US2795716A

US2795716A - Electrical vapor detector

Info

Publication number: US2795716A
Application number: US554235A
Authority: US
Inventors: John A Roberts
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1955-12-20
Filing date: 1955-12-20
Publication date: 1957-06-11
Anticipated expiration: 1974-06-11
Also published as: CH350819A; DE1062957B; GB842897A; FR1075507A; BE553533A; NL102120C; FR70862E

Abstract

842,897. Discharge tubes. GENERAL ELECTRIC CO. Dec. 20, 1956 [Dec. 20, 1955], No. 38875/56. Class 39(1). In a vapour detector in which the atmosphere containing the substance to be detected is passed between a negative collector electrode and a heated positive emitter electrode supported on a ceramic core, and the positive ion current is measured, positive ion emitting material containing an alkali metal glass is interposed between the core and the emitter electrode to supply positive ions when detecting substances such as halogen compounds which have an ionization potential higher than the work function of the emitter electrode surface. The detector shown comprises a cylindrical core 12 of ceramic material preferably having a high alumina content supported on two rods 16 and 17 which are retained in openings 10b and 10c in the end wall of a ceramic housing 10, which end wall also has openings 10d and 10e through which vapour may enter or be discharged. A heater coil 13, e.g. of an alloy of 90% platinum and 10% rhodium, is wound on the core 12 with one end 18 wrapped around the rod 16 and the other end 20 wrapped around the rod 17, and is buried in a coating 15 of positive ion emitting material containing an alkali metal glass known as B-eucryptite (Li2O Al2O3. 2SiO2). A cylindrical emitter electrode 14, preferably of platinum, surrounds and is in contact with the coating 15 and is connected to the rod 16 by a strap 23. A cylindrical collector electrode 24, preferably of platinum, is maintained with its upper flared end 24a engaging a shoulder 10g in the housing 10 by a ceramic washer 25 having a plurality of sloping fingers 25a which engage the lower end of the electrode. The washer 25 is held in engagement with a shoulder 10h by bent portions 16a and 17a of the rods 16 and 17 which are secured to bolts 21 and 22 mounted in a flange 10a at the open end of the housing 10 and forming terminals for the heater coil 13. A conductor 26 having one or two turns 26a wrapped around the electrode 24 is spaced from the electrode 24 and is bent at the end and secured to a terminal bolt 27. The construction is such that the heat loss from the emitter and collector electrodes is kept to a minimum. The emitter electrode attains a temperature between 700 and 925‹C.

Description

June 1957 J. A. ROBERTS 2,795,716

ELECTRICAL VAPOR DETECTOR Filed D90. 20, 1955 John A. Robens by, fdmd His AHorney ELECTRICAL VAPGR DETECTOR Application December 20, 1955, Serial No. 554,235

12 Claims. (Cl. 313-7) This invention relates to improvements in electrical vapor detectors for detecting the presence of certain substances or impurities in gases, such as are disclosed in U. S. Patent 2,550,498, issued April 24, 1951, to Chester W. Rice and assigned to the same assignee as the present invention.

In the Rice patent, there is disclosed apparatus for detecting substances such as certain gases, vapors, smokes and similar matter in an atmosphere by directing a sample of that atmosphere into an electrical discharge device, which permits the suspected substance to induce positive ion formation at a positively charged and heated electrode. The positive ions so formed are collected by a negatively charged electrode to produce an indicating current that increases with the concentration of the substance being detected.

As pointed out in the Rice patent, impurities of two general classes may be detected by the apparatus. The first class includes those substances which have an ionization potential lower than the work function of the positively charged hot electrode, and the second class includes these substances which require a sensitizing material to provide positive ions because the work function of the substance is higher than that of the hot positively charged surface. The present invention is directed toward the provision of an improved vapor detector for detecting the presence of substances of both classes, although it is particularly useful in detecting substances of the second class.

