US3775762A

US3775762A - Gas multiplication ultraviolet detector system for fire detection

Info

Publication number: US3775762A
Application number: US00289713A
Authority: US
Inventors: T Trumble
Original assignee: US Air Force
Current assignee: US Air Force
Priority date: 1972-09-15
Filing date: 1972-09-15
Publication date: 1973-11-27
Anticipated expiration: 1990-11-27

Abstract

A fire detection system using an ultraviolet gas multiplication detector, an amplitude discriminator to differentiate between a fire and cosmic radiation, and a time base discriminator to differentiate between a fire and background noise, provides a sensitive, highly reliable, fire detecting system.

Description

United States Patent [191 340/258 R, 258 B, 258 D, 237 S; 250/372, 374, 388; 313/93, 101; 328/150; 307/100; 321/16 Trumble Nov. 27, 1973 [54] GAS MULTIPLICATION ULTRAVIOLET 3,513,311 5/1970 McAlister et a1. 340/228 R X DETECTOR SYSTEM FOR FIRE 3,634,747 1/1972 Loftus 250/372 X 3,493,304 2/1970 Rouner 250/372 X DETECTION 3,621,262 11/1971 Lecuyer 340/228 R X [75] Inventor; Terry M, Trumble, Dayton, Ohio 3,702,936 11/1972 Erickson 250/388 X K 3,544,792 12/1970 Giltaire 340/228 8 X [73] Ass1gnee: The United States of America as represented by the Secretary of the United States Force, Primary Examiner-Thomas B. Habecker Washington, DC Assistant Examiner-William M. Wannisky [22] Filed: Sept 15 1972 Att0mey1-1arry A. Herbert, Jr. et a1. [21] Appl. No.: 289,713

ABSTRACT [52] US. Cl. 340/228 R, 250/372, 250/388 [5 Cl. A fire detection system using an ultraviolet gas mu|ti Fleld of Search R, plication detector an amplitude discriminator to ferent'iate between a fire and cosmic radiation, and a time base discriminator to differentiate between a fire and background noise, provides a sensitive, highly reliable, fire detecting system.

6 Claims, 6 Drawing Figures 1 GAS MULTIPLICATION ULTRAVIOLET DETECTOR SYSTEM FOR FIRE DETECTION BACKGROUND OF THE INVENTION tube is the type number 42743 Edison tube. In a typical prior art circuit using an ultraviolet gas multiplication tube for fire detection (such as has been used at launch complexes, and in aircraft), the two electrodes of the tube are generally used interchangeably as a cathode or anode by impressing an AC sine wave voltage across them. A current limiting resistor and a relay are series connected with the tube and an AC voltage. When the voltage across the electrodes of the tube is high enough to cause an emitted electron to propogate to the anode, nominally 450 volts then when about 10 photons strike the cathode (for a tungsten cathode), the tube will turn on and current will flow in the series circuit. The tube will then conduct until the AC sine wave swings below approximately 425 volts at which point it turns off. Since in each positive or negative swing of the AC wave, each electrode becomes a cathode,-there will be two times per cycle that the tube will conduct. The relay actuates an alarm circuit indicating that the tube has fired and therefore the possibility of a fire. The foregoing type tube is insensitive to sunlight at sea level and can be used for fire detection out-of-doors. The conventional tube may be used directly for relay or other loads up to approximately 5 watts. Direct current has also previously been used to energize the tube to improve the effective sensitivity of the tube. In which case one electrode is always the cathode and the other always the anode. In this mode of operation, and since the cathode is always positioned electrically above triggering voltage, photon detect time is increased approximately 60%. Optical systems are conventionally used to increase the energy quanta striking the cathode.

Typical examples of UV (ultravoilet) tubes and associated electronic circuits to be found in the prior art are exemplified by the following patents: U.S. Pat. No. 3,207,903, patentees A.T. Abromaitis et al.; U.S. Pat. No. 3,372,279, patentees R. O. Engh et al.; and U.S. Pat. No. 3,425,225, patentee R. O. Engh.

