US2880323A - Diffusion pump and mass spectrometer - Google Patents

Description

M. E. REINECKE ETAL 2,880,323

DIFFUSION PUMP AND MASS SPECTROMETER 5 Sheets-Sheet 1 March 31, 1959 Filed June 3. 1955 MOLECULAR LEAK STANDARD SUPPLY VACUUM PUMP 3o l3 Q S L 'L. ./3a 37 BOOSTER VACUUM PUMP PUMP II INVENTORS. I M. E. REINECKE A. B. BROERMAN WJM March 31, 1959 M. E. REINECKE ET AL 2,880,323

DIFFUSION PUMP AND MASS SPECTROMETER Filed June 5, 1955 5 Sheets-Sheet 2 f4 1 F G. 3

Y 65 64 72 7o 66 y 5 46 69 INVENTORS.

M. E. REINECKE A. a. BROERMAN A7 RNEY March 31, 1959 M. E. REINECKE ET AL 2,

DIFFUSION PUMP AND MASS SPECTROMETER 5 Sheets-Sheet 3 Filed June 5, 1955 INVENTORS. M. E. REINECKE A. B. BROERMAN H JM Mar h 31, 1959 M. E. REINECKE ET AL 2,880,323

DIFFUSION PUMP AND MASS SPECTROMETER Fil ed June 3, 1955 5 Sheets-Sheet 4 PLANT STREAM INPUT I IOO CLEANING SYSTEM VENT FLOWRATOR a4 jlOl .9- F92 [[9 PC 23 I4 i VISCOUS j LEAK *1 5 l5 STANDARD FLUID MOLECULAR e SUPPLY LEAK SAMPLE swn'c L I04 r VACUUM I06 V I05 ION GAUGE DIFFUSION -:\39 4 PUMP i1 '3' WATER no VACUUM BOOSTER PUMP PUMP I09 I32J I30 me as Hla v 123a FIG. 6 113 INVENTORS.

M. E. REINECK'E A. B. BROERMAN BY ATTO NEYS 'March 31, 1959 M, E. RElNECKE ET AL DIFFUSION PUMP AND MASS SPECTROMETER 5 Sheets-Sheet 5 Filed June 3, 1955 CIRCUITS l4 J (ELECTRONIC HEATER\ FIG. 8

IN VEN TORS. M. E. REINECKE A. B. BROERMAN ATTOR EYS.

FIG. 7'

United States Patent DIFFUSION PUMP AND TMASS SPECTROMETER Marvin E..Reinecke and Arthnr.B..Broerman, Bartlesville,

0kla., assignors to Phillips Petroleum Company, a corporation of Delaware Application June 3, 1955, iSeriaLNo. 513,008 10 Claims. '(Cl. 250-419.)

This invention relates to mass spectrometers particularly suited for industrial applications. .In another aspect, it relates to adiftusion pump.

Heretofore, mass spectrometers have been used extensively as laboratory equipment for conducting various types of analyses. .Recently, interest has developed in the industrial application of mass spectrometers as factory .or refinery analytical and/or control instruments. However, where flammable or explosive vapors are present in the atmosphere adjacent the :unit, thereis a distinct explosion hazard. v

:In accordance with this invention, a commercially practical mass spectrometer unit is provided which is suitable for use in hazardous locationaparticularly where flammable or explosive vapors may be present. The mass spectrometer tube .is enclosed wit-hinan explosion-resistant metal housing, and a vacuum conduit is connected to the tube interiorly of the housing and extends outwardly to a valve and difi-usion pump connection whereby the requisite degree of vacuum can be maintained .in the spectrometer tube. Advantageously, this vacuum conduit extends through a special cylindrical flanged section forming a part of the explosion-resistant housing. The vacuum conduit and valve are, in turn, connected to a 'difiusi'on pump having an exterior electrical heating unit which is enclosed within a special explosion-resistant housing from which the electrical leads are withdrawn through a conduit. The mechanical pump, which acts as a fore pump for the diffusion pump, is driven by an explosion-proof electrical motor, as is a pump provided in the sampling system, a part of which is mounted outside the casing. The solenoid valve and other control elements of this sampling system are likewise of explosion-proof construction.

I Accordingly, it is an object of the invention to provide a mass spectrometer unit capable of being used in haz ardous locations where flammable or explosive vapors may be present.

It is a further object to provide a diffusion pump which can be safely used in such locations.

It is a still further object to provide such units in an economical way, and yet provide the requisite degree of protection.

