Vacuum/volume 37/numbers 8/9/pages 643 to 649/l 987 0042-207X/87$3,00 + .OO Printed in Great Britain Pergamon Journals Ltd

The use of hot filament ionization gauges Peter Nash, Division of Mechanical and Optical Metrology, National Physical Laboratory, Teddington, Middlesex, TWl 1 OL W, UK

This paper describes the design and theory of operation of the various types of hot filament ionization gauge. The emphasis of the paper is placed upon aiding the vacuum user to correctly operate and interpret the readings of ionization gauges.

1. Introduction

The hot cathode ionization gauge (ion gauge) has few competitors for the measurement of pressures in the high and

ultra-high vacuum pressure ranges. For a general review of the

problems of accurate vacuum measurement see Tilford’ but in summary: cold cathode Penning ionization gauges, although

robust are inaccurate, the McLeod gauge is operationally difficult and has many other drawbacks including limited

range, and the recently available spinning rotor gauge*~clearly the most repeatable gauge for the high vacuum

region-has a long integration time and a range limited by its

residual reading. Mass spectrometers are attractive in that they give additional information about the gas composition but the

total pressure reading is unreliable and they have unstable sensitivitiesj. Hot cathode ionization gauges are beset with problems too and the user who believes the indications of ion

gauges without a basic understanding of their operation is

likely to be fooled. Many different designs of ion gauge have been developed to try and overcome some of the basic

disadvantages but very few have stayed the course. For most vacuum users the term ion gauge is synonymous with

a nude Bayard-Alpert gauge complete with control box. The

gauge will probably have been provided as part of a large vacuum system and its operation will be taken for granted by the operator

and the gauge readings will, therefore, not be correctly inter- preted. The large body of published work on ionization gauges

will be too widely spread to be useful to those users and this paper attempts to explain the workings of the ion gauge, outline the

major problems and describe ion gauge types which have

successfully surmounted some of them. Finally, a few practical

tips are given. Of course, it is not possible to cover comprehensively this

complex gauge in a short article. Some subjects have had to be treated very briefly and others ignored completely; almost all are considered at a rather basic level. In general, more emphasis has been placed on gauges which are commercially available and on moderately accurate measurements. It is hoped that the list of references is sufficiently comprehensive to allow the reader to find answers to most problems.

2. Principle of operation

The basic ion gauge is the triode gauge shown diagrammatically

in Figure 1. This type of gauge is not the one most commonly

encountered today but the basic principles of operation of ion gauges will be explained with reference to it and then the gauge

variants will be described. The triode gauge, as its name implies, is

essentially a triode radio valve; it has a directly heated cathode or filament, a grid and a collector. Its operation differs from that of a

radio valve in that ions and not electrons are collected by the

collector. The electrons given off by the filament are, due to the bias potentials applied to the electrodes (typically collector 0 V,

grid I50 V and filament 50 V with respect to ground), attracted by the grid. Since this is a semi-transparent object the electrons may

pass through it several times before finally striking it. During their oscillations about the grid the electrons ionize some of the gas

molecules, some of which are then attracted to the collector and measured by the ammeter. This ion current gives a measure of the

pressure in the gauge since the higher the pressure the greater the probability of an electron forming an ion.

The ion current will also be proportional to the electron current

(emission current) and it is usual to define the sensitivity (S) of an ion gauge by

s=z+,/(z-P) (1)

where I+ is the ion current, I- is the electron current and P is the pressure. The gauge sensitivity, which is gas dependent, will

normally be provided by the manufacturer but such a value should not be relied upon (see section 6). The only reliable method

of determining the sensitivity of an ionization gauge is to calibrate it by one of the methods described by Poulter4 (see section 7.8).

Equation (1) should not be expected to hold rigorously, even over limited ranges of emission currents and pressures (see section 2.2).

