795

5.14 Vacuum Sensors

B. G. LIPTÁK

(1969, 1982, 1995),

REVIEWED

BY

J. WELCH

(1995)

R. A. GILBERT

(2003)

Design Variations:

A. MechanicalA1. Liquid manometers (Section 5.9)A2. Capacitance diaphragm manometers (Section 5.7)A3. Quartz helix gauge (Section 5.4)A4. Spinning ball viscous friction gaugeA5. Molecular momentum gaugeA6. Bourdon gaugeA7. Bellows gauge

B. ThermalB1. Pirani (thermistor) gaugeB2. Thermocouple, thermopile gaugeB3. Convectron gauge

C. IonizationC1. Hot cathode (Bayard-Alpert)C2. Hot cathode (Suhultz-Phelps)C3. Cold cathode (Philips)C4. Cold cathode (Redhead)

D. Vacuum gauge calibrationD1. McLeod gaugeD2. Calibration reference tubes

Range:

See Figure 5.14a

Inaccuracy:

A1. Visibility limited to 0.1 mmA2. Usually

±

0.05% or reading at 1 torr, increasing as vacuum rises to about 3% ofreading at 10

−

4

torrA3.

±

0.01% of full scaleA4. Usually

±

3 to 4% of actual readingA5. Usually 1 to 3% of actual readingA6, A7. Limited range (only about 1% of atmosphere)B1, B2, B3. From 2% of reading at point of calibration to over 10%; reading changes

with compositionC. Reading changes with compositionC1, C2. 10% of reading or lessC3, C4. 10 to 20% of reading

Cost:

A1. $150 to $500A2. $400 to $1000A3. Over $5000A4. Sensing head $2000; with controller, $11,000 to $14,000A6. $50 to $100A7. $75 to $300B1. $350 to $700; with controller, $900 to $1100B2. $300 to $800; with controller, $800 to $1200B3. $120; tube and indicator, $500; tube, readout, and controller, $1100 to $1500C1. $150 to $350; tube and controller, $1500C2. $400

PEPI

Flow Sheet Symbol

(Ion or Th)

© 2003 by Béla Lipták

796

Pressure Measurement

C3, C4. $200 to $600D1. $200 to $600D2. $300

Partial List of Suppliers:

Ametek Inc. (A6, A7) (www.ametekapt.com)Cooke Vacuum Products (B, C) (www.cookevacuum.com)Dresser Industrial Instruments (A6, A7) (www.dresserinstruments.com)Dwyer Instruments Inc. (A1) (www.dwyer-inst.com)Edwards High Vacuum International (B, C, D1) (www.edwards.boc.com)Fountain Valley Instruments Inc. Televac Div. (hand-held) (www.telvac.com)Fredericks Co. (www.frederickscom.com)GFV Associates Inc. (B, C) (www.gfvassociates.com)Granville-Phillips Co. (B, C) (www.helixtechnology.com)Hastings (A, B, C, D2) (www.hastings-inst.com)International Pressure Products Leopold Inc. (A6, A8, B1, B2, C)Leybold Inficon Inc. (A) (www.inficon.com)Marshalltown Instruments Inc. (A)Meriam Instrument, a Scott Fetzer Co. (A5) (www.meriam.com)MKS Instruments Inc. (A, B, C, D2) (www.mksinst.com)Moeller Instrument Co. (A5) (www.moellerinstrument.com)Rosemount Inc. Div. of Emerson (A2) (www.rosemount.com)Stokes Div. of PermaH (D1)Uehling Instrument Co. (A5, C1) (www.uehling.com)Vacuum Technology Inc. (C) (www.vacuumtechnology.com)Varian, Vacuum Products Div. (B, C) (www.varianinc.com)Weksler Instruments Corp. (A6, A7) (www.monarchinstrcorp.com)

INTRODUCTION

From a working definition perspective, atmospheric pressureis considered to be 1.01 bar, 14.7 psia, 29.9 in. Hg, 760mmHg absolute, 760 torr, 760,000

µ

m, or 101,300 Pa. Anobvious impact of this statement is the potential for confusiondue to the various unit options people have when it comesto reporting pressure. Fortunately, it is not an issue that theatmospheric pressure at sea level is not one fixed value, noris there any deleterious impact of defining 1 atmosphere ofpressure—the pressure at sea level on a clear cold nonstormyday, to be 1,013,250 dynes/cm

2

. Industrial vacuum measure-ments are expressed in pressure units that reflect a pressurerange and historic preference of the industrial practice. Thus,it is common for vacuum applications that are near atmo-spheric pressure to be described in inches of water or mer-cury. However, it is also expected that the pressure associatedwith a semiconductor metalization industrial application thatuses physical vapor deposition be described in Pascals,microns, or perhaps millitorr units.

The reader should also be prepared to deal with pressureunit selection variations with respect to the pressure unitsthat appear on various vacuum pressure gauges. The unfor-tunate realities when dealing with industrial applications thatrequire a vacuum environment is that there is no universalvacuum gauge that will work for all applications and thatdifferent types of gauges will use different pressure units.The normal measurement ranges for various gauges are sum-marized in Figure 5.14a. A variety of pressure units will beused in this section.

People new to vacuum gauges and vacuum technologyshould also be prepared to deal with potentially confusingvocabulary when describing various pressure situations. Peo-ple with different technical backgrounds may use the termslow vacuum, low pressure, and/or high vacuum, and highpressure imprecisely at different times. The reader shouldrefer to a pressure that is very much lower than atmosphericpressure as a low pressure or a high vacuum environment.For a process situation that is very close to but below atmo-spheric pressure, the terms to use would be low vacuum orhigh pressure. In all situations, when a vacuum is referencedor described, the maximum pressure value being consideredis atmospheric pressure. Therefore, vacuum chambers locatedin Denver, CO, or Geneva, Switzerland, have maximum pres-sure values that are less than 1 atm (760 mmHg).

