pressure defined as 1/760 of an atmosphere. The torr (symbol Torr) is a non- SI unit of Pressure defined as 1/760 of an atmosphere. Inches of mercury, inHg or "Hg is a measuring unit for Pressure. Torr and micron usually indicates an absolute pressure, while inHg usually indicates a gauge pressure.
Atmospheric pressures are usually stated using kilopascal (kPa), or atmospheres (atm), except in American meteorology where the hectopascal (hPa) and millibar (mbar) are preferred. The Standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of Pressure (symbol atm Meteorology (from Greek grc μετέωρος metéōros, "high in the sky" and grc -λογία -logia) is the Interdisciplinary The bar (symbol bar) decibar (symbol dbar) and the millibar (symbol mbar, also mb are units of Pressure. In American and Canadian engineering, stress is often measured in kip. Stress is a measure of the average amount of Force exerted per unit Area. In the United States a kip is a unit of Force that equals 1000 pounds-force, used primarily by architects and engineers of mayapur to measure engineering loads Note that stress is not a true pressure since it is not scalar. In Physics, a scalar is a simple Physical quantity that is not changed by Coordinate system rotations or translations (in Newtonian mechanics or In the cgs system the unit of pressure was the barye (ba), equal to 1 dyn·cm-2. The centimetre-gram-second system ( CGS) is a system of physical units. The barye (symbol Ba) was in France a centimetre-gram-second (CGS unit of Pressure. In the mts system, the unit of pressure was the pieze, equal to 1 sthene per square metre. The metre-tonne-second or mts system of units is a system of Physical units It was invented in France hence the unit names sthène and pièze The pieze is the unit of pressure in the Metre-tonne-second system of units (mts system used e The sthène is the unit of force in the Metre-tonne-second system of units (mts invented in France and used in the Soviet Union 1933 - 1955.
Many other hybrid units are used such as mmHg/cm² or grams-force/cm² (sometimes as kg/cm² and g/mol2 without properly identifying the force units). Using the names kilogram, gram, kilogram-force, or gram-force (or their symbols) as a unit of force is forbidden in SI; the unit of force in SI is the newton (N).
Static pressure is uniform in all directions, so pressure measurements are independent of direction in an immobile (static) fluid. In the design and operation of Aircraft, static pressure is the air pressure in the aircraft’s static pressure system. Flow, however, applies additional pressure on surfaces perpendicular to the flow direction, while having little impact on surfaces parallel to the flow direction. This directional component of pressure in a moving (dynamic) fluid is called dynamic pressure. In Fluid dynamics dynamic pressure (indicated with q, or Q, and sometimes called velocity pressure) is the quantity defined by An instrument facing the flow direction measures the sum of the static and dynamic pressures; this measurement is called the total pressure or stagnation pressure. In Fluid dynamics, stagnation pressure is the Pressure at a Stagnation point in a fluid flow where the kinetic energy is converted into pressure energy Since dynamic pressure is referenced to static pressure, it is neither gauge nor absolute; it is a differential pressure.
While static gauge pressure is of primary importance to determining net loads on pipe walls, dynamic pressure is used to measure flow rates and airspeed. Dynamic pressure can be measured by taking the differential pressure between instruments parallel and perpendicular to the flow. Pitot-static tubes, for example perform this measurement on airplanes to determine airspeed. A Pitot (ˈpiːtoʊ tube is a Pressure measurement instrument used to measure Fluid flow Velocity. The presence of the measuring instrument inevitably acts to divert flow and create turbulence, so its shape is critical to accuracy and the calibration curves are often non-linear.
