US2739283A

US2739283A - High-vacuum device

Info

Publication number: US2739283A
Application number: US272999A
Authority: US
Inventors: Jonathan R Roehrig
Original assignee: Nat Res Corp
Current assignee: National Research Corp
Priority date: 1952-02-23
Filing date: 1952-02-23
Publication date: 1956-03-20
Anticipated expiration: 1973-03-20

Description

March 20, 1956 J. R. ROEHRIG 2,739,233

HIGH-VACUUM DEVICE Filed Feb. 25, 1 52 Power Supply l I 1 {To Vacuum Sysfem AMPLIFIER FIG. I

INVENTOR. JGHNATHAN R. ROEHRIG Y 0M w ATTORNEY United States HIGH-VACUUM DEVICE Application February 23, 1952, Serial No. 272,999

8 Claims. (Cl. 324-33) This invention relates to high vacuum and more particularly to pressure gauges for measuring absolute pressure from atmospheric down to pressures on the order of 10" mm. Hg abs. The present invention is primarily concerned with improvements in gauges of the type described in U. S. Patent No. 2,497,213, issued February 14, 1950 to I. R. 0. Downing.

The Downing gauge comprises a radioactive source whose rate of emission of ionizing agents is substantially constant and is substantially independent of temperature and electrical field therearound. This radioactive material is positioned to radiate ionizing agents into a space between two electrodes so as to ionize gas molecules which are within an ionization chamber holding the two electrodes.

In a preferred embodiment of the Downing gauge the source of ionizing agents is an alpha particle emitter, such as radium, or a beta particle emitter, such as strontium 90. The invention will be more particularly described in connection with the use of such charged particles as ionizing agents, although the general principles are clearly broadly applicable to other types of ionizing agents. by collecting ions produced therein and amplifying and indicating the ion current so collected. The ionization current bears a direct relationship to the composition and absolute pressure of the gas within the gauge. Over a substantial range of pressures this relationship is that of simple proportionality. Departures from this proportional or linear relationship between gas pressure and ionization current are encountered at still higher pres sures. This departure results from the combined effect of two principal causes. First, the number of ion pairs (i. e., one positive ion and one electron) produced by an ionizing particle in traversing a given mass of gas varies significantly as the ionizing particle approaches the end of its range. Secondly, at high pressures the atent The ionization within the chamber is measured concentrations of positive ions and electrons, produced by the ionizing particle, become sufiiciently great so that a significantly large portion of these positive ions and electrons recombine before reaching the collecting electrodes. It is, of course, necessary that the positive ion and a corresponding electron reach their respective electrodes in order to appear as ionization current. Ions recombined in the gas do not contribute to this current.

In designing an ionization chamber for some specific range of operating pressures as, for example, from about 10 mm. to atmospheric pressure, the above considerations may be applied as follows: It is desired that the ion current, which is collected by the electrodes, be taken only from those ion pairs which are produced by ionizing particles which have traveled considerably less than their mean residual range. By mean residual range is meant the mean range (in units of length) of the ionizing particles after they enter the gas whose pressure is to be measured. This range depends somewhat on the composition of the gas and, naturally, is inversely pro portional to the pressure of the gas.

For example, in

2,739,283 Patented Mar. 20, 1956 the case of alpha particles emanating from radium, this mean residual range is the mean range, at the pressure of the gas, of the alpha particles after they have passed through the film of rhodium which is preferably used to confine the decay products (e. g., radon) of radium. Another consideration is that the distance between the two electrodes in the ionization chamber should be as small as possible so as to discourage recombination of the ion pairs. In practice, the above two considerations are amply provided for if the mean eifective limits of the ionization chamber are maintained less than the mean residual range of the ionizing particles at the maximum pressure to be measured. Since the electrodes are within this ionization chamber, the distance between electrodes is considerably less than this particular mean residual range.

Since, at absolute pressure on the order of 1000 mm., the mean residual range of alpha particles, for example, is relatively short (of the order of one inch), a very small effective ionization chamber is required for linear readings in the atmospheric pressure range. On the other hand, the measurement of very low pressures cannot be readily achieved with a very small ionization chamber, since the ratio of dark current to positive ion currents is excessively large. This dark current" is due, at least in part, to alpha particle bombardment of the positive ion collector electrode. At low pressures, the positive ion current generated in the gas is very small, since it is a direct function of the pressure and the limits of the ionization chamber, as expressed (for air) by the equation I=5.25 l0- LNP, wherein I is ionization current in amperes, P is the pressure to be measured in millimeters, N represents the useful activity of the radioactive material in alpha particles per second directed into the ionization chamber, and L represents the mean distance in centimeters between the source of alpha particles and the effective limits of the ionization chamber.

