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( 
 
Bureau of Mines Information Circular/1985 
 
 Design and Operation of Four Prototype 
 Fire Detection Systems in Noncoal 
 Underground Mines 
 
 By William H. Pomroy and Robert E. Helmbrecht 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
 .751 
 
 %INES 75TH AV"!^ 
 
Information Circular 9030 
 
 1/ 
 
 Design and Operation of Four Prototype 
 Fire Detection Systems in Noncoal 
 Underground Mines 
 
 By William H. Pomroy and Robert E. Helmbrecht 
 
 UNITED STATES DEPARTMENT OF THE INTERIOR 
 
 Donald Paul Model, Secretary 
 
 BUREAU OF MINES 
 Robert C. Morton, Director 
 

 ^yw^. 
 
 Gfo^-b 
 
 Library of Congress Cataloging in Publication Data; 
 
 Pomroy, William H 
 
 Design and operation of four prototype fire detection systems in 
 noncoal underground mines. 
 
 (Information circular ; 9030) 
 
 Bibliography: p. 18. 
 
 Supt. of Docs, no.; 1 28.27:9030. 
 
 1. Mine fires. 2. Fire detectors. I. Helmbrecht, Robert. II. Se- 
 ries: Information circular (United States. Bureau of Mines) ; 9030. 
 
 fTN315] 622s [622\8] 84-600372 
 
CONTENTS 
 
 Page 
 
 Abstract I 
 
 Introduction 2 
 
 Fire detection system for a timbered salt mine shaft 2 
 
 Smoke detection system for a multilevel copper mine 6 
 
 Spontaneous combustion fire warning system for a deep silver mine 10 
 
 Computerized fire detection system for an underground trona mine 12 
 
 Conclusions 17 
 
 References 18 
 
 Appendix 19 
 
 ILLUSTRATIONS 
 
 1. Layout of shaft fire detection system 3 
 
 2. Thermistor strip shaft fire detector and junction box attached to shaft 
 
 t imbe r 4 
 
 3. System control panel in headframe building 4 
 
 4. Shaft fire detection system annunciator panel in hoist house 4 
 
 5. Installation of thermistor strip shaft fire detector from a canopy above 
 
 the man cage 4 
 
 6. Annunciator panel during system test showing alarms and digital hotspot 
 
 indicator 5 
 
 7. Mineworthy ionization-type combustion particle (smoke) detector 6 
 
 8. Digital telemetry module mounted inside detector cap 7 
 
 9. Installation of ionization-type combustion particle (smoke) detector at 
 
 the 500-250 ramps 7 
 
 10. Mine map on video display showing location and output of detectors in the 
 
 collar-decline area, the oxide extraction area, and the sulfide extrac- 
 tion area 8 
 
 11. Video display showing three-key function commands 9 
 
 12. Layout of major elements of spontaneous combustion detection system 9 
 
 13. Spontaneous fire warning system detection instruments 10 
 
 14. Draw tube supplying mine air from fan cowling to detection instruments... 11 
 
 15. Strip chart recorder in guard shack 12 
 
 16. Telemetry interface modules adjacent to detection instruments 12 
 
 17. Telemetry interface modules in guard shack 13 
 
 18. Typical chart recording showing elevated CO levels following end-of-shift 
 
 blasts 13 
 
 19. Layout of major elements of trona mine fire detection system 14 
 
 20. Added remote unit A attached to existing remote unit 14 
 
 21. Master station 15 
 
 22. Thermistor line-type detection at conveyor drive 16 
 
 23. Carbon monoxide and smoke detection instruments located downstream of the 
 
 conveyor drive 16 
 
 24. Ultraviolet flame detector in grease niche area 17 
 
 TABLE 
 
 A-1. Temperature versus resistance for thermistor strip shaft fire detector... 19 
 

 UNIT OF MEASURE 
 
 ABBREVIATIONS USED 
 
 IN THIS REPORT 
 
 A 
 
 angstrom 
 
 min 
 
 minute 
 
 c/s 
 
 cycles per second 
 
 nA 
 
 nanoampere 
 
 "C 
 
 degree Celsius 
 
 Q 
 
 ohm 
 
 eV 
 
 electron volt 
 
 ppm 
 
 parts per million 
 
 ft 
 
 foot 
 
 pet 
 
 percent 
 
 °F 
 
 degree Farenheit 
 
 V 
 
 volt 
 
 h 
 
 hour 
 
 V ac 
 
 volt, alternating current 
 
 in 
 
 inch 
 
 V dc 
 
 volt, direct current 
 
 m/s 
 
 meters per second 
 
 W 
 
 watt 
 
 mA 
 
 milliampere 
 
 yr 
 
 year 
 
 mCi 
 
 millicurie 
 
 
 