Many instruments have been invented to measure pressure, with different advantages and disadvantages. A sphygmomanometer ( or blood pressure meter is a device used to measure Blood pressure, comprising an inflatable Cuff to restrict blood flow and a History The first barometer is thought to have been built unintentionally by Gasparo Berti, sometime between 1640 and 1643 An altimeter is an instrument used to measure the Altitude of an object above a fixed level A Pitot (ˈpiːtoʊ tube is a Pressure measurement instrument used to measure Fluid flow Velocity. The examples and descriptions in this article apply strictly to Four-stroke cycle gasoline engines Pressure range, sensitivity, dynamic response and cost all vary by several orders of magnitude from one instrument design to the next. The oldest type is the liquid column manometer invented by Evangelista Torricelli. Evangelista Torricelli ( ( October 15, 1608 &ndash October 25, 1647) was an Italian physicist and mathematician
Hydrostatic gauges (such as the mercury column manometer) compare pressure to the hydrostatic force per unit area at the base of a column of fluid. Hydrostatic gauge measurements are independent of the type of gas being measured, and can be designed to have a very linear calibration. They have poor dynamic response.
Piston-type gauges counterbalance the pressure of a fluid with a solid weight or a spring. For example dead-weight testers used for calibration and Tire-pressure gauges. A tire-pressure gauge is a pressure gauge used to measure the Pressure of Tires on a vehicle
Liquid column gauges consist of a vertical column of liquid in a tube whose ends are exposed to different pressures. The column will rise or fall until its weight is in equilibrium with the pressure differential between the two ends of the tube. A very simple version is a U-shaped tube half-full of liquid, one side of which is connected to the region of interest whilst the reference pressure (which might be the atmospheric pressure or a vacuum) is applied to the other. In general a reference is a relation between objects in which one object designates by linking to another object The difference in liquid level represents the applied pressure. The pressure exerted by a column of fluid of height h and density ρ is given by the hydrostatic pressure equation, P = hgρ. Therefore the pressure difference between the applied pressure Pa and the reference pressure Po in a U-tube manometer can be found by solving Pa − Po = hgρ. If the fluid being measured is significantly dense, hydrostatic corrections may have to be made for the height between the moving surface of the manometer working fluid and the location where the pressure measurement is desired.
Any fluid can be used, but mercury is preferred for its high density(13. Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum 534 g/cm³) and low vapour pressure. For low pressure differences well above the vapour pressure of water, water is a commonly-used liquid (and "inches of water" is a commonly-used pressure unit). Water is a common Chemical substance that is essential for the survival of all known forms of Life. In the design and operation of Aircraft, static pressure is the air pressure in the aircraft’s static pressure system. Liquid column pressure gauges are independent of the type of gas being measured and have a highly linear calibration. They have poor dynamic response. When measuring vacuum, the working liquid may evaporate and contaminate the vacuum if its vapor pressure is too high. Vapor pressure (also known as equilibrium vapor pressure or saturation vapor pressure) is the Pressure of a Vapor in equilibrium When measuring liquid pressure, a loop filled with gas or a light fluid must isolate the liquids to prevent them from mixing. Simple hydrostatic gauges can measure pressures ranging from a few Torr (a few 100 Pa) to a few atmospheres. The torr (symbol Torr) is a non- SI unit of Pressure defined as 1/760 of an atmosphere. (Approximately 1,000,000 Pa)
A single-limb liquid-column manometer has a larger reservoir instead of one side of the U-tube and has a scale beside the narrower column. The column may be inclined to further amplify the liquid movement. Based on the use and structure following type of manometers are used
A McLeod gauge isolates a sample of gas and compresses it in a modified mercury manometer until the pressure is a few mmHg. A McLeod gauge is a scientific instrument to measure very low Pressures down to 10-7 Torr. The torr (symbol Torr) is a non- SI unit of Pressure defined as 1/760 of an atmosphere. The gas must be well-behaved during its compression (it must not condense, for example). The technique is slow and unsuited to continual monitoring, but is capable of good accuracy.
An important variation is the McLeod gauge which isolates a known volume of vacuum and compresses it to multiply the height variation of the liquid column. A McLeod gauge is a scientific instrument to measure very low Pressures down to 10-7 Torr. The McLeod gauge can measure vacuums as high as 10−6 Torr (0. 1 mPa), which is the lowest direct measurement of pressure that is possible with current technology. Other vacuum gauges can measure lower pressures, but only indirectly by measurement of other pressure-controlled properties. These indirect measurements must be calibrated to SI units via a direct measurement, most commonly a McLeod gauge. 