SUMMARY OF THE INVENTION The invention is a fire detecting system that is relatively insensitive to solar radiation and relatively insensitive to normal background radiation such as from a hot engine and nacelle. It will detect the photons coming from a fire (flame) and reliably discriminate between these photons and photons from other sources.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a block diagram of an embodiment of the invention;

FIG. 2 is a more detailed block-schematic diagram of an embodiment of the invention;

FIG. 3a is a representative plot of the voltage across the UV gas multiplication tube during a typical cycle of operation;

FIG. 3b is a representative plot of the current through the UV tube during a typical cycle of operation;

FIG. 4 is a representative plot of the voltage at the discriminator sensing point for typical operation with a small fire present; and

FIG. 5 is a representative plot of the voltage at the discriminatory sensing point for typical operation with a large fire present.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a simplified block diagram of the invention. The preferred ultravoilet gas multiplication tube 11 is a hydrogen-helium, tungsten photcathode tube such as the type42743, or 42262. Tubes with molybdenum cathodes are generally not as desirable due to irregularities in operation caused by contamination. A minimum of approximately 335 volts DC is required for ionization to occur. Voltages up to approximately 1,100 volts may generally be used before self-counting occurs. Self-counting is due. to UV radiation being given off during conduction. The UV radiation then triggers photoelectrons from the photocathode, which in turn causes more ionization and more UV radiation.

As previously implied, DC is preferred to AC for energizing the UV gas multiplication tube. AC excitation does provide for the ionization to go to completion and the tube to be turned off (automatically) between half cycles, but since each electrode, alternates as the cathode, focusing of UV radiation on one small spot is not feasible unless a low duty cycle efficiency is accepted. Also with AC sensitizing the photocathode work function changes as the surface is intermittently cleaned and contaminated, and the sweeping of the ions out of the interstitial space is slower as the voltage swings up and down as in a sine wave, and thus the maximum count rate in the system is lower and sensitivity is lower.

With typical preferred UV gas multiplication tubes the maximum voltage generally considered safely below the self-counting voltage is approximately 900 volts. This is well above the ionization voltage. It is preferred that the DC supply 12 (FIGS. 1 and 2) energizing the UV tube be relatively free of any ripple voltage due to the ripple inducing. instabilities in the sensitizing threshold of the tube. Series resistance as provided by resistors l3, l4, and 15 must be used to limit the amount of current flowing through the UV tube 11 so that the current flowing through a localized area of the photocathode will be limited so as not to induce localized thermal changes in the cathode surface work function, yet the current flow must be great enough to allow replenishment of locally depleted electrons and provide enough energy for the creation of thermally active electrons to cause the initiation of ionization. The actual amount to which the current is limited is-not critical. The range is quite wide. Two milliamperes has been found to be generally a suitable value. Excessively larger amounts of current (i.e., 3 to 4 ma) begin to appreciably degrade the photcathode work function. Operation has been had with as low a current as 6 microamperes, however this 6 microamperes is generally considered too low for practical devices, as well as not providing enough energy for consistent ionization formation. A current value positioned within the range from approximately 250 microamperes to 2 milliamperes during ionization is suitable with the 2 milliampere value being generally preferred.

With AC excitation of the UV tube turn off is automatic, with DC sensitizing excitation turn off must be provided by the shunting circuit 16 extinguishing the tube at a determined time after ionization occurs, as provided by the delay circuit 17. It has been found that the tube will completely ionize (and start regulating) approximately 2 milliseconds after strong UV radiation exposure and that once the tube is strongly ionized it takes approximately 2 milliseconds of voltage removal from the tube to assure that the tube will be, and remain, extinguished (with no UV radiation present) upon the reapplying of the high voltage potential.

In the circuit shown in FIG. 1, high voltage substantially constant DC potential source 12 supplies the energizing potential for the UV gas multiplication detector tube 11 so that it will ionize and conduct current when ultraviolet radiation strikes it. Resistor is the principal current limiting resistor, limiting the current, in conjunction with

