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Electronic Maintenance
This comprehensive INVOLVE® multimedia training program was produced in association with the Instrument Society of America (ISA). This five lesson program trains participants in Me maintenance of electronic instruments, including pressure, temperature, flow, level, and weight transmitters as well as transducers, recorders, annunciators, and analog electronic controllers.
- Pressure and Temperature Transmitters
- Flow Transmitters
- Level and Weight Transmitters
- Transducers, Recorders, and Annunciators
- Electronic Controllers
- Pressure and Temperature Transmitters
Description:
This lesson introduces electronic transmitter maintenance focusing on pressure and temperature transmitters. The lesson describes the components of a typical pressure or temperature transmitter, their functions, adjustments, inspections, and repairs. Procedures for isolating the faulty component in a transmitter are also demonstrated.
Objectives:
- Identify components in a disassembled electronic differential pressure or electronic temperature transmitter
- Test the power supply for the transmitter in an electronic pressure transmitter
- Adjust the sensor zero and replace the electronics module of the pressure transmitter
- Isolate malfunctions to either the sensor or circuitry portion of a differential pressure transmitter
- Verify that a sensor is properly grounded
Swap circuit boards in a differential pressure transmitter - Replace the sensor assembly of a DP transmitter
- Identify the faulty component in a thermocouple transmitter
- Test outputs and repair an RTD
Swap a defective board to calibrate a malfunctioning RTD
Flow Transmitters
Description:
This lesson introduces the inspection and repair of electronic flowmeters by demonstrating maintenance procedures for vortex shedding, turbine, magnetic, and mass electronic flowmeters. The lesson describes typical flow transmitter components, their functions, common malfunctions, and procedures for isolating a faulty component.
- Objectives:
- Test and replace the amplifier unit, sensor, and bluff body of a vortex shedding flowmeter
- Test and replace
- The preamplifier unit, coil and other necessary components of a turbine flowmeter
- Test and replace the coil, electrodes, and circuit board in a magnetic flowmeter
- Jumper the appropriate terminals to simulate zero output and check the flowmeter output in an installed mass flowmeter
- Test and replace the sensor and circuit boards in an installed mass flowmeter
Level and Weight Transmitters
Description:
This lesson describes the operation, applications, and maintenance of ultrasonic, capacitance, conductivity, and radiation level detectors. The lesson also explains the functions and operation of weighing systems.
Objectives:
Describe the applications and operation of ultrasonic level detectors and their use in both point and continuous measurement applications
Troubleshoot and maintain ultrasonic level detection systems
Describe the applications and operation of radiation level detectors and their use in both point and continuous measurement applications
Explain the safety considerations when maintaining radiation level detectors
Describe the applications and operation of capacitance and conductivity level detectors in both point and continuous measurement applications
Recognize safety considerations for the use of level probes with flammable and/or explosive materials
Identify the maintenance procedures for capacitance level detection systems
Describe the applications and operation of a strain gage load cell as well as considerations for load cell calibration
Describe the applications and operation of a belt conveyor scale as well as how to test and calibrate it
Transducers, Recorders, and Annunciators
Description:
This lesson teaches routine maintenance requirements and calibration procedures for transducers, recorders, and annunciators. The lesson provides a basic understanding of the functions of I/P, P/I, and E/I transducers, multipen and multipoint recorders, and annunciators. The lesson also outlines how to identify and troubleshoot problems in these instruments.
Objectives:
Identify and describe the function of electronic transducers
Identify how I/P transducers work
Identify troubleshooting steps for pneumatic and electronic function on I/P transducers
Identify the steps for continuity tests on I/P transducers
Identify coil replacement steps for I/P transducers
Identify calibration steps for P/I and E/I transducers, identify motor replacement steps for the chart drive on a multipen recorder
Identify the function of drive gears on a multipen recorder and how to clean them
Identify installation steps for a new drive cable on a multipoint recorder and check for proper operation
Identify the function of drive wire resistors on multipoint recorders and how to clean and inspect them
Calibrate multipen and multipoint recorders
Define the function of annunciators and troubleshoot them
Electronic Controllers
Description:
This lesson presents routine maintenance requirements and calibration procedures for electronic controllers. The lesson shows how controller circuitry works and how to adjust and calibrate each of its component sections: the display, the alarm circuitry, and the control circuitry.