The gauge sensitivity for nitrogen will be of the order of 0.1 Pa-’ and the emission current will generally be in the range 0.1 mA-10 mA. It is, therefore, easy to deduce that at a system pressure of 10e5 Pa the gauge output will be a current ofthe order of 1 nA. This very small output should always be born in mind when using ion gauges.

643

Peter Nash: The use of hot filament ionization gauges

E -_.-I -1 I

0 50 100 150 200 250 300

Electron energy (eV)

lomzatfon cross sectlon (orbltrary units)

Figure 2. Ionization cross section ofa range of gases in arhltrary units from Rapp and Englander Golden’.

Figure I. Diagram of a triode ionization gauge and simplified electrical circuit.

2.1. Gas sensitivity. Clearly the ion current will be proportional to the efficiency of ion formation of the electron molecule interac- tions and this will be a function of the electron energy and the gas species. The ionization cross section, in arbitrary units, as a function of electron energy for a range of gases5 is given in Figure 2. When considering the behaviour of gauges in detail it should be remembered that the creation of ions by electron impact is complex and, for example, ionization by loss of a single electron produces ions of low energy while fragmentary ions and ions desorbed from gauge surfaces may have much higher energies. It can be seen that the ionization cross sections differ by a factor of more than 6 for the gases shown at electron energies of 150 eV (typical of those in ionization gauges). Ionization gauge sensitivi- ties for different gases will clearly vary by a similar amount. However, it is not possible to obtain accurate theoretical relative gas sensitivities for ionization gauges from data such as those shown in Figure 2, since, amongst other factors, the electron energy will vary throughout its trajectory. Several authors’-’ have published empirical tables of relative sensitivities but these should not be relied upon for accurate work since the precise values will vary significantly between ‘identical’ gauges and more so for different gauge types, filament materials and operating potentials.

2.2. Low pressure limit. Soft X-ray photons produced by the collision of electrons with the grid will, if they are incident on the collector, in turn cause the emission of photo electrons. To the ion current amplifier this electron emission is indistinguishable from the collection of gas phase ions but is pressure independent and, therefore, gives rise to a residual indication which is known as the X-ray limit. For a given gauge it will be a function of the gauge geometry, the electron energy and the electrode materials and surface conditions. The X-ray limit will vary considerably depending upon the cleanliness of the electrode surfaces but for a typical triode gauge the X-ray limit is about 10m6 Pa.

644

An additional contribution to the residual indication of the

gauge is produced by electron stimulated desorption of molecules and ions from the grid ‘“.l’ This too will be strongly affected by the cleanliness of the gauge but in a reasonably clean triode gauge it is the X-ray effect which dominates.

2.3. High pressure limit. Obviously at very high pressures the filament will burn out, but at much lower pressures the gauge becomes non-linear due to two main causes, multiple collisions and space charge effects. When the electron path lengths become comparable with their mean free paths, the ionization efficiency will drop as energy is lost in non-ionizing impacts. In addition, as the ion current becomes comparable with the electron current, the true emission current may not be maintained at a constant value in the presence of the ion current and the current due to electrons from the ionized molecules. The latter current will not have as high a probability of producing ions as the primary emission current. Ions and electrons are collected by the electrodes and will form currents in the various circuits shown in Figure 1. Secondly. a positive space charge may form, as the rate of ion formation increases, which can decrease ion collection efficiency.

These, and other effects, result in high pressure behaviour which differs significantly between gauge types. Triode gauges normally become non-linear above 10 ’ Pa.

3. Gauge variants

3.1. Bayard-Alpert gauge. The most significant alteration to the triode ionization gauge to date was that made by Bayard and Alpert’ ’ who, in order to reduce the X-ray limit, made some simple but highly effective alterations to the electrode configu- ration In a Bayard-Alpert (BA) gauge the filament is outside the grid structure and the collector is a fine wire on the axis of the grid. A photograph of a BA gauge is shown in Figure 3. The gauge operates in exactly the same way as the triode gauge but the area of the collector is now very much reduced and X-rays emitted from the grid have a much reduced chance of striking the collector. As a result the X-ray limit is reduced by a factor of about 100 over the triode gauge.

