US3028490A - Apparatus responsive to the composition of a gaseous medium - Google Patents

Description

R. GUILLEUX April 3, 1962 APPARATUS RESPONSIVE TO THE COMPOSITION OF A GASEOUS MEDIUM Filed Feb. 10, 1958 United States Patent APPARATUS RESPONSIVE TO THE COMPOSITION OF A GASEOUS MEDIUM Robert Guilleux, 'Chatillon sous Bagneux, France, assignor to Societe a Responsabilite Limitee la Detection Electronique Francaise, Paris, France, a French society Filed Feb. 10, 1958, Ser. No. 714,341 Claims priority, application France Oct. 24, 1957 8 Claims. (Cl. 250-435) This invention relates to apparatus responsive to the composition of a gas, of the type wherein variations in said composition induce corresponding variations in the ionizing current flowing through an ionizing chamber exposed to the gas being tested. Apparatus of this type have applications in various fields, e.g. as fire detectors, control of furnace combustion, automatic control of chemical processes, and the like. A general object of this invention is to provide improved apparatus of this type which will be simple, accurate and reliable and will have a more rapid response than obtainable heretofore;

In a known form of apparatus of the type specified, there is provided a first ionization chamber through which the gas is caused to flow and containing a sourc of radiations such as a small body of radio-active material in the path of said gas and an ion-collecting electrode or anode positioned beyond said source so that the ions generated in the gas flow by the radiation source are collected by said electrode. The rate of ionization of the gas and the rate of re-combination of the ions therein being functions of the gas composition, the potential of said electrode is' itself a function of the composition of the gas stream. A secondary ionization chamber defining a sealed enclosure containing a reference gas therein, has an electrode thereof connected in series with the output electrode of the primary chamber, so as to provide on a junction common to both said electrodes a reference potential which may be made to represent the departure of the condition of the gas being tested from a reference condition. This potential can be utilized in a suitable controller transducer circuit to provide an indication of the tested gas composition or to operate any desired controlled means, such as warning apparatus and equipment adapted to restore the gas to the prescribed condition. This potential may also be similarly utilized to provide an indication of both the tested gas composition and to operate any desired controlled means.

In many cases the primary ionization chamber is required to be installed at a location remote from the space in which the gaseous atmosphere is to be controlled, and is connected with said space by way of a conduit, and a fan is provided for impelling the gas stream from the space to the chamber. Difiiculties have been encountered in the past in installations of this sort because, if the rate of flow of the gas through the conduit is slow, there is a delay or leg in the response of the apparatus corresponding to the time required for the gas to pass from the space to the chamber and such delayed response can have serious consequences as in cases where the system is used, for example, as a fire-alarm or for automatic process control. If on'the other hand the rate of flow of the gas through the conduit is increased it is foundthat the ions are irregularly entrained with the gas flow and are not uniformly collected by the collector electrode in the primary ionization chamber so that the ionization current becomes substantially dependent on the flow velocity of the gas instead of being a measure of the composition of the gas. The adjustment of the apparatus becomes a delicate matter and the operation of the system is unreliable. Y

It is an object of the invention to eliminate this difficulty and to provide apparatus of the specified type which will reliably deliver ancutput which is at all times a substan- 3,028,490 Patented Apr. 3, 1962 tially accurate measure of the condition being tested, while at the same time having a comparatively rapid response even in cases where the apparatus is installed at a location remote from the space to be supervised.

The invention provides an electrode in the form of a screen grid which is carried to a negative voltage potential relative to the ion collector electrode and is disposed beyond the latter in the direction of flow. There is provided in this way an electrostatic barrier which acts to repel the negative ions towards the collector electrode thereby ensuring that all said ions are collected regardless of the rate of gas flow, so that uniform indications are produced.