Various other objects, advantages and features of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

Figure 1 is a schematic diagram of the mass spectrometer system; H

Figure 2 is a vertical sectional view, partially in eleva tion, of the mass spectrometer, difiusion pump and associated elements;

- Figure 3 is a vertical 'view, partially in elevation, of the bottom portion of the diffusion pump;

Figure 4 is a front elevational view, partially in section, of a modified mass spectrometer system;

. Figure 5 is a side elevational view of the modification of Figure 4;

2,880,323 Patented Mar. 31, 1959 Figure 6 is a schematic flow diagram of the mass spectrometer system of Figures 4 and 5; I

Figure 7 is a vertical sectional view of a modified -diffusion pump heater assembly; and

Figure 8 is a schematic circuit diagram illustrating a feature of the invention.

Referring now to Figure 1, a mass spectrometer tube 10 is enclosed within an explosion-resistant metal housing 11 hereafter described in detail, this tube being connected to a sampling system generally indicated by reference numeral 12 and an exhaust system generally indic'ated by reference numeral 13. This tube can advant'a'geously be of the type shown in copending application of M. C. Burk entitled Ion Source, Serial Number 412,790, filed February 26, 1954, now U.S. Patent 2,792,500, and the associated electronic circuits described in that copending application are mounted interiorly of the housing 11.

. The sampling system 12 includes an inlet line 13a which is connected through a viscous leak '14 and a molecular leak 15 to the interior of the tube 10. The inlet line 13a is further connected by a solenoid valve 16 of explosion-proof construction to a sample line 17 which is incorporated a flow controller 18 and a valve 19. The solenoid valve 16 is also arranged to connect the sample line 17 to a pipe 20 which leads to a vent conduit 21 whereby sample vapors bypassing the mass spectrometer during the standardization cycle hereafter referred to are safely vented.

The inlet line 13a is further connected by an explosion proof solenoid valve 22 to a standard fluid line 23 which incorporates a fiow controller 24 and is connected to a standard fluid source 25. w

The inlet line, adjacent the molecular leak 15, is fun ther connected to an exhaust line 27 which leads through a special tubing section 28 of proper diameter and length to regulate the pressure at the molecular leak 15 to a desired value. It was found that provision of a needle valve instead of the tubing section 28 caused undesirable molecular flow in the viscous flow line. The special tubing section 28 is, in turn, connected to a rotary vacuum pump 30 driven by an explosion-proof electrical motor 31. Just upstream of the viscous leak 14, the inlet line 13a is bypassed to the vent conduit 21 through a flow controller 32.

In the operation of the sample system, valves 16 and 22 are operated alternately. During an indicating cycle, sample flows through lines 17 and 13a to the interior of the tube where it is ionized by electrons emitted from an incandescent filament, and ions of a particular mass are selectively accelerated toward a collector electrode which is connected to an indicating circuit showing the concentration of ions of a selected mass in the sample stream. Either alternatively or in conjunction with the indicating function, the output can be applied to control apparatus to control one or more process variables to maintain a predetermined concentration of ions of a selected mass number in the sample stream. During the standardizing cycle, solenoid valve 16 is actuated to divert the flow of sample fluid through pipe 20 to the vent conduit 21 and to cause standard fluid to flow through line 23, valve 22, and line 13a to the interior of the tube. During this standardizing cycle, the gain of an amplifier associated with the tube is varied to compensate for any drift which might have occurred during the preceding indicating cycle.

Both the sample fluid and standard fluid llo'w through the viscous leak 14 and molecular leak 15. The flow 0 through leak 14 is viscous, that is, the path length through the leak is long compared with the mean free. path of the molecules in the stream so that flow through.

3 the leak depends upon the viscosity of the fluid, the pressure, and the dimensions of the opening.

At the molecular leak 15,-the opening is of molecular dimensions, and flow therethrough depends upon the geometry of the system and the molecular weight of the molecules rather than upon viscosity of the fluid. A fractionation effect occurs in the molecular leak, molecules of lower molecular weight being preferentially transmitted through the opening defined thereby. However, since the flow out of the tube is molecular flow, as distinguished from viscous flow, this fractionation efiect does not vary the relative proportions of components of different molecular weight entering and leaving the tube.