DESIGN AND OPERATION OF FOUR PROTOTYPE FIRE DETECTION 
 SYSTEMS IN NONCOAL UNDERGROUND MINES 
 
 By V/illiam H. Pomroy ^ and Robert E. Helmbrecht^ 
 
 ABSTRACT 
 
 Fires in underground metal and nontnetal mines pose a threat to the 
 safety of underground miners and to the productive capacity of this Na- 
 tion's mines. Contaminated air (smoke, carbon monoxide, and other 
 products of combustion) is the primary life safety hazard created by a 
 mine fire. The most reliable defense against the hazard posed by the 
 rapid spread of contaminated air underground is early warning fire de- 
 tection and rapid evacuation. This Bureau of Mines report describes the 
 design and operation of four prototype early warning fire detection sys- 
 tems, for underground noncoal mines, presently undergoing prolonged in- 
 mine testing by the Bureau. The systems are described within the con- 
 text of the underground mine environment. 
 
 ^Supervisory mining engineer. 
 '^Stuaent trainee (electrical engineering). 
 Twin Cities Research Center, Bureau of Mines, Minneapolis, MN. 
 
INTRODUCTION 
 
 Fires in underground noncoal mines pose 
 a serious threat to the safety of un- 
 derground miners and to the productive 
 capacity of this Nation's mines. The 
 potential for loss of life and interrup- 
 tions to production due to fires is often 
 underestimated, even within the mining 
 community and despite documented fire 
 statistics to the contrary. 
 
 This misconception was addressed by a 
 senior Mine Safety and Health Administra- 
 tion official (J_) : -^ 
 
 Over the years there has devel- 
 oped a generally accepted opinion 
 that major disasters from fires 
 do not occur in noncoal mines. 
 The Sunshine Mine disaster should 
 have erased that opinion. Fires 
 in mines are not unusual. We have 
 a continuing history of fires in 
 North American mines. 
 
 Historically, timber has been the 
 primary source of fuel in the major 
 mine fires, and mine operators at 
 properties that do not use timber 
 for ground support tend to believe 
 they do not have a potential fire 
 problem. Let us point out now that 
 there are other sources of fuel 
 for combustion. So long as inter- 
 nal combustion engines, electrical 
 equipment, lubricant storage, fuel 
 stores, combustible hydraulic sys- 
 tems, warehousing of combustible 
 solvents, combustible ventilation 
 tubing, and timber support are nec- 
 essary to our mining systems, the 
 potential of mine fires remains. 
 Indeed, since 1965, over 150 fires 
 accounting for 119 fatalities have been 
 reported in underground noncoal mines 
 (_2 ) . Countless millions of dollars have 
 been spent on rescue and recovery, equip- 
 ment repair and replacement, and mine 
 
 rehabilitation. In addition, mines shut 
 down by fires have been forced to forego 
 hundreds of millions of tons of mineral 
 production. 
 
 Contaminated air is the primary life 
 safety hazard in an underground mine 
 fire, accounting for over 78 pet of mine 
 fire deaths since 1945 (3^). Ventilation 
 streams carry smoke, carbon monoxide, and 
 other toxic fire gases to areas of the 
 mine remote from the fire itself, thereby 
 exposing miners who may be widely scat- 
 tered throughout the working to toxic 
 fire gases. 
 
 One means of defense against the hazard 
 posed by the rapid spread of contaminated 
 air is early warning fire detection and 
 rapid evacuation. The data show that 
 fires detected within 15 min of develop- 
 ment result in little or no damage to the 
 mine in 73 pet of all cases (2). Effec- 
 tive, reliable detection systems, capable 
 of detecting fires at their early, or 
 even incipient stages, can significantly 
 improve mine safety by ensuring adequate 
 time for mine personnel to follow appro- 
 priate emergency procedures. Since 1968, 
 however, only about one-third of reported 
 fires were detected within 15 min ( 2_) . 
 
 This Bureau of Mines report describes 
 the design and operation of four proto- 
 type fire detection systems developed for 
 noncoal underground mines. Results of 
 prolonged in-mine testing and system de- 
 sign refinements will be presented in a 
 subsequent report. 
 
 The body of this report contains four 
 sections, each describing oae of the pro- 
 totype detection systems. The detection 
 instruments employed in this research 
 program, several of which are common to 
 more than one detection system, are de- 
 scribed in detail in the appendix. 
 
 FIRE DETECTION SYSTEM FOR A TIMBERED SALT MINE SHAFT 
 
 Although all mine fires present the 
 potential for disaster, fires in mine 
 shafts are particularly hazardous because 
 rapid and safe egress of miners and prop- 
 
 -^Underlined numbers in parentheses re- 
 fer to items in the list of references 
 preceding the appendix. 
 
 er mine ventilation can be seriously im- 
 paired by shaft fires. A recent study of 
 mine fires shows that since 1950, about 
 9.7 pet of all mine fires occurred in 
 shafts, but about 17.6 pet of all mine 
 
 fire fatalities were attributed to shaft 
 fires (2). 
 