Examples of substances of the first class are the alkali metals. When an atom of one of these metals strikes a hot surface, whose electron work function is greater than the ionization potential of the atom, an electron is taken from the atom which then evaporates as a positive ion. This theory, which is now well known, was explained by Langmuir and Kingdon in a number of publications, for example, in Science, vol. 57 (1923), page 58, and in Physical Review, vol. 21 (1923), page 380.

Examples of substances of the second class are the halogens and their compounds, which have an ionization potential greater than the electron work function of the hot positively charged electrode. Although the precise theory of operation ofsuch detection devices in detecting substances of the second class is not completely understood, it is believed that the positive ions are provided by impurities in the core material on which the heated electrode is supported. The impurities are believed to diffuse outwardly from the core to form a sensitizing surface film on the heated positively charged electrode. The substance to be detected then cleans the electrode surface of the impurity and causes the impurity to be ionized by the hot surface and attracted to the negatively charged electrode. It has been found that the amount of ion producing impurities available in the core material becomes exhausted in a relatively short time, Therefore, it is necessary to replace tha portion of the detector or to restore it to life in one manner or another.

Accordingly, it is an object of the present invention to hired States Patent Q provide a vapor detector embodying a positive ion source that provides a relatively long life as compared to that of the device disclosed in the Rice patent.

It is known that the efliciency of vapor detectors of the type presently being considered may be markedly decreased through loss of heat from the interior of the detector. Such loss decreases the ease with which positive ions are formed and, hence, decreases the positive ion current flowing between the electrodes of the device that indicates the presence and amount of the substance being detected. Therefore, another object of the present invention is to provide an improved construction to reduce heat loss from the interior of the detector.

A further object of the invention is to provide an improved construction for electrical vapor detectors that greatly simplifies the manufacturing and assembly processes.

In accordance with the invention, there is provided an electrical vapor detector comprising a housing containing an emitter electrode supported on a core, an ion emitting material containing an alkali metal glass interposed between the core and the emitter electrode, and means for heating the emitter electrode and the ion emitting material. A collector electrode is mounted in spaced relationship to the emitter electrodeand the housing is adapted to permit the passage between the two electrodes of the atmosphere containing the substance to be detected.

In operation, suitable electrical means are provided to energize the heating means, and to maintain the emitter electrode at a positive potential and the collector electrode at a negative potential. It is believed that, when the emitter electrode and the material containing an alkali metal glass are heated to the proper temperature, positive ions are emitted from the alkali metal glass material, diffuse through the positively charged emitter electrode, and are attracted to the negatively charged collector electrode. When a substance to be detected passes over the hot surface of'the emitter electrode, the positive ion current flow to the collector electrode is increased by an amount depending on the concentration of the substance to be detected.

The material containing an alkali metal glass provides a positive ion source having quite a long life, relative to those sources previously utilized. In addition, the core is preferably constructed of a material having a high alumina content, which also provides positive ions and further increases the life of the device.

The various elements comprising the ion emitting and collecting structures are mounted within the housing with a minimum amount of surface area contact through which heat might be transferred to the outer atmosphere. The electrical connections to the various elements are so arranged as to provide poor heat transmission paths, and, in some cases, also serve as mounting means for retaining the elements in position. The housing is so constructed that it cooperates with the various elements contained therein to automatically position them properly as the device is assembled.

For a better understanding of the invention, together with further objects and advantages thereof, reference is made to the following description taken in conjunction.

with the accompanying drawing, in which:

Fig. 1 is a perspective view of a vapor detector construction in accordance with the invention, with a portion broken away to show the internal construction; and

Fig. 2 is a schematic diagram showing a typical electrical circuit with which the vapor detector of the invention may be utilized.

Referring now to Fig. 1 of the drawing, it is seen that the improved vapor detector of the invention includes. a ceramic housing having a substantially cylindrical hollow interior and having a flange or lip 10a formed on one end thereof. Located within the hollow interior of the housing is an emitter structure 11 comprising a core 12, and heater means such as a coil 13 wound on the core 12. A cylindrical'emitter electrode 14 preferably made of platinum, is supported by the core 12 outside the heater 13.