VACUUM GAUGE CLASSIFICATIONS

There are a variety of ways to categorize vacuum gauges.Grouping gauges by operational principle is one commonway. Thus, the Bourdon tube and the bellows gauge areclassified as mechanical gauges, but the spinning elementgauge is know as a momentum transfer gauge. Figure 5.14aprovides a list of vacuum gauges and their operating rangegrouped by operational principle. Vacuum gauges that func-tion because of a mechanical principle typically perform adifferential pressure measurement.

Gauges that operate within the same pressure range canalso be grouped together. Pirani gauges work in the samegeneral pressure range as thermocouple (TC) gauges but the

© 2003 by Béla Lipták

5.14 Vacuum Sensors

797

ion gauge would not be grouped with these two. A third wayto categorize vacuum gauges is to identify if they actuallymake a direct or an indirect pressure measurement. Thus, adirect vacuum pressure measurement gauge will provide a newmeasurement signal value simply because the pressure beingmeasured changes. The indirect pressure measurement gaugeis actually monitoring some other parameter of the system thatproportionally responds as the pressure of the system changes.A vacuum gauge that uses a TC as the pressure-sensing elementis one example of an indirect pressure gauge. The increase ordecrease in pressure near the thermocouple does not directlyalter the electromagnetic force generated by that TC. However,that pressure change does reflect a change in the molecularpopulation near the TC. This molecular density change affectsthe heat transfer near that TC, which, in turn, alters the valueof voltage, measured across the TC.

Finally, it is also important to identify if the vacuum gaugeperforms an absolute pressure measurement. Similar to pressuregauges designed for above atmospheric pressure measurementapplications, vacuum pressure gauges may provide an absoluteor gauge pressure reading. Naturally, it is important for instru-mentation and controls personnel to know which type of pres-sure measurement is being taken. The adjustment from gaugeto absolute pressure is straightforward and may be provided asan electronic correction within the gauges. Elastic elementbased vacuum gauges are simple and inexpensive examples. If

the vacuum is measured against an internal vacuum reference,the instrument is called an absolute pressure sensor; when thevacuum is compared to the barometric pressure, the instrumentwill measure gauge pressure and is sometimes identified as acompound pressure gauge. Examples of absolute pressure vac-uum sensors include the copper-beryllium alloy bellows ele-ments shown in Figures 5.3a, 5.3b, and 5.3c. Diaphragm-typevacuum gauges (Figure 5.5f ) use hermetically sealed, thin-walled, copper-beryllium alloy, circular capsules. Similarly tothe diaphragm and bellows elements, thin-walled, multiturnspiral Bourdon tube elements can also be used to measurevacuum when a second reference Bourdon tube, containing anear absolute zero vacuum, is also provided as reference.

MECHANICAL VACUUM GAUGES

Manometers

Manometers (Section 5.9) that use a liquid working fluid rep-resent the classical differential pressure measurement device.They are simple and inexpensive and some of their designvariations are also shown in Figures 5.9a, 5.9c, 5.9h, and 5.9i.The simplest design for a vacuum pressure measurementapplication is a U-tube with one leg connected to an evacuatedreference chamber, while the other is exposed to the unknownprocess pressure. The difference between the two column

FIG. 5.14a

Ranges of the different types of vacuum gauges.

Bourdon Tube

Bellows

Diaphragm

Momentum

Quartz

Pirani

Thermocouple

Convectron

Capacitance

Viscous Friction

Bayard-Alpert

Philips

Redhead

Pressure in Torr

10+2 100 10−2 10−4 10−6 10−8 10−10

Medium VacuumRange

ionizationgauge

mechanicalgauge

thermalgauge

0.110 0.001

© 2003 by Béla Lipták

798

Pressure Measurement

heights indicates the process vacuum. The precision of readingthe manometer is limited to about 0.1 mm when detected bythe naked eye. Inclining the manometer tube and using lower-density filling fluids (low-vapor-pressure oils) can improve thesensitivity of the readout to about 10 mtorr. When the unitsare operated at constant temperatures and are provided withmicrometer readouts, vacuums down to the 1 mtorr level canbe detected. Manometers are not used in vacuum related pro-cesses that require medium to high vacuum or a clean vacuumenvironment. This limitation reflects the possible vaporizationof the filling fluids at high vacuums or at high temperatures.Density variation and light refraction can also make it difficultto read the gauge. Although liquid manometers have limiteduse in today’s process environment, the term manometer hasbecome ubiquitous. It is now generally applied to vacuumgauges that make differential measurements, but do not nec-essarily contain a working fluid.

Capacitance Manometers

The capacitance-type pressure detectors are examples ofmanometers that do not have a moving fluid as a sensor element.Capacitance manometers have already been described in con-nection with Figure 5.7i. These gauges provide high precisiondifferential measurements because of their ability to detectextremely small diaphragm movements. The thinnest dia-phragms are capable of measurement down to the 10

–5

torrlevel, while for the thicker diaphragms the measuring rangecan be extended to atmospheric or higher pressures.

A single capacitance manometer can report vacuum pres-sures values over a four- or five-decade range. Their inaccuracyin units of percent reading rises from about 0.1 to about 3% asthe vacuum rises from 1.0 torr to 10

4

torr. These devices, alsocalled micromanometers, are differential pressure detectors thatuse a flexible diaphragm in an electrical capacitance circuit.One plate of the capacitor is the pressure-sensitive diaphragmin the bridge circuit. The DC balancing voltage connectedacross the plates of the capacitor exerts an electrostatic force.When a pressure change occurs, the diaphragm deflects and theelectrostatic force restores the balance that opposes the initialdiaphragm movement. The magnitude of this rebalance voltageis a measure of the pressure exerted on the flexible diaphragm.