Carbon monoxide, carbon dioxide, and 
 submicron particulate detectors used sep- 
 arately or in combination, have been used 
 successfully to provide early warning of 
 shaft fires (4^) , however these detectors 
 require occasional maintenance and cali- 
 bration. Industrial-grade thermal fire 
 detection devices are generally charac- 
 terized by high reliability and durabil- 
 ity but low maintenance requirements, 
 even when used under the harshest condi- 
 tions. Clearly, these attributes are de- 
 sirable for mining applications. 
 
 One limiting feature of thermal detec- 
 tors is that they rely on convected ther- 
 mal energy for response. The distance 
 between the detector and the fire, the 
 relative spatial orientation and place- 
 ment of the detector relative to the 
 fire, and local air currents profoundly 
 affect detector performance. Thus, in 
 order to provide for large area coverage, 
 numerous closely spaced detectors are 
 required. A common example of thermal 
 detection in the mining industry is the 
 typical conveyor belt fire detection 
 system mandated for underground coal 
 mines (5^). Spot-type, thermal detectors, 
 spaced at 125-ft intervals along the en- 
 try provide early warning of a belt fire. 
 
 A string of spot-type detectors arrayed 
 in a similar manner in a mine shaft is a 
 feasible approach to shaft fire detec- 
 tion, however, the use of a line-type de- 
 vice would offer superior performance. A 
 line-type device senses the heat from a 
 fire at any point along its length. It 
 can be thought of as spot-type detection 
 in the limiting case where the distance 
 between adjoining detectors equals zero. 
 
 Limited success has been achieved using 
 fusible contact line-type thermal sensors 
 in shafts (^) . However, fusible contact 
 line-type sensors are subject to occa- 
 sional false alarms and considerable ef- 
 fort may be required to restore the de- 
 tector to proper operation following an 
 alarm, especially if the contact occurs 
 in a section of the shaft for which ac- 
 cess is difficult. An alternative to the 
 fusible contact detector is thermistor 
 strip. 
 
 A prototype thermistor strip fire de- 
 tection system for mine shafts was devel- 
 oped by the Bureau and installed in the 
 
 1,200-ft main production shaft of a salt 
 mine in Detroit, MI. The system was in- 
 stalled along the entire length of the 
 shaft (fig. 1). The detector is de- 
 scribed in detail in the appendix. 
 
 The system provides two alarm tempera- 
 ture settings, permitting a prealarm at a 
 lower temperature and an alarm at a high- 
 er temperature. The system also includes 
 a hotspot indicator that pinpoints the 
 location of the overheated area and pro- 
 vides digital readouts of the distance 
 between the shaft collar and the hotspot. 
 
 The system has three main subsystems: 
 the sensor element in the shaft, the 
 system control panel in the headframe- 
 crusher building, and an alarm annun- 
 ciator panel in the hoist house. The de- 
 tector is positioned roughly in the cen- 
 ter of the two-compartment shaft. It is 
 attached at each timber set with special 
 mounting brackets, thereby providing sup- 
 port for the detector at approximately 
 4-ft intervals. The detector is divided 
 into two zones, the upper zone and the 
 lower zone, with the two thermistor ca- 
 bles joined in a junction box at the 
 shaft midpoint (fig. 2). The system con- 
 trol panel (fig. 3) contains all control 
 circuits, backup power supply, and means 
 for calibrating and troubleshooting the 
 system. The annunciator panel (fig. 4), 
 within sight of the hoist operator, pro- 
 vides a green lamp indicating normal 
 system operation, visual and audible 
 indication of prealarm and alarm condi- 
 tions, and a digital display of the hot- 
 spot location. 
 
 Headframe and crusher building 
 
 System control panel 
 Ho ist hou se ./^ y^ 
 
 Surface 
 
 FIGURE 1. - Layout of shaft fire detection system. 
 

 
 FIGURE 2. - Thermistor strip shaft fire detector 
 and junction box attached to shaft timber. 
 
 V 
 
 
 5?%^' 
 
 '^<¥«9!«Si5 
 
 sj/;'-.'''. 
 
 
 ^^.' .'^' 
 
 FIGURE 3. - System control panel in hecdframe 
 building. 
 
 FIGURE 4. - Shaft fire detection system annun- 
 ciator panel in hoist house. 
 
 FIGURE 5. - Installation of thermistor strip shaft 
 fire detector from a canopy above the man cage. 
 Cage doors are open for illustration purposes only. 
 