While it is possible to use a dual ionization chamber with two separate sources of ionizing agent activity, and suitable switching means for obtaining linear readings over a wide pressure range, this dual system has the disadvantage that two separate sources of ionizing agent (e. g., radium) must be employed. Since the radioactive materials utilized in these Downing gauges constitute an appreciable amount of the total cost of such gauges, the use of two radium plaques, for example, in a single gauge is disadvantageous from the standpoint of economics and health.

- Accordingly, a principal object of the present invention is to provide an improved pressure gauge of the Downing type which has a linear response over a wide range of pressures from atmospheric pressures to pressures on the order of 10 mm. Hg abs.

Another object of the present invention is to provide such a gauge which employs only a single source of ionizing agent activity.

Still another object of the invention is to provide a gauge of the above type which includes a small ionization chamber within a larger chamber, this smaller ionization chamber being electrically isolated from the large chamber to prevent migration of positive ions from one chamber to the other.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the apparatus possessing the construction, combination of elements and arrangement of parts which are exemplified in the following detailed disclosure, and the scope of the application of whichwill be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings wherein:

Fig. l is a diagrammatic, schematic diagram showing one ptefetxcdem dim nt mihe; invention; and

F sa lnlar d f a menta y view otaw tien. of s 1- I [In the pre n invention t ere pr vided a-fitst cles: o d n n a e ti ly lar e ion za ion ch mber. an a. first. col cto lec ro h this arge. hamb r- M aa ar inc ud d. or m ain ng a pot ntial differenc be wee the t o. le trod s s t atfinos ti i n a ollec ed y One ofv he lec e n t e. p t cles (i ns. an ectro s). r collected; y the. o er of the el c rod s- In a preferred em di th nven ion the e res o i izin a nt t t isa alpha particl emitte s. such a r m; w ich will. r ate positive ions as a i ect fiu r i n Qt p es ure.- Th fir l ct o (wh rl; define t e. on za on. hamb r) is. ma ntained a a potential V1, and a second electrode is maintained at a pot nt a t e Pot ial v2 beinsn eati e. re ative to V1 n de hat the e n electrod may c llect he Posit v nn W thout ntent, t lim t the nvention i l e Par smhr y de r bed in conne t on. w th th use of such an alpha emitter as the source of ionizing agent activity and with the various electrodes arranged sothat the collection of positive ions is used to measure pressure.

The gauge also includes a third electrode, which delines a second ionization chamber, this second, chamber being relatively small with respect to the, first ionization chamber. This second ionization chamber is also provided with a collector electrode, the fourth electrode in the gauge, The source of ionizing: agent activity (e. g., radium) is positionedso that the alphaparticles emitted therefrom traverse both the large and small ionization chambers. In a preferred embodiment ofi the invention the radium is positionedwithin, or; closely adjacent, the small ionization chamber and its alpha particlestraverse the small ionization chamber before entering the large ionization chamber. The alpha particles then pass through the electrode defining'the small ionization charm her, and then traverse the larger ionization chamber.

With this arrangement of elements, the collection of positive ions is usedto' indicate the pressure within the gauge. The electrode which defines the smaller ionization chamber is permeable to alpha particles andis maintained at a potential V3 (preferably equal= to V1) to preventmigrationof positive ions from onechambcnto the other. In thisconnectionit isparticularly important thatmigration ofpositive ions from the larger to the smaller chamber be avoided when the smaller chamber is being used to measure relatively high -pressures.