The core 12 is substantially cylindrical in shape and preferably is made of a ceramic material having a high alumina content. The heater coil 13 is of conventional type for such applications and may conveniently comprise an alloy containing approximately 90% platinum and 10% rhodium. In practice, it is necessary for the emitter electrode 14 to attain a temperature of 700 C. to 925 C. The exact temperature required for the emitter electrode to function properly may vary from one type of construction to another, and is generally determined empirically.

As was previously mentioned, it is believed that, when detecting the presence of substances having a greater electron work function than the work function of the hot platinum electrode 14, alkali metal impurities in the ceramic core 12 provide positive ions. It is apparent that the number of such impurities in the core is limited. Therefore, in accordance with the invention, a coating 15 of positive ion emitting material is provided on the core 12. The coating 15 substantially surrounds the heater coil 13, so that in effect the coil is buried in the ion emitting material, and the platinum electrode 14 has its inner surface in contact with the positive ion producing layer 15. Thus, the space between the core 12 and electrode 14 is substantially filled with the positive ion emitting material. It has been found that various compounds of the alkali metal glasses serve satisfactorily as positive ion emitting sources. Such glasses are described by Blewett and Jones in the Physical Review, volume 50, page 464, 1936. Blewett and Jones found that the ternary system consisting of the alumino-silicates of the alkali metals (Li, Na, K, Rb, Cs) gave the most satisfactory positive ion sources. It has been further discovered that the alkali metal glass known as b-eucryptite (lLizO- lAlzOs-ZSiOz) is an especially good emitter. In preparing one of these materials, for example, b-eucryptite, for use in the present invention, proper amounts of LizCOaAl(NOs)a, and SiOz are thoroughly mixed with the proper amount of water and fused in a temperature of approximately 1400 C. The glass-like material thus obtained is then powdered and mixed with a suitable ceramic cement in the desired proportion, and the mixture coated on the core 12 and heater 13 and allowed to set. Of course, coatings utilizing others of the alkali metal glasses may be prepared in a similar appropriate manner. Such a coating provides a. positive ion source of great efiiciency and extends the useful life of the vapor detector in which it is used to a degree heretofore unattainable without replacing the ion source or reactivating it in some manner.

Referring again to Fig. 1, it is seen that the emitter structure 11 is supported on a pair of

rods

16 and 17 which extend axially through the housing 10 and lengthwise through the core 12 with a friction fit therein. The upper ends of the

rods

16 and 17 are retained within openings 10b and 100, respectively, provided in the end wall of the housing 10. The construction of the vapor .detector of the invention is greatly simplified in that electrical connections to the heater coil 13 are provided by the mounting means for the emitter structure 11; that is one end of the heater coil 13 is wrapped about the rod 16 above the core, as at 18, and the other end of the coil is wrapped about the rod 17 below the core, as at 20. The lower ends of the

rods

16 and 17 may be bent outwardly and secured to

bolts

21 and 22, respectively, to provide electrical terminals for the heater coil 13 and to properly position the emitter structure 11 axially within the housing10. Of course, lateral positioning of the emitter structure 11 depends on the locations of the .4

rods

16, 17 relative to the core 12 and housing 10, and preferably the core 12 and electrode 14 are coaxial with the housing 10. Each of the

bolts

21, 22 extend through the flange 10a on the housing and is countersunk into the upper surface of the flange. Of course, each

bolt

21, 22 is provided with a cooperating nut, only one of which, the nut 22a, is seen in the drawing, to fasten securely the ends of the

rods

16, 17, to the housing, as well as fasten electrical leads from an energizing voltage source (not shown in Fig. 1). A strap 23 is provided to connect electrically the cylindrical platinum electrode 14 to one of the

rods

16, 17, in this case, the rod 16.