Peter Nash: The use of hot filament ionization gauges

Further reduction of the surface area of the collector of the BA

gauge by reducing the diameter of the collector wire will, of course, further reduce the X-ray current. However, there are two problems with this approach; the reduction in mechanical

strength of the wire and the drop in sensitivity of the gauge due to the difficulty in collecting those ions with a high tangential velocity about the collector . l3 Gauges with collectors 10 pm in

diameter and claimed residual pressures of ‘well below’ lo- lo Pa have been developed for use at CERN14.

3.2. High pressure gauges. Many different patterns of high pressure gauge exist-some are simply small BA or triode gauges with low-temperature filaments (see section 5.2wthers such as the Schultz--Phelps gauge l5 have a reduced sensitivity and thus, a smaller space charge effect as a result of using a single pass of electrons. These gauges are not normally used for accurate work as other gauges, such as the capacitance manometer16, can be used instead. They are, however, useful as a reasonably robust gauge which operates over a wide and commonly encountered pressure range.

3.3. Uhv gauges. Many other attempts have been made to further reduce the X-ray limit below that obtained in the BA gauge. Gauges based upon the modulator” and extractor” principles are available commercially and the principles of these gauges are explained below.

In the modulator gauge an additional ‘collector’ is used to attempt to separate the gas phase ions from the others19-21. The extra electrode is switched between collector and grid potentials (other modulation potentials are possible with the same principle of operation22~23 ) and the resulting collector currents are noted. When the modulator electrode is at grid potential the gauge is largely unaffected by its presence and the measured ion current is given by

z+ =z,‘+z:

where the subscripts denote the gas phase and residual currents. Now if the modulator is switched to collector potential the residual current to the collector will be unaffected but some of the gas phase ions will be attracted to the modulator electrode, hence,

Z,‘=(l-k)Z; +-I;

and, eliminating I,’ between the two equations

Z; =(I+ -Z,‘)/k

where the subscript m denotes modulated collector current and k is the modulation coefficient. The ion current resulting from gas phase molecules is reduced by about 40% when the potential on the additional electrode is modulated. It is normally assumed that the modulation factor is invariant with pressure and may be determined at high pressure where the residual current is insignificant. The indicated pressure, for any ion current, may then be calculated from equation (4). Unfortunately the assump- tion that the residual current is unaffected by the modulation of the gauge is untrue and it is only possible to gain a factor of about l&100 on the low pressure limit using this techniquez4.

In the extractor gauge the collector is re-sited below the grid structure. The ions formed within the grid are extracted, usually using an additional electrode, to the vicinity of the collector. Several different variations of gauge on this theme have been described,25-28 but limited work has been carried out into their performance due to the difficulties of calibrating uhv gauges.

Figure 3. Photograph of a Bayard-Alpert ionization gauge.

3.4. Transfer standard gauges. A considerable amount of work has been carried out in attempts to produce stable reference gauges. Limited space prevents this important topic being discussed here and the reader is referred to papers by Genschz9, Bills3’, Reich31, Sutton and Poulter3’, and Choumoff and Iapteff”..

An intercomparison of national standards of pressure in the high vacuum range organised by the Community Bureau of Reference (BCR) of the Commission of the European Communi- ties demonstrated that an ionization gauge can provide a transfer standard with a repeatability around + 2%34. However, the ion gauges used in this exercise were of a complex design and operated according to a detailed procedure.

4. Gauge control boxes

Ionization gauges need external electronics to carry out three functions.

(i) Provision of stable electrode potentials. (ii) Filament heating and emission current control.