Referring now to Figs. 1 and 2, therein shown one preferred embodiment of theinvent-ion which-is particularlyqadapted' for use with a radium plaque'which serves as a source of ionizing agent.- activity. Inthesedi gures; the gauge comprises a housing lowwhi'ch is sufficiently rugged towithstand; evacuation. This housing-or casing preferably serves asan electrode (having the. potential V1) and defines therewithin an ionization: chamber: 12. Within the chamber. 12 is a. collector electrode 14; arranged to be maintained at a potential -vm more negative than V1 so as, to collect the positive. ions. A. second ionizationFchamber-defining electrode-:16; is positioned within.- the chamber 12, this second electrode, 16. being at a potential V2. which preferably. equals. Y1. Within theionization chamber 20; defined bv the 562036: electrode 16 there is provided another positiverioni. collector lse: w e-.,1: ic smai a n da'atam en iatJYn-prefsm v qual; to. 1 vAs llust an he-electrode 1. pr ierably compr es rli ra tv f ho pr haped .rusmh rs which ser e. a a gri Or p ta s as n u rmitti g passagetof high e ergx alph partic e t st-vi e tor n positive.

pair of conducting strips 23, these conducting strips 23 being secured to the housing 10. This radium plaque 22 is preferably of the type described in the above mentioned Downing patent and is arranged so that it is in equilibrium with its immediate decay products.

The two positive-

ion collector electrodes

14 and 18 are preferably supported by

insulators

24 and 26, respectively, and are also conncctedto electrical leads 25 and 27, respectively.

As seen in Fig. 1, the electrical circuit for measuring the positive-ioncurrents collected by the two

electrodes

14 and 18 comprises an amplifier, generally indicated at 30., This. amplifier includes a: meter 32- preferably calibrated directly in increments of pressure, and is arranged to give a direct indication of the absolute pressure within the chamber 12. The amplifier 30 is preferably of the type described by N. P. Moody in Rev. Sci. Instruments 22, No.. 4., 236 (19,51).. In. conjunction with. this amplifier circuit, there is preferably provided a range-shifting mechanism for reading low pressures, in.- termediate. pressures and high pressures. This rangeshifting mechanism preferably includes a resistance net- WQlZk comprising resistances vR1,,R2', R-3 and R4 and a plurality of thermally operated relays RY-1,,RY-2 and RY-ii. In one preferred. embodiment, the various resistauces. have the following values:

R-I=1,,O00: megohms R' :2=.l'0,000. megohrns has: 100,000 megohms. RA: 1,000 megohms The relays RY-l, RY-2 and RY-3 are connected to pressure range selector buttons B1,, B2, and B respectively, so that the appropriate range can be selected by the operator. A zero set selector button B is also provided, this selector button being arranged to close. relays RY-1' and" RY-3, although the circuitry for accomplishing this is not shown,

The above resistance network, comprising resistances R-'1, R-2, R-3, and R-4', serves as. a particularly desirable arrangement for selectively feeding, to .a single, amplifier, equivalent voltages corresponding to. the widely .di-fi f'erent ionization currents from the two ion collectors. At low gas pressures, the ionization current from coll'ector 14; alone is used the current. from collector 118 being grounded through RY -l'. The ionization current from collector T4 is relatively small and the highresistance R-S serves as the input resistance to the amplifier. This arrangement of input resistors serves for ahundredfold; range of pressure, say 121 to P2. For still higher pressures, as P; to P3, the relay RY -2 is closed and R-l' serves as the input resistance to the amplifier. By this means, a range of four. decades (i. e., Ba=P1X10 isavailable. If the linear range of-the-largt v chamber 12 and its collector-electrode 14 cover morethan this pressure range of four decades, additional lower valuedresistors and associated relays (not shown) can beplaced in parallel with R--1 and, RY-Z across the input to the amplifier. At still higher gaspressures, as P; to P theionization current from collector 18 alone is used, the ionization currentcollected by collector" 14- being grounded by relay R*Y+3; relays RY-l andRY-2 being open. The ionization current from collector. 18 isrelatively large and the relatively low resistance 'R-4 serves as the input resistance;

Since at-the high; pressures above- P4- the ion current collected by electrode 18' is inherently large, for com veniently constructed sizesof chamber 20, his necessary that; the input: resistance by which el'ectrode- 181s connected to ground} he not-- too high. This is achieved, as showmby theguse fresi'stor R-4, with-resistorsR-S and Ri-Z'pdCtllJji as avoltage divider in a three-resistor circuit I of satisfactorily low-netinputresistance pnsappr a hinathegr d 6.- Ashillustrated, -ri artiu1a; v

There are severaljadvantag'es to theaise oftheabove arrangement: Eirst, the number of componentsconnected to the-amplifier input is reduced to amiuiinum, thusp-re serving the extremely high-resistance leakage path necessary for proper operation. Second, by the same token, the input shunt capacitance is kept low, permitting rapid response to pressure variations. Third, the use of thermally operated relays is made possible, whereby stray magnetic fields deleterious to the amplifier operation may be avoided.