The materials used in single capacitance manometerdetectors include Monel, Inconel, high-nickel alloys, andceramics. The single-sided design illustrated in Figure 5.14bis corrosion-resistant because only one side of the diaphragmis exposed to the process vapors. Some models can operateat temperatures up to 750

°

F (400

°

C). The two main sourcesof error are nonlinearity and temperature effects. As an alter-native to temperature compensation, the sensor can also beheated and operated at a constant temperature.

Quartz Helix Vacuum Gauge

The quartz helix vacuum gauge is an example of a mechanicalvacuum gauge that makes a differential pressure measurementwithout a working fluid element. The quartz helix element

and the mirror used to detect its deflection have already beendiscussed in connection with Figure 5.4i. When used for vac-uum detection, two quartz Bourdon elements are formed intoa helix. The reference tube contains a sealed vacuum, whilethe measuring tube is connected to the unknown processvacuum. The pressure difference between the two (at constanttemperature) results in an angular deflection, which isdetected optically. The optical readout is used to eliminatefriction effects and to guarantee high resolution (about onepart in 100,000). The main advantages of this detector are itsprecision and the corrosion resistance of quartz; however, itis an expensive gauge.

Viscous Friction of Spinning Ball

At high vacuums, viscosity and friction are pressure-depen-dent. This instrument detects vacuum pressures down to 10

7

torr by detecting the deceleration caused by molecular frictionon a levitating and spinning ball in a magnetic field. As suggestedin Figure 5.14c, the vacuum is measured in this instrument by

FIG. 5.14b

One-sided capacitance manometer detector can also be used oncorrosive services.

1

FIG. 5.14c

Spinning ball element type vacuum gauge.

High VacuumReference Cavity

Dual Electrodes

CapacitanceSignals

Tensioned InconelDiaphragm

Process Pressure

Permanent Magnets

Permanent Magnets

Suspended BallRotationDrivingCoils

SuspendedBall RotationDriving Coils

Suspended Ballin Sampling Tube

Gas Entry Portfrom VacuumChamber

Top (Bottom)VerticalStabilizationCoils

© 2003 by Béla Lipták

5.14 Vacuum Sensors

799

first driving the ball until it reaches a rotational speed of about425 r/s. When the drive is turned off, the rotational speed dropsas a function of the viscous friction of the process vapors.When the speed has dropped to 405 r/s, the ball is acceleratedagain. The pressure of the gas in the gauge is related to thetime that it takes for the speed to drop from 425 to 405 r/s.As the process pressure decreases, the time needed to reachthe lower rotational speed increases.

This instrument has several process advantages. Sinceits wetted parts are made of stainless steel, the gauge issuitable for corrosive services. It is also suited for operationat baking temperatures up to 750

°

F (400

°

C). When cali-brated, the inaccuracy (uncertainty) can be as low as 1.5%of the reading. Without calibration, the uncertainty is 4%or more.

Molecular Momentum Vacuum Gauges

Molecular momentum type gauges have two basic workingparts: a rotating and a restrained cylinder. The gas moleculesfrom the vacuum chamber come in contact with the rotatingcylinder (at a constant speed of 3600 rpm), experience amomentum change, and are set in motion in the direction ofrotation. These molecules acquire energy from contact withthe spinning cylinder, and then, in turn, strike and transferthat energy to the restrained cylinder. The pressure measure-ment is possible because these molecular collisions move therestrained cylinder a distance that is proportional to the energytransferred and represents a function of the number of gasmolecules in that space. The number of molecules is relatedto the absolute pressure of the gas. The pointer attached tothe restrained cylinder indicates the gas pressure on the scale,somewhat similar to the operation of the viscous drag gasdensity sensor in Chapter 6.

The energy transferred in momentum transfer gauges isnot just related to the number of molecules (pressure) andthe velocity of molecules. The molecular weight of the gasis also a factor. Thus, the full-scale range of the gaugedepends on the type of gas to be detected. For air, the rangeis 20 to 10

−

3

mmHg (2.7 to 1.3

×

10

−

4

kPa), while for hydro-gen, the maximum reading on the instrument is 280 mmHg(37 kPa). Thus, molecular momentum transfer vacuum gaugeshave to be calibrated for each application.

Molecular momentum transfer gauges give continuousdirect readout, but are not usually available as a signal trans-mitter for remote indication or control. The inaccuracy of theunit is between

±

5 and 25%, with accuracy decreasing atlower pressures. Additional inaccuracy can be caused byprocess temperature variations, which in the range of 50 to100

°

F (28 to 56

°

C), can amount to 2%. External vibrationin the range of 50 cps should be protected against by the useof bellows couplings on the process connection. The gauge isnot damaged by exposure to atmospheric pressure, but thesample from the process has to be kept clean, free of dust, oil,or other particles.

Mechanical Linkage Vacuum Gauge

The only vacuum gauges that operate by a mechanical link-age are the ones used for vacuum measurements, such as theBourdon gauge and the bellows gauge. The Bourdon gaugeis also know as a C tube gauge and operates by the curvingand uncurving of a hollow tube that is connected to theprocess under vacuum. The movement of this “C” shapedtube alters the position of a needle pointer connected to theend of the sensing tube by a ratchet and spring mechanism.Although inexpensive and robust, the Bourdon gauge willdemonstrate a memory effect and does not fit into automateddata recording or process control situations. In addition, ithas a limited range and is not used in vacuum processes thatoperate at pressures below 1 torr.

The Bellows vacuum gauge is another example of amechanical linkage type gauge. In this case, the bellowsexpands or contracts based on the pressure difference acrossthe inside and outside of the bellows unit. The gauge readoutis also the result of a mechanical interaction between thebellows and the needle pointer. These gauges do not haveelectronic readouts.