resistors

13 and 14, to approximately 2 milliamperes during ionization. When the UV tube 1 l ionizes and current commences to flow, the potential on line 18 rises initiating the time delay circuit 17. Approximately 2 milliseconds time delay after current has commenced to flow has been found to be an optimum (but not critical) time to permit the tube to fully ionize. This 2 millisecond period is represented by the time interval 31 in FIGS. 3a and 312. At the end of the 2 millisecond delay the shunting circuit 16 is triggered which removed the voltage from the UV tube, i.e., it shunts to ground the voltage across the UV tube (and resistors 13 and 14). As previously stated a shunting duration of approximately 2 milliseconds is preferred. During this shunting interval substantially the full output of the power source 12 appears across resistor 15. This two millisecond shunting period is represented by the time interval 32 in the representative plot of the voltage across the UV tube shown in FIG. 3a. The voltage on line 18 is at ground potential until the tube fires, at which time it commences to rise. It rises until the 2 milliamperes of current flow is reached or some lower value if the UV tube extinguishes before saturation occurs. The voltage on line 18 at some determined point, usually a few volts such as 6 to 8 volts, initiates the time delay 17. The voltage is not critical, it is dependent upon the triggering voltage requirement of the delay circuit. The same voltage on line 19 feeding the amplitude discriminator 20 may be used to trigger the delay circuit or even a lower voltage may be used. (In such cases either a single potentiometer would be used, or

potentiometers

13 and 14 would be interchanged.) The 2 millisecond delay must start within the time period after ionization has started and before saturation of the tube reached. The preferred time being shortly after ionization commences, as shown graphically in FIG. 3a. The amount of current flowing through the UV tube is a function of whether ionization is caused by UV photons to which the photocathode is sensitive or by cosmic radiation which can directly ionize the gas in the tube. Generally, full ionization of the tube will occur whenever any UV photons (from a fire) strike the cathode. Weak ionization will occur from cosmic particles passing through the gas of the tube except when such particles happen to pass extremely close to the photocathode. In such instances apparently UV induced ionization occurs and full ionization takes place. Small fires produce strong (maximum) ionization pulses separated considerably in time (relatively). Large fires produce the maximum number of pulses possible (within the limitations of the equipment), all of which are also complete ionization maximum current pulses. As previously indicated, occasionally large pulses of solar radiation, such as a cosmic particle, occur that by themselves, i.e., one pulse, cannot be distinguished from the UV radiation that occurs from a fire. It has been found that typically over a long period of time the probability of such a pulse occuring at sea level is one pulse per five minutes of time interval. It is to be understood that such pulses are of a random nature, time wise, and some variations do occur with variations in altitude, longitude, latitude, and the presence of solar flares. An empirical study has shown that in one week typically only three of these cosmic particle pulses will occur in any one millisecond period that will produce the full amplitude pulse such as a UV fire generated conduction pulse.

By using an amplitude discriminator 20 to sense the magnitude of the electrical pulsed passed by the UV tube 11 the small random non-fire pulses are eliminated, and by using a time discriminator 21 to effectively accommodate the discrimination of the large pulses that randomly occur, naturally, a highly reliable fire detection system is provided. For optimum reliable operation it has been found that when three pulses exceeding a predetermined value occur in 40 milliseconds, a fire is in all reasonable probability present, and if three pulses of this magnitude do not occur in the given 40 millisecond period after the first pulse, that counting interval (40 milliseconds) is discarded. Then a new 40 millisecond period is commenced with the next pulse that occurs that is over the predetermined magnitude, and counted as the first count in the new 40 millisecond period. The selection of the 40 millisecond time interval over which to count pulses represents an approximate safety factor of 2, yet it is not too short a time period as to make it difficult to acquire an adequate number of UV photons to strike the photocathode. Thus a very small flame can be detected by merely adding one count during the 40 milliseconds while the other two counts are background counts. In addition after a large fire starts, where saturation pulses from the UV tube occur every 4 millisecond time interval, the delay in indicating the fire in the worst condition is approximately 13 milliseconds; one pulse in a prior 40 millisecond period that contained only one previous pulse and three successive pulses in the present period. This is sufficiently fast for automatic extinguishing equipment to operate before the fire has materially progressed, when automatic equipment is connected in place of, or in addition to, the visual alarm indicator 22. An output from the time discriminator 21 on line 23 occurs when the third pulse in any 40 millisecond period occurs. This fires the silicon controlled rectifier 24 lighting fire indicator alarm 22 which in this embodiment will stay illuminated until the momentary push button reset switch 25 is actuated. As previously indicated, in place of or in addition to the visual alarm 22, conventional fire extinguishing apparatus may be activated by the current flowing through the SCR after it is triggered. FIG. 3b shows a typical plot 35 of the current flowing through the UV tube 11 and the resistors 13, and 14 (but not 15) for fast complete ionization to current limiting of two milliamperes where the voltage across the UV tube is as shown 33 in the plot of FIG. 3a. It is to be noted that if a large fire is present that as soon as the high voltage potential is reapplied to the UV tube that it will essentially immediately ionize again and the action will be repeated as shown by the dotted line 34, with another corresponding pulse of current 36. With a small fire a few milliseconds of delay will occur between pulses.

