US3851321A - Conveyor fire alarm system - Google Patents

Abstract

D R A W I N G
1. A FIRE DETECTION AND ALARM DEVICE COMPRISING: 1. A FIRE DESTRUCTIBLE CONDUIT MEANS FOR DISPOSITION IN A SPACE WHERE FIRE IS TO BE DETECTED, 2. FLUID PRESSURE MEANS OPERABLY CONNECTED TO A FIRST END OF THE SAID CONDUIT MEANS FOR SUPPLYING PRESSURIZED FLUID TO SAID CONDUIT MEANS, 3. FLUID RELEASE MEANS OPERABLY CONNECTED TO THE OPPOSITE END OF THE SAID CONDUIT MEANS AND RESPONSIVE TO THE FLUID PRESSURE THEREIN FOR RELEASING FLUID FROM THE SAID CONDUIT MEANS WHEN THE FLUID PRESSURE EXCEEDS A PRESET VALUE, 4. LOW PRESSURE DETECTION MEANS OPERABLY CONNECTED TO THE SAID CONDUIT MEANS FOR DETECTING AND SIGNALLING FLUID PRESSURES IN SAID CONDUIT MEANS BELOW A PRESET VALUE, 5. A FIRST LOW PRESSURE ALARM MEANS OPERABLY CONNECTED AND RESPONSIVE TO SAID LOW PRESSURE DETECTION MEANS FOR GIVING AN ALARM WHEN THE FLUID PRESSURE IN SAID CONDUIT MEANS FALLS BELOW THE SAID PRESET LOW VALUE, 6. FLUID PRESSURE CONTROL MEANS OPERABLY CONNECTED TO SAID FLUID PRESSURE MEANS FOR INTERMITTENTLY CAUSING A FLOWOF PRESSURIZED FLUID TO SAID CONDUIT MEANS, THE OPERATION OF THE SAID FLUID PRESSURE CONTROL MEANS BEING INDEPENDENT OF THE PRESSURE IN THE SAID CONDUIT MEANS, 7. HIGH PRESSURE DETECTION MEANS OPERABLY CONNECTED TO THE SAID CONDUIT MEANS FOR DETECTING AND SIGNALLING FLUID PRESSURES IN SAID CONDUIT MEANS ABOVE A PRESET HIGH VALUE, AND 8. A HIGH PRESSURE ALARM MEANS OPERABLY CONNECTED AND RESPONSIVE TO SAID HIGH PRESSURE DETECTION MEANS FOR GIVING AN ALARM WHEN THE FLUID PRESSURE IN AID CONDUIT RISES ABOVE THE SAID PRESET HIGH VALUE.

Description

[54] couvizvoe rim: ALARM SYSTEM [76] Inventor: Thomas C. Stitt, Proctorville, Ohio [22] Filed: Sept. 25, 1972 [21] Appl. No.: 291,546

[52] US. Cl .l 340/229, 73/357, 116/103,

137/557 [51] lint. Cl. G081) 17/04 [58] Field of Search 340/229, 227, 240, 242, 340/243; 73/357; 137/554, 557; 116/65, 103

[56] References Cited UNITED STATES PATENTS 3,380,430 4/1968 Hogel 137/557 X 2,972,132 2/1961 Putney 340/229 905,378 12/1908 Smith 340/229 969,414 9/1910 Shepherd 340/229 1,219,723 3/1917 Gracey et a1. 340/229 2,493,351 l/1950 Jones 340/229 3,014,206 12/1961 Slavin 340/229 3,119,368 l/l964 Barnard 340/229 UN X [451 Nov,26,;1 974 Primary Examiner-John W. Caldwell Assistant Examiner-Scott F. Partridge Attorney, Agent, or Firm-John W. Malley et a1.