The system was completely installed by 
 a three-person crew over a 4-day period 
 in April 1982. The detector was in- 
 stalled from a platform above the skip 
 (fig. 5) by interconnecting ten 120-ft 
 detector segments end to end. Because 
 this shaft is the main mine exhaust, the 
 air is laden with salt. This highly cor- 
 rosive atmosphere is detrimental to the 
 operation of electrical systems, necessi- 
 tating great care in hermetically sealing 
 each detector segment interconnection 
 with a silicone adhesive-sealant. All 
 external parts of the detector wire and 
 connections are stainless steel which has 
 been further protected with a corrosion- 
 resistant Teflon'^ fluorocarbon polymer 
 jacket. The control panel and annuncia- 
 tor panel are housed in dust-tight enclo- 
 sures. Following installation, the sys- 
 tem was functionally tested. At a known 
 elevation in the shaft, a propane torch 
 was used to heat a section of the detec- 
 tion cable. The prealarm and alarm func- 
 tions operated properly and the hotspot 
 indicator displayed the correct elevation 
 (fig. 6). 
 
 The system was operated continuously 
 from April 1982 until June 1983 without 
 hardware failure. Once during that peri- 
 od, a lightning strike at the headframe 
 structure caused a momentary alarm, how- 
 ever, the system returned to the normal 
 operating mode without further incident. 
 These preliminary test results are sig- 
 nificant because they indicate that the 
 hardware and installation precautions are 
 suitable for this worst case corrosive 
 environment . 
 
 In June 1983, mine officials reported 
 a system failure. A technician was 
 dispatched to the mine to inspect the 
 system, determine what repairs and/or 
 equipment replacement were required, and 
 recommend system modifications (if any) 
 needed to avoid future similar problems. 
 
 The technician found that four 120-ft 
 detector segments in the shaft had been 
 ripped from their mountings by an object 
 
 ^Reference to sjjecific products does 
 not imply endorsement by the Bureau of 
 Mines . 
 
 FIGURE 6. - Annunciator panel during system 
 test showing alarms and digital hotspot indicator. 
 
 protruding from the skip. All four seg- 
 ments needed to be replaced. As a pre- 
 caution to prevent further damage in the 
 future, it was recommended that a 1/8- 
 in-diam stainless steel messenger cable 
 be installed in the shaft parallel to the 
 detector and flush with the timber sets. 
 The detector could then be removed from 
 the mounting brackets and attached to the 
 messenger. This mounting arrangement 
 would (1) draw the detector closer to the 
 timber sets so that it will be less like- 
 ly to become entangled with objects pro- 
 truding from the skip, (2) permit the 
 detector to be secured at intervals 
 closer than the 4-ft spacing of the tim- 
 ber sets, and (3) provide greater overall 
 strength to the installation because the 
 stainless steel messenger cable is much 
 stronger than the detector. 
 
 Repairs to the damaged portion of the 
 detector have been delayed because of a 
 production shutdown at the mine. Re- 
 placement of the damaged segments and 
 installation of the messenger will be 
 effected and testing continued upon re- 
 opening of the mine. 
 
SMOKE DETECTION SYSTEM FOR A MULTILEVEL COPPER MINE 
 
 One of the earliest products of In- 
 cipient combustion Is submlcrometer sized 
 particulates, or smoke (6^). Smoke de- 
 tection systems that are capable of re- 
 liably detecting these particulates are 
 extremely valuable because fires can be 
 detected before they reach the flam- 
 ing combustion stage. With the aid of 
 such systems, emergency procedures, such 
 as personnel evacuation and fire fight- 
 ing efforts, can be undertaken at the 
 earliest opportunity, often before the 
 fire poses a direct threat. A complete 
 prototype smoke detection system was 
 designed, fabricated, and Installed In 
 an Arizona underground copper mine for 
 prolonged testing and evaluation. 
 
 The system consists of 10 detection 
 Instruments (fig. 7). Each detector is 
 equipped with a digital telemetry module 
 (mounted in detector cap, figure 8) to 
 convert the detector's analog output to a 
 digital word for transmission of the de- 
 tector value along with a unique address 
 and verification words to the system con- 
 trol unit. A microcomputer system con- 
 trol, a line Interface control module for 
 communication to the computer through an 
 industry standard protocol (RS232C), a 
 disk drive to store the control program 
 and detector output records, a color vid- 
 eo display with graphics to highlight 
 alarms , and a printer to provide hard 
 copy of alarm and fault messages are also 
 provided. 
 
 The detectors are linked to the system 
 control by a single-pair closed loop 
 telemetry circuit. Connecting outsta- 
 tions in a closed loop minimizes cable 
 costs and installation time and provides 
 a redundant signal path for uninterrupted 
 signal transmission in case of a broken 
 telemetry line. 
 
 The system was completely Installed by 
 a 4-person crew over a 1-week period 
 (fig. 9). Minor debugging was required 
 following installation because of prob- 
 lems with several telemetry modules, how- 
 ever, the necessary repairs were effected 
 on-site during the week following the 
 Installation. 
 
 The system operated for approximately 
 1 yr with a simplified control program 
 while the final version of the control 
 software was developed and debugged. 
 During this period, system operation was 
 limited to a video display of real-time 
 detector outputs and an audible alarm and 
 printout whenever any detector output ex- 
 ceeded its individually programed alarm 
 threshold. 
 