Current limiting as shown in FIG. 3b does not occur for weaker excitations of the UV tube as previously stated. It has been determined, for the embodiment being described, that current pulses having amplitudes of less than approximate 1.6 milliamperes are not generally representative of a fire, and that in all reasonable probability less than three pulses exceeding this value of 1.6 milliamperes in any one 40 millisecond time period are not indicative of a fire.

FIG. 2 is a more detailed block-schematic diagram of an embodiment of the invention. The high voltage DC constant potential supply 12 provides the approximately 900 volt DC potential for the gas multiplication UV tube 11. In typical embodiments of the invention for aircraft usage for detecting fires in engine nacelles, the aircraft primary power sources are 115 volt, 400 hertz, and 28 volt direct current. Thus, the primary power source 50 may be the aircrafts 115 volt, 400 hertz voltage. It is to be understood that the high voltage supply 12 may be any conventional, essentially constant, voltage source that will supply 900 volts DC of 2 milliamperes. Batteries may be used for many applications of the invention In this embodiment the shunting switching transistor 51 must have at least a 900 volt emitter-collector voltage rating, emittercollector current capabilities of at least approximately four milliamperes, relatively low emitter-collector resistance in the on" state, relatively high emittercollector resistance in the off state, and switching times of approximately 1 microsecond. The 2 millisecond time delay may be provided by any suitable conventional delay circuit, as may the two millisecond pulse circuit 53 be any suitable conventional timed pulse circuit. Depending on the characteristics of the particular components utilized in practicing the invention, conventional amplification may be used between the pulse circuit 53 and the switchingtransistor 51. A typical example of a solid state integrated circuit assembly that may be used for the time delay 52 and the pulse circuit 53 is a .I or N dual-in-line or retriggerable, monostable multivibrator with clear such as type SN 74123. One section of the integrated circuit (such as the A" section) with appropriate timing capacitance and resistance is used for the delay circuit and the other section (such as the B section) with appropriate resistance and capacitance timing elements may be used for the 2 millisecond control pulse to the switching transistor. (R= k ohms, C 0.25 MF are examples.) Such circuits and equivalent circuits both integrated and separate components are well known in the art of those practicing this invention.

The amplitude discriminator 20 provides amplitude discrimination by allowing only pulses that exceed in current the approximate 1.6 milliampere value previously mentioned to be counted by the time discriminator. This is accomplished in the embodiment shown in FIG. 2 by zener diode 53 and the associated potentiometer l3 and the transistor 54 and its circuitry. In a typical embodiment a 4 volt zener diode 53 has proven satisfactory. The arm of the potentiometer 13 is adjusted so that when the 1.6 milliamperes flow through the UV tube a potential of four volts is present between the potentiometer arm and ground. When the limited value of current (two milliamperes in this embodiment) flows the potential at the arm of potentiometer 13 is approximately 5 volts. Thus, only potentials over the 4 volts will pass any potential through the zener diode and the emitter follower transistor to provide a pulse to be counted. Obviously other values of potentials with corresponding associated values of zener breakdown voltages may be used to sense current flows in the range enumerated.

FIG. 4 shows a train of typical voltage pulses appearing at the arm of potentiometer l3, in the particular embodiment being described in detail, when a small fire is present. ,Only the pulses exceeding the 4 volt value 60 are counted. Thus, voltage pulses 61, 62, and 63.are not passed by the zener diode and are ignored. Pulse 64 does exceed the 4 volt value, in fact it is a current limited pulse at the 5 volt level, thus a pulse (in this case a l-volt pulse) is passed to the time discriminator and a 40 millisecond counting period is commenced approximately at the time indicated by line 70. Now this pulse and any magnitude of pulse over the 4-volt value in this 40 millisecond period will be counted as additional counts by the time discriminator.

Pulses

65 and 66 also exceed the 4-volt level and are thus counted. The presence of the third pulse 66 in the 40 millisecond time base interval is indicative of a tire and the fire alarm is initiated approximately at time represented by line 67.

