Topics in
... Chemical Instrumentation
Edited by
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S. Z . LEWIN, N e w York University, N e w York 3, N.Y.
1hese articles, most of which are to be contributed by guest authors, are intended lo serve the readers of this JOURNAL bu callino attention lo new developments i n the theory, design, or availability of ciemic~iaboralwy instmmenlalion, o i by presenting useful insights and ezplanations of topics that are of practical importance to those who use, ,or teach the use of, modern instrumentation and instncmental techniques.
XII. Pressure Measurement.
Part Three
Seymour T . Zenchelsky, School of Chemistry, Rutgers, The State University, New Brunswick, New Jersey
The preceding parts of this article dealt with liquid-column, meehrtnicitl, and thermal gauges. This part concludes the article.
mer derive their designation from the fact that electrons are furnished by
Ionization Gauges
of eieckons from the (cold) cathode.
Several different kinds of ionization gauges are available, and, in combination, they cover the range of pressures from atmospheric down to 10-LSmm Hg. Thus these gauges extend the range of measurement to pressures far below those within the capability of the devices already described. I n fact, ionization gauges are indispensable for the measurement of pressures below lo-' mm Hg. Principle of Operation. All ionimtion gauges depend upon the measurement of an electric current which results from the ionization of s gas whose pressure is being determined, but the various types differ principally in the method of ion production. For, in order to convert a gas molecule into apmitive ion, i t is necessary to remove an electron by supplying energy equal to the imzizatimz potential of the molecule involved. And this energybetween 4 and 25 electron-volts--may he introduced in a variety of ways, some of which will be described below. When the requisite energy is supplied a t a constant rate, the rate of ion production is also constant, and almost exactly proportional to the pressure of the gas. Thus when the ions and electrons are collected under the influence of a, constant potential gradient, the observed current is proportional to the rate of production, and hence t o the gas preaaure. Ion currents are measured by means of conventional electrometer circuits. Ion Production. Two methods are commonly used for introducing the energy necessary t o ionize the gas:eledran bombardment ar bombardment by nuclear radiation. Ionization gauges which utilize the first mebhod fall into two classes: hotfilament gauges and cold-cathode gauges. The for-
between the electrons and gas molecules. This is accomplished by surrounding the gauge with a magnetic field in order t o deflect the electrons. Their resultant ~ a t h is thus cycloidal, rather than linear, thereby increasing their transit time (and hence their probability of collision with gas molecules). Cold-cathode gauges are also designated Philips gauges or Penning gauges.
Figure 37. Simple hot-filament goge. A-fllament B-onode C-ion collector D-ion-current mekr E-rptem connection
T h e gauges which employ radioactive sources to produce ionization are less f r e quently used than the others because they cover only the range of pressures from g' to They 'Ontain either tritium compounds (beta partides) or radium salts (alpha particles) as the source of ionizing radiation. I n both cases the energies of the particles are far
in excess of the requisite ionization energy. Hot-Filament Gauges. The simplest kind of hot-filament gauge is an ordinary triode as shown in Fieure 37. Variations in design are chieay directed toward the following objectives: (1) prolonging element life, (2) minimiziigthe inconvenience of filament burnout, and (3) extending the lower pressure-limit. Pursuit of the first two objectives has resulted in the use of filaments made of various materials, or of double filaments to permit rapid changeover without removal of the gauge tube. The third objective has led to changes in electrode design and spacing.
Figure 38. Eiion Vocuum Goge Contro1,Model CVG-5. This goge covers the r m g e 10-3 mm I i g to 2 X 10-lo mm Hg in six decades. It co+ ni% outgarring and filament-protection sircuitr. Recorder output is available a t the reor of the cabinet. Other models are availmble.
The lower pressure-limit is governed by the magnitude of the observed residual current, which is completely independent of the gas pressure. This current results from the ejection of photoelectrons, from the ion-collector electrode, by X-rays which are produced when electrons strike the anode. By means of changes in the physical shape and placement of the eleotrodes, the number of X-ray photons reaching the coIlector electrode m n be decreased. Thus Bayard, Alpert, and Nottingham were able to decrease the lower pressure-limit from mm Hg to lo-" mm Hg. Lafferty further lowered this limit t o approximately 10-'"m Hg by adding a n external magnetic field in order to increase the number of collisions between electrons and gas molecules. All components of -the boLfilament gauge must be freed f, adsorbed gases prior to use. T~~~is aceomplished by heating *he tube invelope to 450°C and using either electron hombardment or resistive heating for the eleetrades, A of the hoefilament is its suscepti,,ilitv to burn(Continued m page A860)
Volume 40, Number I I , November 1963
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Chemical Instrumentation
Figure 39. Consolidofed Vacuum Corporation, GIC-016 lonizotion Goge Tube. It utilize. the Bayord-Alpert canrtruction ond incorporotet two filaments for rapid changeover in core of
burnou:.
out in the presence of even rnoderute pressures of air; thus many gauges pnrvide for automatic cutoff of the filamenl voltage when t,he pressure rises s h v e s presel level.
Figure 40. NRC Equipment Corporation, Model 533 Ion G a g e Tube. This tube features a platinum moting inride the envelope for electrortotic shielding. It also has o dual filornent.