A cylindrical collector electrode 24, preferably made of platinum, is also mounted within the housing 10. Preferably, the collector electrode 24 is mounted. coaxially with the emitter electrode 14 by means to be later described, and is spaced outwardly therefrom to permit the passage therebetween of the vapor containing the substance to be detected. The housing 10 is open at one end and its end wall is provided with a pair of openings 10d and lite through which vapor may enter or be discharged.

The hollow interior of the ceramic housing 10 is provided with a portion 10 of increased internal diameter which is engaged by a flare 24a formed on the upper end of the cylindrical collector electrode 24. A shoulder 10g formed above the portion 107 of the housing 10 helps position the electrode 24 axially within the housing.

The electrode 24 is retained within the housing by means of a ceramic washer 25 provided with a plurality of downwardly sloping internally extending fingers 25a. The washer 25 bears against a shoulder 10h formed at the open end of the housing 10 and is retained in position by the bent portions 16a, 17a of the

rods

16, 17.

Electrical connection to the cylindrical collector electrode 24 is provided by a conductor 26 which is wrapped about the electrode 24 for one or two turns near the end wall of the housing 10, as at 26a. The conductor 26 extends downwardly alongside but spaced from the electrode 24, and may be bent outwardly and secured to a bolt 27 in the same manner as the

rods

16 and 17 are secured to

bolts

21 and 22, as previously described.

It has been found that efiicient operation of a vapor detector of the type described requires that the collector electrode be maintained at a relatively high temperature along with the emitter electrode. Sutficient heat is supplied to the collector electrode by the heater coil to attain such operation if loss of heat from the collector electrode can be minimized. It is pointed out that the physical connection between the electrode 24 and the conductor 26 obtained by wrapping the conductor about the electrode for only one or two turns provides poor heat transfer between the two members. Furthermore, by making the conductor 26 long, the conduction of heat from the electrode 24 to the atmosphere outside the housing 10 is minimized. The amount of heat lost from the electrode 24 is further reduced by the use of the ceramic washer 25 having fingers 25a which contact the electrode 24 in only very small areas.

By mounting the emitter structure 11 on the

rods

16 and 17, which also serve as electrical connections, heat loss from the emitter structure is kept to a minimum, while the construction and assembly of the device is maintained extremely simple.

Fig. 2 illustrates a typical conventional electrical circuit in which the improved vapor detector of the invention may be utilized. The heater coil 13 may be energized from the secondary winding of a transformer 28 whose primary winding may be connected to a conventional source of alternating current (not shown). The positive side of a conventional direct current source 30 may be connected to one end of the heater coil 13 and to the emitter electrode 14, and the negative side of the direct current source 30 may be connected through a direct current microamrneter 31 to the collector electrode 24."

In operation, a vapor suspected of bearing a substance whose presence and amount are to be determined is introduced into the housing and between the

cylindrical electrodes

14 and 24. If the suspected substance is present, the number of positive ions emitted from the hot emitter electrode 14, which is connected to the positive side of the direct current source 20, is increased. These positive ions are attached to the negatively charged collector electrode 24, thus causing positive ion current to flow between the two

electrodes

14 and 24, the magnitude of which is indicated by the direct current microammeter 31. The theory of operation of such devices is set forth in the Rice patent previously mentioned, and the detector of the present invention operates on substantially those same principles. However, it has been discovered, as previously pointed out, that by coating the positive ion emissive layer on the core 11 the life of the detector is greatly extended, and by providing a construction that minimizes heat loss from the interior of the vapor detector its etficiency is also greatly improved.