When a large flash fire occurs as shown in FIG. 5 the two following pulses will occur immediately after the first, i.e., as soon as the voltage shunt extinguishing the UV tube is removed it immediately refires, and the alarm is indicated at time 68 approximately 9 milliseconds after the onset of the flame. The 40 millisecond counting period commenced approximately at the time indicated by line 71.

The time discriminator 21 comprises a 40 millisecond time base 55 and a count 3 counter 56. At the third count an output is present on line 57 which triggers the SCR 24 into conduction initiating alarm 22 which provides a continuous indication until the reset button 25 is pushed breaking the circuit to the SCR. If the fire is out the indicator will stay off, if it is not the indicator will substantially immediately (within a maximum of approximately 9 milliseconds) re-present the indication. The circuit 55 providing the 40 millisecond time base may be, for example, a convention one shot multivibrator, such as one section of the conventional integrated circuit module SN 74l23, with appropriate resistance-capacitance values (such as 40K ohms and 2mF), or any other conventional time interval circuit. (In connection with the use of the integrated circuits mentioned herein as examples, reference is made to sections 6 and 9 of The Integrated Circuits Catalog for Design Engineers, First Edition, published by Texas Instruments, Inc.)

It has been found preferrable to use a down counter 56 to count the three pulses which will provide a continuous output after the third pulse. A three-count ring counter or three-count shift register may be used; they are not generally as desirable due to an output being provided only for approximately the two millisecond period (until the fourth pulse comes along) when a large fire is present. Of course, this may be changed to provide a continuous output after the third count (in the time base) by utilizing an inhibit circuit comprising an AND gate at the input to the counter with the output of the counter inverted to provide the second input to the AND gate. Thus, when an output is present from the counter, signals on line 58 will be inhibited by the AND gate from entering the counter. An example of a solid state integrated circuit suitable for a down counter with clear (reset) is the type SN 74193 with associated well known conventional circuitry.

To further explain the operation of the time discriminator with reference to FIG. 2, the potentiometer 13 is set in correlation with the zener diode 53 to provide the previously mentioned threshold voltage so that substantially only pulses resulting from ultraviolet radiation will exceed the threshold value and provide a pulse on line 59. When'a pulse appears on line 59 transistor 54 turns on and the pulse from the emitter of transistor 54 turns on the one-shot multivbrator 55 whose output is a 40 millisecond wide pulse. Simultaneously the counter receives the pulse and counts l (down from three). If a second pulse arrives on line 58 in 40 milliseconds the counter counts 2, (down from three, i.e., down to l") if the second (or third) pulse is not received in the 40 millisecond period the one shot multivibrator returns to its normal state at the end of the 40 millisecond period and resets the counter (to three if a down counter is used, to zero if an up counter is used). If a third pulse arrives within the 40 milliseconds the counter 56 provides an output on line 57 triggering the SCR allowing current to flow from the conventional 28 volt DC power source through the alarm indicator 22 providing the alarm. Once the SCR is turned on (triggered) the alarm remains activated until the circuit is broken by manually operating the momentary pushbutton switch 25.

It is to be understood that conventional amplification may be utilized in the system wherever necessary to meet the particular requirements of the circuits being used to provide the operations indicated. Such structures and the utilization thereof are well known to those skilled in the art.

The tubes, time intervals, and pulse requirements as set forth in the specifically detailed embodiment are considered those that will be generally preferred. For other type ultraviolet detector tubes having different characteristics, or for apparatus designed for a particular environment, such as high altitudes, those skilled in the art will readily adapt the principles of the circuits disclosed herein to meet the different requirements. For example, with some other ultraviolet tubes it has been found desirable that the time base of the time discriminator be extended to encompass a larger time interval than the forty milliseconds and that the number of counts for fire indication occurring in that period be increased to a greater number than three, in order to provide a high degree of certainty that a fire will be correctly indicated over the increased number of background counts occurring in the apparatus.

I claim:

l. A fire detecting system comprising:

a. a gas multiplication ultraviolet tube for receiving ultraviolet radiation;

b. means for supplying a high voltage direct current potential to the said tube to provide ionization of the tube when the tube receives the said radiation;

c. means for extinguishing the ionization of the said tube after a predetermined interval of time from the onset of ionization;

d. means for sensing the current flow through the said tube and providing a voltage potential responsive to the magnitude of the said current;

e. an amplitude discriminator, cooperating with the said voltage potential provided by the current sensing means, for providing an output each time the said voltage potential exceeds a predetermined threshold value;

f. a time discriminator cooperating with the output of the said amplitude discriminator for providing an output signal when more than two signals are received from the said amplitude discriminator in a predetermined time period; and

g. means responsive to the output of the said time discriminator for indicating the presence of a fire.