 The final version of the control soft- 
 ware provides, in addition, video graph- 
 ics of the detector locations on color 
 mine maps (fig. 10), simple instructions 
 and three-key coded function commands 
 (fig. 11), and alarm, fault, and trouble- 
 shooting messages. 
 
 FIGURE 7. - Mineworthy ionization-type com- 
 bustion particle (smoke) detector. 
 
FIGURE 8. - Digital telemetry module mounted inside detector cap. 
 
 FIGURE 9. - Installation of ionization-type combustion particle (smoke) detector at the 500-250 ramps. 
 
1188 te«L 
 
 BECXIfCS _ 
 
 FIGURE 10. - Mine mapon videodisplay showing 
 location and output of detectors in {A) the collar- 
 decline area, (6) the oxide extraction area, and 
 (C) the sulfide extraction area. 
 
 Following the on-screen prompts and us- 
 ing the simple three-key commands, opera- 
 tors can display one of three mine maps 
 covering the system, change any sensor 
 alarm threshold, display current alarm 
 thresholds, display 72-h sensor history 
 in tabular or graphic form, and manually 
 control the printer. 
 
 During the first 3 months of system 
 operation (using the simplified con- 
 trol program) numerous false alarms were 
 issued. The detector-mounted digital 
 
 telemetry modules, which are susceptible 
 to low voltage conditions, were found 
 to be the cause. Boosting line voltage 
 slightly corrected the problem. 
 
 The system has operated for approxi- 
 mately 7 months with the final version 
 of the control software. Minor debug- 
 ging has been required; however, during 
 this period, three abnormal events (smok- 
 ing rubber drive pulleys on two pumps and 
 a smoking electrical controls enclosure) 
 were detected by the system. 
 
.TrrTnt 
 
 THIS IS A MINE FIRE SEMSIIIS st^tn 
 DISPLAY AND CONTROL TEII SItOKE 
 
 SPREAD THROUGHOUT THE If I ME Vlf* 
 ED I GATED TELEPWHC LIME. ftyTCI5$Tir 
 ALARHS HILL INDICATE eCCESSIVE PftRTICU 
 LATE LEVELS BY A STEADY TWE ftMO^SCREEK 
 DISPLAY SHOUING THE HIIIE AREA AFFECTED 
 
 6^ 5^84 18=46= 6 
 
 IF PRINTER COPY IS 'iCEBED TYPE IK CPl 
 AS FOURTH CHARACTER FOLLOHING THREE 
 CHARACTER COH^AhD PRINTABLE REPORTS 
 
 V vrf 
 
 SENSOR ALfiR^PUlHl 
 
 iii-m 
 
 SEM = SU 
 
 .-- nPi 
 
 IDE EXT 
 LFIDE EKT 
 
 Mh' 
 
 bNTE 
 
 HHAHD. HIT RETURN 
 
 FIGURE n. - Video display showing three-key function commands. 
 
 Telemetry 
 modules 
 
 Detection 
 ■instruments 
 
 Draw tube 
 
 Exhaust fan 
 
 Main mine ventilation 
 exhaust borehole 
 
 Guard house 
 
 FIGURE 12. - Layout of major elements of spontaneous combustion detection system. 
 
10 
 
 SPONTANEOUS COMBUSTION FIRE WARNING SYSTEM FOR A DEEP SILVER MINE 
 
 By definition, spontaneous combustion 
 is the outbreak of fire in combustible 
 material that occurs without application 
 of direct flame and is usually caused by 
 slow oxidation processes under conditions 
 restricting the dissipation of heat. 
 Historically, several disastrous noncoal 
 mine fires have been attributed to the 
 spontaneous combustion of wood or of wood 
 and ores in remote, inactive or sealed 
 areas of mines (7). Overall, spontaneous 
 combustion accounts for only about 2 pet 
 of all underground metal and nonmetal 
 mine fires. However, the potential seri- 
 ousness of spontaneous combustion fires 
 is understated by this statistic. Fire 
 fighting, mine rescue and recovery, and 
 related operations are complicated by the 
 difficult access to the remote areas 
 where spontaneous combustion fires occur. 
 Since 1950, over half of all underground 
 metal and nonmetal fires lasting longer 
 than 24 h were caused by spontaneous com- 
 bustion. Associated with the spontaneous 
 combustion problem are still other inci- 
 dents involving ignition of sulfide 
 dusts, combustion of AN-FO heated by sur- 
 rounding hot ground surfaces, and the 
 continuous heating of mines utilizing 
 backfill with a high sulfide content. 
 