In known apparatus of the kind described, the ionization chamber usually comprises a cathode in the form of a conductive cylinder coated with radio-active material,

, e.g. a salt of radium, and an anode in the form of a wire conductor disposed axially of the cathode cylinder. It is found that ionization chambers so constructed are apt to respond to stray or parasitic radiation fields from external sources, and it is therefore another object of this invention to remove this additional cause of unreliability. According to an aspect of the invention, this object is achieved by the provision of an ionization chamber comprising a conductive outer casing maintained at a fixed reference potential, e.g. ground, and a coaxial inner cylinder of insulating material serving as a support forthe electrodes of the chamber.

In one desirable embodiment the invention provides apparatus of the type specified comprising a first ionization chamber including a conductive outer cylinder maintained at ground potential, an insulating inner cylinder coaxial therewith, a cathode, a radiant source and an anode all supported in the chamber from said insulating cylinder, a screen grid supported from said insulating cylinder in a downstream position relative to said anode and maintained at an adjustable potential negative with respect to the anode potential, means for delivering gas to be tested through said chamber, a scaled reference ionization chamber having a cathode electrically connected to a common junction with theanode of the first chamber, so as to provide at said junction a potential depending on the composition of said gas, and control circuit means connected with said junction and responsive to said potential for operating output apparatus in accordance therewith.

In accordance with a further feature of the invention, the control circuit means preferably includes a multielectrode electron tube comprising a cathode, a first grid positively biassed with respect to the cathode and a second or control grid beyond the first grid biassed negatively so as to maintain the tube beyond cut-off, the potential from the above mentioned junction being applied to said value.

The above and other objects, features and advantages of the invention will appear from the ensuing description of an exemplary embodiment thereof illustrated in the accompanying drawings wherein:

FIG. 1 is a diagrammatic view, partly in simplified axial cross section and partly in schematic form, of apparatus according to the invention; and

FIG. 2 shows fragmentarily a modified form of the ionization chamber according to the invention.

Referring to FIG. 1, an improved ionization chamber 1 comprises an outer tubular member 4 of electrically conductive material through which a stream of gas to be tested, i.e. air from an enclosure that is to be supervised for fire hazard, is blown by means of an electrically operated fan 3. It will be understood that the fan 3 may be disposed at a point remote from the ionization chamber 1 at the far end of a conduit connecting said chamber with the premises being supervised. The ends of the tubular member 4 have protective screens 5 and 6 secured across them for preventing the ingress of dirt and other foreign objects into the ionization chamber. Internally lining the tubular shielding member 4 is a tubular member 7 made of insulating material and serving as a support for the electrodes in the chamber.

The said electrodes include a cathode 8, which may be provided in the form of a grid, an ion collector electrode or anode 9 which is supported from member 7 by means of an insulator support 10, and a screen grid electrode 11. Supported adjacent to the cathode 8 on the side thereof toward the anode 9 is a source of radiations, such as a strip 12 coated with radium salt or other source of predominantly alpha particles. To control the radiation emitted by the source 12 there is provided a rod 13 made of suitable metallic material for arrestin the radiations, and adjustable in position in front of the emissive surface of the source 12, operable from outside the chamber by means of a knurled head and screw threads thereon. In order to prevent the formation of a condensed film of moisture over the emissive surface which would interfere With the emission, a heating filament 14 is provided near said surface and parallel thereto and is connected with a source of voltage so as to be raised to a temperature slightly above ambient.

The screen grid electrode 11 is connected with a negative potential relatively to the anode 9. The function of electrode 11 is to repel back towards the collector anode 9 any negative ions that may be carried with the gas flow past said anode, and to collect any positive ions formed in the gas flow due to bombardment by the alpha particles. In this way the degree of ionization and the ionizing current are maintained substantially constant between electrodes 8 and 9 for high and variable values of the gas flow velocity. Optimum operating conditions are obtained by a proper adjustment of the voltage difference between electrodes 9 and 11, as well as by appropriate shaping, dimensioning and spacing said electrodes. To facilitate this latter adjustment the electrode 11 is mounted in a slidable sleeve 15 the axial position of which within the insulating support member 7 can readily be adjusted by means of a knob 16 formed at the Outer end of a stem projecting through a helical guiding slot 17 formed in said member. With the above adjusting facilities it is found possible to obtain a desired ionizing current which will remain stable over a comparatively wide range of variations of the gas flow velocities, for any given composition of the gas.