(iii) Ion current measurement. Most gauges are operated from commercial ion gauge control boxes which perform all these functions and will normally also include outgassing facilities. The required electronics are not complex35-37, but need to be robust. For the highest accuracy work specially constructed electronics may be used with provision made for monitoring the electrode potentials and emission current. The emission current can be monitored by measuring the voltage across a resistor in the grid circuit. It should be noted that, if this method is adopted, there will be a contribution to the current resulting from gas molecule ionization which becomes

645

Peter Nash; The use of hot filament ionization gauges

significant at higher pressures. Commercial digital pica-ammeters for the measurement of the ion current are available with 100 pA ranges and measurement uncertainties of less than 1%38.

Control boxes made by two different manufacturers used with the same ion gauge may give different indications, even if they have been correctly calibrated (electrically), since the manufac- turers may use different electrode potentials and emission currents. Manufacturers’ standard conditions vary considerably since they are to some extent arbitrary. The emission current may be adjustable using a front panel control and its value will then be apparent to the user. However, the electrode potentials are generally set at values that will only be found in the instruction booklet.

Some ionization gauge control boxes do not maintain the emission current at a constant value but measure it and scale the ion current measurement accordingly. Other designs of control box switch the emission current over a wide range depending upon the pressure range selected. Both of these methods can result in significant measurement errors due to the imperfect linearity of

ionization gauges. Many gauge control boxes have logarithmic scales and,

therefore, make use of logarithmic amplifiers. These amplifiers are not normally very accurate and on a gauge control box with decade ranges and a logarithmic scale large discrepancies will be encountered. Some ion gauge controllers have only a single logarithmic scale with a very wide range which can only be read with limited accuracy.

Some functions of commercial gauge control boxes are readily checked and the discerning user requiring good measurements will do so. The electrode potentials may readily be checked for stability and accuracy using a voltmeter. The emission current may be checked as mentioned above. Providing the electrometer is of the feedback type so that the input terminal is maintained at ground potential it may be checked by providing a known input current from a voltage source via a high value resistor. Alternatively a current source may be used.

5. Gauge construction

5.1. Mechanical construction. Two basic mechanical variants are encountered, the ‘nude gauge’ and the glass tubulated gauge. Nude gauges are constructed straight on a flange with no tubulation and are the current fashion; they have the advantage that their electrode structure can be totally within the volume where the pressure measurement is required. However, they have several drawbacks: they are delicate and it is very easy to damage them when they are being installed; the potential around the gauge will not be well defined and shifts in gauge sensitivity may result. The glass tubulated gauge is self-contained but has the disadvantage that the restricted conductance between the gauge and the system may cause measurement errors (such as the well known Blears effect39). Some gauges have a conducting film on the inside of the glass envelope in order to define its potential. If this screen is left floating the gauge will behave unpredictably. The envelope of an unscreened gauge will rapidly charge up to a stable potential although erratic behaviour has been reported40,4’.

It is important that there is a very high insulation resistance between pins on the gauge lead-through, particularly that of the collector. Gauges should be designed so that metal evaporated from the filament does not produce leakage paths. Glass lead throughs may become conducting at very high temperatures, for example, during baking at very high temperatures.

646

5.2. Filament materials. Simple tungsten filaments perform well and are recommended for transfer standard use but they may produce various problems due to their high operating tempera- tures. The common difficulties are: chemical reactions on the filament. in particular reactions between the gas and carbon impurities in the tungsten and molecular dissociation, and outgassing of nearby surfaces. At extreme uhv the vapour pressure of tungsten may present a problem4’. A further limitation is the short life of tungsten filaments at high operating pressures. These difficulties may be surmounted by using low work function filaments; thorium. or lanthanum may be used (but see section 6. I ). Materials for ionization gauge filaments have been reviewed by Gear”“.

5.3. Electrode material. Electrodes may be constructed from molybdenum, nickel or tungsten. Grids are normally fabricated from photo-etched sheet, woven mesh or wound from wire. Gold or platinum electrodes have been used notably in transfer

standard gauges (section 3.4).