THERMAL VACUUM GAUGES

Heat transfer is a useful characteristic of a gas with respectto the operational basis for a grouping of vacuum gauges.If a heated element with constant power input is placed ina vacuum environment, the surface temperature of that ele-ment will be a function of the heat conductance of theprocess gas, which also relates to the pressure of the processgas near the heating element. Thermal vacuum gauges con-sist of three basic elements: a heater, a temperature sensor,and a compensator for process temperature variations. Thereare two basic designs, depending on the type of temperaturesensor used: the resistance wire and the TC. In general,thermal vacuum gauges measure pressures down to 1 mtorr(10

−

3

mmHg, 0.13 Pa) absolute pressures. Although this limitcan be extended with special designs involving specialamplifiers and liquid nitrogen cooling around the gauge tubeto reduce radiation losses, an ion gauge is the single gaugeof choice when the pressure drops below 10

−

4

torr. Usualapplications for thermal vacuum gauges include refrigera-tion, vacuum furnaces, freeze-drying, air conditioning, phar-maceutical productions, and the manufacture of thermal foodcontainers.

Thermal vacuum gauges may be orientation sensitive.Once attached to the vacuum system and calibrated, thepressure readings from the gauge are dependent on thatphysical location and orientation. If the gauge is remountedat the same location but rotated from its original position,the gauge may need to be recalibrated. Most thermal vacuumgauge manufacturers will indicate if their gauge is positionsensitive and also indicate on the gauge housing the recom-mended gauge orientation.

© 2003 by Béla Lipták

800

Pressure Measurement

Pirani Vacuum Gauge

The Pirani gauge is a popular example of a thermal vacuumgauge. In this device, a constant current heats a filamentthat is cooled by the gas present in the vacuum environment.Since the amount of heat transferred changes with thenumber of molecules, the equilibrium temperature, whichis detected as the resistance of the metal wire, ribbon, orthermistor element, is a function of the gas pressure nearthe wire. Figure 5.14d outlines the components of opera-tion for the Pirani gauge. The heating and sensing elementsare often combined as part of a Wheatstone bridge typesensing circuit. A second resistance wire, which is enclosedin a reference vacuum, is used to compensate for processtemperature variations. The readout device detects theamount of current or voltage that is necessary to return thebridge circuit to balance after a change in the vacuum beingmeasured.

The standard pressure-sensing range for a Pirani gaugeis between 10

−

3

and 1 mmHg (0.13 to 133 Pa) absolute. Theinaccuracy of the gauge is about 2% at the calibration pres-sure and

±

10% over the operating range. Maximum outputsignal is about 0.1 mA at full-scale reading.

Pirani gauges are relatively inexpensive and convenientto use. They are not used for reliable pressure measurementshigher than 1.0 torr, because at higher pressures the thermalconductivity of gases does not decrease with the decreasein pressure. The scale of the Pirani gauge is linear between10

−

3

to about 2

×

10

−

2

torr. Above these pressures it becomesroughly logarithmic. The two scales, when provided, areusually calibrated in terms of air. However, the gauge is gasdependent since different gases have different thermal con-ductivity. Therefore, the Pirani gauge is composition-depen-dent and needs to be calibrated for the process gas in theapplication.

Pirani gauges are not highly accurate. As they depend onthe detection of thermal conductivity, the surface conditionsand emmissivity of the heated elements do affect the reading.As these surfaces are coated, oxidized, or carbonized, theirtemperatures rise and the sensor is likely to read high by asmuch as 2

×

10

−

2

torr. This represents a very large error atthe low end of the pressure range. Another substantial errorsource is changes in the bridge voltage.

Thermocouple Vacuum Gauge

As shown in Figure 5.14e, the single TC detector consists ofa wire heated by the passage of constant AC or DC current.A TC is welded to the center of this heated filament, therebyproviding means to measure the temperature of the filamentdirectly. An opening in the tube is provided for connectionto the vacuum system being measured.

In operation, the constant current passing through theheater wire is in the order of 20 to 200 mA, and the TC sensordevelops a full-scale output in the order of 20 mV DC. Forany constant value of current through the filament, the tem-perature increases as the pressure in the tube is reduced. Thetemperature detected by the TC depends on the thermal con-ductivity of the gas surrounding the junction. For the same gas,thermal conductivity is related to pressure when the pressureis at or below 1.0 torr. Like the Pirani gauge, TC gauge responseis process gas dependent and the gauge must be calibratedfor specific applications.

The available readout devices are as varied as the receivinginstruments that measure TC output signals. Sensitive millivoltmeters and potentiometers are common choices. At higherranges, such as 5 to 10

−

2

mmHg (665 to 0.13 Pa), dual scalesare usually used, while for the standard range, 0.1 to 10

−

3

mmHg(133 to 0.13 Pa), a single scale is sufficient. The readinginaccuracy at midscale is

±

2%, while

±

10% covers the fullrange. On multistation installations, a single readout devicecan serve several TC gauges through a manual selector switch.Because all filaments are on at the same time, the readingsare instantaneous.

The filament temperature is kept below 400

°

F (250

°

C) toreduce the possibility of the sample gases decomposing or form-ing a deposit on the gauge tube elements. Compensation can

FIG. 5.14d

Resistance wire (Pirani) vacuum detector.

Compensating Cell

VoltageSupply

Measuring Cell

Process

Pressure

OutputVoltage

FIG. 5.14e

Single thermocouple vacuum gauge.

D.C.Potentiometer

Readout

Thermocouple

Heater

ProcessConnection

Hot Junctionis Welded to the

Midpoint of the Heater

110V, 60C A.C.

© 2003 by Béla Lipták

5.14 Vacuum Sensors

801

be provided for process temperature variations, which other-wise would introduce an error by affecting the filamenttemperature. This is achieved by the use of a reference gaugethat has been fully evacuated and sealed. The same currentand voltage are applied to both the measuring and the ref-erence gauges. The temperature of the two heater wires isthen compared, and the difference is used as a measure ofprocess vacuum. The reference tube compensates for ambi-ent temperature changes because the two cells are at thesame temperature. Generally, the inherent low accuracy ofthese sensors does not make it worthwhile to compensatefor ambient temperature variations.