The device of the invention is useful in detecting substances of the class comprising the halogens and compounds thereof as well as substances having a lower work function than the hot surface of the platinum emitter electrode 14. A partial list of some of the halogen com pounds to which the device is sensitive is as follows: chloroform (CI-ICls), ferric chloride (FeCla), hydrochloric acid (HCl), bromobenzene (CsHsBI'), xylene hexafiuon'de (C8H4F6), bromine (Brz), sodium hypochlorite (NaClO), methyl iodide (CHsI), monochlorobenzene (CeHsCl), carbon tetra chloride (CC14), tetra chlorobenzene (CsHaCl-r), paradibromobenzene (CsHrBrz) methylene chloride (CHzClz), paradichlorobenzene (CsH4C12), butyl bromide (CH3(CH2)2CH2Br), methyl chloride (CHsCl), Freon #12, chlorodifluoro meth ane (CHClFz), and trichloroethylene (CHClCClz).

The device has been found to be extremely useful in detecting leaks in closed systems containing refrigerants and the like of the halogen class, and finds use in detecting leaks in a vacuum system where the detector may be mounted inside the system and the leak searched out by going over the outside of the system with a halogen compound.

While a particular embodiment of the invention has been illustrated and described, various changes and modifications therein may be made by one skilled in the art. Therefore, it is intended in the appended claims to cover all such changes and modifications as fall within the true scope and spirit of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A detector for a finely divided atmospheric substance comprising a core, a layer of material containing an alkali metal glass coated on said core, a first electrode in contact with said layer of material, heater means for heating said layer of material and said first electrode, and a second electrode spaced apart from said first electrode to permit the passage of said atmospheric substance therebetween.

2. A detector for a finely divided atmospheric substance comprising a core, a layer of material containing b-eucryptite coated on said core, a first electrode in contact with said layer of material, means for heating said layer of material and said first electrode, and a second electrode spaced apart from said first electrode to permit the passage of said atmospheric substance therebetween.

3. A detector for a finely divided atmospheric substance comprising a ceramic core of high alumina content material, a layer of material containing an alkali metal glass coated on said core, a first platinum electrode carried by said core outside said layer of material, heater means for heating said core, said layer of material and 6. said first platinum electrode, and a second electrode spaced apart from said first electrode to permit the passage of said atmospheric substance therebetween.

4. A detector for a finely divided atmospheric substance comprising a substantially cylindrical ceramic core of high alumina content material, a layer of material containing an alkali metal glass coated on said core, a substantially cylindrical first platinum electrode carried by said core outside said layer of material, heater means for heating said core, said layer of material and said first platinum electrode, and a second substantially cylindrical electrode spaced apart from said first electrode to permit the passage of said atmospheric substance there between.

5. A detector for a finely divided atmospheric substance comprising a hollow housing, a substantially cylindrical ceramic core of high alumina content material mounted within said housing, a layer of material containing an alkali metal glass coated on said core, a substantially cylindrical first platinum electrode carried by said core outside said layer of material, heater means for heating said core, said layer of material and said first platinum electrode, and a substantially cylindrical second electrode mounted with said housing and spaced apart from said first electrode, said housing being adapted to permitthe passage of said atmospheric substance between said first andsec'ond electrodes.

6. A detector for a finely divided atmospheric substance comprising a housinghaving a substantially cylindrical hollow interior, a substantially cylindrical ceramic core of high alumina content material mounted Within said housing, a layer of material containing an alkali metal glass coated on said core, a substantially cylindrical first platinum electrode carried by said core outside said layer of material, heater means mounted on said'core for heating said core, said layer of material and said first platinum electrode, and a substantially cylindrical second electrode mounted within said housing and spaced apartfrom said first electrode, said housing, core, and electrodes being substantially coaxial and said housing beingadapted to permit the passage of said atmospheric substance between said electrodes.

7. A detect-or for a finely divided atmospheric substance comprising a housing having a hollow interior, a core mounted within said housing, a first electrode mounted within said housing surrounding said core and spaced therefrom, heating means carried by said core for heating said first electrode to an ion emissive tempera ture, a material containing an alkali metal glass substantially filling the space between said core and said first electrode, and a second electrode mounted within said housing surrounding said first electrode and spaced therefrom, said housing being adapted to permit the passage of said atmospheric substance between said first and second electrodes.