6. The stability of ionization gauges

The sensitivity of ionization gauges (as defined by equation (1)) will obviously vary between gauge designs and for different operating conditions. Of more importance to a gauge user is how the sensitivity of a given gauge is likely to vary from the value given by the manufacturer and how much it will change over the gauge’s lifetime. More crudely does he need to have his gauge calibrated and if so how often.

Several laboratories have carried out measurements on hatches of similar gauges’,j4 4h and the work is reviewed in the last reference. Large variations were found even though the gauges were operated under good conditions; gauges visually identical to those in a previously tested batch of gauges showed different characteristics. The results can, therefore, only be used to give an indication of what range of performance can be expected. Measured sensitivities have been found to show large departures from manufacturers’ values (greater than 40%). The standard deviation of the sensitivities of individual gauges about the mean of a group of nommally identical gauges is generally around 15%.

The situation is even more difficult for long term sensitivity where less figures are available and more variables exist. Figure 4 gives calibration data for a gauge which was returned to NPL for calibration with its control box over a number of years. The gauge was used as an industrial transfer standard and can, therefore, he assumed to have been carefully treated. The data gives an indication of the considerable changes in sensitivity which may occur with time.

The cause of these instabilities cannot be ascribed to any one particular source; some of the many possible causes are discussed below.

6.1. Filament. The emission current from the filament of an ionization gauge will be maintained at a constant value hut the efficiency ofion formation ofan electron will vary depending upon its trajectory. The initial paths of several electrons in a BA gauge calculated by Pittaway are shown in Figure 5. Although the potential within the grid varies, spatially, between grid and collector potentials since the collector is of small diameter the field will he roughly constant except in its immediate vicinity. It can. therefore, be seen that electrons with long paths within the grid will have higher chances of producing an ion than those with short paths.

Peter Nash: The use of hot filament ionization gauges

1 2 1979

1977 1 1

1976 10

09

t - 1g75 i

10-L

I

10-I

I

10-Z

Pressure (Pa)

I

10-l

Figure 4. Calibration of a commercial ionization gauge and control box used as an industrial secondary standard.

The paths of electrons may be altered by three causes. Firstly, changes in the position of the filament causing changes in the field close to the filament. This may be brought about in a given gauge by thermal cycling or rough handling. Manufacturing variations will result in sensitivity changes within a batch of gauges. The

latter effect has been analysed for one particular gauge type by multivariate statistics48. Secondly there may be changes in the directions of preferential emission from the filament due to particular crystallographic orientations or to local variations in temperature or surface contamination. Large variations in the radial emission of electrons from a filament have been reported by McComas49 and Tilford has noted local temperature variations in low work function filaments which he attributes to poor coating adhesion. Finally, there may be changes in the potential distribution around the gauge due to a nude gauge being mounted in tubulations of different sizes, for example. This is the cause of the variations in sensitivity of a tubulated gauge with a screen which is left floating (section 5.1). Tilford has also observed changes in gauge sensitivity when the DC heating current in the filament is reversed. The changes are presumably caused by the small, localized potential changes.

All these electron orbit effects may be minimised by good gauge design with careful positioning of the filament or use of a reflector behind the filament5’ a design refinement which has been largely

ignored.

6.2. Grid and collector. Gopolaraman et al. have shown the importance of the surface conditions of the grid and collector upon gauge sensitivity5’. Changes in surface contamination changes the efficiency of emission ofelectrons, ions or neutrals due to ion or electron impact. To take one specific case Messer52*53 has shown that the reduction in sensitivity of a BA gauge could be explained by the removal of the surface layers of the collector which are contaminated by carbon and oxygen in a new gauge.