 Recognizing the hazard of spontaneous 
 combustion, the Bureau initiated a re- 
 search program in 1978 to develop a 
 spontaneous combustion fire warning sys- 
 tem for deep metal mines (8^). The pro- 
 gram began with a comprehensive search 
 of the published literature for reports 
 dealing with spontaneous combustion in 
 mines. In addition, industry experts 
 were contacted in an effort to acquire 
 relevant unpublished research findings. 
 This data base, together with the re- 
 sults of a series of laboratory experi- 
 ments of the spontaneous heating char- 
 acteristics of various sulfide ores, 
 timber, and other mine combustibles, 
 supported the development of a conceptual 
 design for a spontaneous combustion fire 
 warning system. The studies indicated 
 that the earliest warning of a spontane- 
 ous heating event could be achieved by 
 sensing for long-term trends in the lev- 
 els of carbon monoxide, carbon dioxide, 
 sulfur dioxide, oxygen, and temperature. 
 
 A prototype system comprising appropri- 
 ate detection instruments (details in ap- 
 pendix), telemetry, and recorders was 
 designed, fabricated, laboratory tested, 
 and in-mine field tested at two mines. 
 
 Initial in-mine testing was conducted 
 in 1979 in an underground copper mine in 
 Arizona. Results of this phase of the 
 in-mine testing program are described in 
 reference 8. Follow-on testing of a 
 slightly modified system (the Arizona 
 studies indicated that sulfur dioxide de- 
 tection could be eliminated) was initi- 
 ated in 1981 at an underground silver 
 mine in Idaho. 
 
 This system was installed on surface in 
 an emergency escape hoist house at the 
 collar of the mine's main ventilation ex- 
 haust borehole (fig. 12). The detection 
 
 FIGURE 13. - Spontaneous fire warning system 
 detection instruments. 
 
11 
 
 instruments, housed in a corrosion- 
 resistant fiberglass enclosure (fig. 13), 
 were supplied mine air through a draw 
 tube linking the enclosure with the ven- 
 tilation fan cowling (fig. 14). The sys- 
 tem was linked by hard wire to strip 
 charts in a guard house approximately 
 2,000 ft from the borehole (fig. 15). 
 
 Signal transmission is provided by a 
 mineworthy telemetry system. Telemetry 
 interface modules are located at each end 
 of the telemetry line; i.e., in the 
 emergency escape hoist house (fig. 16) 
 and the guard house (fig. 17). The sys- 
 tem accepts 1- to 0-V inputs for cur- 
 rent loops from the detectors. Each wire 
 pair can accommodate from 1 to 48 chan- 
 nels. Operating on a balanced line prin- 
 ciple, and incorporating special line 
 filters and protection networks, the sys- 
 tem is noise immune and interference 
 free. 
 
 The oxygen analyzer used in the earlier 
 test program experienced excessive drift. 
 
 Consequently, it was replaced by a simi- 
 lar unit from a different vendor. How- 
 ever, this detector also suffered exces- 
 sive drift and was removed approximately 
 1 week after installation. A second 
 electrochemical cell carbon monoxide de- 
 tector was later installed and connected 
 to the former oxygen analyzer's telemetry 
 channel. This redundancy provided an 
 opportunity to observe tracking between 
 the two carbon monoxide detectors. 
 
 After 12 months of system operation, 
 the remaining detectors and telemetry 
 system were functioning properly. Chart 
 recordings indicated up to 15-ppm excur- 
 sions in carbon monoxide values following 
 end-of-shift production blasts (fig. 18). 
 The two CO detectors track very closely, 
 both at low levels and following the pro- 
 duction blasts (the traces are slightly 
 offset to facilitate data analysis). 
 These readings have been validated by 
 analyses of air samples collected at the 
 time of the CO readings. 
 
 FIGURE 14. - Draw tube supplying mine air from fan cowling to detection instruments. 
 
12 
 
 COMPUTERIZED FIRE DETECTION SYSTEM FOR AN UNDERGROUND TRONA MINE 
 
 The trona mines of southwestern Wyoming 
 are similar in layout and operation to 
 deep room-and-plllar coal mines. Typi- 
 cally encompassing thousands of acres, 
 utilizing numerous production sections 
 and miles of belt conveyor, they are so 
 large that physical monitoring of all 
 mine equipment and operations is not 
 feasible. 
 
 Fire detection in these and similar 
 metal and nonmetal mines is very diffi- 
 cult because many potential fire hazard 
 areas are not under continuous, or even 
 periodic, observation. The Bureau has 
 developed a fire detection system for 
 
 settings and conditions of this type and 
 in-mine tests of the system were under- 
 taken at a Wyoming trona mine. 
 
 The system represents the addition of a 
 fire detection capability to an existing 
 computerized monitoring and control net- 
 work the mine had installed previously as 
 an aid to production. The system moni- 
 tors and controls the conveyors, ventila- 
 tion fans, mine pumps, power substations, 
 production shafts and hoists, and under- 
 ground bunker ore levels. Fire detection 
 system costs were significantly reduced 
 by taking full advantage of in-place te- 
 lemetry and associated control equipment. 
 
 FIGURE 15. - Strip chart recorder in guard shack. 
 
 FIGURE 16. - Telemetry interface modules 
 adjacent to detection instruments. 
 
