US3506831A - Apparatus for reconditioning tubes operating with townsend avalanche - Google Patents

Description

Apnl 14, 1970 L. F.. GILBERT 3,506,331

I APPARATU$"FOR ,RECONDITIONING TUBES OPERATING WITH TOWNSEND AVALANCHE Filed Dec. 29. 1966 I a l I I I I l l I l I I 1 I I I I I I I I I I I I I 1 1- I I I 1 .J

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I I I I l I I I l I I I INVENTOR. LYMAN E GILBERT United States Patent M U.S. Cl. 25083.6 Claims ABSTRACT OF THE DISCLOSURE A system and process for restoring to its original work function .an electron tube that operates with a townsend avalanche and which has had its work function altered because of electrode contamination. In the system and method the electrodes are heated for a short time by passing a current through the tube to raise the electrode temperature to a value such that the contaminates will be evaporated from the electrode surface and will condense on the envelope of the tube.

SUMMARY OF THE INVENTION Electron tubes that operate with a townsend avalanche are designed originally with a particular work function such that they respond to a particular energy spectrum. Illustrative of such a tube is the glow discharge tube that may be employed in flame scanner systems with the tube having an energy sensitive spectrum range of 2000* to 3300 angstroms such that it responds only to energy emitted by the flame and not to energy emitted by glowing refractory, slag or the like. During the operation of such tubes it has been found that contaminates gradually deposit on the electrodes (one or both of them) so that the characteristics of the tube change with the tube eventually being no longer suitable for its intended operation. These contaminates may be impurities from the glass or quartz envelope of the tube such as sodium and silica compounds or they may be impurities from the tungsten or other electrodes themselves such as copper or tungsten oxide impurities which may come from the inner portion of the electrode to the surface thereof. These contaminates will change the work function of the tube so that the energy spectrum to which the tube responds will be changed. Thus in the case of the glow discharge tube of the flame scanner the tube will begin to respond to energy sources other than the flame and be less sensitive to the flame energy source.

It has been found that this ditficulty can be overcome and the tube reconditioned so that its Work function is returned generally to the original value by heating the electrodes of the tube to drive the contaminates from the electrode surface and deposit them on the tube envelope.

This is preferably accomplished with the tube in situ i.e., Within the circuit in which it is intended to operate, for exarrfple in the flame scanner circuit. The heating is accomplished by passing a current through the tube that is considerably greater than that ordinarily passed through it with this current being maintained only for a very short time such as one to ten seconds. This heats the electrodes without substantially heating the tube envelope and so that the temperature of the electrodes is elevated to a high value sufficient to evaporate the contaminates from the electrode surface. The tube envelope is maintained at a sufliciently low value so that these contaminates will condense on the surface of the envelope. The circuit in which the tube is connected is preferably provided with an automatic control device to periodically effect this heating and accordingly decontamination of the electrodes 3,506,831 Patented Apr. 14, 1970 ICC such that the tube is maintained generally at its original work function.

DESCRIPTION OF THE DRAWING The single drawing figure is in the nature of a circuit diagram showing one environment in which the present invention may be utilized.

DETAILED DESCRIPTION Referring to the drawing, there is illustrated therein a circuit for a flame scanner which is generally similar to that of my U.S. Patent 3,286,093 of Nov. 15, 1966, with the exception in the illustrative arrangement the tube that operates with townsend avalanche has pulsating DC rather than AC applied to it and has a current limiting resistor connected across the tube electrodes. (Reference to said patent is made for details of the theory of operation.) In this circuit the tube 10 operating with townsend avalanche is a glow discharge tube having spaced electrodes 36 and 37 and receiving its electric potential from the source 12 by means of the transformer 14. This transformer 14 forms part of the electric circuit into which the tube is connected and this circuit includes the diode 16 connected to one electrode of the tube through the fuse 18 with the other electrode of the tube being connected with the other terminal of the secondary of transformer 14. This connection includes, resistor 20, capacitor 22 and resistor 24. The step-up transformer 14 is thus effective to apply a DC supply to the tube 10 and which, for example, may have a value of between 300 and 750 volts.