[57] ABSTRACT A fire destructible flexible hose is disposed along the length of a conveyor, or like machine, to be protected and is pneumatically pressurized. A pressure relief valve maintains a preset pneumatic pressure in the hose, and pneumatic pressure is applied to the hose in a timed cycle, independent of the pressure within the hose. Pressure-sensitive switches in communication with the pneumatic fluid in the hose signal either a low pressure or a high pressure in the hose and actuate alarm devices. Low pressure may be caused by a fire along the length of the conveyor burning through the destructible hose or severance of the hose or loss of electric power. An obstruction on the hose will cause high pressure in the hose and signal an alarm. A battery powered auxiliary alarm system is also provided as a back-up for the primary system in the event of a malfunction thereof.

17 Claims, 3 Drawing Figures coal we a can: vdf P/r'e'ss 012E 9 Ma 7'0 12 REL/AU V/PL V6 I CONVEYOR F IRE ALARM SYSTEM INTRODUCTION AND BACKGROUND The present invention relates to a fire alarm system and to a method of detecting and signalling fires. The invention is particularly concerned with detection and signalling of fires occurring in an underground mine, especially along parts of the mine which are infrequently attended, such as in shafts having belt conveyors and the like.

Fire in mine shafts is a particularly dangerous occurrence and the early detection and extinguishment of a fire is of utmost importance. However, in certain parts of mines, fire may go undetected for substantial periods of time due to the infrequency of mine personnel in those parts. For example, current mining operations might take place in a part of a mine shaft which is distant from the entrance thereof. Typically, a belt conveyor will move the mined material from the current mining operation, through the mine shaft and deposit the material at the entrance of the mine shaft, from which the material is subsequently removed. The mine shaft through which the material is moved and, accordingly, the length of the conveyor belt, can easily be several thousand feet and presents a considerable area in a mine where personnel infrequently visit. A fire along the conveyor belt in this situation could go undetected for a substantial period of time and produces most serious results before being discovered.

In view of this particularly dangerous situation, the art has sought to provide fire detection and alarm devices for conveyor belts of the foregoing these While there devices operate with varying degrees of efficiency and reliability, any malfunction of the device could have serious repercussions and, accordingly, it is important that such devices provide sufficient fail-safe features that malfunctions of the device will not allow fire to go undetected. The art, however, has not heretofore provided fire detection and alarm devices of this nature which have sufficient fail-safe features that the devices can operate relatively unattended. Since constant attention to the devices is not always practical, it would be, of course, desirable to provide a fire detection and alarm device with such fail-safe features that constant attention to the device is not required.

OBJECTS OF THE INVENTION It is therefore an object of the invention to provide a fire detectionand alarm devices which are so constructed that a malfunction of the device will be signalled by the device itself. It is a further object to provide such devices wherein only infrequent attention thereto is required and with which relatively little maintenance is necessary. It is a further object to provide a method of detecting and providing an alarm for fire, in the above-noted environments. Other objects will be apparent from the following disclosure and claims BRIEF SUMMARY OF THE INVENTION Briefly stated, the invention provides a device having a destructible flexible hose which is disposed along the length of a conveyor, or like machine, to be protected and which is pneumatically pressurized by a mechanical pneumatic compressor. At the end of the hose opposite the compressor, a pressure relief valve maintains a preset pneumatic pressure in the hose. The compressor applies pneumatic pressure to the hose in a timed cycle and independent of the pressure within the hose. The pressure relief valve functions to maintain the preset pressure by release of the pneumatic fluid beyond that required to maintain the preset pressure. Pressuresensitive switches in communication with the pneumatic fluid in the hose signal either an abnormally low pressure or an abnormally high pressure in the hose and actuate alarm devices, e.g., indicating lights and stop operation of the conveyor.

Abnormally low pressure can be caused by a tire along the length of the conveyor burning through the destructible hose and allowing the pressure therein to drop. Similarly, an inadvertent severance of the hose will activate the low-pressure switch, which functions as a fail-safe measure for the detection and alarm device. Similarly, a loss of electrical power to the compressor will actuate the low-pressure switch and additionally function as a fail-safe device.