13 
 
 FIGURE 17. - Telemetry interface modules in guard shack. 
 
 FIGURE 18. - Typical chart recording showing elevated CO levels following end-of-shift blasts. 
 
14 
 
 The system is designed to monitor heat, 
 CO gas, submicron particulates (smoke), 
 and UV radiation levels (flame) at spe- 
 cific locations in the mine in order to 
 detect fires at the earliest possible 
 time. 
 
 The fire detection system addition con- 
 sists of four components: a submaster 
 station and three remote outstations in- 
 terfaced to fire detectors (fig. 19). 
 The three remotes. A, B, and C, are phys- 
 ically attached to the existing remote 
 units (fig. 20) and utilize the existing 
 remote unit telemetry system to transmit 
 information from the various sensors to 
 the master station. The master station 
 microprocessor (fig. 21) processes the 
 data, initiates alarms and warnings, and 
 sends the processed information to the 
 submaster station to be displayed. The 
 processed information is recorded in the 
 form of paper hard copy on a printer for 
 mine company records. 
 
 Remote unit A is located near a belt 
 motor and grease niche area. Fire haz- 
 ards in this area would include various 
 combustible liquids and greases stored in 
 the grease niche and overheating of belt 
 drive motors. Thermistor line-type heat 
 sensors were mounted above the conveyor 
 belt and over the belt motors to detect 
 overheating (fig. 22). CO and smoke de- 
 tectors were mounted downwind of the belt 
 
 Remote 
 unit A 
 
 Remote 
 unit B 
 
 iSi 
 
 
 motors (fig. 23). The grease niche area 
 is being monitored by two UV detectors 
 (fig. 24). 
 
 Remote unit B is interfaced with the 
 same type of sensors as remote unit A 
 with the exception of the two heat sen- 
 sors. Remote unit C is interfaced to 
 the same types of detectors as remote 
 unit A with the exception of the two UV 
 detectors. 
 
 All analog detector signals (smoke 
 and CO) are converted to digital 
 form by an analog-to-digital converter 
 and are transmitted to the submaster 
 along with the various contact clo- 
 sures and dc signals already in digital 
 
 FIGURE 19. - Layout of major elements of 
 trona mine fire detection system. 
 
 FIGURE 20. - Added remote unit A attached 
 to existing remote unit. 
 
15 
 
 FIGURE 21. - Master station. 
 
 form. The analog-to-digital conversion 
 of the analog signals is accomplished 
 by an incremental charge balancing 
 technique. 
 
 The telemetry system within the remote 
 units transmits all signals received from 
 the fire detectors to the master station 
 via FSK tone (frequency shift key tone 
 modulation) transmission when called by 
 the microprocessor located in the master 
 station. 
 
 The transmitted data are temporarily 
 stored and analyzed by the control micro- 
 processor located in the master station. 
 The submaster station serves as an alarm 
 
 setpoint control for the microprocessor, 
 a present time status display of pro- 
 cessed data, and an alarm annunciator. 
 The alarm set point controls are digital 
 thumb wheels that are used to set the de- 
 sired alarm level for each measured vari- 
 able. When the telemetered signal ex- 
 ceeds the set thumb wheel alarm level, 
 the microprocessor initiates an alarm at 
 the submaster station, which corresponds 
 to the remote unit area and type of de- 
 tector experiencing an alarm. Deactiva- 
 tion of the alarm is automatic when the 
 telemetered variable drops below the 
 setpoint level. The microprocessor also 
 
16 
 
 FIGURE 22. - Thermistor line-type detection at conveyor drive 
 
 FIGURE 23. - Carbon monoxide and smoke detection 
 instruments located downstream of the conveyor drive. 
 
 senses which contact closures are open or 
 closed and transmits the proper alarm or 
 normal mode to the submaster station. 
 
 All data received by the submaster sta- 
 tion from the microprocessor in the mas- 
 ter are recorded by a printer on paper. 
 An alarm-status logger is provided to 
 record sensor status of the various re- 
 mote units in the mine for the mine com- 
 pany's records. The printer provides an 
 easy to read formatted output. CO levels 
 and smoke particle levels are recorded 
 automatically at 1-h intervals for a 
 period of 24 h and are summarized at mid- 
 night of each day. Any alarm conditions 
 will cause the printer to record data 
 instantly at 10-min intervals until the 
 alarm condition subsides. 
 
 The system has operated continuously 
 since its installation in 1981 without 
 hardware or software failure. 
 
17 
 
 FIGURE 24. - Ultraviolet flame detector in grease niche area. 
 
 CONCLUSIONS 
 
 The elapsed time between the onset of a 
 fire and its detection is critical be- 
 cause fires tend to grow in size and in- 
 tensity with time. Early fire detection 
 and warning permit the initiation of a 
 mine's emergency plan (evacuation, fire 
 fighting, etc.) while the fire is still 
 small, or ideally, while it is still in 
 the incipient stage. Fire detection and 
 
 warning systems, utilizing sensitive 
 heat, flame, smoke, and gas analyzers, 
 provide the most rapid and reliable indi- 
 cation of a developing fire. Testing of 
 prototype equipment in a variety of mine 
 settings has highlighted both deficien- 
 cies and advantages of various detection 
 instruments and telemetry systems. 
 