8. A detector for a finely divided atmospheric substance comprising a housing having a hollow interior, a core mounted within said housing, a first electrode mounted within said housing surrounding said core and spaced therefrom, heating means carried by said core for heating said first electrode to an ion emissive temperature, a material containging b-eucryptite substantially filling the space between said core and said first electrode, and a second electrode mounted within said housing sur* rounding said first electrode and spaced therefrom, said housing being adapted to permit the passage .of said atmospheric substance between said first and second electrodes.

9. A detector for a finely divided atmospheric substance comprising a housing having a substantially cylindrical hollow interior, a pair of electrically conductive rods secured to said housing and extending axially therethrough, a substantially cylindrical ceramic core mounted on said rods within said housing, electrical heater means carried by said core and electrically connected between said rods, a layer of material containing an metal glass coated on said core and in contact with said heater means, a first substantially cylindrical electrode carried by said core in contact with said layer of material, and a second substantially cylindrical electrode mounted within said housing and spaced apart from said first electrode, said housing being adapted to permit the passage of said atmospheric substance between said first and second electrodes.

10. A detector for a finely divided atmospheric substance comprising a hollow ceramic housing having one end wall and being open at the other end with an external flange formed at said open end, a pair of electrically conductive rods extending through the hollow interior of said housing, said rods being retained at one end within openings in said end wall and secured to said flange attheir other end to provide electrical terminals, a ceramic core mounted on said rods within said housing, electrical heater means carried by said core and electrically connected between saio rods, a layer of material containing an alkali metal glass coated on said core and in contact with said heater means, a first substantially cylindrical electrode carried by said core in contact with said layer of material, means electrically connecting said first electrode to one of said rods and a second substantially cylindrical electrode mounted within said housing and spaced apart from said first electrode, said housing being adapted to permit the passage of said atmospheric substance between said first and second electrodes.

11. A detector for a finely divided atmospheric substance comprising a hollow ceramic housing having one end wall and being open at the other end with an external flange formed at said open end, a pair of electrically conductive rods extending through the hollow interior of said housing, said rods being retained at one end within openings in said end wall and secured to said flange at their other end to provide electrical terminals, :3. ceramic core mounted on said rods within said housing, electrical heater means carried by said core and electrically connected between said rods, a layer of material containing an alkali metal glass coated on said core and in contact with said heater means, a first substantially cylindrical electrode carried by said core in contact with said layer of material, means electrically connecting said first electrode to one of said rods, a second substantially cylindrical electrode mounted within said housing and spaced apart from said first electrode, said housing being adapted to permit the passage of said atmospheric substance between said first and second electrodes, and an electrical conductor electrically connected at one end to said second electrode near the end wall of said housing and 'secured'at its other end to said flange to provide an electrical terminal.

12. A detector for a finely divided atmospheric substance comprising a hollow ceramic housing having one end wall and being open at the other end with an external flange formed at said open end, a pair of electrically conductive rods extending axially through the hollow interior of said housing, said rods being retained at one end within openings in said end wall and having portions at their other end ben't outwardly and secured to said flange to provide electrical terminals, a ceramic core mounted on said rods Within said housing, electrical heater means carried by said core and electrically connected between said rods, a layer of material containing an alkali metal glass coated on said core and in Contact with said heater means, a first substantially cylindrical electrode carried by said core in contact with said layer of material, means-electrically connecting said first electrode to one of said rods, :1 second substantially cylindrical electrode Within said housing and spaced apart from said first electrode, a ceramic Washer within said housing resting on the bent portions of said rods and supporting said second electrode at spaced points, said housing being adapted to permit the passage of said atmospheric substance between said first and second electrodes, and an electrical conductor electrically connected at one end to said second electrode near the end wall of said housing and secured at its other end to said flange to provide an electrical terminal.

No references cited.