Objectives:
Identify the features and functions of controllers
Describe and compare pneumatic and electronic controllers
Identify the signal path through a control circuit
Describe the function of resistors, comparators, proportional band amplifiers, integral amplifiers, differentiating amplifiers, summing amplifiers, and the transducer
Visually identify indicators on electronic controllers as well as set point, process, output, and alarms
Visually identify controls on electronic controllers as well as set point control, auto-manual selector swITC Learningh, and manual/valve control
Identify appropriate equipment and demonstrate procedures for calibrating and troubleshooting display indicators
Identify appropriate test points and demonstrate procedures for calibrating and troubleshooting alarm indicators
Identify appropriate equipment for calibrating control circuits and calibrate proportional, integral, and derivative zero on the control circuit
Identify appropriate equipment for troubleshooting control circuits
Electronic Maintenance
This comprehensive INVOLVE® multimedia training program was produced in association with the Instrument Society of America (ISA). This five lesson program trains participants in Me maintenance of electronic instruments, including pressure, temperature, flow, level, and weight transmitters as well as transducers, recorders, annunciators, and analog electronic controllers.
Pressure and Temperature Transmitters
Flow Transmitters
Level and Weight Transmitters
Transducers, Recorders, and Annunciators
Electronic Controllers
Pressure and Temperature Transmitters
Description:
This lesson introduces electronic transmitter maintenance focusing on pressure and temperature transmitters. The lesson describes the components of a typical pressure or temperature transmitter, their functions, adjustments, inspections, and repairs. Procedures for isolating the faulty component in a transmitter are also demonstrated.
Objectives:
Identify components in a disassembled electronic differential pressure or electronic temperature transmitter
Test the power supply for the transmitter in an electronic pressure transmitter
Adjust the sensor zero and replace the electronics module of the pressure transmitter
Isolate malfunctions to either the sensor or circuitry portion of a differential pressure transmitter
Verify that a sensor is properly grounded
Swap circuit boards in a differential pressure transmitter
Replace the sensor assembly of a DP transmitter
Identify the faulty component in a thermocouple transmitter
Test outputs and repair an RTD
Swap a defective board to calibrate a malfunctioning RTD
Flow Transmitters
Description:
This lesson introduces the inspection and repair of electronic flowmeters by demonstrating maintenance procedures for vortex shedding, turbine, magnetic, and mass electronic flowmeters. The lesson describes typical flow transmitter components, their functions, common malfunctions, and procedures for isolating a faulty component.
Objectives:
Test and replace the amplifier unit, sensor, and bluff body of a vortex shedding flowmeter
Test and replace
The preamplifier unit, coil and other necessary components of a turbine flowmeter
Test and replace the coil, electrodes, and circuit board in a magnetic flowmeter
Jumper the appropriate terminals to simulate zero output and check the flowmeter output in an installed mass flowmeter
Test and replace the sensor and circuit boards in an installed mass flowmeter
Level and Weight Transmitters
Description:
This lesson describes the operation, applications, and maintenance of ultrasonic, capacitance, conductivity, and radiation level detectors. The lesson also explains the functions and operation of weighing systems. Objectives:
Describe the applications and operation of ultrasonic level detectors and their use in both point and continuous measurement applications
Troubleshoot and maintain ultrasonic level detection systems
Describe the applications and operation of radiation level detectors and their use in both point and continuous measurement applications
Explain the safety considerations when maintaining radiation level detectors
Describe the applications and operation of capacitance and conductivity level detectors in both point and continuous measurement applications
Recognize safety considerations for the use of level probes with flammable and/or explosive materials
Identify the maintenance procedures for capacitance level detection systems
Describe the applications and operation of a strain gage load cell as well as considerations for load cell calibration
Describe the applications and operation of a belt conveyor scale as well as how to test and calibrate it
Transducers, Recorders, and Annunciators
Description:
This lesson teaches routine maintenance requirements and calibration procedures for transducers, recorders, and annunciators. The lesson provides a basic understanding of the functions of I/P, P/I, and E/I transducers, multipen and multipoint recorders, and annunciators. The lesson also outlines how to identify and troubleshoot problems in these instruments.