It is also important that the filament current and voltagebe carefully maintained for stable calibration. Filaments shouldnot become dull or tarnished due to contamination becausethis would cause radiation losses affecting calibration. Oneway to overcome this problem is to precoat the filaments sothat further contamination would have no effect. Unfortu-nately, precoating tends to impair gauge sensitivity. However,several precoated TCs connected in series, a thermopile, pro-vide extra potential output that compensates for sensitivitylosses due to filament precoating.

Thermopile Vacuum Gauge

To increase sensitivity, thermopiles can be used to detectheater temperature. A thermopile vacuum gauge is a seriesof TCs, and a typical design is shown in Figure 5.14f. In thisdesign, the TCs (A and B) are heated by low-voltage alter-nating current so that the heater and the temperature-sensingfunctions are combined in the same noble (noncorrosive)

metal thermopile. A change in process pressure results in achange of thermopile temperature causing a new DC outputfrom the TCs. A third unheated thermocouple (C) is includedin the circuit to compensate for operating process temperaturevariations. This couple is the same size as the heated ones,but it is connected in opposite polarity. A change in processtemperature develops voltages in all the TCs, but the transienteffects are equal and opposite in the heated and unheatedelements. Therefore, compensation is achieved.

Thermopile vacuum gauges have the same features andaccessories normally associated with a single TC gauge.However, the following additional characteristics shouldbe noted. First, because of the noble metals used in the ther-mopile, oxidation of the couples does not occur. Second, theoperating temperature of the heated thermopile is lower thanthat of the hot filament used in the single couple design.Therefore, the probability of sample gas decomposition ordeposit formation is remote. Third, thermopile gauges thatwithstand several thousand PSIG overpressures are avail-able. Fourth, the gauges are corrosion resistant, rugged underdemanding conditions, and provide stable calibrations. Thecold junctions are kept at ambient temperature by heavymounting studs. Thus, the amplified electromagnetic forcegenerated between the hot and cold junctions tends to be astable signal that reflects the temperature change at the hotjunctions. Finally, accurate readings are obtained in therange of 10

−

1

to 10

−

3

mmHg (13 to 0.13 Pa), but extendedcoarse detection is feasible over a broader range from 100mmHg (13 KPa) to 10

−

4

mmHg (1.3

×

10

−

2

Pa) absolutepressures.

Convectron Vacuum Gauge

A convectron gauge is similar to thermal conductivity typevacuum detectors, except that it provides a wider (six-decade)range, from 10

−

3

to 1000 torr within a single gauge. The heatloss sensor is a temperature-compensated gold-plated tung-sten wire that detects both conduction and convection coolingeffects. At lower pressures (higher vacuums), it operates asa thermal conductivity vacuum gauge, while at higher pres-sures (in excess of 1.0 torr), it depends on convective coolingas the sensing principal. In this pressure regime, the processgas molecules are circulated in the sensor tube by gravita-tional forces.

In most respect, the features and limitations of convectronvacuum gauges are similar to the Pirani and TC gauges, butin addition to the extended range, there are some other nota-ble differences. These include a lower sensor wire tempera-ture, which is likely to reduce the coating and carbonizationon the sensor wire surface. The gauge tube is stainless steeland can be baked to up to 300

°

F (150

°

C), when not operating.The maximum ambient or operating temperature is 122

°

F(50

°

C). The bridge circuit is an integral part of the gaugetube, which is vibration and position-sensitive (it must behorizontal). The gauge is calibrated for nitrogen, and mustbe recalibrated when used on other gases.

FIG. 5.14f

Multiple thermocouple (thermopile) vacuum gauge.

D.C.Readout

HeatedThermocouples

Process Connectionto Vacuum System

UnheatedCompensatorThermocouple

110V, 60C A.C.

A C B

© 2003 by Béla Lipták

802

Pressure Measurement

IONIZATION VACUUM GAUGES

Ionization vacuum detectors have been available since 1916.However, practical, reliable, and sturdy ionization vacuumgauges did not come into common use until the 1930s. Today,ionization gauges are routinely used to detect pressure levelsfrom 10

−

4

to 10

−

9

torr. All ionization vacuum gauges detectan electric current that is generated from the ionization ofthe gas whose pressure is being measured. Ion gauges aredistinguished by the method applied in producing the ions.To convert a gas molecule into a positive ion, an electronmust be removed. If an atom or a molecule is supplied withenergy equal to its ionization potential, an outer orbit electronwill escape and a positive ion will be created. The approxi-mate energy values required for ionization range from 5 to30 eV. If this energy is supplied at a constant rate and suffi-cient atoms and molecules are available, the ions will beproduced at a similarly constant rate. This ion stream isdirected to the gauge’s cathode, and the current that flowsthrough that cathode is proportional to the pressure of thegas in the gauge. The initial ion current is also proportionalto molecular weight. The ionic current generally rises withmolecular weight. In any case, calibration is usually donewith dry nitrogen or air. The various designs for ionizationgauges can be grouped by the method used to generate anddirect the ion current. One broad grouping for ion gaugesincludes hot cathode and the cold cathode gauges.

Hot Cathode Ionization Gauges

In the Bayard-Alpert hot cathode filament vacuum gauge illus-trated in Figure 5.14g, the ionization energy is supplied byelectron bombardment. The electrons are derived from thermi-onic emission from the hot filament. As these electrons areattracted to and passing through the helix shaped grid, theyacquire kinetic energy. When the electrons finally collide withthe gas molecules from the vacuum system, positive ions are

produced. These ions are then attracted to the negativelycharged collector electrode in the center of the grid to form anion current. At constant accelerating voltage, the number ofions formed is proportional to the gas pressure if it is below10

−

3

mmHg (0.13 Pa). Variations on the design include cappingthe end of the grid to prevent electron escape. A fine wirecollection electrode reduces the x-ray-generated photo current.A spare emission filament is also often provided. At higherpressures, the relationship between plate current and pressureis not linear. This is because the mean free path becomes soshort that an ionized molecule may pick up a free electron tobecome a molecule before it reaches the collector plate.