Since any obstruction in or on the hose between the compressor and the relief valve will render the hose useless for detecting fire between the obstruction and the relief valve, a high-pressure switch will activate to signal an alarm, e.g. an indicating light, in the event an obstruction blocks off the pressure relief valve and al lows pressure to build in the hose. Here again, a failsafe feature is provided.

A battery powered auxiliary alarm system is also provided as a backup for the primary system in the event of a malfunction thereof. This also serves as a fail-safe device.

The various functions, as described above, are integrated and performed by an alarm circuit which can control operation of the compressor motor, the conveyor motor and alarm and signalling devices, as well as internal testing for malfunction of the device.

DETAILED DESCRIPTION OF THE INVENTION The invention can best be understood by reference to the drawings which illustrate a preferred embodiment of the invention where:

FIG. 1 is a diagrammatic illustration of the arrangements of the components of the present fire detection and alarm system;

FIG. 2 is a diagram of the electrical and mechanical connections operating to form the alarm and malfunction circuit of FIG. 1; and

FIG. 3 is a detailed diagram of circuit B of the alarm and malfunction circuit of FIG. 2.

Turning now to FIG. I in detail, a predetermined pneumatic pressure is maintained in destructible, flexible hose 1 by means of pressure supplied by compressor 2 driven by compressor motor 3. So long as a predetermined pneumatic pressure range is maintained in hose 1, pressure switches 4 and 5 will remain closed and the normal function of the fire alarm system will be maintained. A visual indication of the operating pressure within the hose is provided by pressure gauge 6. However, if the pressure in the hose drops below a preset minimum pressure, for example, when a tire on the conveyor melts through the hose, then low pressure switch 4 opens and the alarm system is actuated, which gives an alarm signal and may disrupt power both to compressor motor 3 and conveyor motor 7. Of course, the same result would take place if the hose is inadvertently broken, and as such, indicates a malfunction requiring immediate repair.

Any obstruction in or on the hose, such as a piece of coal falling from conveyor 9 onto the hose, can block pneumatic communication in the hose. Thus, a total obstruction midway of the hose length would prevent the hose from signalling a fire between the obstruction and pressure relief valve 8. In the case of such malfunction, pneumatic pressure applied by compressor 2 will continue to rise in hose 1 until the maximum preset pneumatic pressure is reached and high-pressure switch is actuated. Pressure increases since the obstruction blocks the escape of pneumatic fluid from pressure relief valve 8. The compressor applies pneumatic pressure to the hose on a timed cycle and independent of pressure in the hose, which provides an independent check of the proper function of the detection and alarm device. A bleed orifice 10 may also be provided to allow a slow escape of pneumatic fluid.

Thus, if additional fluid pressure is not provided to the hose for any reason (e.g. break down of the compressor) which could not, therefore, signal an obstruction on the hose, the bleed will, in time, reduce the pressure in the hose to below the minimum and the low pressure switch will activate and signal an alarm. A check valve 12 may be used with bleed 10 to provide a more predictable bleed time, or bleed l0 and check valve 12 may be eliminated and the natural bleed back through the compression may be used to effect the foregoing described low-pressure alarm.

Alarm and malfunction circuit 11 integrates the responses which may be obtained from the various components of the system and actuates signalling and alarm devices as well as stopping power to compressor motor 3 and conveyor motor 7. Additionally, the circuit has an integrated self-checking system for periodically testing for system malfunctions.

The alarm and malfunction circuit is shown in FIG. 2 and comprises a control circuit, with subcircuits A, B and C, and a battery circuit. One function of the control circuit is to control operation of the conveyor motor circuit.