Both the viscous leak and molecular leak are required because it is impractical to go from a region of atmospheric pressure through a molecular leak directly to the interior of the tube. Thus, the pressure is substantially reduced by the viscous leak, so that there is viscous flow downstream thereof through the exhaust line 27, which viscous flow is not affected by the small amount of material passing to the interior of the tube through the molecular leak 15. This combination of molecular and viscous leak provides a practical and efficient way of introducing a representative sample of the stream passing through inlet line 17 or standard fluid line 23 to the interior of the tube.

Material leaves the tube through a conduit 35, flow in which is governed by the molecular weight of the molecules and geometry of the system, rather than the viscosity. This material passes through the exhaust system 13 which includes a globe valve 36 with a bellows sealed stem, a battle assembly 39 and a diffusion pump 38. The high pressure side of the diffusion pump is connected by a conduit 37 to a booster pump 40 and a rotary vacuum pump 41, the latter pump being preferably driven by the motor 31 which is explosion-proof as previously noted. These units remove the gases being pumped by the diffusion pump.

The diffusion pump has a plurality, for example three, of jets through which a diffusion pump oil is ejected at high velocity, the oil being vaporized by an external electrical heater, the baffle system 39 preventing this diffusion pump oil from entering the tube 10 with the exception of a small amount which gets into the system by evaporation from the surfaces of the baffle. The difluslon pump can be provided with a pump oil fractionation system, and other auxiliary components as those skilled in the art will readily understand, and the combination of pumps provides a high order of vacuum interiorly of the tube 10.

Preferably and advantageously, the tube 10 has a metal envelope so that no breakage will occur with resultant discharge of flammable or explosive vapors into the interior of the housing 11. However, important advantages of the invention are realized where the tube has a glass envelope.

The construction of the explosion-proof housing and the tube and associated electrical circuitry is shown in detail by Figure 2. It will be noted that a metal frame 45 having a base 46 carries an end plate 47 having an opening 48 therein to which the tube 10 can be inserted or withdrawn, this opening being closed and sealed by a cap 49. Secured to the plate 47 is a flanged cylindrical member 50 which, in turn, is secured by bolts to a generally cylindrical flanged housing section 51 having a cylindrical body portion 52 with a dome-shaped integral end section 53. The members 47, 50 and 51 defining the housing are formed from explosion-resistant metal, such as steel, and the joints between the parts are substantially vapor tight. This is in accordance with specifications which provide for breathing" at the joints but do not allow flame propagation therethrough.

The'tube 10 is secured by a mounting fixture 54 to a support 55 which is positioned below a shelf 56 carrying electronic equipment 57 associated with the tube. The

tube envelope defines a cylindrical outlet section 58 which,

in turn, is connected and sealed to a flanged pipe 59 extending radially through the cylindrical section 50 and welded thereto to form a vapor tight seal. The protruding end of the pipe 59 is connected through a vacuum globe valve 60 and a horizontal conduit, not shown, to the low pressure side of the diffusion pump 38, the high pressure side of which is connected to a conduit 61 leading to the booster pump 40 and rotary vacuum pump 41 of Figure 1.

The body of the difiusion pump 38 is defined by a generally cylindrical casing 62 having a lower end plate 63 to which an annular electrical heater element 64 is secured by a nut 65 and a bolt 66.

The heater 64, in turn, is enclosed by an annular housing 67 having its upper end welded at 68 to the lower portion of the casing 62. An end plate or disc 69 is secured to the bottom of the housing member 67 so that a joint which permits breathing but prevents flame propagation is formed.

The terminals of the heating element, one of which is indicated at 70, are connected to leads 71, 72 which extend through a metal pipe or conduit 73. The latter conduit extends through the side wall of the housing section 67 and forms a vapor tight seal therewith.

It will be evident that we have achieved the objects of our invention in providing a complete mass spectrometer unit suitable for use in hazardous locations where flammable or explosive vapors may be present. In particular, although the enclosure defined by the elements 47, 50 and 52 is essentially vapor tight, some flammable or explosive vapors may leak into the interior of this enclosure. Should these vapors be ignited by a spark from the electronic equipment in the enclosure, the walls thereof effectively confine the explosion so that it is not transmitted to the area outside the equipment. Of course, this will only occur rarely, and it has been found that the enclosure effectively performs the desired function of confining or minimizing any flame or explosion which may occur so that it does not affect the plant exteriorly of the instrument.

In fact, in testing equipment of the present type, explosive mixtures were purposely introduced to the interior of the enclosure and an explosion was produced therein.