The accuracy of a hot cathode ionization gauge is poorbecause the number of gas molecules to be measured is verysmall. At an absolute pressure of 10

−

5

mmHg (1.3

×

10

−

3

Pa),the inaccuracy would be about 10%. The vacuum rangedetectable by the hot cathode gauge is 10

−

3

to 10

−

11

mmHg(1.3

×

10

−

1

to 1.3

×

10

−

9

Pa). The minimum span of thereadout device is one decade, and it can be furnished withfive or six ranges. Range switching can be automatic ormanual from the instrument’s front panel.

The sensitivity of the hot cathode gauge is 100

µ

A/

µ

m(10

−

3

mmHg) pressure. The readout device can be combinedwith thermocouple readout to extend its coverage to vacuumsin the range of 1 to 10

−

3

mmHg (133 to 0.13 Pa). This is acommon practice, but a protective relay circuit is also fur-nished to prevent filament burnout by keeping it off at pressureshigher than 10

−

3

mmHg. Besides multirange indicators, thereadout device can provide signals for actuate electrical controland alarm circuits. When several ionization sensors areinvolved, the output signals can be multiplexed to reduce thenumber of signal detectors and pressure monitors, reducingtotal system cost.

The hot filament vacuum gauge provides a wide pressurereading range with fast response. Its application is limited togases that will not decompose on the hot filament. The glassionization tube, by its nature, is subject to mechanical dam-age. However, gauges for industrial process applications arecommercially available. The filament current is controlled insuch a way that a constant flow of electrons is emitted fromit. In some designs, calibration is maintained by controllingthe grid charge so that if emissivity of the filament is decreas-ing, the grid receives a correspondingly greater charge tomaintain the entire circuit in equilibrium.

The limitations of the hot-cathode design are related tothe high filament temperatures (about 4000

°

F, or 200

°

C)involved. At such temperatures, the incandescent filamenttends to deteriorate and is also susceptible to chemical attack.The sorption of gas by the hot filaments and the degassingthat follows is another potential error source. These effectsare addressed by alternative gauge designs that use largediameter tubes and minimize in vacuum connection lengths.

As suggested above, various modified Bayard-Alpert hotcathode vacuum gauges are available from different iongauge manufacturers. One example is the Schultz and Phelpsmodification shown in Figure 5.14h. This gauge covers the

FIG. 5.14g

Hot cathode ionization vacuum gauge. (Courtesy of Bayard-Alpert.)

Negatively Biased PositiveIon Collection Electrode

Gas Entry Portfrom VacuumChamber

Hot ElectronEmitting Filament

Helix-ShapedAcceleration Grid

SpareFilament

© 2003 by Béla Lipták

5.14 Vacuum Sensors

803

vacuum range of 1 to 10−5 torr and can be obtained in anintegrated unit with the Bayard-Alpert unit, when a widervacuum range needs to be covered.

Cold Cathode Ionization Gauges

The cold cathode vacuum gauge is a composition sensitivegauge traditionally known as Philips gauges, after its firstmanufacturer, or a Penning gauge after F.M. Penning, whooutlined its operating principle in 1937. The basic differencebetween cold and hot cathode gauges is in the method by

which ions are produced. In the hot filament unit, the elec-trons are derived from thermionic emission. In the cold cath-ode design, a high-potential field withdraws the electronsfrom the electrode surface. Because the rate of electron emis-sion is lower in the cold cathode units, the collision frequencybetween gas molecules and electrons would also be lower ifthe electrons traveled in a straight path. To increase the pathlength of the electrons, a magnetic field is created around thetube to deflect the electrons. Thereby, the emitted electronsspiral as they move across a magnetic field to the anode(Figure 5.14i). This spiraling action greatly increases theelectron path of travel and the corresponding chance of elec-tron collision with gas molecules from the vacuum chamber.The overall result is greater sensitivity of the cold cathodegauge than that of the hot cathode.

The inaccuracy of the cold cathode unit is about 20% atan absolute pressure of 10–5 mmHg (1.3 × 10−3 Pa). The gaugeresponse is gas molecular weight dependent and its detectablevacuum range is from 10−2 to 10−7 mmHg (1.3 to 1.3 × 10−5 Pa).As with the hot cathode gauges, the minimum span is onedecade and readout devices are available with one, two, orthree ranges, which are selected automatically or by manualaction at the front of the instrument. The gauge sensitivity is5 mA/µm micron pressure.

There are several advantages of the cold cathode gaugerelative to the hot cathode gauges. First, they cost less. Sec-ond, they are more robust and do not burn out as easily. Third,they do not subject the process gas to thermal stress. Coldcathode gauge disadvantages include lower accuracy, nonlin-ear output signal, and gas take-up caused by the high-voltageoperation. In order to remove polymerized organic contami-nants, periodic cleaning of the electrodes and the vacuumchamber is required.FIG. 5.14h

Hot cathode vacuum gauge. (Courtesy of Schultz-Phelps.)2

Exhaust System

High PressureGauge

ElectronCollector

Filament

IonCollector

FIG. 5.14i Philips cold cathode ionization vacuum gauge. (The electrons travel from the cathode to the anode through a path of multiple spiraloscillations which increase the opportunity for them to encounter and ionize molecules.)

Micro-AmmeterReads theVacuum

~4000 V

~1500 GaussMagnetic Field

+ −Anode

Cathode

Cathode

ProcessVacuum

© 2003 by Béla Lipták

804 Pressure Measurement

Similar to the hot cathode gauge, manufacturers haveoffered modified designs for the cold cathode gauge. Oneexample is a modification developed by Paul Redhead. In theRedhead gauge, a cylindrical electrode is configured aroundthe anode that has a probe shape at the center axis. Thisarrangement is like an inverted magnetron: the electrons ion-ize the gas molecules while they are spiraling in toward thecentral anode, while the gas ions are collected on the cylin-drical cathode surface. The Redhead gauge operates at around5000 V in a 1000-Gauss magnetic field and can detect vacu-ums in the 10−6 to 10−12 torr range.