According to a modification of the same inventive concept, the repelling or stabilizing electrode 11 instead of being in the form of a grid as shown, may be provided in the form of an emissive filament or cathode operated to create a space charge in the area beyond (to the right of) the collector anode 9 with similar beneficial effects to those indicated above for the case where electrode 11 is a grid.

A reference ionization chamber 2 is provided in the form of a sealed enclosure containing a body of gas having predetermined characteristics, and including a cathode 20, a radio-active source adjacent thereto, and an ion collector anode 21. The cathode 20 is connected to the anode 9 of the primary or detecting ionizing chamber 1 to provided a common junction 19 constituting the voltage take-off or output terminal of the detector system. A source 18 of D.-C. voltage is shown as providing the necessary power for polarizing the respective electrodes of the system. As shown, the cathode 8 and screening electrode or grid 11 of the primary chamber are both connected to the negative terminal of source 18 by way of a resistor. The anode 21 of the reference chamber is connected to the positive terminal by way of a resistor. Thermistor devices or resistors having a high negative thermal coeificient of resistance are shown at 22 and 23 as connected between the primary chamber cathode 8 and the positive pole of the source 18 for compensation of effects of temperature variations on the circuit charac teristics. In operation, the adjustments are so made that the reference ionization chamber 2 is operated substantially at saturation. The voltage variations at the junction 19 are accurately representative of the variations in composition of the gas flowing through the primary chamber 1, i.e. are representative of the departure of the condition of the gas in chamber 1, from the reference condition of the gas present in chamber 2. The use of the reference chamber 2 in conjunction with the primary detector chamber 1 provides a comparison effect which substantially eliminates the influence of disturbing factors such as variations in temperature and pressure, since such disturbing factors can be made to affect the gases in both chambers equally.

The voltage appearing at junction 19 is applied to an output or control circuit which is so designed as to prevent any back-action from said circuit upon the voltage at junction 19 liable to disturb the proper operation of the detector system. For this purpose there is preferably used a multi-electrode tube 26 including a cathode 27, a first grid 28 and a second or control grid 29. The tube 26 may be a conventional multi-electrode tube including further electrodes, such as a plate, in addition to those mentioned, in which case such additional electrode or electrodes would not be used.

The cathode 27 is connected to the negative terminal of an adjustable D.-C. source 30, which negative terminal is desirably grounded and simultaneously provides the reference voltage for the shielding member 4 of the primary ionizing chamber 1. A connection from said negative terminal of source 30 to the adjustable tap of a potentiometer resistance connected across the source 18, as shown, permits of adjusting the various afore-mentioned polarizing potentials relatively to said reference voltage.

The first or output grid 28 of tube 26 is connected to one end of the winding of a milliammeter instrument 31 the other end of which is connected with the positive terminal of source 30. The second grid 29, which constitutes a control electrode of the tube 26, is connected to the junction 19.

The bias potentials for tube 26 are so adjusted that the control grid 29 is at all times biassed to a voltage negative relative to cathode 27, so that the tube 26 is at all times cut off, that is to say, there is no electron current in the tube 26 to the control grid 29 but only to the output grid 28. However, the variations of the negative voltage on control grid 29 as derived from the junction 19 generate within tube 26 a varying inhibitor field so that the current applied from grid 28 to the milliammeter winding constitutes an amplified replica of the voltage variations at point 19.