7. Practicalities

7.1. Required measurement. In any vacuum measurement it is important that some thought be given to why the measurement is necessary and that a realistic assessment of the required

Filament

Grid

Figure 5. Initial paths of several electrons in a Bayard-Alpert gauge from Pittaway4’.

measurement accuracy is made. In laboratory conditions measurements with an uncertainty of + 10% will be difficult to achieve and measurements with an uncertainty of + 1% are likely to be impossible. Under conditions met in vacuum processing much larger measurement uncertainties will be equally hard to attain.

Some systems with large gas sources and high pumping speeds will have molecular velocity distributions which are not Maxwel- lian; there will be so called gas beaming effects. A nude gauge in a gas beam will give a very different reading to that of a gauge a short distance away. The two gauge readings can both be useful but give different information.

The range over which pressure measurements are required will, of course, directly affect the selection of gauge type. In many cases an ionization gauge will be used in conjunction with other types of gauge so that the required range may be covered. Uhv gauges may look more ‘high-tee’ and certainly cost more than high vacuum gauges but this does not mean that they will perform better in all vacuum ranges. In addition gauges are needed mostly during plant malfunction where the system pressure will be higher rather than lower than the design pressure.

7.2. Flanges. The two basic designs of nude and glass tubulated gauge have already been pointed out. Modern nude gauges are available on ‘Conflat’ flanges and in a few cases on ‘Klein’ flanges. Glass tubulated gauges can be obtained without flanges or already sealed onto ‘Conflat’ flanges via a glass to metal seal. Any leak, real or virtual, is of course to be strenuously avoided; small gas sources close to gauges can cause large measurement errors. ‘Conflat’ flanges are reliable and have no polymer components. To avoid filament sag and to prevent the gauges accumulating ‘rubbish they should be mounted vertically with their electrical connections uppermost.

7.3. Electrical connections. It has already been pointed out that the output from an ionization gauge is a very small current. Even relatively high resistances can shunt a significant proportion of this current and it is, therefore, important to use good quality leads carefully maintained. The area around the collector pin on the gauge must be kept clean. Flexing of coaxial collector cables can produce significant noise currents. For this reason and also to avoid straining the lead-through seals, all leads should be

647

Peter #ash: The use of hot filament ionization gauges

adequately fixed. In contrast to the minute ion currents the filament heating current is large, up to 10 amps, and badly made connections or dirty contacts can get very hot. The voltage drop down connecting leads may also be significant. Gauges which have been calibrated without control boxes should have the electrode potentials checked at the gauge head.

7.4. Outgassing. As has been pointed out in sections 2.3 and 6.3, the cleanliness of an ionization gauge has a considerable effect upon its performance. The filament of a gauge is rapidly outgassed when first turned on. The radiant heat from the filament then causes outgassing from nearby surfaces. These effects are readily seen from the pressure indicated by an ionization gauge immediately after switch on. However, the heating effect of the filament is not sufficient to clean the grid and collector surfaces

properly although at moderate pressures the collector may be cleaned by ion bombardment. In some gauges (typically those originating in the USA) the grid is in the form of a simple helix to allow a current to be passed through the grid to provide ohmic heating. A second outgassing method, which may also be used to clean the collector at the same time, is that of electron bombardment. The grid and collector are biased at around 600 V and bombarded with electrons from the filament, an emission current of a few tens of milliamps being sufficient to heat the electrodes to a dull red. Normally half an hour is sufficient to clean the gauge thoroughly. The outgassing is best carried out either while the rest of the apparatus is being baked or before it has fully cooled to avoid degassed products adsorbing onto the walls of the system. A third method of outgassing a gauge is to simply run in while the apparatus is being baked. Provided high temperature leads are available this method has a lot to commend it.

The residual current in a gauge will be significant at low pressures and it will not be reproducible. A manufacturer’s figure should be treated with caution since at best it will represent the X- ray current of the gauge in very clean conditions; in a BA gauge the current due to electron-stimulated desorption of ions from the grid can be more by at least an order of magnitude.