Aneroid gauges are based on a metallic pressure sensing element which flexes elastically under the effect of a pressure difference 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 fluid. For this reason, they are often called mechanical gauges in modern language. Aneroid gauges are not dependent on the type of gas being measured, unlike thermal and ionization gauges, and are less likely to contaminate the system than hydrostatic gauges. The pressure sensing element may be a Bourdon tube, a diaphragm, a capsule, or a set of bellows, which will change shape in response to the pressure of the region in question. The deflection of the pressure sensing element may be read by a linkage connected to a needle, or it may be read by a secondary transducer. The most common secondary transducers in modern vacuum gauges measure a change in capacitance due to the mechanical deflection. Gauges that rely on a change in capacitances are often referred to as Baratron gauges.
A Bourdon gauge uses a coiled tube, which, as it expands due to pressure increase causes a rotation of an arm connected to the tube. A rotation is a movement of an object in a circular motion A two- Dimensional object rotates around a center (or point) of rotation
In 1849 the Bourdon tube pressure gauge was patented in France by Eugene Bourdon. This article is about the country For a topic outline on this subject see List of basic France topics. Eugène Bourdon (b Paris, France, April 8, 1808, d Paris September 29, 1884) was a Watchmaker and Engineer
The pressure sensing element is a closed coiled tube connected to the chamber or pipe in which pressure is to be sensed. As the gauge pressure increases the tube will tend to uncoil, while a reduced gauge pressure will cause the tube to coil more tightly. This motion is transferred through a linkage to a gear train connected to an indicating needle. A mechanical linkage is a series of rigid links connected with joints to form a closed chain or a series of closed chains This is the page for mechanical Gears For other uses see Gear (disambiguation For the gear-like device used to drive a roller chain see Sprocket The needle is presented in front of a card face inscribed with the pressure indications associated with particular needle deflections. In a barometer, the Bourdon tube is sealed at both ends and the absolute pressure of the ambient atmosphere is sensed. Differential Bourdon gauges use two Bourdon tubes and a mechanical linkage that compares the readings.
In the following pictures the transparent cover face has been removed and the mechanism removed from the case. This particular gauge is a combination vacuum and pressure gauge used for automotive diagnosis:
A second type of aneroid gauge uses the deflection of a flexible membrane that separates regions of different pressure. An artificial membrane, also called a synthetic membrane, is a membrane prepared for separation tasks in Laboratory and industry The amount of deflection is repeatable for known pressures so the pressure can be determined using by calibration. The deformation of a thin diaphragm is dependent on the difference in pressure between its two faces. The reference face can be open to atmosphere to measure gauge pressure, open to a second port to measure differential pressure, or can be sealed against a vacuum or other fixed reference pressure to measure absolute pressure. The deformation can be measured using mechanical, optical or capacitive techniques. Ceramic and metallic diaphragms are used.
For absolute measurements, welded pressure capsules with diaphragms on either side are often used. The torr (symbol Torr) is a non- SI unit of Pressure defined as 1/760 of an atmosphere.