During the 1990s, a combination ionization gauge wasdesigned. This gauge is basically a cold cathode unit but isfurnished with a hot cathode serving to trigger the dischargefrom the cold cathode. This design extends the detectablepressure range to 10 decades, or from 10−4 to 10−14 mmHg(1.3 × 10−2 to 1.3 × 10−12) absolute.

VACUUM GAUGE CALIBRATION

Vacuum gauge calibration is an issue for two reasons. First,a single vacuum gauge type does not work over the totalvacuum pressure range. Second, most vacuum gauges do notdirectly measure pressure, and the pressure reading providedby the gauge is dependent on a property of the gas moleculesbeing measured. Some common ways to calibrate vacuumgauges are provided below.

McLeod Vacuum Gauges

The McLeod vacuum detector, or barometer gauge, is theclassic reference or calibration gauge. Figure 5.14j shows oneversion of this gauge. In this design, the unit is stationary,and a piston is used to trap the rarefied gas. The filling of thegauge with mercury is done through the process connection.This improved design does not use a dead-ended capillary,and, therefore, the problems associated with keeping the cap-illary clean are eliminated. The diagram on the left side ofFigure 5.14j shows the unit just prior to the taking of ameasurement. As the piston with micrometer adjustment ismoved up, the rarefied gas is trapped when the mercuryreaches point D; thus, the initial volume (V1) is the volumebetween points A and D. When the instrument is connectedto the vacuum system, the mercury level in the reservoir isbelow point D to allow trapped gases to be liberated. As thepiston is moved up, the mercury fills the large bulb up topoint C. At this point, a reading can be taken on the dualscale if the pressure to be detected is in the mmHg range. Ifthe vacuum is higher, the piston is moved further up, increas-ing the compression ratio until the mercury reaches point Babove the small bulb. In this case, the reading is taken on themicron side of dual scale.

McLeod gauges can cover the vacuum range between 1and 10−6 mmHg (133 and 1.3 × 10−4 Pa). At pressures below10−4 mmHg (1.3 × 10−2 Pa), the reading accuracy is limited

by capillary effects. These gauges are laboratory instrumentsthat measure on a sampling rather than on a continuous basis.They are not recommended for industrial installations.

Calibration Reference Tubes

A relatively new method of vacuum gauge calibrationinvolves the use of calibration reference tubes. These tubesare available from most gauge manufacturers for applicationwith their specific gauges. These reference tubes are funda-mentally evacuated, sealed vacuum tubes accurately cali-brated to precisely simulate an operating pressure environ-ment of the gauges to be calibrated. The tubes permit quickand easy gauge recalibration by simply connecting the gaugeto be calibrated to the appropriate reference tube and adjust-ing the gauge’s pressure reading to reflect the value statedon the reference tube.

VACUUM CONTROLLERS

Vacuum gauges are one part of a vacuum process instrumentsystem. In the complete instrument system, the process vac-uum pressure is compared to a desired set point value, and,if necessary, a corrective signal is sent to a final controlelement. The vacuum controller is the component of the systemthat will alter a control signal to adjust the vacuum pressurewhen a process disturbance occurs. Process controllers donot have to have a pressure readout system; they only require

FIG. 5.14j Piston McLeod vacuum gauge.

To Vacuum System

Trap

Small Bulb

Well

Large Bulb

Capillary

DualScale

Reservoir

Piston

Before Measurement

As a Reading is Taken

AB

C

D

© 2003 by Béla Lipták

5.14 Vacuum Sensors 805

a way to determine if the current vacuum is at the desiredset point condition. Three types of vacuum controllers arebriefly discussed.

Aneroid Manostats

The aneroid manostat illustrated in Figure 5.14k is a self-contained bellows-type vacuum controller. The bellows arefully evacuated to provide a zero absolute pressure referenceunaffected by barometric changes. The spring is temperature-compensated, and its tension is adjustable over the entire rangeof the controller, which is 1 to 60 in.Hg (3.4 to 200 kPa)absolute pressure. The sensitivity of setting is about 0.5 mmHg(0.07 kPa) and the inaccuracy of control is 2% of set pressure.

The aneroid manostat works at above and below atmo-spheric process conditions. When a below-atmospheric pres-sure is to be maintained, connections are made both to thevacuum source (usually a vacuum pump) and to the controlledsystem. The spring tension is set for the desired set pressure,and expansion or contraction of the bellows moves the valveport to control the airflow through the manostat. The manostatis primarily designed for dead-ended service, but it will handlesmall flows. For example, at a setting of 300 mmHg (2000 kPa)without any restrictions in the vacuum source line, it will pass0.2 ft3 (6 l) of air each minute. When an above-atmosphericpressure is to be controlled, the filtered air supply is not atmo-spheric any longer, but connected to a pressurized supply, andthe vacuum source connection is left open to vent the unit.

Cartesian Diver Regulators

Cartesian divers are self-contained pressure, or vacuum, reg-ulators operating on principles somewhat similar to those ofthe aneroid manostats. As shown in Figure 5.14l, the set pres-sure for this controller is sealed in under the diver. The processpressure acts on the outside of the diver causing it to sink or

rise as pressure varies. If the unit is to control a vacuum process,a vacuum pump (or other vacuum source) is connected to theunit. A process pressure increase causes the diver to sink,opening the control port and connecting the vacuum pump tothe process to lower its pressure. If the unit is installed forpositive pressure control, a pressure source, not a vacuumsource, is connected to the control port, which is closed by anincrease in process pressure and opened by its reduction.

This device is capable of maintaining process pressuresbetween 1 mmHg absolute to 100 psia (0.13 to 690 kPa) toan approximate inaccuracy of 0.1% of set point. The unit isavailable in both glass and metal, requires no external powersource, and is simple to operate or to change its set pressure.Because of its limited flow capacity, it can control smallvolume systems only.