7.5. Gauge pumping. Clean ion gauges may be significant sinks of gas. The causes of this pumping effect are various. Widely differing figures have been quoted and are reviewed by Berman54; this is in

part due to the difficulties of measuring small pumping speeds but more significantly is probably due to the many parameters that will affect the pumping action. Amongst the most important parameters are the cleanliness of the gauge, the gauge design, the filament type, and the gas species.

It must also be noted that many ion gauges will not be clean, as assumed above, and may be significant sources of gas, particularly when the filament is first turned on.

Measurement errors will be minimised when the gauge has a high conductance connection to the apparatus where the pressure measurement is required.

7.6. Ambient conditions. For measurements carried out in laboratory conditions the effects of variations in ambient temperature are small. Work studying this effect has generally shown that it is not as large as would be predicted from theoretical considerations, i.e. the sensitivity varying inversely with the square root of the envelope temperature (which is assumed to dominate the gas temperature)5”m58.

Magnetic fields have a strong and rather unpredictable effect on the gauge sensitivity by changing the trajectories of the charged

64%

particles. Work on this problem has been necessary for gauges used on fusion experimentssgmh’.

7.7. Transient measurements. Gauge calibrations are normally carried out under quasi-static conditions. Two obvious limi- tations to the accurate measurement of transients exist in the limited conductance of any gauge tubulation and the time constant of the electrometer. However. the time taken for surface changes to reach equilibrium may be longer by several orders of magnitude. Tilford’ quotes from the small amount of available experimental work in this area.

7.8. Calibration. As discussed in section 6 the sensitivities of ionization gauges vary from gauge to gauge and, therefore, any gauge used for a measurement with any pretence to accuracy (around 50% or better) should have some sort of calibration check. The subject of calibration is dealt with in another article in this issue but the reader is also referred to Poulter’. A few practical remarks are appropriate however.

The commonest type of calibration is a direct comparison with a reference gauge of some description. Very often, due to the reference gauge or simply expendience, a single calibration point at a relatively high pressure will be used and equation (I ) is then assumed to hold over 5 or more decades of pressure. Two major sources of error should be checked if this procedure is adopted. Firstly, the ionization gauge may be non-linear around the calibration pressure (particularly if it is designed for uhv oper- ation). An initial check over a wider pressure range should. therefore, be carried out. Secondly, the ionization gauge will become non-linear at lower pressures due to residual currents. If these are likely to be a problem they will need to be checked regularly as they can vary significantly from day to day. The only reliable way to do this is to ensure that the system on which the gauge is used has a base pressure considerably below that at which pressure measurements are required. This is often impossible, of course, since the base pressure may itself be a required measure- ment.

The most accurate types of calibration are performed against primary standards ofpressure. The highest available accuracy will only be obtained if the gauge is used in a similar fashion to which it was calibrated. Thus, careful thought should be given to calibration methods and any calibration certificate should be carefully studied.

Calibrations are normally carried out for nitrogen and results quoted as ‘nitrogen equivalent’. The limitations of this practice should not be overlooked. The gauge indication will vary as a function of gas species. For example, a gauge will have a different effective sensitivity at system base pressure, where the residual gas could be largely hydrogen, to operating pressure where the process gas will dominate. The magnitude of this effect will vary from gauge to gauge6’.

8. Conclusion

The indication of an ionization gauge cannot be taken at face value and the likely effects of such factors as outgassing, gauge pumping and residual currents must be considered. Dependable measurements, even of only moderate accuracy, will only be obtained at the expense of considerable effort. There is no way around this by replacing ion gauges with either mass spectr- ometers where calibrations against ion gauges are often recom-

Peter Nash; The use of hot filament ionization gauges

mended or by more expensive types of ion gauge where the cost has probably gone into digital displays or uhv compatibility.

Acknowledgements

I should like to thank my colleagues at NPL for their patient reading of my manuscript.

References

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