In FIG. 2, power is supplied to the control circuit by way of fuses and switch 21. Note particularly that switch 21 also energizes the battery circuit. It will also be seen that the control circuit, battery circuit and conveyor motor circuit are electrically isolated from each other but are, however, mechanically interlocked as shown by the dashed lines. When switch 21 is closed, the indicator light 22 in subcircuit A will burn, indicating that the control circuit is powered. If desired, a pneumatic alarm (e.g. a horn) operating on a reserve pneumatic storage tank could be activated on loss of power in the control circuit by placing an activating relay switch in circuit A, either in series with or parallel to the illustrated indicator light.

Subcircuits B and C are energized, after closing switch 21, by depressing the push-button of reset switch 23. Note that reset switch 23 is physically connected to (or activates corresponding) reset switches 23a and 23b. Depressing reset switch 23 also energizes circuit B, as more fully explained hereinafter. By holding reset switch 23 depressed for a sufficient length of time, if required. relay coil 24 will be energized and close normally open switch 24a, as well as conveyor motor circuit switch 24b. The circuit through coil 24 passes from the hot line through switch 25a or switch 26, depending upon the particular portion of the cycle of the multiple cam drive timer at which the reset switch is depressed, as explained hereinafter. The F igure shows switch 26 to be closed, although switch 25a could be closed and switch 26 could be open. In any event, by holding down reset switch 23 for a sufficient time, coil 24 will be energized through either switch 26 or 25a, high pressure switch 5 (normally closed) and reset switch 23. Since coil 24 closes switch 24a, a circuit is also made across the compressor motor by way of reset switch 23a, while depressed, and also or subsequently by way of cam actuated switch 27, as more fully explained hereinafter. After the compressor motor has operated for a short period of time, hose 1 will be sufficiently pressurized that preset low pressure switch 4 (normally open) is closed (as shown in the drawing). Thereafter, reset switch .23 can be released and coil 24 remains energized by a circuit from the hot line through switch 250 or 26, switch 5, low pressure switch 4 and relay switch 240, which is also closed by operation of coil 24.

Thus, closing switch 24a powers the single cam drive timer motor 28, the multiple cam drive timer motor 29 and the compressor motor 3, as well as the conveyor motor 7. It is also most evident that if anything disrupts the power to coil 24, then switches 24a, 24b, and 24c will open, disrupting the power to both timer motors 28 and 29, as well as the compressor motor 3 and the conveyor motor 7. Further, it can also be seen that in this event, since switch 240 is also opened, the alarm system cannot cycle but must be manually reset by reset switch 23, thus demanding attention by mine personnel.

Considering both FIGS. 1 and 2, should the pressure in hose 1 drop below a preset minimum, low pressure switch 4 (FIGS. 1 and 2) will open as discussed above. Thus, as can be seen from the control circuit of FIG. 2, when low pressure switch 4 opens, the circuit to coil 24 is broken, which in turn opens switches 24a, 24b, and 240 with the result discussed above. However, since either one of switches 25a and 26 (depending on the position of the cams as discussed hereinafter) will be closed, and normally closed high-pressure switch 5 will be closed, a circuit from the hot line to the ground line will be made through alarm device 30 and the current flow will be sufficient to actuate that device, since the device will then no longer be shunted by the circuit through switches 4 and 240. In series with alarm device 30 is a resistor 31 of sufficient resistivity to prevent shunting of coil 24 when in a circuit with reset switch 23 or switches 4 and 246.

While the alarm device 30 is diagrammatically shown as an indicator light, it is most obvious that any desired alarm or indicator could be used, including lights, whistles, horns, sirens or otherwise. Similarly, alarm device 30 could accommodate additional relays for controlling other equipment or telemetering the alarm throughout various parts of the mine.