2. The apparatus as claimed in claim 1 wherein the said predetermined time period of the time discriminator is approximately forty milliseconds.

3. The apparatus as claimed in claim 2 wherein the said predetermined time interval for extinguishing the ionization of the ultraviolet tube after the onset of ionization is approximately 2 milliseconds.

4. A fire detecting system comprising:

a. a gas multiplication ultraviolet tube for receiving ultraviolet radiation;

b. means for providing a direct current potential to the said tube to provide ionization of the tube when it receives ultraviolet radiation;

c. means for providing approximately a 2-millisecond time interval;

d. means for initiating the said 2-millisecond time interval approximately at the onset of the said ionization of the ultraviolet tube;

e. means cooperating with the said 2-millisecond time interval and the said direct current potential for substantially removing the said direct current potential from the ultraviolet tube at the end of the said 2-millisecond time interval;

f. means for sensing the current flow through the said ultraviolet tube and providing an output when the said current exceeds a predetermined value;

g. means for providing approximately a 40- millisecond time interval;

h. means cooperating with the output of the said means for sensing current, for initiating the said 40- millisecond time interval when the said current sensing means provides an output;

i. a counter responsive to the said output of the means for sensing the current through the ultraviolet tube cooperating with the said 40-millisecond time interval for providng an output upon the occurrence of a third count in the said 40-millisecond time interval; and

j. means responsive to the output of the said counter for indicating a tire.

5. The apparatus as claimed in claim 4 wherein the said predetermined time interval that the direct current potential is removed from the ultraviolet tube is approximately 2 milliseconds.

6. The apparatus as claimed in claim 5 wherein the said gas multiplication ultraviolet tube is a hydrogenhelium, tungsten photocathode type tube and the said means for sensing current through the tube provides an output when the said current exceeds approximately 1.6 milliamperes.

In =0: 1: k

Claims

1. A fire detecting system comprising: a. a gas multiplication ultraviolet tube for receiving ultraviolet radiation; b. means for supplying a high voltage direct current potential to the said tube to provide ionization of the tube when the tube receives the said radiation; c. means for extinguishing the ionization of the said tube after a predetermined interval of time from the onset of ionization; d. means for sensing the current flow through the said tube and providing a voltage potential responsive to the magnitude of the said current; e. an amplitude discriminator, cooperating with the said voltage potential provided by the current sensing means, for providing an output each time the said voltage potential exceeds a predetermined threshold value; f. a time discriminator cooperating with the output of the said amplitude discriminator for providing an output signal when more than two signals are received from the said amplitude discriminator in a predetermined time period; and g. means responsive to the output of the said time discriminator for indicating the presence of a fire.

4. A fire detecting system comprising: a. a gas multiplication ultraviolet tube for receiving ultraviolet radiation; b. means for providing a direct current potential to the said tube to provide ionization of the tube when it receives ultraviolet radiation; c. means for providing approximately a 2-millisecond time interval; d. means for initiating the said 2-millisecond time interval approximately at the onset of the said ionization of the ultraviolet tube; e. means cooperating with the said 2-millisecond time interval and the said direct current potential for substantially removing the said direct current potential from the ultraViolet tube at the end of the said 2-millisecond time interval; f. means for sensing the current flow through the said ultraviolet tube and providing an output when the said current exceeds a predetermined value; g. means for providing approximately a 40-millisecond time interval; h. means cooperating with the output of the said means for sensing current, for initiating the said 40-millisecond time interval when the said current sensing means provides an output; i. a counter responsive to the said output of the means for sensing the current through the ultraviolet tube cooperating with the said 40-millisecond time interval for providng an output upon the occurrence of a third count in the said 40-millisecond time interval; and j. means responsive to the output of the said counter for indicating a fire.

6. The apparatus as claimed in claim 5 wherein the said gas multiplication ultraviolet tube is a hydrogen-helium, tungsten photocathode type tube and the said means for sensing current through the tube provides an output when the said current exceeds approximately 1.6 milliamperes.