The

insulators

24 and 26, shown in considerable detail in Fig. 2, are preferably provided with guard rings 40 and secondary insulators 42 so as to provide a very low effective leakage current for these insulators. These guard rings 49 are preferably connected to an electrical lead 44 which is at about the same potential as the potentials V2 and V4 of the electrical leads 25 and 27. This guard ring 40 thus serves to shield the two output leads 25 and 27 from the positive potential on the housing 10. The circuit diagram of Fig. 1 also shows a preferred relationship of the potentials V1 and V3 to the potential of the nearby conducting objects considered as ground. As seen, V1 and V3 are preferably connected directly to ground to eliminate shock hazard and the possibility of malfunction from accidental grounding of the chamber 10. As is apparent from Fig. 2 the position of the radium plaque 22 is such that alpha particles emanating therefrom pass through the small ionization chamber 20 defined by the grid electrode 16. The alpha particles pass between the gaps in the electrode 16 and then enter the large ionization chamber 12. For measuring high pressures the positive ions created within the small ionization chamber 20 are collected by electrode 18. The small size of the chamber 120 permits linear response of the gauge at high pressures due to the fact that the dimensions are less than the mean residual range of the alpha particles, even at pressures on the order of 1,000 mm. Hg abs. In order to maintain this linearity, the distance between the source of alpha particles and the limits of the ionization chamber 20 are preferably kept below about 0.25' centimeters. The fact that the electrode 16 is at the potential V3 substantially prevents the migration of positive ions, generated outside of the ionziation chamber 20, from entering the ionization chamber 20 and thus destroying the linearity of the pressure measurement.

When the pressures become below about mm. Hg abs., the collector electrode 14, in the large ionization chamber 12 is used for collecting the positive ions gen- 'erated therewithin by the alpha particles which pass through the openings in the electrode 16.

In the operation of the gauge ilustrated in Fig. 1 it is connected to a vacuum system by means of a suitable fitting 28. If the vacuum system is at atmospheric pressure, the selector button B3 is pushed so that only the positive ion current collected by the collector 18 is fed to the amplifier. When the pressure in the vacuum system has been reduced to below about 10 mm. Hg abs, the selector buttons Bl and B2 are actuated so that now only the positive ion current collected by the collector 14 is fed to the amplifier. The fact that both relays RY-1 and RY-2 are closed, however, permits reading of relatively high ionization currents. When the pressure drops below about 0.10 mm. Hg abs, the selector button B2 is deenergized so that very small ionization currents, which correspond to the low pressures, can be read. When it is desired to obtain a zero-set reading, selector button B0 is actuated, this serving to close the relays RY-l and RY-3 and preventing the feeding of any positive ion currents into the amplifier.

f While the invention has been described particularly with respect to a preferred modification thereof, numerous alternative embodiments may be employed without departing from the invention. For example, the small chamber may be positioned outside of the large ionization chamber 12 in an auxiliary housing fastened to housing 10. Additionally, the housing 10 may be made longer and narrower, if desired, and the small ionization chamber may be formed in one end of this long narrow housing.

"6 Equally, it is not essential that the radium plaque 22 be positioned so that it actually defines one side of the small ionzation chamber. However, it is desired that the small ionization chamber and the radium plaque be so related that the alpha particles emitted from the radium plaque pass through the small ionization chamber before passing into the large ionziation chamber. Additionally, the plaque can separate the two ionization chambers so that radiation emitted from one side of the plaque enters one chamber while radiation from the other side of the plaque enters the other chamber. This arrangement is less desired since a radium plaque having equal emissivity from both sides is more diflicult to prepare. Equally, the electrode 16 which defines the small ionization chamber may take a number of difierent shapes. It can be a wire mesh or any suitable arrangement which will not unduly hinder the passage of alpha particles therethrough. If desired, electrode 16 can be at a more positive potential than the housing 10. Equally, the housing 10 may be a grid-like casing positioned within a second, more rugged, vacuum-tight housing. However, this is not necessary with the particular circuit shown, and merely adds to the insulating problems. r

While radium has been described as a preferred source of ionizing agent activity, numerous other materials such as beta ray emitters (e. g., strontium or gamma ray emitters (e. g., filtered radiation from radium) may be employed. Gamma ray emitters are least desirable since their ionization efficiency is least and their health hazard is greatest. In those cases where the ionization agents are other than alpha particles, suitable modifications in the circuitry may be employed. For example, where beta rays are the ionization agents their use may require ditferent input resistors since the ionization produced by beta rays is less than the ionization produced by alpha particles.

