Pressure measurement
Many techniques have been developed for the measurement of pressure and vacuum. Instruments used to measure pressure are called pressure gauges or vacuum gauges
A manometer is a pressure measuring instrument, usually limited to measuring pressures lower than atmospheric. It 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 broadly divided into two categories: high and low vacuum (and sometimes ultra-high vacuum). Many of the different techniques used to measure these categories have an overlap at some point in the pressure range. By combining several different types of gauge it is possible measure system pressure from 10 mbar down to 10e-11 mbar. [1]
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Zero reference
Pressure measurements may be expressed relative to various zero references.. Absolute pressure of a fluid is referenced against a perfect vacuum. Gauge pressure is referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Atmospheric pressure is typically about 100 kPa at sea level, but is variable with altitude and weather. A standard value of atmospheric pressure has been defined to be 101.325 Pa. If the absolute pressure of a fluid stays constant, the gauge pressure of the same fluid will vary as atmospheric pressure changes. For gauge pressures several times larger than atmospheric pressure, this variation is small as a percentage of reading and may be ignored. Differential pressure is the difference in pressure between two points.
Examples of absolute pressure measurements include barometric pressure, altimeters, and the Manifold Absolute Pressure (MAP) sensor used in the engine control systems of modern fuel-injected automobiles. Examples of gauge pressure measurements include the tire-pressure gauge and sphygmomanometer. 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.
Gauge pressure of vacuum is usually indicated and expressed without a negative sign, so it is equal to the atmospheric pressure minus the absolute pressure
Units
The SI unit of pressure is the pascal (abbreviation Pa). Atmospheric pressures are usually stated using its decimal multiple kilopascal (kPa), where 1 kPa is close to 1.0% of Earth's atmospheric pressure at sea level. In meteorologic reports, hPa or mbar are the commonly used units. In vacuum systems, the equivalent units torr and millimeter of mercury (mmHg) are also used, with 1 torr equaling 133.3223684 Pa above an ideal vacuum.
Other vacuum units occasionally encountered in the literature include micrometers of mercury, the barometric scale, or as a percentage of atmospheric pressure in bars or atmospheres. Low vacuum is measured in the United States also in inches of mercury (inHg) below atmospheric pressure. "Below atmospheric" means that the absolute pressure is equal to the atmospheric pressure (29.92 inHg) minus the vacuum pressure in inches of mercury. (This is effectively a gauge pressure.)Thus a vacuum of 26 inHg is equivalent to an absolute pressure of 29.92 inHg − 26 inHg = 4 inHg.
Dynamic pressure
Static pressure is uniform in all directions, so pressure measurements are independent of direction in an immobile (static) fluid. 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. 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. 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. The presence of the measuring instrument inevitably acts to divert flow and create turbulance, so its shape is critical to accuracy and the calibration curves are often non-linear.
Applications
Instruments
Hydrostatic
They are helpful because the deflection of the manometer is not dependent upon the type of gas being measured, unlike thermal and ionization vacuum gauges.
Hydrostatic gauges (such as the mercury column manometer) 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. The simplest design is a closed-end U-shaped tube, one side of which is connected to the region of interest. Any fluid can be used, but mercury is preferred for its high density and low vapour pressure. Simple hydrostatic gauges can measure pressures ranging from 1 Torr (100 Pa) to above atmospheric. 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. 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.[2]
piston
For example dead-weight tester used for calibration. Tire-pressure gauge
liquid column
A simple mercury or other liquid manometer can be used to measure pressures from atmospheric down to a few mmHg. Care must be taken to ensure that vapour from the working liquid does not contaminate the vacuum system.
- Useful range: above about 1 torr (roughly 100 Pa)
The oldest type is the liquid-column manometer. A very simple version is a U-shaped tube half-full of liquid where the measured pressure is applied to one side of the tube whilst the reference pressure (which might be of the atmosphere) is applied to the other. The difference in liquid level represents the applied pressure. It is quite easy to make a manometer. For low pressure differences, water is a commonly-used liquid (and "inches of water" is a commonly-used pressure unit). For larger pressure differences, the greater density of mercury makes it more useful.
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. Liquid-column manometers can be used to measure small differences between great pressures.
McLeod gauge
A McLeod gauge isolates a sample of gas and compresses it in a modified mercury manometer until the pressure is a few mmHg. 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.
- Useful range: above 10-4 torr [3] (roughly 10-2 Pa)
Aneroid
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.
Bourdon
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.
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In 1849 the Bourdon tube pressure gauge was patented in France by Eugene Bourdon.
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. 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:
- the left side of the face, used for measuring manifold vacuum, is calibrated in centimetres of mercury on its inner scale and inches of mercury on its outer scale.
- the right portion of the face is used to measure fuel pump pressure and is calibrated in fractions of 1 kgf/[square centimeter|cm² on its inner scale and pounds per square inch on its outer scale.