Objectives:
Identify and describe the function of electronic transducers
Identify how I/P transducers work
Identify troubleshooting steps for pneumatic and electronic function on I/P transducers
Identify the steps for continuity tests on I/P transducers
Identify coil replacement steps for I/P transducers
Identify calibration steps for P/I and E/I transducers, identify motor replacement steps for the chart drive on a multipen recorder
Identify the function of drive gears on a multipen recorder and how to clean them
Identify installation steps for a new drive cable on a multipoint recorder and check for proper operation
Identify the function of drive wire resistors on multipoint recorders and how to clean and inspect them
Calibrate multipen and multipoint recorders
Define the function of annunciators and troubleshoot them
Electronic Controllers
Description:
This lesson presents routine maintenance requirements and calibration procedures for electronic controllers. The lesson shows how controller circuitry works and how to adjust and calibrate each of its component sections: the display, the alarm circuitry, and the control circuitry.
Objectives:
Identify the features and functions of controllers
Describe and compare pneumatic and electronic controllers
Identify the signal path through a control circuit
Describe the function of resistors, comparators, proportional band amplifiers, integral amplifiers, differentiating amplifiers, summing amplifiers, and the transducer
Visually identify indicators on electronic controllers as well as set point, process, output, and alarms
Visually identify controls on electronic controllers as well as set point control, auto-manual selector swITC Learningh, and manual/valve control
Identify appropriate equipment and demonstrate procedures for calibrating and troubleshooting display indicators
Identify appropriate test points and demonstrate procedures for calibrating and troubleshooting alarm indicators
Identify appropriate equipment for calibrating control circuits and calibrate proportional, integral, and derivative zero on the control circuit
Identify appropriate equipment for troubleshooting control circuits
Pressure Measurement
Pressure measurement
It has been suggested that Difference between gauge and absolute pressure be merged into this article or section. (Discuss) The construction of a bourdon tube gauge, construction elements are made of brass 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 could also be referring to a pressure measuring instrument, usually limited to measuring pressures near to atmospheric.
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). 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. [1] Contents * 1 Zero reference * 2 Units * 3 Dynamic pressure * 4 Applications * 5 Instruments o 5.1 Hydrostatic + 5.1.1 Piston + 5.1.2 Liquid column + 5.1.3 McLeod gauge o 5.2 Aneroid + 5.2.1 Bourdon # 5.2.1.1 Mechanical details + 5.2.2 Diaphragm + 5.2.3 Bellows + 5.2.4 Secondary transducer o 5.3 Thermal conductivity + 5.3.1 Two wire + 5.3.2 Pirani (one wire) o 5.4 Ionization gauge + 5.4.1 Hot cathode + 5.4.2 Cold cathode * 6 Calibration * 7 Dynamic transients * 8 History * 9 European (CEN) Standard * 10 See also * 11 Patents * 12 External links * 13 References
Zero reference
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: * Absolute pressure is zero referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure. * Gauge pressure is zero referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted. * Differential pressure is the difference in pressure between two points.
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. 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 101.325 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.
Units
Pressure Units pascal (Pa) bar (bar) technical atmosphere (at) atmosphere (atm) torr (Torr) pound-force per square inch (psi) 1 Pa ≡ 1 N/m2 10−5 1.0197×10−5 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 14.5037744 1 at 98,066.5 0.980665 ≡ 1 kgf/cm2 0.96784 735.56 14.223 1 atm 101,325 1.01325 1.0332 ≡ 1 atm 760 14.696 1 torr 133.322 1.3332×10−3 1.3595×10−3 1.3158×10−3 ≡ 1 Torr; ≈ 1 mmHg 19.337×10−3 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 = 10.197×10−6 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). 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. 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 [1]. 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. 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 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. Natural gas pipeline pressures are measured in inches of water, expressed as '"WC' ('Water Column'). Scuba divers often use a manometric rule of thumb: the pressure exerted by ten metres depth of water is approximately equal to one atmosphere. In vacuum systems, the units torr, micrometre of mercury (micron), and inch of mercury (inHg) are most commonly used. 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. In American and Canadian engineering, stress is often measured in kip. Note that stress is not a true pressure since it is not scalar. In the cgs system the unit of pressure was the barye (ba), equal to 1 dyn·cm-2. In the mts system, the unit of pressure was the pieze, equal to 1 sthene per square metre. 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 .