Analog Electronic Controllers

Continuous reading, relay action, analog gauge, electroniccontrollers are also available and used for vacuum control.These instruments provide dependable control with simplic-ity and economy. The desired set point is established bypositioning the gauge need set point indicator at the desiredvalue. The system’s integrated vacuum gauge is stable andrugged and provides reliable measurements over the 1 to 50torr range commonly provided. If the pressure-indicatingneedle passes the set point, the relay is activated. The relayaction is typically available as 115 V 5A SPDT @VAC nor-mally open or normally closed contacts. The relay automat-ically drops out when set point vacuum value returns.

Mass Flow Controllers

Many vacuum related processes require vacuum control, whileprocess gases are admitted on a continuous basis. A (PID)process control loop for such circumstances is possible witha mass flow controller. Vacuum process mass flow controllers

FIG. 5.14k Aneroid manostat.

ControlledVacuumSensing

Line

EvacuatedReferenceBellows

Filtered Air

ValvePort

ControlSpring

Connection to Vacuum Source

FIG. 5.14l Cartesian diver.

Set PressureConnection

Seal Fluid

Air Supplyfor Positive

PressureService

ToVacuum Pump

or OtherVacuum Source

Vent

Restriction

ControlPort Diver

Process

Connection

© 2003 by Béla Lipták

806 Pressure Measurement

usually detect the temperature change as the gas passesthrough a small, typically 3-in. flow chamber. The temperatureis monitored with a TC gauge, and chambers can have streamsplitter bypass elements to allow adaptation to different pro-cess gas flow ranges. The differential temperature measure-ment signal becomes the input to an electronic PID controllerand the resulting control signal is sent to the final controlelement.

There are several final control element choices for massflow controllers dedicated to vacuum process gas control. Thesolenoid proportional valve is one widely used cost effectivecontrol value that is coupled to a mass flow controller. Thisfinal control element uses a solenoid coil, a spring-supportedarmature, plug, and orifice. The unit can be configured withan elastomer sealed valve. The assembly also usually containsboth external seals, which isolate the gas flow path fromatmosphere, and an internal seal for the valve seat. All metalversions of the value are available when the process gas isalso corrosive.

Other control value options include a pieazoelectric con-trol valve and moving coil actuator valve. In the former, astack of piezoelectric elements drives the value. This valvetype has a fast response time with high valve action force.The counter valve action can be spring or diaphragm driven.Piezo valves are all constructed from metal. For the lattervalve option, the actuator is a moving coil that surrounds apermanent magnet. As current passes through the coil, it isdisplaced. The displacement is transferred by a pivot arm anddiaphragm action to the wetted volume on the other side ofthe diaphragm. This type of valve action produces a low valvesweep volume. This all-metal construction valve is good forcontamination-sensitive applications.

References

1. Welch, J., “Capacitance Manometers,” Measurements and Control,December 1989.

2. Melling, R.J., “Ionization Vacuum Gauge Measures Absolute Pres-sure,” Instruments and Control Systems, September 1964.

Bibliography

Alvesteffer, W.J., Jacobs D.C., and Baker, D.H., “Miniaturized Thin FilmThermal Vacuum Sensor,” Journal of Vacuum Science and Technology,Vol. 13(6), 1995, p. 2980.

von Beckerath, A., Eberlein, A., Julien, H., Kerstein, P., and Kreutzer, J.,WIKA Handbook on Pressure and Temperature Measurement, U.S. ed.,Lawrenceville, GA: Wika Instrument Corp., 1998.

Brombacher, W.G., “40 Years of Pressure Measurement,” Instruments andControl Systems, September 1967.

Comber, J. and Hockman, P., “Pressure Monitoring: What’s Happening?”Instruments and Control Systems, April 1980.

Delajoud, P. and Girard, M. “The Need for Evaolution in Standards andCalibration to Improve Process Measurement and Control of Low MassFlow,” 1996 National Conference of Standards and Laboratories Work-shop and Symposium, Monterey, CA, 1996.

Dyer, S.A., Wiley Survey of Instrumentation and Measurement, New York:John Wiley & Sons, 2001.

Elliott, T.C., “Temperature, Pressure, Level, Flow: Key Measurements inPower and Process,” Power, September 1975.

Hall, J., “Monitoring Pressure with Newer Technologies,” Instruments andControl Systems, April 1979.

Harvey, G.F., “ISA Transducer Compendium,” Instrumentation, Systems,and Automation Society, Research Triangle Park, NC, 1969.

Hughes, T.A., “Pressure Measurement,” EMC series, downloadable PDF,Instrumentation, Systems, and Automation Society, Research TrianglePark, NC, 2002.

Instrumentation, Systems, and Automation Society, “Industrial Measure-ment Series: Pressure” (video VHS, PAL & NTSC), Research TrianglePark, NC, 2002.

Instrumentation, Systems, and Automation Society, “Pressure: Indicatorsand Transmitters,” CD-ROM, Research Triangle Park, NC, 2002.

Johnson, D., “Pressure Sensing: It’s Everywhere,” Control Engineering,April 2001.

Marrano, S.J., “How to Choose and Apply Pressure Transmitters,” Control,March 2000.

O’Hanlon, J.F., User’s Guide to Vacuum Technology, 2nd ed., New York:John Wiley & Sons, 1989.

Roper, D.L. and Ryans, J.L., “Select the Right Vacuum Gauge,” ChemicalEngineering, March 1989.

Sullivan, J.J., Journal of Vacuum Science and Technology, Vol. 3, 1985, p.1721.

Sullivan, J.J. and Uttaro, F.L., “Performance and Calibration of CapacitanceDiaphragm Gauges,” Measurement Science Conference, Irvine, CA, 1985.

Tompkins, H.G., Vacuum Technology: A Beginning, New York: AmericanVacuum Society, AVS Education Committee Book Series, July 2002.

© 2003 by Béla Lipták