In gauges intended 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 such as water or the liquid metal mercury suspended by a vacuum. History The first barometer is thought to have been built unintentionally by Gasparo Berti, sometime between 1640 and 1643 Water is a common Chemical substance that is essential for the survival of all known forms of Life. Mercury (ˈmɜrkjʊri also called quicksilver or hydrargyrum, is a Chemical element with the symbol Hg ( Latinized hydrargyrum This vacuum means "absence of matter" or "an empty area or space" for the cleaning appliance see Vacuum cleaner. ) This bellows configuration is used in aneroid barometers (barometers with an indicating needle and dial card), altimeters, altitude recording barographs, and the altitude telemetry instruments used in weather balloon radiosondes. An altimeter is an instrument used to measure the Altitude of an object above a fixed level A barograph is a recording Aneroid barometer. It produces a paper or foil chart called a barogram that records the Barometric pressure over A weather or sounding balloon is a Balloon (specifically a type of High altitude balloon) which carries instruments aloft to send back information A radiosonde ( Sonde is French for Probe) is a unit for use in Weather balloons that measures various atmospheric parameters These devices use the sealed chamber as a reference pressure and are driven by the external pressure. Other sensitive aircraft instruments such as air speed indicators and rate of climb indicators (variometers) have connections both to the internal part of the aneroid chamber and to an external enclosing chamber. The airspeed indicator or airspeed gauge is an instrument used in an Aircraft to display the craft's Airspeed, typically in knots, to the The term Variometer also refers to a type of tunable Electrical Transformer A variometer (also known as a rate-of-climb indicator
This is also called a capacitance manometer, in which the diaphragm makes up a part of a capacitor. A change in pressure leads to the flexure of the diaphragm, which results in a change in capacitance. These gauges are effective from 10−3 Torr to 10−4 Torr.
Thermal Conductivity gauges rely on the fact that the ability of a gas to conduct heat decreases with pressure. In this type of gauge, a wire filament is heated by running current through it. An electrical filament is a thread of Metal, usually Tungsten, which is used to convert Electricity into light in Incandescent light bulbs (as developed A thermocouple or Resistance Temperature Detector (RTD) can then be used to measure the temperature of the filament. In Electrical engineering and industry thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal Potential Resistance thermometers, also called resistance temperature detectors ( RTD s are Temperature Sensors that exploit the predictable change in This temperature is dependent on the rate at which the filament loses heat to the surrounding gas, and therefore on the thermal conductivity. A common variant is the Pirani gauge which uses a single platinum filament as both the heated element and RTD. Marcello Stefano Pirani ( July 1, 1880 – January 11, 1968) was a German Physicist, known for his invention of the These gauges are accurate from 10 Torr to 10−3 Torr, but they are sensitive to the chemical composition of the gases being measured. The torr (symbol Torr) is a non- SI unit of Pressure defined as 1/760 of an atmosphere.
One wire coil is used as a heater, and the other is used to measure nearby temperature due to convection. Convection in the most general terms refers to the movement of molecules within Fluids (i
A Pirani gauge consists of a metal wire open to the pressure being measured. The wire is heated by a current flowing through it and cooled by the gas surrounding it. Joule heating is the process by which the passage of an Electric current through a conductor releases Heat. If the gas pressure is reduced, the cooling effect will decrease, hence the equilibrium temperature of the wire will increase. The resistance of the wire is a function of its temperature: by measuring the voltage across the wire and the current flowing through it, the resistance (and so the gas pressure) can be determined. Electrical resistance is a ratio of the degree to which an object opposes an Electric current through it measured in Ohms Its reciprocal quantity is Electrical resistance is a ratio of the degree to which an object opposes an Electric current through it measured in Ohms Its reciprocal quantity is The volt (symbol V) is the SI derived unit of electric Potential difference or Electromotive force. Electric current is the flow (movement of Electric charge. The SI unit of electric current is the Ampere. This type of gauge was invented by Marcello Pirani. Marcello Stefano Pirani ( July 1, 1880 – January 11, 1968) was a German Physicist, known for his invention of the
Thermocouple gauges and thermistor gauges work in a similar manner, except a thermocouple or thermistor is used to measure the temperature of the wire. In Electrical engineering and industry thermocouples are a widely used type of temperature sensor and can also be used as a means to convert thermal Potential A thermistor is a type of Resistor with resistance varying according to its Temperature.
Ionization gauges are the most sensitive gauges for very low pressures (high vacuums, AKA "hard" vacuums). The torr (symbol Torr) is a non- SI unit of Pressure defined as 1/760 of an atmosphere. Many techniques have been developed for the measurement of Pressure and Vacuum. They sense pressure indirectly by measuring the electrical ions produced when the gas is bombarded with electrons. Fewer ions will be produced by lower density gases. The calibration of an ion gauge is unstable and dependent on the nature of the gases being measured, which is not always known. They can be calibrated against a McLeod gauge which is much more stable and independent of chemistry. A McLeod gauge is a scientific instrument to measure very low Pressures down to 10-7 Torr.