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 turbulence, so its shape is critical to accuracy and the calibration curves are often non-linear.
Applications
* Sphygmomanometer * Barometer * Altimeter * Pitot tube * MAP sensor
Instruments
Many instruments have been invented to measure pressure, with different advantages and disadvantages. 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 (a vertical tube filled with mercury) manometer invented by Evangelista Torricelli in 1643. The U-Tube was invented by Christian Huygens in 1661. Hydrostatic 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 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. Liquid column The difference in fluid height in a liquid column manometer is proportional to the pressure difference.
H=\frac{P_a-P_o}{g \rho} 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 while the reference pressure (which might be the atmospheric pressure or a vacuum) is applied to the other. 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 (13.534 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). 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. 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. (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[2] 1. Simple Manometer 2. Micromanometer 3. Differential manometer 4. Inverted differential manometer A McLeod gauge, drained of mercury
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) 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.[4] 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. Gauges that rely on a change in capacitances are often referred to as Baratron gauges.
Bourdon Membrane-type manometer 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 combination pressure and vacuum gauge (case and viewing glass removed) Indicator Side with card and dial Mechanical Side with Bourdon tube 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/cm² on its inner scale and pounds per square inch on its outer scale. [edit] Mechanical details 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
: 1. Stationary end of Bourdon tube. This communicates with the inlet pipe through the receiver block. 2. Moving end of Bourdon tube. This end is sealed. 3. Pivot and pivot pin. 4. Link joining pivot pin to lever (5) with pins to allow joint rotation. 5. Lever. This an extension of the sector gear (7). 6. Sector gear axle pin. 7. Sector gear. 8. 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. 9. Hair spring to preload the gear train to eliminate gear lash and hysteresis.
Diaphragm A pile of pressure capsules with corrugated diaphragms in an aneroid barograph. 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 by using 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 [5] (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.
This type of gauge was invented by Marcello Pirani. 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 [6] (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. 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. Thermionic emission 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) 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. 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.[7]
Hot cathode Bayard-Alpert hot cathode ionization gauge A hot cathode ionization gauge is mainly composed of three electrodes all acting as a triode, where the cathode is the filament. The three electrodes are a collector or plate, a filament, and a grid. The collector current is measured in picoamps by an electrometer. 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.
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).
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. 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 * Faraday cup * Microchannel plate detector with Faraday cup * Quadrupole mass analyzer with Faraday cup * Quadrupole mass analyzer with Microchannel plate detector Faraday cup * ion lens and acceleration voltage and directed at a target to form a sputter gun. In this case a valve lets gas into the grid-cage. See also: Electron ionization
Cold cathode 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. The major difference between the two is the position of the anode with respect to the cathode. Neither has a filament, and each may require a DC potential of about 4 kV for operation. Inverted magnetrons can measure down to 1x10-12 Torr. 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.
It has been suggested that Difference between gauge and absolute pressure be merged into this article or section. (Discuss) The construction of a bourdon tube gauge, construction elements are made of brass 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 could also be referring to a pressure measuring instrument, usually limited to measuring pressures near to atmospheric.
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). 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. [1] Contents * 1 Zero reference * 2 Units * 3 Dynamic pressure * 4 Applications * 5 Instruments o 5.1 Hydrostatic + 5.1.1 Piston + 5.1.2 Liquid column + 5.1.3 McLeod gauge o 5.2 Aneroid + 5.2.1 Bourdon # 5.2.1.1 Mechanical details + 5.2.2 Diaphragm + 5.2.3 Bellows + 5.2.4 Secondary transducer o 5.3 Thermal conductivity + 5.3.1 Two wire + 5.3.2 Pirani (one wire) o 5.4 Ionization gauge + 5.4.1 Hot cathode + 5.4.2 Cold cathode * 6 Calibration * 7 Dynamic transients * 8 History * 9 European (CEN) Standard * 10 See also * 11 Patents * 12 External links * 13 References
Zero reference
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: * Absolute pressure is zero referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure. * Gauge pressure is zero referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted. * Differential pressure is the difference in pressure between two points.