The source 30 is indicated as being adjustable, e.g. by means of a voltage divider or the like schematically indicated by the arrow across the source, and the voltage is so adjusted that in the normal or reference condition of the gas being tested the pointer of the milliammeter 31 indicates the central zero of the scale. Contacts 32 and 32 are provided at the opposite ends of the scale with which the pointer is adapted to make contact When it is displaced beyond either end of a pre set range. Assuming for example the case where the system is used as a fire detector, in which case the gas flowing through the ionization chamber 1 is atmospheric air from the space being supervised, then should the air-flow be contaminated with smoke or combustion gases, the ions emitted by the source 12 are retarded and tend to recombine more rapidly so that the internal resistance of the chamber -1 increases. The potential at point 19 rises and the current through milliammeter 31 also rises. On the pointer engaging contact 32, a connection is established from a positive voltage source with which said pointer is connected to the base 34 of a power-transistor 35, thereby arresting the flow of current from the emitter 33 to the base, and hence also arresting the current in the collector electrode 36 of said transistor. A relay winding 37 connected in the collector circuit is thereby deenergized and the relay contacts 3839 move to their normally closed position; this completes an energizing circuit for an alarm siren 40 and a warning signal lamp 41. As shown, a similar circuit including a power-transistor, relay and alarm devices is connected to a contact 32 provided at the opposite end of the milliammeter scale, symmetrically with the circuit just described; the components thereof are designated by the same references primed.

It will be noted that should defective operation occur in any part of the system described, the milliammeter pointer would be displaced to its rightward or leftward end position thereby operating the warning signal means and attracting attention to the defect.

To disable the system a push-button switch 42 is used depression of which connects the pointer of the milliammeter to a negative polarity without requiring operation of a switch to cut off the energizing circuit for said pointer.

FIG. 2 illustrates a modified construction of the primary ionizing chamber, which can be substituted for the chamber 1 in FIG. 1. The modified chamber shown in FIG. 2 is of symmetrical construction and includes, to either side of the ion collector anode 9 two pairs of radiation sources 24a, 24b on one side and 25a, 25b on the other side each emitting alpha-particles in opposite directions. Such a chamber can be exposed to gas flow in either direction indiscriminately. Assuming for example the gas is flowing at a certain velocity in the direction indicated by the arrows f then any of the ions formed between source 2412 and the anode 9 and entrained without being captured by the anode are compensated for, at least in part, by those ions that are formed from the source 24a and carried towards the anode 9. Since the rate of recombination of the ions depends on the velocity of gas flow, it will be seen that should the flow velocity increase thereby increasing the amount of ions produced by source 24b that are lost by entrainment with the gas, at the same time the number of ions produced by the particles emitted by source 240 and collected by the anode 9 also increases so that satisfactory compensation obtains. Such an ion1z ing chamber is especially well-suited for use in circumstances where the gas flow is apt to occur at variable velocities and is even liable to reverse in direction, as is the case in large drafty halls such as warehouses, hangars and the like.

The detecting and supervising system of the invention s susceptible of receiving a wide number of different applications especially owing to the number of different adjustments of which it is capable. It can readily be constructed in the form of a compact portable unit. Alternatively it may be installed at a central supervising station and a rotary distributor may be provided whereby the atmosphere in a plurality of different locations may be cyclically supervised by means of a common detector unit. In such case, a common electrically operated fan may be provided for delivering the gas to be controlled from a plurality of conduits into the ionizing chamber.

If preferred, a plurality of similar detecting units may be provided in the respective spaces to be supervised, all connected electrically to a central station including the control, indicating and alarm circuitry.

The system provides a means of supervising the variations in composition of a gaseous mixture, and such variations may be recorded, e.g. as concentrations of a predetermined constituent in cases where only a single constituent of the mixture is variable. This may be the case in installations for supervising the combustion gases from a furnace or the like, exhaust gases of combustion engines or from jet engines, e.g. during bench tests, and similar applications.

Another possible application of the system is for the supervision and control of the degree of atmospheric pollution in underground highways and highway tunnels, buildings containing various kinds of pressure gas equipment, mines (including firedamp control), and the like.

The system can also serve for controlling a reducing gas atmosphere against the possible ingress of atmospheric air (or other gases) in various apparatus such as treating furnaces, A.-C. generators operated in an atmosphere of hydrogen, and so on.

Although a single embodiment of the invention and a modification thereof have been particularly described and illustrated, it will be evident that various components thereof can be added, omitted, substituted and modified without exceeding the scope of the invention.