While it is preferred that positive ions be collected to indicate pressure, negative ions can be equally collected by suitable reversal of polarity in the gauge and by modifying the amplifier.

Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description, or shown in the accompanying drawing, shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A gas pressure gauge operable at pressures above atmospheric pressure without damage thereto and comprising an electrode which defines an ionization chamber, an ion collector within the ionization chamber, means for maintaining said ion collector at a potential different than said electrode, a second electrode defining a second and smaller ionization chamber which communicates with said first ionization chamber, a second ion collector positioned in said smaller ionization chamber, means for maintaining a potential diflerence between said second electrode and said second ion collector, said second electrode being maintained at a potential which will repulse ions of the type collected by said second ion collector to prevent migration of said ions into said smaller ionization chamber, a single radioactive source whose rate of emission of ionizing agents is substantially constant and substantially independent of temperature and electric field therearound, said second electrode being substantially permeable to said ionizing agents, said radioactive source being positioned to radiate ionizing agents into said two ionization chambers, means conductively connected to said collectors for amplifying current created by the collection of ions at either of said collectors, and means connected to the output of the amplifying means for in dicating the magnitude of said amplified currents and thus the gas pressure within said ionization chambers.

2. A gas pressure gauge operable at pressures above atmospheric pressure without damage thereto and comprising an electrode which defines an ionization chamber, an ion collector within the ionization chamber, means for maintaining said ion collector at a potential different than said electrode, a second electrode defining a second and smaller ionization chamber which communicates with said first ionization chamber, a second ion collector positioned in said smaller ionization chamber, means for maintaining a potential difference between said second electrode and said second ion collector, said second .elec-. trode being maintained at a potential which will repulse ions of the type collected by said second ion collector to prevent migration of said ions into. said smaller ionization chamber, a single radioactive source whose rate of emission of ionizing agents is substantially constant and substantially independent of temperature andelectric field therearound, said second electrode being substantially permeable to said ionizing agents, said radioactive source being positioned to radiate ionizing agents into said two ionization chambers, the limits of said smaller ionization chamber being less than the mean residual range of said ionizing agents at the maximum pressure to be measured, means conductively connected to said collectors for ampli fying current created by ions collected at either of said collectors, and means connected to the output of the amplifying means for indicating the magnitude of said amplified currents and thus the gas pressure within said ionization chambers.

3. A gas pressure gauge. operable at pressures above atmospheric, pressure without damage thereto and comprising an electrode which defines an ionization chamber, an ion collector within. the ionization chamber, means. for maintaining said ion collector at a potential which is negative with respect to said electrode a second electrode defining a second. and smaller ionization chamber which communicates with said first ionization chamber, a second ion collector positioned in said smaller ionization chamber, means for maintaining a potential difference between said second electrode and said second ion collector so that said second ion collector collects positive ions; said second electrode being maintained positive with respect; to said first collector so as to prevent migration of positive ions into said smaller ionization chamber, a single radioactive source whose rate of emission ofijonizing agents is substantially constant and substantially independent of temperature and electric field therearoupd', said second electrode being substantially permeable to said ionizing agents, said radioactive source being positioned to radiate ionizing agents into saidtwo ionization cham' bers, means conductively connected to said collectors for amplifying current created by the positive ions collected at either of said collectors, and means connected to the output of the amplifying meansfor indicating the magnitude of said amplified currents and thus the gas pressure within said ionizationchambers.