This explosion was not sufliciently severe that it would damage sensitive tube and electronic circuit elements in the enclosure. Moreover, no spark or flame was transmitted to the region outside the enclosure.

The same remarks apply to the enclosure of the electrical heater element 64 on the difiusion pump. Were this not provided, a spark occurring in the heater leads might ignite vapors in the atmosphere surrounding the instrument and cause a dangerous fire or explosion, which is effectively prevented by provision of the metal housing 67, 69 and its associated conduit 73 carrying the electrical leads to the heater element.

Finally, it should be pointed out that even if the envelope of tube 10 breaks, assuming it is formed from glass or other breakable material, and flammable vapors from the sample line are discharged into the interior of the housing, igniting of the vapors and any resulting ex-' plosion will still be effectively confined within the region defined by the members 47, 50 and 52.

Referring now to Figures 4 to 8 inclusive and par ticularly to Figure 6, we have shown a modified mass spectrometer system which is similar in some respects to that described by Figures 1 to 4 inclusive and in which the corresponding parts are indicated by like reference numerals.

In this system, the sample before being admitted to the line 17 passes through a sample cleaning system 100. Here, a solenoid valve 101 is provided to connect either line 17 or 23 to the line 13 and a second solenoid valve 102 connects line 17 to the vent 21, the same function but being the valves 16, 22 of Figure 1.

these valves having arranged diflercntly than In this modification, the tube has an explosionproof cover or housing 103, and the interior of the tube communicates through aline 104 and valve 105 with the main diffusion pump 38 vand its associated batfie 39.

An 1011 gage 106 surrounded by an explosion-proof cover 107 communicates with the conduit 104, and a vacuum switch 108 also communicates with the interior of conduit 104 and thus with the interior of the tube 10. T1118 vacuum switch is arranged to be actuated when the pressure in the tube rises above a predetermined va ue. The main diifusion pump 38 issurrounded by a water 1&Ck6t 109 having a thermal switch 110 in thermal contact therewith, this switch being actuated when the water jacket temperature rises above a predetermined value.

The heater assembly at the bottom of the main diffusion pump, indicated generally by reference numeral 11, is of modified construction, as shown by Figure 7. Referring to this figure, it will be noted that housing 112 is suitably secured, as by welding, tothe lower end of the cylindrical housing 62 of the main diffusion pump 38.

An annular heater 113 is secured to the bottom of the casing 62 by a nut 114 threaded to a member 115 which defines an oil drain 117 at the bottom of the unit, a sealing ring 116 being provided at the lower end of the member 115 and being held in place by a bolt 117. As in the structure of Figure 3 the heater leads 119 extend through an opening 120 in the side of the housing 112 which can receive a suitable metal conduit.

Insulating strips 121, 122 are mounted interiorly of the housing 112, and a thermal switch 123 is mounted in a recess 124 defined by the insulating strip 122 and a plate 125 secured to the lower end of the housing, the leads 126 of the thermal switch extending through the opening 120.

The thermal switch 123 is actuated when the housing temperature rises above a predetermined value.

The high pressure side of the diffusion pump is con nected by a conduit 128, Figure 6, and a flame arrestor 129, which can be of the capillary tube type or of any other suitable type, to a booster diffusion pump 130 having a housing assembly 111a at its lower end of the same construction as described in connection with Figure 7, this unit including a heater 113a and a thermal switch 123a. The high pressure side of the booster dilfusion pump 130 is connected by a conduit 131 to a rotary vacuum pump 132.

Referring now to Figure 8, it will be noted that the switches 108, 110, 123 and 123a each have one grounded terminal, and have their other terminals connected, respectively, to the operating windings of a series of relays 133a to 133d, each operating winding further being connected to ground through a suitable current source repre sented by the respective batteries 134a to 134d.

Each relay has a holding circuit, and these holding circuits are defined by the respective normally open contact sets 135a to 135d and reset switches 136a to 136d.

The relays further have the respective sets 137a to 137d of normally closed contacts, all of which are connected in series with a power plug 138 and a fuse 139 so that a current is supplied to the heaters 113, 113a and to the amplifier and associated electronic circuits 140 of the mass spectrometer unit when all of the contacts 137 are closed. However, opening of any one of these contact sets interrupts the supply of current to the heater and electronic circuits. Moreover, when any of the relay 133 is actuated, its associated holding circuit maintains the relay in energized condition until it is reset by actuation of the associated reset switch 136.