Thermionic emission generate electrons, which collide with gas atoms and generate positive ions. Thermionic emission is the flow of Charge carriers from a surface or over some other kind of Electrical potential barrier caused by thermal vibrational energy An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge The ions are attracted to a suitably biased electrode known as the collector. In Electrical engineering, the term bias has the following meanings A systematic deviation of a value from a reference value The current in the collector is proportional to the rate of ionization, which is a function of the pressure in the system. Hence, measuring the collector current gives the gas pressure. There are several sub-types of ionization gauge.
Most ion gauges come in two types: hot cathode and cold cathode, a third type exists which is more sensitive and expensive known as a spinning rotor gauge, but is not discussed here. A cathode is an Electrode through which (positive Electric current flows out of a polarized electrical device In the hot cathode version an electrically heated filament produces an electron beam. The hot filament Ionization gauge, sometimes called a hot filament gauge or hot cathode gauge, is the most widely used vacuum (negative pressure measuring The electrons travel through the gauge and ionize 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 cold cathode version is the same, except that electrons are produced in a discharge created by a high voltage electrical discharge. A cold cathode is an element used within some Nixie tubes Gas discharge lamps Gas filled tubes and Vacuum tubes Cold cathodes do not Cold Cathode gauges are accurate from 10−2 Torr to 10−9 Torr. The torr (symbol Torr) is a non- SI unit of Pressure defined as 1/760 of an atmosphere. Ionization gauge calibration is very sensitive to construction geometry, chemical composition of 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 vacuums will usually be unpredictable, so a mass spectrometer must be used in conjunction with the ionization gauge for accurate measurement. 
A hot cathode ionization gauge is mainly composed of three electrodes all acting as a triode, where the cathode is the filament. The hot filament Ionization gauge, sometimes called a hot filament gauge or hot cathode gauge, is the most widely used vacuum (negative pressure measuring A triode is an electronic amplification device having three active electrodes A cathode is an Electrode through which (positive Electric current flows out of a polarized electrical device The three electrodes are a collector or plate, a filament, and a grid. An electrical filament is a thread of Metal, usually Tungsten, which is used to convert Electricity into light in Incandescent light bulbs (as developed The collector current is measured in picoamps by an electrometer. An electrometer is an electrical instrument for measuring Electric charge or electrical Potential difference. The filament voltage to ground is usually at a potential of 30 volts while the grid voltage at 180–210 volts DC, unless there is an optional electron bombardment feature, by heating the grid which may have a high potential of approximately 565 volts. Electron ionization ( EI, formerly known as electron impact) is an Ionization technique widely used in Mass spectrometry, particularly for The most common ion gauge is the hot cathode Bayard-Alpert gauge, with a small ion collector inside the grid. A glass envelope with an opening to the vacuum can surround the electrodes, but usually the Nude Gauge is inserted in the vacuum chamber directly, the pins being 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 even low vacuum while hot. The measurements of a hot cathode ionization gauge are always logarithmic.
Electrons emitted from the filament move several times in back and forth movements around the grid before finally 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). Electron ionization ( EI, formerly known as electron impact) is an Ionization technique widely used in Mass spectrometry, particularly for The number of these ions is proportional to the gaseous molecule density multiplied by the electron current emitted from the filament, and these ions pour into the collector to form an ion current. An ion is an Atom or Molecule which has lost or gained one or more Valence electrons giving it a positive or negative electrical charge Since the gaseous molecule density is proportional to the pressure, the pressure is estimated by measuring the ion current.