The glow discharge tube 10 preferably has a work function such that it responds to an energy spectrum range of between 2000 and 3300 angstroms. In operation the tube is located such that it directly views the flame that is to be under surveillance. Energy from the flame thus passes through the quartz envelope of the tube so that a photon of proper energy content and proper wave length, will trigger a townsend avalanche of the tube. The portion of the circuit designated generally 26 may be termed a transmitter and is effective to produce a pulse signal upon avalanche occurring in the tube. This signal is separated from the potential source supplied to the tube with resistor 24 and capacitor 28 comprising the signal separator network. The capacitor 22 and resistor 20 are for the purpose of increasing the time duration of the short pulse produced at avalanche.

The pulse signals thus produced and separated by the transmitter are coupled into the receiver circuit 34, which is identical to that of previously identified Patent 3,286,- 093, to produce an output signal as a result of the device detecting the presence of a flame.

During the operation of the tube 10 it has been found that one or both of the electrodes progressively become contaminated by impurities depositing on the surfaces thereof (one when DC is applied to the tube and both when AC is applied to the tube). These contaminates may come from the surface of the envelope of the tube, for example, they may be sodium or silica compounds found in the quartz or glass, and they may also be impurities found in the tungsten which work to the surface of the electrode. In the illustrative example the tube may be provided with a pair of spaced tungsten electrodes in an atmosphere of hydrogen. In the the manufacture of the tube particular efforts is may to eliminate impurities. However, notwithstanding such efforts all impurities cannot be removed and those that remain gradually deposit on the electrodes during operation of the tube. As these deposits increase, the work function of the tube progressively changes such that it no longer responds to or is no longer restricted to the original spectral response and therefore the tube becomes ineffective for its intended purpose.

In order to prevent this from happening there is provided, in the illustrative circuit, means for periodically heating the electrodes of the tube in a manner which will remove these contaminates. This is achieved by connecting electrode 36 to the power supply through resistance 40 and switch 42. Thus in closing the switch 42 tube is connected across the secondary of the transformer with only fuse 18 and resistance 40 being in series with the tube. The purpose of resistance 40 is to sufficiently limit the current so as to not blow the fuse 18. The resistance 40 may thus be eliminated and in its stead a bypass provided around the fuse 18 with this bypass being completed simultaneously with the closing of the switch 42.

The power supply for the tube 10 is a partially filtered DC by means of the capacitor 33. However, even if the power supply were an unfiltered DC (pulsating 60 c.p.s.), tube 10, once triggered, would continue to conduct with switch 42 closed when the voltage dropped to zero because the heavy current prevents the recovery of the tube. The current flow through the tube determines the degree of ionization of the gases therein and, therefore, markedly affects the recovery time of the tube. Thus once the tube is triggered into heavy conduction it will continue conducting while switch 42 is closed. Therefore the time of conduction can be controlled by switch 42.

Switch 42 is a time operated switch such that it closes periodically for a short time depending upon the characteristics of the tube and the circuit into which it is connected as well as upon the operating conditions to which the tube is subjected. The depositing of contaminates upon the electrodes of the tubes is influenced by the temperature at which the tube operates with the contamination being accelerated at higher temperatures. The time requirement for removal of the contaminates is dependent upon the temperature to which the electrodes are heated, with the higher the temperature the faster will be the removal. Illustrative of the time required is between 1 and 10 seconds and illustrative of the temperature to Which the electrodes may be heated is between 1400 and 4500 F., with tungsten electrodes. The temperature rise of the electrodes must be sufliciently rapid so that the electrodes are heated to their desired temperature where the contaminates will evaporate from the electrodes while the envelope of the tube is not substantially increased in temperature but remains at a temperature below the condensation temperature of the vaporized contaminates. Thus, the contaminates vaporize from the electrodes and condense upon the inner surface of the envelope whereby the electrodes are returned to their pure state that they originally had and the work function of the tube is that originally provided. It will be understood that in this process of removing contaminate from the electrodes that they must not be elevated to such a high temperature as to damage the electrodes. Thus in the case of tungsten the temperature should not be above 4500 F.