A similar result to the foregoing is also obtained if an obstruction is placed in or on the hose in the manner discussed above. This obstruction actuates high pressure switch 5 to open the same, which will deenergize coil 24 and open switches 24a, 24b, and 240 in the same manner discussed above. Since switch 5 now no longer shunts resistor 32 (switch 5 being open), a circuit is also made through alarm device 30 with the same results as noted above. Of course, the voltage to alarm device 30 will be less in this case than in the case discussed above in connection with the low pressure switch, since an additional voltage drop takes place across resistor 32. Thus, this difference in voltage may be used, if desired, to distinguish a high pressure alarm from a low pressure alarm and indeed both from a battery failure in connection with resistor 43, as explained hereinafter. These voltage differences may be used as simply differences in intensities of the alarm signals or be used to close voltage-sensitive relays for activating distinguishable alarm devices. Also, in this regard, resistor 32 may be replaced by an appropriate resistortype signal device such as an indicator light. This will, of course, specifically indicate a high pressure malfunction. Further, if desired, by so choosing the resistivity of such a signalling device, the voltage drop across the signalling device will not be sufficient to cause coil 24 to drop out switches 24a, 24b and 240, so that the high pressure malfunction will only signal an alarm without shutting down the conveyor and fire alarm system. The same may be used in connection with a signalling device to replace resistor 43, as explained hereinafter.

The foregoing operation of the alarm and control devices is periodically monitored on a timed sequence by I subcircuit B. That subcircuit contains cam-activated switches and for preferred flexibility in operation there are two cam activated switches 33 and 34 driven by, respectively, signal cam drive timer motor 28 and multiple cam drive timer motor 29. The operation of this preferred cam switch arrangement is shown in detail in FIG. 3.

Turning now to F IG. 3, cam switches 33 and 34 are indicated by the dotted lines surrounding the components thereof. When coil'24 is energized and switch 24a is closed in the manner discussed above, single cam drive timer motor 28 will be energized and will revolve cam 35. During the revolution of cam 35, cam switch 36 will close (as shown). This will energize multiple cam drive timer motor 29. Of course, with further revolution of cam 35, switch 36 will be opened and motor 29 will be deenergized. Thus, cam switch 33 simply serves as a timer for applying power, periodically, to multiple cam drive timer motor 29. This allows some adjustment in the time that motor 29 is energized by the shape and speed of cam 35. If less flexibility in this regard is required, then single cam timer motor 28 and cam switch 33 can simply be eliminatedand motor 29 can be connected directly to the power line. Of course, for even greater time flexibility, a plurality of v cam motors and switches in series could be used instead of single motor 28 and single switch 33. Thus, even with standard clock motors the time periods could vary widely.

In any regard, when motor 29 is energized, it drives cam 37 which operates switch 27 for periodically powering compressor motor 3. This, periodically, applies pneumatic pressure to hose 1, in the manner described above, e.g., every 4 hours.

Multiple cam drive timer motor 29 also turns cams 39 and 40 which actuate, respectively, switch 41 (in the battery circuit) and switch 26 (in circuit C of the control circuit). Cams 39 and 40 are arranged so that cam 39 closes switch 41 prior to cam 40 opening switch 26.

As can be seen from the battery circuit, when switch 41 is closed, a circuit through battery 42 and coil 25 is made. Coil 25 closes switch 25a of circuit C of the control circuit. Thus, both switches 26 and 25a are closed at the same time for a short period if the system is functioning properly. However, if battery 42 is not sufficiently charged, coil 25 will correspondingly not be sufficiently energized and switch 25a will not be physically closed. Thus, when cam 40 opens switch 26, both switches 26 and 25a will be open, breaking the circuit to coil 24, which in turn will open switches 24a, 24b and 240, cutting off the cam motors, compressor motor, conveyor motor and actuating alarm 30. Here again, the alarm can actuate devices as desired, and as described above. If, however, the battery has sufficient power to close switch 25a, then the alarm system will function normally and no alarms or other disruption will occur. Thereafter, cams 39 and 40 further revolve and cam 40 again closes switch 26 and subsequently cam 39 opens switch 41, returning the circuit to the condition prior to the battery test, as described.