The low pressure sensitivity of hot cathode gauges is limited by the photoelectric effect. Electrons hitting the grid produce x-rays that produce photoelectric noise in the ion collector. This limits the range of older hot cathode gauges to 10-8 Torr and the Bayard-Alpert to about 10-10 Torr. Additional wires at cathode potential in the line of sight between the ion collector and the grid prevent this effect. In the extraction type the ions are not attracted by a wire, but by an open cone. As the ions cannot decide which part of the cone to hit, they pass through the hole and form an ion beam. This ion beam can be passed on to a
See also: Electron ionization
There are two subtypes of cold cathode ionization gauges: the Penning gauge (invented by Frans Michel Penning), and the Inverted magnetron, also called a Redhead gauge. Electron ionization ( EI, formerly known as electron impact) is an Ionization technique widely used in Mass spectrometry, particularly for A cold cathode is an element used within some Nixie tubes Gas discharge lamps Gas filled tubes and Vacuum tubes Cold cathodes do not Many techniques have been developed for the measurement of Pressure and Vacuum. Frans Michel Penning ( 12 September 1894, Gorinchem – 6 December 1953, Utrecht) was a Dutch Physicist. The major difference between the two is the position of the anode with respect to the cathode. An anode is an Electrode through which Electric current flows into a polarized electrical device A cathode is an Electrode through which (positive Electric current flows out of a polarized electrical device Neither has a filament, and each may require a DC potential of about 4 kV for operation. Direct current ( DC) is the unidirectional flow of Electric charge. The volt (symbol V) is the SI derived unit of electric Potential difference or Electromotive force. Inverted magnetrons can measure down to 1x10-12 Torr. The torr (symbol Torr) is a non- SI unit of Pressure defined as 1/760 of an atmosphere.
Such gauges cannot operate if the ions generated by the cathode recombine before reaching the anodes. If the mean-free path of the gas within the gauge is smaller than the gauge's dimensions, then the electrode current will essentially vanish. A practical upper-bound to the detectable pressure is, for a Penning gauge, of the order of 10-3 Torr.
Similarly, cold cathode gauges may be reluctant to start at very low pressures, in that the near-absence of a gas makes it difficult to establish an electrode current - particularly in Penning gauges which use an axially symmetric magnetic field to create path lengths for ions which are of the order of metres. In ambient air suitable ion-pairs are ubiquitously formed by cosmic radiation; in a Penning gauge design features are used to ease the set-up of a discharge path. For example, the electrode of a Penning gauge is usually finely tapered to facilitate the field emission of electrons.
Maintenance cycles of cold cathode gauges is generally measured in years, depending on the gas type and pressure that they are operated in. Using a cold cathode gauge in gases with substantial organic components, such as pump oil fractions, can result in the growth of delicate carbon films and shards within the gauge which eventually either short-circuit the electrodes of the gauge, or impede the generation of a discharge path.
Pressure gauges are either direct- or indirect-reading. Hydrostatic and elastic gauges measure pressure are directly influenced by force exerted on the surface by incident particle flux, and are called direct reading gauges. Thermal and ionization gauges read pressure indirectly by measuring a gas property that changes in a predictable manner with gas density. Indirect measurements are susceptible to more errors than direct measurements.
When fluid flows are not in equilibrium, local pressures may be higher or lower than the average pressure in a medium. These disturbances propagate from their source as longitudinal pressure variations along the path of propagation. This is also called sound. Sound pressure is the instantaneous local pressure deviation from the average pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water. A hydrophone (Greek "hydro" = "water" and "phone" = "sound" is a Microphone designed to be used underwater for recording or listening The effective sound pressure is the root mean square of the instantaneous sound pressure over a given interval of time. In Mathematics, the root mean square (abbreviated RMS or rms) also known as the quadratic mean, is a statistical measure of the Sound pressures are normally small and are often expressed in units of microbar.
John H. , Moore; Christopher Davis, Michael A. Coplan and Sandra Greer (2002). Building Scientific Apparatus. Boulder, CO: Westview Press. ISBN 0-8133-4007-1.
"Vacuum Techniques". The Encyclopedia of Physics (3rd). (1990). Edited by Robert M. Besançon. Published by Van Nostrand Reinhold, New York