4. A gas pressuregauge operable at pressures above atmospheric pressure. without damage thereto and com: prising an electrode which defines an ionization chamber, an ion collector within theionization-chamber, means for maintaining said ion collector at a potential which is negative with respect to said electrode, a second electrode defining a second and smaller ionization chamber which communicates with saidYfirst-ionization chamber, a second ion collector positioned in saidsmaller ionization chamber, means for. maintaining a potentialdifference" between said second electrode and said second ion collector so that said second ion collector collects positive ions,sai d second electrodebeing maintainedpositive with respect to said first collector so as. to prevent migration ofpositive ions into said smaller ionization.- chamber, asource of ionizing agents comprising-radium positioned to radiate alpha particles into saidtwoionizationchambersi, said sec: ond electrode beingsubstantially permeable to alpha particles, means conductively" connected to said collectors for amplifying current created by the positive ions collected at. either of saidcollectors, and means connected to. the output of the amplifying means for indicating the magnitude of said amplified currents and thus the gas pressure within said ionization chambers.

5, A gas, pressure, gauge operable at pressures aboye atmospheric pressure without damage thereto and comprising an electrode which defines an ionization chamber, an ion collector Within the ionization chamber, means for maintaining said ion collector at a potential which is negative with respect to said electrode, a grid within said ionization chamber, said grid defining a second and smaller ionization chamber, a second ion collector posi-v tioned in said smaller ionization chamber, means for maintaining a potential difference between said grid and said second ion collector so that said second ion collect collects positive ions, a source of ionizing agents comprising radium positioned to radiate alpha particles into said two ionization chambers, said grid being-constructed so as, not to interfere with the majority of alpha particlesemittcd from said source, and means for maintaining said grid at a potential substantially preventing passage of positive ions into said smaller ionization chamber, said radium being positioned so that some of the alpha par-. ticles emitted therefrom pass through said smaller ionization chamber and into said larger ionization chamber, means conductively connected to said collectors for amplifyingcurrent created by the positive ions collected at either of said collectors, and means connected to the output ofthe amplifying means for indicating the magnitude of said amplified currents and thus the gas pressure within said ionization chamber.

6. Agas pressure gauge operable at pressures above atmosphericpressure without damage thereto and prising; an electrode which defines an ionization chambeg, an ion; collector within the ionization chamber, meaps for maintaining saidion collector at a potential which is negative with respect to said electrode, a second elect-rode defining a second and smaller ionization chamber which communicates with said first ionization chamber, a sgee 0nd ion colleotorpositioned in said smaller ionizati nchamber, means for maintaining a potential difference between said second electrode and said second ion collecs tor so that said second ion collector collects positive ions, at least aportion of said second electrode comprising a grid-like member separating said two ionization cl' atn-v hers, said grid-like member being maintained at a potential which is positive with respect to said two collector electrodes, said grid-like member permitting equalization of pressure: within said two ionization chambers, asourceof ionizing agents comprising radium positiouedto radiate alpha particles into said two ionizationch-ambers, means conductively connected to said collectons for amplifying current created by the positive ions collected at either of said collectors, and means connected to theoutputof the amplifying means for indicating the magnitudelofsaid' amplified currents and thus the gas pressure within said ionization chambers.

7'. The pressure gauge of claim 1 wherein said means for amplifying. current created by collection of ions comprises .a resistance network for selectively feeding, to a single amplifier, equivalent voltages corresponding.- to ionization currents of widely different amplitudes from the two collectors, said resistance network being arranged so tliat'at low gas pressures the relativelyv small ionization current from the first ion collector develops a'voltage across a relativoly'hig'hinput resistance while at high gas pressures the'relatively large ionization current from second ion collector develops an equivalent voltage across'a relatively low input resistance, and a voltage divider betweensaid'low input resistance and said amplifier, at least part-of said voltage divider comprising said input-Ii a 8,- App us-for measuring the pressure of a gas by nization produced in the gas, saidapp-aratus c9mprlsmg.-.twoioncollectors and means conductive'ly conuect'ed to'said" collectors for amplifying ion currents greases collected by either of said collectors, said amplifying means comprising a resistance network for selectively feeding, to a single amplifier, equivalent voltages corresponding to ionization currents of widely difierent arnplitudes from the two collectors, said resistance network being arranged so that at low gas pressures the relatively small ionization current from the first ion collector develops a voltage across a relatively high input resistance while at high gas pressures the relatively large ionization current from the second ion collector develops an equivalent voltage across a relatively low input resistance, a voltage divider between said low input resistance and said amplifier, at least part of said voltage divider comprising said high input resistance, and means connected to the output of the amplifier for indicating the magnitude of said amplified currents and thus the gas pressure to be measured.

References Cited in the file of this patent UNITED STATES PATENTS