What I claim is:

1. In apparatus continuously responsive to the variations in composition of a gaseous fluid, a tubular outer member of electrically conductive material, a tubular inner member of insulating material defining an ionization chamber, means for delivering a stream of gaseous fluid through said inner member, a cathode electrode, a radioactive source adjacent the cathode, an ion-collecting anode electrode spaced from the cathode and a screen electrode positioned downstream from said ion-collecting anode spaced from the anode, means supporting said electrodes from said inner member Within said chamber, means applying a fixed reference potential to said outer member, electric supply means applying operating potentials to said electrodes, the potential applied to the screen electrode being negative relative to said anode, and circuit means connected to said anode and responsive to the variations in ions collected thereby from said stream gheireby to respond to the composition of said gaseous 2. In apparatus continuously responsive to the composition of a gaseous fluid, a tubular outer member of conductive material, a tubular inner member of insulatmg material, spaced cathode, anode and screen electrodes supported in and from said inner member, means applymg a fixed reference potential to said outer member, electric supply means applying operating potentials to said electrodes to maintain the screen electrode negative relative to the anode, means for delivering a stream of said fluid through the inner member to be ionized by said source, a sealed reference chamber containing a body a of reference gaseous fluid therein, a cathode and anode and a radioactive source within said sealed chamber, means applying an operative potential to said last mentioned anode, a junction connected both to said first mentioned anode and to said second mentioned cathode, and circuit means connected to said junction and responsive to the potential thereat as determined by the variations in composition of said stream of gaseous fluid relative to the composition of said gaseous fluid in the reference chamber.

3. In apparatus as claimed in claim 1, means for adjusting the spacing from said anode to said screen electrode.

4. In apparatus as claimed in claim 1, an electric heater element adjacent said source for preventing moisture condensation on an emissive surface of said source.

5. Apparatus as claimed in claim 2, wherein said circuit means comprises a multielectrode tube having a.

cathode, a first grid and a second grid, means biassing the first grid positively and biassing the second grid negatively relative to the cathode so as to maintain said tube in a cut-off condition, means connecting said second grid 'to said junction'and means connected to said first grid to respond to the electrical output therefrom.

6. In apparatus as claimed in claim 2, a multielectrode tube in said circuit means including a cathode, a first grid and a second grid, means biassing the first grid posieluding a winding connected to said first grid, a displaceable member operable by current in said winding and limit stop means operable by said pointer on predetermined displacement thereof from a null position, a transistor having a base, a collector and an emitter, means applying an operative bias to said base; relay means connected to said collector, a load operated by said relay means, and means connecting said limit contact means with said base bias means for modifying said bias to operate said load.

7. In apparatus as claimed in claim 6, means for adjusting the range of said predetermined displacement of said pointer from null position to modify said bias and operate said load, including means whereby said pointer is displaced beyond said range in case of defective operation of said apparatus.

8. In apparatus continuously responsive to variations in composition of a gaseous fluid, a tubular outer member 'of electrically conductive material, a tubular inner memher of insulating material defining an ionization chamber open at both ends for free circulation therethrough, an ion-collecting anode electrode within said tubular inner member, two pairs of radioactive sources one pair on each side or said anode and arranged to emit ionizing radiation in opposite directions, means for comparing the rate of ion collection by said anode, said means being electrically connected to said anode and to said outer member, and circuit means connected to said anode and responsive to the variations in ions collected thereby from the gas inside of said ionization chamber whereby to respond to the composition of the gas.

References Cited in the file of this patent UNITED STATES PATENTS 2,594,777 Hicks Apr. 29, 1952 2,641,710 Pompeo et al June 9, 1953 2,702,898 Meili Feb. 22, 1955 2,761,976 Obermaier et al. Sept. 4, 1956 2,817,768 Shamos Dec. 24, 1957 2,883,555 London Apr, 21, 1959 FOREIGN PATENTS 389,722 Great Britain Sept. 21, 1933 OTHER REFERENCES Serial No. 376,930, Peycelon et al. (A.P.C.), published May 25, 1943.

Images