Mechanical details
Stationary parts:
- A: Receiver block. This joins the inlet pipe to the fixed end of the Bourdon tube (1) and secures the chassis plate (B). The two holes receive screws that secure the case.
- B: Chassis Plate. The face card is attached to this. It contains bearing holes for the axles.
- C: Secondary Chassis Plate. It supports the outer ends of the axles.
- D: Posts to join and space the two chassis plates.
Moving Parts:
- Stationary end of Bourdon tube. This communicates with the inlet pipe through the receiver block.
- Moving end of Bourdon tube. This end is sealed.
- Pivot and pivot pin.
- Link joining pivot pin to lever (5) with pins to allow joint rotation.
- Lever. This an extension of the sector gear (7).
- Sector gear axle pin.
- Sector gear.
- Indicator needle axle. This has a spur gear that engages the sector gear (7) and extends through the face to drive the indicator needle. Due to 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 that of the spur gear, any motion of the Bourdon tube is greatly amplified. A small motion of the tube results in a large motion of the indicator needle.
- Hair spring to preload the gear train to eliminate gear lash and hysteresis.
Diaphragm
A second type of aneroid gauge uses the deflection of a flexible membrane that separates regions of different pressure. 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.
- Useful range: above 10-2 torr [4] (roughly 1 Pa)
For absolute measurements, welded pressure capsules with diaphragms on either side are often used.
Shape:
- Flat
- corrugated
- flattened tube
- capsule
Bellows
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.) 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. 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.
Secondary transducer
- resistive (strain gauge)
- inductive
- capacitive - The deflection of the piston is often one half of a capacitor, so that when the piston moves, the capacitance of the device changes. This is a common way (with proper calibrations) to get a very precise, electronic reading from a manometer, and this configuration is called a capacitive manometer vacuum gauge.
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.
- piezoelectric/piezoresistive
Thermal conductivity
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. A thermocouple or Resistance Temperature Detector (RTD) can then be used to measure the temperature of the filament. 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. These gauges are accurate from 10 Torr to 10−3 Torr, but they are sensitive to the chemical composition of the gases being measured.
two wire
One wire coil is used as a heater, and the other is used to measure nearby temperature due to convection.
Pirani (one wire)
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. 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.
Thermocouple gauges and thermistor gauges work in a similar manner, except a thermocouple or thermistor is used to measure the temperature of the wire.
- Useful range: 10-3 - 10 torr [5] (roughly 10-1 - 1000 Pa)
Ionization gauge
Ionization gauges are the most sensitive gauges for very low pressures (high vacuums, AKA "hard" vacuums). 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.
Thermionic emission emissions generate electrons, which collide with gas atoms and generate positive ions. The ions are attracted to a suitably biased electrode known as the collector. 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.
- Useful range: 10-10 - 10-3 torr (roughly 10-8 - 10-1 Pa)
Ion gauges come in two types: hot cathode and cold cathode. In the hot cathode version an electrically heated filament produces an electron beam. 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. Cold Cathode gauges are accurate from 10−2 Torr to 10−9 Torr. 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.[6]
Hot Cathode
Cold Cathode
Calibration
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.
- Dead weight tester
- McLeod
- mass spec + ionization
Dynamic transients
- frequency response
- resonance
- microphones
History
European (CEN) Standard
- EN 472 : Pessure gauge - Vocabulary.
- EN 837-1 : Pressure gauges. Bourdon tube pressure gauges. Dimensions, metrology, requirements and testing.
- EN 837-2 : Pressure gauges. Selection and installation recommendations for pressure gauges.
- EN 837-3 : Pressure gauges. Diaphragm and capsule pressure gauges. Dimensions, metrology, requirements and testing..
See also
Patents
- U.S. Patent 2960867 : W. R. Valcourt : "Overflow valve for a manometer "
- U.S. Patent 4779464 : Nevin L. Schwien : "Manometer"
- U.S. Patent 4683756 : Michael A. Ciminelli & James L. Derleth : "Low cost manometer"
External links
References
- ^ www.vacgen.com Introduction. Accessed 15 April 2006.
- ^ Beckwith, Thomas G., Roy D. Marangoni and John H. Lienhard V (1993). “Measurement of Low Pressures”, Mechanical Measurements, Fifth Edition, Reading, MA: Addison-Wesley, 591-595. ISBN 0-201-56947-7.
- ^ Techniques of high vacuum
- ^ Product brochure from Schoonover, Inc
- ^ VG Scienta
- ^ "Vacuum Techniques". The Encyclopedia of Physics (3rd edition): pp. 1278-1284. (1990). Ed. Robert M. Besançon. Van Nostrand Reinhold, New York. ISBN 0-442-00522-9.
John H., Moore, Christopher Davis, Michael A. Coplan and Sandra Greer (2002). Building Scientific Apparatus. Boulder, CO: Westview Press. ISBN 0-8133-4007-1.</ref>
Categories
Vacuum | Pressure gauges