Since the battery circuit operates, mainly, as an independently powered alarm back-up circuit, although of a simplified configuration as opposed to the control circuit, it may not be desirable that the entire system, including the conveyor, be shut down in the event of a low powered battery. As an alternate, a low battery could be signalled by an alarm device which is actuated if the current or voltage (e.g. voltage) of the battery drops below a preset minimum. in this regard, coil 25 would simply be replaced by such a device and switch 25a, switch 26, resistor 43 and cam 40 would be eliminated. If the low voltage alarm device which replaces coil 25 is also of a low current drainage configuration, then cam 39 and switch 41 can also be eliminated. However, such a simplified configuration does exhibit the possibility that any failure in the low voltage alarm device replacing coil 25 would not be detectable and.

for this reason, the system illustrated in FIG. 2 is the preferred embodiment.

Also, as described above in connection with resistor 32, resistor 43 could be replaced by an alarm or signalling device of such resistivity that the voltage drop thereacross is not sufficient to cause coil 24 to drop out switches 24a, 24b and 240. Thus, in the same manner, only an indicator, elg. a light, could signal a low battery without shutting down the conveyor and alarm systems.

signal a low pressure in hose 1 in the manner described I above. In the event of a low pressure in hose 1, a circuit is made through the low pressure switch and alarm indicators 45 and 46 (shown as a light and bell, respectively) will be powered and give an alarm. Here again, other alarm devices as well as telemetering devices may be used in conjunction with or in lieu of devices 45 and 46. Once actuated, these alarms will persist until reset button 23b is depressed in conjunction with closing reset switches 23 and 23a in commencing the starting sequence.

As can be seen from the foregoing, a malfunction of the apparatus for supplying pneumatic pressure to the which severs the hose. Additionally the pneumatic pressure, which is the primary motive force for detection and signalling, is supplied to the device in a manner independent of the pressure in the hose by the cam system. Thus, no self-compensation within the device can allow a malfunction to go undetected. An independently powered back-up battery system will signal a low pressure condition, which is the primary purpose of the device. Of course, the back-up battery system does not make the internal checks of the primary control circuit, but the battery circuit is designed only to operate during emergency conditions where some malfunction of the primary device has taken place. Of course, this malfunction will have been signalled by the control circuit.

In view of the foregoing, it is clear that the objects of the invention have been met and that a fail-safe fire detection and alarm device requiring low maintenance and infrequent attention is provided by the present invention. While the invention has been disclosed with regard to specific elements for purposes of illustration, it is quite apparent that equivalent elements performing the same function can be used in the practice of the invention. Thus, the hose can be any conduit means which is distructible by fire, including plastics, rubbers, metals and the like. Additionally, any fluid pressure means, aside from a compressor, may be used, e.g., pressurized bottled gas, vaporizable liquids, etc. vOf course, in these latter instances, the means of controlling the intermittent addition of the pressurized fluid into the conduit will be other than a conventional motor control means when a compressor is used. For example, the control means for intermittently introducing pressurized fluid into the conduit may be a. motor controlled valve and the like. Also, a pressure release valve can be replaced by any fluid release means, including a motor valve and the like.

Further, the invention has been illustrated with pressure-sensitive switches, but any low pressure detection means, such as a gas-filled bulb and the like, can be used for transmitting a low-pressure signal. Likewise, the high-pressure detection means may be other than a pressure sensitive switch. It is only necessary that these means be capable of monitoring the fluid pressure in the conduit and generating a signal when the pressure in the conduit falls below a predetermined value or rises above a predetermined value in the case of the low pressure monitoring device and the high pressuring monitoring device, respectively.

Ofcourse, other equivalent means for performing the presently disclosed functions will be immediately apparent to those skilled in the art and those equivalent means'are intended to be embraced by the following claims.