In operation, it will be noted that switch 108 closes whenever a predetermined degree of vacuum does not exist within the tube 10, Figure 6. Thus, should vacuum be lost during operation, or should the filament inadvertently be turned on before evacuation of the tube when flammable or explosive vapors might be present, switch 108 is closed and the heater and electronic circuitsare broken 'so that no spark can be .set off with resultant danger ,of an explosion.

Moreover, if the water supply to the jacket I109 fails, switch is actuated to interrupt the supply of current to the heater and electronic circuits. Finally, should the temperature within the housing 111 or 'lllarise above ,a predetermined value, say 450 B, one of the switches 123 or 123a is energized to interrupt the flow of current to the heater and electronic circuits. Accordingly, we have provided an interlocking safety system to prevent or greatly minimize explosion hazards should the vacuum in the tube fail, the diifusion pump temperatures 'rise to a dangerous value, or the flow of water throughthe diffusion pump cooling jacket be interrupted.

Referring now to Figures 4 and 5, it will be noted that the cylindrical housing 53 encloses the electronic circuits and amplifier in an explosion-resistant housing. The tube 10, instead of being mounted in the housingJ53 {is supported by a shelf within an explosion-resistant housing 151. If desired and preferably, electronic circuits making up a preamplifier can beincorporated within the upper part 152 .of the housing.

Within the housing and surrounding the tube is a sheath 153 of insulating material, and the tube is mounted vertically so that the tube outlet is horizontal, rather than vertical as in the system of Figure 2. The entire apparatus is supported by a base 154 upon which are mounted the booster diffusion pump 130, the main diffusion pump 38, the flame arrestor '129 and the rotary vacuum pump 132. L

In this structure, the housing 151 is of relatively small volume, for example, about 200 cubic inches.

Where the tube is formed from glass, it is evident that it may burst if an explosion would occur within the housing 151. To minimize the pressure buildup due to an explosion, this housing is made to fit closely around the tube because a small volume will not produce as high an explosion pressure as a large enclosure. Moreover, if the tube bursts, the relatively large volume of the diffusion pump will allow expansion of the gasesinto the evacuated pump chamber. Thus, the pressure buildup inside the diffusion pump 38 will be small (not more than about 90 pounds per square inch). The diffusion pump is of explosion-proof construction to prevent flame from escaping and to withstand pressure surges. The flame arrestor 129 prevents flame from passing beyond the dilfusion pump and also clamps pressure surges so that the pressure buildup in the booster and rotary pumps is not excessive.

Alternatively, a venting section or flame arrestor is placed in the explosion-proof cover around the tube to allow blowdown to the atmosphere without allowing flames to escape.

While the invention has beendescribed in connection with present, preferred embodiments thereof, 'it is to be understood that this description is illustrative only and is not intended to limit the invention.

We claim:

1. In a mass spectrometer, in combination, an explo sion-resistant housing, a mass spectrometer within said housing including a tube, a main diffusion pump connected to said-tube through said housing, a booster diffusion pump connected to said main diffusion pump', a heater connected to the housing of each diffusion pump, a housing enclosing each such heater, a thermal switch in each housing which is actuatable when the housing: temperature rises abovei-a predetermined value, meansfor supplying an electrical current to said heater, and means for interrupting said supply of current upon actuation of either switch.

2. In a mass spectrometer, in combination, an explosion-resistant housing, a mass spectrometer within said housing including a tube, a main difiusion pump connected to said tube through said housing, a booster dif fusion pump connected to said main diffusion pump, a heater connected to the housing of each diflusion pump, a housing enclosing each such heater, a thermal switch in each housing which is actuatable when the housing temperature rises above a predetermined value, means for supplying an electrical current to said heater, a water jacket surrounding the housing of said main diffusion pump, a thermal switch in thermal contact with said water jacket and actuatable when the temperature therein rises above a predetermined value, means for supplying an electrical current to each of said heaters, and means for interrupting the flow of current through said heaters upon opening of any one of said thermal switches.

3. In a mass spectrometer, in combination, an explosion-resistant housing, a mass spectrometer within said housing including a tube, a main diffusion pump connected to said tube through said housing, a booster diffusion pump connected to said main diffusion pump, a heater connected to the housing of each ditfusion pump, a housing enclosing each such heater, a thermal switch in each housing which is actuatable when the housing temperature rises above a predetermined value, means for supplying an electrical current to said heater, a water jacket surrounding the housing of said main diffusion pump, a thermal switch in thermal contact with said water jacket and actuatable when the temperature therein rises above a predetermined value, means for supplying an electrical current to each of said heaters, means including a relay connected to each thermal switch to interrupt the flow of current to said heaters upon actuation of any thermal switch, and a holding circuit connected to each relay.