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. 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 101.325 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.
Units
Pressure Units pascal (Pa) bar (bar) technical atmosphere (at) atmosphere (atm) torr (Torr) pound-force per square inch (psi) 1 Pa ≡ 1 N/m2 10−5 1.0197×10−5 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 14.5037744 1 at 98,066.5 0.980665 ≡ 1 kgf/cm2 0.96784 735.56 14.223 1 atm 101,325 1.01325 1.0332 ≡ 1 atm 760 14.696 1 torr 133.322 1.3332×10−3 1.3595×10−3 1.3158×10−3 ≡ 1 Torr; ≈ 1 mmHg 19.337×10−3 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 = 10.197×10−6 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). 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. 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 [1]. 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. 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 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. Natural gas pipeline pressures are measured in inches of water, expressed as '"WC' ('Water Column'). Scuba divers often use a manometric rule of thumb: the pressure exerted by ten metres depth of water is approximately equal to one atmosphere. In vacuum systems, the units torr, micrometre of mercury (micron), and inch of mercury (inHg) are most commonly used. 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. In American and Canadian engineering, stress is often measured in kip. Note that stress is not a true pressure since it is not scalar. In the cgs system the unit of pressure was the barye (ba), equal to 1 dyn·cm-2. In the mts system, the unit of pressure was the pieze, equal to 1 sthene per square metre. 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 .
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 turbulence, so its shape is critical to accuracy and the calibration curves are often non-linear.
Applications
* Sphygmomanometer * Barometer * Altimeter * Pitot tube * MAP sensor
Instruments
Many instruments have been invented to measure pressure, with different advantages and disadvantages. 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 (a vertical tube filled with mercury) manometer invented by Evangelista Torricelli in 1643. The U-Tube was invented by Christian Huygens in 1661. Hydrostatic 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 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. Liquid column The difference in fluid height in a liquid column manometer is proportional to the pressure difference.
H=\frac{P_a-P_o}{g \rho} 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 while the reference pressure (which might be the atmospheric pressure or a vacuum) is applied to the other. 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 (13.534 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). 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. 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. (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[2] 1. Simple Manometer 2. Micromanometer 3. Differential manometer 4. Inverted differential manometer A McLeod gauge, drained of mercury
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) 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.[4] 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. Gauges that rely on a change in capacitances are often referred to as Baratron gauges.
Bourdon Membrane-type manometer 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 combination pressure and vacuum gauge (case and viewing glass removed) Indicator Side with card and dial Mechanical Side with Bourdon tube 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/cm² on its inner scale and pounds per square inch on its outer scale. [edit] Mechanical details 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
: 1. Stationary end of Bourdon tube. This communicates with the inlet pipe through the receiver block. 2. Moving end of Bourdon tube. This end is sealed. 3. Pivot and pivot pin. 4. Link joining pivot pin to lever (5) with pins to allow joint rotation. 5. Lever. This an extension of the sector gear (7). 6. Sector gear axle pin. 7. Sector gear. 8. 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. 9. Hair spring to preload the gear train to eliminate gear lash and hysteresis.
Diaphragm A pile of pressure capsules with corrugated diaphragms in an aneroid barograph. 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 by using 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 [5] (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.
This type of gauge was invented by Marcello Pirani. 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 [6] (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. 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. Thermionic emission 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) 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. 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.[7]
Hot cathode Bayard-Alpert hot cathode ionization gauge A hot cathode ionization gauge is mainly composed of three electrodes all acting as a triode, where the cathode is the filament. The three electrodes are a collector or plate, a filament, and a grid. The collector current is measured in picoamps by an electrometer. 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.
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).
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. 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 * Faraday cup * Microchannel plate detector with Faraday cup * Quadrupole mass analyzer with Faraday cup * Quadrupole mass analyzer with Microchannel plate detector Faraday cup * ion lens and acceleration voltage and directed at a target to form a sputter gun. In this case a valve lets gas into the grid-cage. See also: Electron ionization
Cold cathode 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. The major difference between the two is the position of the anode with respect to the cathode. Neither has a filament, and each may require a DC potential of about 4 kV for operation. Inverted magnetrons can measure down to 1x10-12 Torr. 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.
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