What is claimed is:

1. A fire detection and alarm device comprising:

1. a fire destructible conduit means for disposition in a space where fire is to be detected;

2. fluid pressure means operably connected to a first end of the said conduit means for supplying pressurized fluid to said conduit means;

3. fluid release means operably connected to the opposite end of the said conduit means and responsive to the fluid pressure therein for releasing fluid from the said conduit means when the fluid pressure exceeds a preset value;

4. low pressure detection means operably connected to the said conduit means for detecting and signalling fluid pressures in said conduit means below a preset value;

5. a first low pressure alarm means operably connected and responsive to said low pressure detection means for giving an alarm when the fluid pressure in said conduit means falls below the said preset low value;

6. fluid pressure control means operably connected to said fluid pressure means for intermittently causing a flow of pressurized fluid to said conduit means, the operation of the said fluid pressure control means being independent ofthe pressure in the said conduit means;

7. high pressure detection means operably connected to the said conduit means for detecting and signalling fluid pressures in said conduit means above a preset high value; and

8. a high pressure alarm means operably connected and responsive to said high pressure detection means for giving an alarm when the fluid pressure in said conduit rises above the said preset high value.

2. The device of claim 1 wherein there is provided a second low pressure alarm means operably connected to a low pressure detection means, said second low pressure alarm means being independent of the first said low pressure alarm means; and

an alarm control means for periodically disrupting the operable connection of the said first low pressure alarm means with the said low pressure detection means and periodically disrupting the operable connection of the said second low pressure alarm means with the said low pressure detection means, and wherein the operable connections of both the first and second alarm means are not disrupted at the same time.

3. The device of claim 1 wherein the low pressure detection means is operably connected to said conduit means at a position near the said first end thereof.

4. The device of claim 1 wherein the high pressure detection means is operably connected to said conduit means at a position near the said first end thereof.

5. The device of claim 1 where a fluid bleeding means is operably connected to said conduit means at a position near the first end thereof for allowing a slow escape of fluid from the conduit means.

6. The device of claim 1 wherein the fluid pressure means is a pneumatic compressor.

7. The device of claim 6 wherein the pneumatic compressor is intermittently operated by a motor.

8. The device of claim 7 wherein the fluid pressure control means is a motor control means.

9. The device of claim 8 wherein the motor control means is operated on a time cycle.

10. The device of claim 2 wherein the said first and second low pressure alarm means are operated by independent power sources.

ll. The device of claim 10 wherein one of said low pressure alarms is operated by a battery.

12. A method of detecting and providing an alarm for fire comprising:

1. introducing pressurizing fluid at a first end ofa fire destructible conduit which is disposed in a space where fire is to be detected;

2. releasing said fluid from the opposite end of the conduit as required to maintain a pressure in the conduit at or below a predetermined value;

3. monitoring the fluid pressure in the said conduit with a low pressure monitoring device which generates a low pressure signal when the fluid pressure in the conduit falls below a predetermined value;

4. monitoring the fluid pressure in the said conduit with a high pressure monitoring device which generates a high pressure signal when the fluid pressure in the conduit rises above a predetermined value;

5. intermittently introducing additional pressurized fluid into the said conduit; and

6. controlling the intermittent addition of pressurized fluid into the conduit by a means independent of the fluid pressure in the conduit.

13. The method of claim 12 wherein periodically a first low pressure signal actuated alarm device is connected in communication with the said low pressure monitoring device and a second low pressure signal actuated alarm device is disconnected from communication with the said low pressure monitoring device in such a manner that both the first and second low pressure signal actuated alarm devices are not disconnected at the same time.

14. The method of claim 12 wherein a slow bleed of fluid pressure from said conduit is produced at a posi- I tion near the first end of the conduit.

15. The method of claim 12 wherein the intermittent introduction of pressurized fluid is on a timed cycle.

16. The method ofclaim 12 wherein the said first and second alarm devices are powered by independent power sources.

17. The method of claim 16 wherein one of the said power sources is a battery.

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