US2806991A - Electrical vapor detector - Google Patents

Description

Sept. 17, 1957 w. c. wHrrE ELECTRICAL VAPOR DETECTOR Filed DSG. 24, 1953 Inventor: william c. white, by )QJ J.

His Attorney.

United States Patent O ELECTRICAL vAPoR DETECTOR William C. White, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application December 24, 1953, Serial No. 400,339

6 Claims. (Cl. 324-33) My invention relates to improvements in electrical vapor detectors for the detection of certain substances or impurities in gases and is particularly adapted to detect the presen-ce of hydrogen, infiammable gases, reducing gases or vapors containing hydrogen. A vapor for purposes of this disclosure is considered to be any diffuse matter which may consist of solid particles, liquid particles, matter in gaseous state or a mixture of matter in the above states. A device in which my invention may be utilized is disclosed in U. S. Patent 2,550,498, issued April 24, 1951, to Chester W. Rice and assigned to the assignee of my present invention.

In the Rice patent, there are disclosed methods and apparatus for detecting substances such as certain gases, vapors, smokes, and similar matter in an atmosphere by directing a sample of an atmosphere suspected of containing a concentration of the substance to be detected into an electrical discharge device under conditions which permit the substance to induce positive ion formation at a more positively charged and heated electrode, the positive ions so formed being collected by a negatively charged electrode to produce an indicating current which increases with the concentration of the substances. Devices of this type have been found to be' qualitatively selective at atmospheric as well as at lower or greater pressures; that is, they respond only to certain types of substances such as the alkali metal compounds or other substances having ionization potentials less than the electron Work function ofthe electrodes, the halogens or compounds of either.

Modifications in the system disclosed by the Rice patent appear in U. S. Patent 2,652,532, issued September 15, 1953, to Paul D. Zemany and application Serial No. 305,883, led August 22, 1952, by Paul D Zemany, both of which are assigned to the assignee of my present invention.. Zemany discloses an apparatus which is particularly adapted to detect halogens at relatively low pressures.

Each of the above-mentionedy detectors depends upon enchancement of the ion emission from a heated element when vapors containing substances having ionization potentials less than the electron work function of the electrodes or halogens are present. These devices are not particularly sensitive to vapors containing hydrogen and it is therefore desirable that a device be available which is sensitive to this chemical element or its compounds.

A principal object of my invention is, therefore, to provide an element which issensitive to vapors containing hydrogen or a compound of hydrogen. kAnother object of my invention is to provide an element, the electrical conductivity of which is altered by the presence of hydrogen;

An object of my invention is to provide a vapor detectingv apparatus which is selectively sensitive to hydrogen and which will indicate a change in the concentration of hydrogen or hydrogen compounds in a sample of vapor.

A further vobject of my invention is to provide an element for the detection of hydrogen or inammable gas which is operated at such a low temperature that the vhazard of burning or explosionis minimized. It is also 2,806,991 Patented Sept. 1 7, i1957 an object of my invention to provide a new and improved method of producing an element which has variable conductance in the presence of hydrogen.

A still further object of my invention is to provide an element which is selectively sensitive to hydrogen and which may be easily adapted for use in conventional vapor detectors which are frequently used as devices for determining the location of leaks.

According to one important aspect of my invention, more particularly described hereinafter, there is provided apparatus for detecting the presence of a gas, particularly gases such as natural gas, reducing gases, water vapor, or vapors containing hydrogen, comprising an electrical conductive device, the conductivity of which is variable in the presence of hydrogen, and which includes a core of refractory material impregnated with a salt having the property of high electrolytic conductivity.

Other objects and features of my invention will be apparent from the following description taken in connectionV with the accompanying drawing in which Fig. 1 is a partly broken-away and partly sectionalized view of a form of vapor detector according to my invention and Fig. 2 is a partly broken-away and partly sectionalized view of an apparatus utilizing the vapor detector illustrated in Fig. l.