It is desirable and extremely convenient to provide an automatic means within the circuit that utilizes the avalanche-operating tube to periodically raise the temperature of the electrodes as previously described to remove contaminates from them. In the illustrative flame scanner circuit in one environment, it has been found that if switch 42 is closed for approximately 7 seconds each hour during the period that the tube 10 is viewing a flame, the work function of the tube remains generally at its original and desired value and thus the electrodes of the tube are maintained generally free of contaminates.

The improved process of the invention can, if desired, be carried out on the bench and need not be carried out in the circuit in which the tube is employed. The tube may be connected into a separate and special circuit for reconditioning and an artificial source of excitation used to provide the energy for triggering the tube into avalanche, such as an ultra-violet lamp in the case of the 2000-3000 angstrom tube previously mentioned. In this separate circuit a sufficient voltage may be applied to the tube for a suflicient length of time such that the current passing through the tube will heat the electrodes as previously described and thus evaporate from the electrodes surface the contaminates deposited thereon.

The voltage developed across the secondary 14 of the transformer may be approximately 700 volts and as previously mentioned switch 42 may close for approximately 7 seconds once each hour, the current then passing through tube 10 at the closing of switch 42 is between 400 and 600 ma. and the electrodes of the tube are heated to a dull red during this time.

Accordingly, it will be appreciated that a novel system and process has been devised whereby tubes, such as Geiger-Mueller tubes and glow discharge tubes, operating with a townsend avalanche, have contaminates removed from the tube electrodes so as to maintain the tube at or return the tube to its desired work function.

It is to be understood that the invention is not limited to the specific embodiment herein illustrated and described but may be used in other ways without departure from its spirit and that various changes can be made which would come within the scope of the invention which is limited only by the appended claims.

I claim: 4

1. In an electrical circuit utilizing a glow discharge tube subjected to energy to periodically fire said tube and combination of means to periodically connect the tube across a potential and for a period of time such that upon firing the tube a current will pass through the electrodes thereof of a magnitude to heat the same to a sufficiently high value to evaporate the contaminates from the surface thereof with the environment in which the tube is operating being such that the envelope is maintained at a temperature below the condensation temperature of these contaminates whereby the contaminates are condensed upon the envelope surface.

2. A flame detector including an electric circuit having a tube responding to energy of a flame to cause a townsend avalanche to occur in the tube thereby producing a pulse signal, means effectively responsive to this pulse signal to produce an output signal, said circuit including a source of potential across which the tube is connected with there being means in the circuit to limit the current flow therethrough, and means effective to connect said tube across said potential in such a manner that upon firing of the tube a current flow much greater than normal is obtained through the tube electrodes so as to heat the same to a sufficiently high value to evaporate contaminates from the surface thereof.

3. The organization of claim 2 including a time operated switch means in bypass relation with at least some of said current limiting means in said circuit.

4. A flame scanner circuit comprising in combination a glow discharge tube adapted to be subjected to energy from a flame for triggering the tube into avalanche, an electric circuit into which said tube is connected and including a source of potential, means providing an output signal developed incident to said tube being subjected to energy of a flame, and means operative to automatically periodically heat said electrodes to a sufficiently high temperature to evaporate contaminates therefrom comprising a time operated switch means for connecting said tube across said potential.

5. A flame detector comprising in combination an electric circuit including a source of potential and a tube operating with townsend avalanche and being fired upon receiving energy from a flame, with the avalanche of the tube producing a pulse signal in the circuit, said circuit including an impedance across which a voltage drop is developed by the current established at avalanche and means to periodically effect an increase in current through said tubes to heat the electrodes thereof to a sufliciently high value to evaporate contaminates therefrom, this last 5 6 named means including a time operated switch means 3,103,589 9/1963 Howling 313-93 X connected in parallel with said impedance. 3,286,093 11/1966 Gilbert 250-83.3 X

References Cited JAMES W. LAWRENCE, Primary Examiner UNITED STATES PATENTS 5 C. R. CAMPBELL, Assistant Examiner 1,906,653 5/1933 Spencer 316-26 2,746,831 8/1952 Chapman 31626 US 3,038,993 6/1962 Masuda 313-45 X 25083.3; 3162, 26, 28

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