Yarictus lrot-filament gauges are avnilable from t,la,se companies already listed in Part Two, under 7'hwrnal Gages. Nost. o f these incurporate s number of the design features mentioned above. They gonerally use the Bityard-Alpert type g:tnpe tube and indieat,e pressure on x linear s d e , with several decades provhled by means of it sn-it,cl>. Most models, r,thrr tlmn the least expensive, provide xutomatic filament-l,ornr,ut protection as well as prwisions for outgassing the electrodes and remrder oonnertion. Typiral rsngrs arc IO-Vtr, 1 0 F , or to 10-'", rnnr
(Conlintred on p o p ,4852)
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Chemical instrumentation
Figure 41. Consolidated Vocvvm Corporation. Type GIC-1 IOA ionization Vocuvm Gage. It cover. the 1 0 - a mm ng to mm ~g on linear rcole in tlve decoder. In addition. the range from 2 mm Hg t o O-SHg is covered b y a thermal gage. More elaborate models ore olro available.
.
Hg, u s u d y covered in six decades. Some models also include a thermocouple gauge so as to extend the high pressure limit t o one mm Hg. Only a. few of the available models are shown in Figures 38, 30, 40, 41, and 42.
Figure 42.
NRC Equipment Corporation, Model
763 Combination Ultra High Vacuum Goge
Control. This goge covers the ronge from lo-' mm Hg to lo-"' mm Hg in sir lineor decades. i t also hor an integral thermocouple gage to extend the ronge up to 2 mm Hg. Thk model porrer.er filoment-protection and outgosing circuits or well as recorder terminolr. Other models are avoiloble.
Cold-Cathode Gauges. These gauge8 (also called discharge gauges) are more rugged than the hot-filament types. The gauge tubes are generally made of metal and they can be cleaned or repaired easily. There are no filaments t o burn out upon accidental exposure to high pressure. On the other hand, cold-cathode gauges arc not easily degassed and are subirct to contamination by breakdown-prnducts from the gas.
Figure 43. F. J. Stoker Corporation, Model DG-10 Cold Cathode Type Dirchorge Tube. A unique feature o f this g o g e is the ear* of replacement of the goge-tube elements and their ease of cleaning. The awociated g a g e control permitr coverage of the range from lo-' mm Hg t o lo-' mm Hg.
(Continueda page A864)
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Journal o f Chemical Education
Chemical Instrumentation The lower pressure-limit of a n ordinary discharge gauge is ahout lo-' mm Hg (for reasons identical with those given for the simple hot-filament gauge). This
Figure 44. Conrolidoted Vacuum Corporation, GPH-001 Digcharge Vacuum G o g e Tube. The permanent-magnet collar ir encored in plartic "to protect against lor$ in strength due t o cantact with_rurrounding steel obiectr."
limit was dropped t o lo-'? mm H g by Redhead, who redesigned the gauge tube and used crossed magnetir and electric fields externally in order to decrease t,he X-ray background. I
-xt
Figure 45. Conrolidded Vacuum Corporotion, Type GPH-100A Dixhorge Vowum Gage. Thir control, "red with the GPH-001 g a g e tube, provide. o range from 2.5 X mm Hg to lo-' mm Hg on three nonlinear rcales. More eloborote models are ovoilable.
Man," models are available cummercidly. Same are equipped fur mulbistation operation as well as for recording and control. Some of these me shown in Figures 43, 44, 45, ;tnd!46.
.
...,. .,. -",,.,..,,,,-,,.,,-. .. ~
Figure 46. NRC Equipment Corporation, Model 7 5 2 Redhead UHV Ionization Gage Control with 5 5 1 G a g e and Magnet. An extremely sensitive gage, "ring crossed electric m d magnetic Relds, Thir model provides a range from lo-' t o 10-'%m Hg on a lineer rcole of nine decades. Recorder terminolr ore provided and no outgassing of the elettrodes is required.
(Contznaed m page AA866)
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Chemical Instrumentation Nuclear-Emission Gauges. The only commercially available gauge uaing a radioactive source to produce ionization is the NRC Equipment Corporation, Model 553 Alphatron gauge. I t covers the range from atmospheric pressuredown to lo-" mm Hg in seven decades on linear scale. This gauge is shown in Figure 47. Although the source of mdioactivity is enclosed, the ordinary precautions must be ohserved. (Some states require licensing of all radioact,ive sources above a specified minimum.) The chief advantages of this type of gauge are the linearity of reading8 a t the high-pressure end of the scale as well as its ruggedness.
Figure 47. NRC Equipment Corporation, Model 530 Alphatron Vocvum Goge. This gage covers the ronge from atmospheric pressure down to lo-' mm Hg in seven linear rcales.
All of the above gauges are calibrated to read directly for dry sir, but-as with the thermal gauges-they require a carrection for other gases.
Bibliography (1) DUSHMAN, S., AND LAFFERTY, J. M., Editors, "Scientific Foundstions of Vacuum Technique," Second Ed., John Wiley and Sans, Inc., 1962, New York. Chapter 5. (2) REILLY,J., AND RAE,W. N., "PhysicnChemical Methods," V d . I, Fifth Ed., D. Van Nostrand Co., Inc., 1953, New York. Chapter VI. (3) LION, K. S., "Instrumentation in Scientific Research," McGraw-Hill Book Co., Inc., 1959, New York, pp. 101-117. 14) Consolidated Vacuum Camoration. "High Vacuum Gauges,"'~ulletin 9-1, Rochester, N. Y.
The Editors will he pleased to discuss possible contributions to this series with potential authors who feel they have a paper of this type on an appropriate theme. A prospective author is asked to check annual indexes to note topios already treated and to be familiar with the general style and format of the papers. Inquiries should he addressed either to Professor S. Z. Lewin, New York University, New York 3, N.Y. or to The Editor, W. F. Kieffer, Dept. of Chemistry, Room 6-123, Massachusetts Institute of Teehnology, Cambridge 39, Mass.
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