Referring now to Fig. 1 of the drawing there is shown one form of detector for detecting the presence of hydrogen according to the invention. The detector comprises a double helical Wire heater winding 3 wound on a cylindrical ceramic core 1 which has been impregnated with a solution of sodium hydroxide, and an electrode 2 inserted into tight fitting holes in core 1 which acts as the cold electrode aswell as the upperV support for the ceramic core 1. Arms 4 and metal tube 5 in conjunction with ceramic base 6 form a support structure for core 1, and tube 5 also acts as a guide for incoming samples of gas. Tube 5, which may be constructed of sheet metal such as nickel, is inserted into a hole in the ceramic base 6. Power supply 7 provides heater current for the winding 3 and is connected across binding posts 8 and 9. High voltage power supply 10is coupled in series with current sensitive indicator 11 and across binding posts 9 and 12. Binding post 12 is conductively connected to tube 5 by lead 13 and thereby to the cold electrode 2 through support arms 4. A series circuit is formed consisting of power supply 10, indicator 11, binding post 12, the cold electrode 2, a portion of core 1 and the heater wire 3.

A core of any suitable ceramic material may be used with satisfactory results provided it will act as a carrier of or is sufficiently porous so that it may be impregnated with a salt of high electrolytic conductivity. For the purposes of this disclosure a salt having a resistance in the range of l to l0 ohms per centimeter cube, at 18 C., when in an aqueous solution consisting of 20 grams of solute to grams of solution is considered to have a high electrolytic conductivity. The core must be impregnated with a highly conductive salt such as NaOH, NaF, or LiCl since the active hydrogen sensitive element is the impregnating salt. The optimum operating voltage and temperature varies with the -impregnating compound used.

Sodium hydroxide (NaOH) has proved particularly satisfactory for the detection of inflammable gases since it is highly conductive and sensitive at relatively low temperatures and operating voltages and, therefore, is used in the specific embodiment described in this application. I use a low fired alumina core of high purity and soak it by total immersion in a water solution of sodium hydroxide (NaOH) for about 24 hours. It is then removed from the NaOH solution and given a brief surface Wash in water. The core is then air dried for approximately 24 hours in an oven at about 15.0 5, The heater consists of a mil commercial platinum or platinum-iridium alloy wire wound as tightly as possible in the form of a double helix.

In order to obtain an element of optimum sensitivity it is desirable .to age it, one method being .as follows. Heater current, raised slowly to a maximum of 1%. arnperes, is applied for a period .of ten minutes so as to drive out any moisture in the core slowly and without injuring the core. The heat is increased gradually and during this period voltage from source is not applied. The heater current is then reduced to a value somewhere in the neighborhood of 0.9 ampere, for a 5 mil heater wire, and a D.C. voltage of approximately 3.00 volts is applied between the heater and outer cylinder .connection preferably such that the outer cylinder 5 and cold electrode 2 4are negative with respect vto the lament 3. Since there may be residual moisture in the core, the high voltage is applied to the core through a protective resistance of approximately 100,000 ohms. The unit is allowed to operate in this manner for a period .of approximately 24 hours after which it is ready for use .as .a vapor detector.

Referring to Figure 2 of the vdrawing there will be seen a complete apparatus for utilizing the vapor detector element of my invention. The components of Fig. 1, which ,appear in Fig. 2, are identified by the same reference numbers. Tube 14 is connected to the intake of a centrifugal blower 15 and tube 16 is connected to the discharge of the blower and to jacket 17.. Centrifugal blower 15 is driven by motor 18 which may receive energy through leads 19 from any suitable power source. Jacket 17, which may be made yof any suitable material either rigid or flexible, is supported by circular bracket 20 and forms an entrance hole into which tube 16 may be inserted. A flexible hose attachment may be coupled to tube 14 so that discrete areas such as welded :or sealed joints may be inspected for possible leaks.

In the operation of this particular embodiment of my invention the heater voltage source 7 is .adjusted for an output of approximately 10 volts and .85 ampere and the 'high voltage supply 10 is adjusted to a voltage somewhere between 275 and 325 volts preferably with the cold electrode 2 negative with respect to the heater 3. Although the interelectrode voltage must be -carefully adjusted and controlled, the heater voltage may be varied by selection of .a suitable heater wire. Satisfactory sensitive elements have been vbuilt with heater wire as small as 3.5 mils in .diameter and as large as l0 mils in diameter. The vapor to 'be :checkedfor the presence of hydrogen is intro- .duced through ,tube 14, blower 15, tube 16 and into the region between the core 1 Vand the cylinder 5 as'indicated by the arrows. lf .traces of hydrogen, natural gas, alcohol vapor or reducing gases are present the interelectrode Vresistance of the core 1Y drops very Aapprecia'bly and thus causes a relatively large change in the current iiowing in the high voltage series circuit which may be read on indicator 11. The change in current iiow may be utilized Vto give a visual indication such as on indicator 11, or to actuate yany other 'electrically controlled device. Forexample, an alarm system could be actuated when the -concentration of hydrogen or compounds containing hy- Vdrogen rises above a predetermined level.