4. In a mass spectrometer, in combination, an explosion-resistant housing, a mass spectrometer within said housing including a tube, a diffusion pump connected to said tube through said housing, a heater connected totthe housing of said difiusion pump, a housing enclosing said heater, a thermal switch in said housing which is actuatable when the housing temperature rises above a predetermined value, means for supplying an electrical current to said heater, a vacuum switch communicating with the interior of said tube and actuatable when the tube pressure rises above a predetermined value, and means responsive to the actuation of said vacuum switch to interrupt the supply of current to said heater.

5. In a mass spectrometer, in combination, a general cylindrical explosion-resistant housing having a flanged open end, a flanged cylinder formed of explosion-resistant material secured to the open end of said housing, an end plate sealing the open end of said cylinder, a mass spectrometer within said housing including a tube, a vacuum conduit extending radially through said cylinder and connected to said tube interiorly of said cylinder, means sealing the area between said cylinder and said vacuum conduit, said conduit having a flanged end protruding from said cylinder, a flanged vacuum valve secured to said flanged end, a diffusion pump connected to said vacuum valve, said diffusion pump including a casing, an exterior housing enclosing the lower end of said casing and formed from explosion-resistant metal, a cylindrical heater disposed in contact with the lower end of said casing and positioned within said housing, a conduit communicating with said housing, leads extending through said conduit into said housing and connected to said heater, and means sealing the area between said housing and said casing.

6. The apparatus of claim 5 wherein the mass spectrometer tube has an explosion-resistant metal envelope.

7. A mass spectrometer including, in combination, an explosion-resistant metal housing, a mass spectrometer tube within said housing, a vacuum conduit extending through said housing and connected to said tube, a diffusion pump mounted exteriorly of said housing and connected to said vacuum conduit, a mechanical pump connected to said ditfusion pump, a sample inlet line, a flow controller in said line, a source of standard material, a standard fluid line connected to said source, a flow controller in said standard fluid line, an inlet line having a viscous leak therein and a molecular leak communicating with said tube, said inlet line extending through said housing, a vacuum pump connected to said inlet line downstream of said molecular leak, a vent conduit, solenoid valve means connecting said sample line selectively to said vent conduit and to said inlet line upstream of said viscous leak, and selectively connecting said standard fluid line to said inlet line upstream of said viscous leak, said solenoid valves being of the explosion-proof type, and an explosion-proof electric motor driving said vacuum pumps.

8. An explosion-proof difl usion pump including, in combination, a casing having a flat-bottomed section, diffusion pump mcchanism in said casing, an explosion-resistant housing secured to said casing and enclosing the fiat-bottomed section thereof, a cylindrical electrical heater secured to said flat-bottomed section exteriorly thereof and positioned within said housing, a conduit extending through said housing, and leads extending through said conduit and connected to the terminals of said heater. I

9. An explosion-proof diffusion pump which comprises, in combination, an upstanding cup-shaped casing, diffusion pump mechanism in said casing, a generally cylin-.

drical housing having a cylindrical section welded to the sides of said casing adjacent the lower end thereof, said housing enclosing the lower end of said casing, a plate sealing the lower end of said housing, an annular electrical heater positioned against the outside of said cas-.

ing, a bolt securing said heater to said casing, a conduit extending through the side walls of said housing, and leads extending through said conduit into the housing and connected to the terminals of said heater.

10. An explosion-proof diffusion pump including, in combination, a casing having a flat-bottomed section, ditfusion pump mechanism in said casing, an explosionresistant housing secured to said casing and enclosing the flat-bottomed section thereof, a cylindrical electrical heater secured to said flat-bottomed section exteriorly thereof and positioned within said housing, a conduit extending through said housing, leads extending through said conduit and connected to the terminals of said heater, and a thermal switch positioned in said housing, and actuatable when the temperature therein rises above a predetermined value.

References Cited in the file of this patent UNITED STATES PATENTS 2,024,726 Ehrenfeld Dec. 17, 1935 2,318,786 Korte et al. May 11, 1943 2,368,492 Ralston Jan. 30, 1945 2,702,869 Felici Feb. 22, 1955 2,768,584 Nicol et al. Oct. 30, 1956

Images