The change in conductance of the impregnated ceramic core is believed to involve the phenomena of electrolysis and a form of polarization. Initially relatively heavy currents pass at low voltage but after a few hours of aging these currents decrease to a very low value, in the order of a few micro amperes. If the interelectrode `voltage is reversed, then relatively heavy currents again pass for short periods of time until the polarizing effect is again established. This same effect again occurs when the polarity is returned to its original condition. The conductivity of the element is temperature sensitive. `In the case of alumina impregnated with sodium hydroxide, the sensitivity to hydrogen is absent or very slight at room temperature and the conductivity .and sensitivity increase as the core temperature is raised. An optimum operating temperature has proved to be 300 C.-450 C., since at higher temperatures the interelectrode current becomes high and erratic.

It has been observed that if another cold connection is made to the end of the ceramic core close to electrode 2, this connection can be used interchangeably with the connection afforded by electrode 2. It makes no difference which `of these cold connections is used nor is the current in the high voltage circuit changed when the two are connected together. The resistance between these two cold connections is quite low and shows no appreciable change when exposed to gas. It would thus appear that the phenomena which gives the element its great sensitivity to hydrogen resides at the junction between the hot platinum heater wire and the hot ceramic immediately in contact with it and that the change in conductivity is a function of both the interelectrode voltage and temperature of the impregnated ceramic core as well as the presence of certain gases.

It has also been observed that if the core is heated by a separate means, such as a hot platinum wire, to approximately the same temperature, i. e. 300 C.-450 C., as that obtained with the heater, the response to certain gases is about the same. This indicates that the temperature of the ceramic core and its impregnating compound is the important factor rather than the temperature of the heater wire. It should be noted that my invention operates on the principle of change in conduction or conductivity between the two electrodes making contact with the impregnated core. The change of current is not due to ion emission since the heater Wire is operated at a very low temperature which is well below visible red heat.

T he element -of my invention has proved to be highly Selective to the presence of inflammable gases, reducing gases, and gases or vapors containing the element hydrogen. The element is also quite responsive to alcohol vapor even in such mixtures as would be obtained with perfumes and liquors. There is some response to nitrogen and ammonia but none to oxygen or carbon dioxide and further there is little or no response to halogen compound vapors. It should be noted, that my invention may take other forms than the specific embodiment of the figures -of the drawing. A gas `sensitive element according to my invention may take any physical form provided there is an yeffective hot and cold electrode sep- -arated by a portion of treated ceramic with an apparatus connected between the two electrodes for measuring change in conductivity in the presence of vapors.

While I have shown and described a particular embodiment of Amy invention, it will be obvious to those skilled in the art that Avarious changes and modifications can be made without departing from Vthe invention, and I, therefore aim in `the appended claims to cover all such `changes and modifications as fall within the true spirit and scope of the invention.

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

1,. A detector for selectively indicating the presence of hydrogen comprising a tubular .ceramic core treated with a sodium salt solution, a heater wound on said core, a power source coupled to the heater to heat it below visible red heat, .a conductive electrode connected to said core but separated from said heater by a portion of the core, a voltage source coupled between said conductive electrode and said heater to maintain the conductiveelectrode at a negative potential with respect to `said heater, the current -tiow between the filament and the conductive element being caused to vary by the presence .of hydrogen or compounds .containing hydrogen.

v2. A vapor detector Vcomprising .a tubular lceramic core which has been impregnated with an alkali metal salt solution, aY-heater tightly wound about said core, a conductive member .connected to one end of said core and conductively coupled toa 4metal cylinder surrounding said core, means vcoupled to said heater for heating it to a temperature below visible red heat, a source of high voltage energy coupled between said metal cylinder and said heater to maintain the cylinder and conductive member at a high negative potential relative to the heater, indicating means coupled between the heater and the conductive member and means for introducing vapor into the region between the core and metal cylinder, the core becoming highly conductive in the presence of hydrogen so that current flows between the filament and support member and the change in current flow is indicated on said indicating means.

3. A vapordetector comprising a ceramic core impregnated with a salt having high electrolyte conductivity, a first electrode contacting a first portion of said core, a .second electrode contacting a second portion of said core, means for passing an electric current through said first electrode to cause heating of the portion of said core contacted thereby, and means for applying a voltage between said first and said second electrodes to cause a current to flow therebetween, said current increasing substantially in the presence of a hydrogencontaining vapor.

4. A vapor detector comprising a ceramic core impregnated with a salt having high electrolyte conductivity, a first electrode contacting a first portion of said core, a second electrode comprising a filamentary winding about the outer surface of said core, means for passing an electric current through said lamentary winding to cause heating of the portion of said core contacted thereby, and means for applying a voltage between said first and said second electrodes to cause a current to flow therebetween, said current increasing substantially in the presence of a hydrogen-containing vapor.

5. A vapor detector comprising a ceramic core impregnated with a salt having high electrolytic conductivity, a first electrode contacting a first portion of said core, a second electrode comprising a filamentary platinum wire winding about the outer surface of said core, means for passing an electric current through said electrode te cause heating of the portion of said core contacted thereby, and a source of voltage connected between said first and said second electrodes to cause a current to flow therebetween, said current increasing substantially in the presence of a hydrogen-containing vapor.

6. A vapor detector comprising a ceramic core impregnated with sodium hydroxide, a first electrode contacting a first portion of said core, a second electrode comprising a filamentary winding wound about the exterior surface of said core, a first source of voltage connected to said filamentary winding to cause a heating current to ow therethrough to heat the contacted surface portion of said core, and a second source of voltage applied between said first electrode and said second electrode to cause an operating current to ow therebetween, said operating current increasing substantially in the presence of a hydrogen-containing vapor.

References Cited in the file of this patent UNITED STATES PATENTS 931,594 Gelstharp Aug. 17, 1909 1,421,720 Roberts July 4, 1922 2,064,651 Fiene Dec. 15, 1936 2,316,872 Kernen Apr. 20, 1943 2,550,498 Rice Apr. 24, 1951 2,595,943 Heinemann May 16, 1952 2,647,234 Pear July 28, 1953 2,714,094 McNally July 26, 1955 FOREIGN PATENTS 683,511 Great Britain Nov. 26, 1952 685,744 Great Britain Jan. 7, 1953

Claims (1)

1. 2. A VAPOR DETECTOR COMPRISING A TUBULAR CERAMIC CORE WHICH HAS BEEN IMPREGNATED WITH AN ALKALI METAL SALT SOLUTION, A HEATER TIGHTLY WOUND ABOUT SAID CORE, A CONDUCTIVE MEMBER CONNECTED TO ONE END OF SAID CORE AND CONDUCTIVELY COUPLED TO A METAL CYLINDER SURROUNDING SAID CORE, MEANS COUPLED TO SAID HEATER FOR HEATING IT TO A TEMPERATURE BELOW VISIBLE RED HEAT, A SOURCE OF HIGH VOLTAGE ENERGY COUPLED BETWEEN SAID METAL CYLINDER AND SAID HEATER TO MAINTAIN THE CYLINDER AND CONDUCTIVE MEMBER AT HIGH NEGATIVE POTENTIAL RELATIVE TO THE HEATER, INDICATING MEANS COUPLED BETWEEN THE HEATER AND THE CONDUCTIVE MEMBER AND MEANS FOR INTRODUCING VAPOR INTO THE REGION BETWEEN THE CORE AND METAL CYLINDER, THE CORE BECOMING HIGHLY CONDUCTIVE IN THE PRESENCE OF HYDROGEN SO THAT CURRENT FLOWS BETWEEN THE FILAMENT AND SUPPORT MEMBER AND THE CHANGE IN CURRENT FLOW IS INDICATED ON SAID INDICATING MEANS.

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