REFERENCES 
 
 1. Riley, R. E. Lessons We Can Learn 
 From the Sunshine Mine Fire. Pres. at 
 Am. Min. Congr. Annu. Meeting, Denver, 
 CO, Sept. 10, 1973, 14 pp.; available 
 f rom W. Fomroy, BuMines, Minneapolis, MN. 
 
 2. Baker, R. M. , J. Nagy, and L. B. 
 McDonald. An Annotated Bibliography of 
 Metal and Nonmetal Mine Fire Reports 
 (contract J0295035, The Allen Corp. of 
 America). BuMines OFR 68(1)-81, 1980, 64 
 pp.; NTIS PB 81-223729. 
 
 3. FMC Corp. Mine Shaft Fire and 
 Smoke Protection System (contract 
 H0242016). BuMines OFR 24-77, 1975, 407 
 pp.; NTIS PB 263577. 
 
 4. Griffin, R. E. In-Mine Evaluation 
 of Underground Fire and Smoke Detectors. 
 BuMines IC 8808, 1979, 25 pp. 
 
 5. U.S. Code of Federal Regulations. 
 Title 30 — Mineral Resources; Chapter I — 
 Mine Safety and Health Administration, 
 
 Dep. of Labor; Subchapter — Coal Mine 
 Safety and Health; Part 75 — Mandatory 
 Safety Standards — Underground Coal Mines; 
 Subpart L — Fire Protection; Sec. 75.1103- 
 4 — Automatic Fire Sensor and Warning De- 
 vice Systems; Installation; Minimum Re- 
 quirements; July 1, 1984. 
 
 6. Litton, C. D. Product of Combus- 
 tion Fire Detection in Mines. Chapter in 
 Underground Metal and Nonmetal Mine Fire 
 Protection. BuMines IC 8865, 1981, pp. 
 28-48. 
 
 7. Ninteman, D. J. Spontaneous Oxi- 
 dation and Combustion in Sulfide Ores 
 in Underground Mines. BuMines IC 8775, 
 1978, 36 pp. 
 
 8. Stevens, R. B. Improved Spontane- 
 ous Combustion Protection for Underground 
 Metal Mines (contract H0282002, ESD 
 Corp.). BuMines OFR 79-80, 1979, 262 
 pp.; NTIS PB 80-210461. 
 
19 
 
 APPENDIX 
 
 THERiMISTOR STRIP SHAFT 
 FIRE DETECTOR 
 
 The thermistor strip detection system 
 selected for the salt mine shaft was the 
 Alison Control A888-M106 Fire Detection 
 System. 
 
 The control unit is housed in a Nation- 
 al Electrical Manufacturers Association 
 (NEMA) 12 enclosure. A separate annunci- 
 ator is provided in a NEMA 9 enclosure. 
 The system provides two independent, ad- 
 justable levels of alarm (prealarra and 
 alarm) that are annunciated at both the 
 control unit and annunciator. The loca- 
 tion of the hotspot is also indicated in 
 feet above or below ground level at the 
 annunciator. 
 
 The sensor is completely supervised. 
 An abnormal condition is indicated at the 
 control unit and annunciator if an open 
 or short occurs anywhere along the entire 
 length of the sensor. All interconnec- 
 tions between the control unit and annun- 
 ciator are also supervised. 
 
 The A888-M106 system requires 115±10 V 
 ac input power. The maximum power dissi- 
 pation is 300 W. 
 
 The detection cable operates on 24 V dc 
 generated by an internal f erroresonant 
 power supply. Should the system lose ac 
 input power, the power supply is auto- 
 matically disconnected and standby bat- 
 teries (located at the bottom of the con- 
 trol unit) are automatically switched in. 
 The batteries are sufficient to power the 
 system for 24 h in standby followed by 1 
 h in alarm. The system contains a bat- 
 tery charger that automatically maintains 
 the batteries fully charged when ac power 
 is present. 
 
 The annunciator is powered from the 
 control unit at 24 V dc and is serviced 
 by the control units backup batteries. 
 
 The sensor is composed of thirty 40-ft 
 sections of Alison 9090-100 continuous 
 thermistor cable. This cable consists of 
 stainless steel tubing containing a spe- 
 cially formulated ceramic thermistor 
 core. A center wire is imbedded in the 
 core and runs the entire length of the 
 sensor. 
 
 The sensor center-wire-to-case resist- 
 ance exhibits a negative temperature 
 coefficient. This means that as the 
 temperature increases, the resistance 
 of the sensor decreases exponentially. 
 It is this decrease in resistance that 
 is sensed by the alarm instrumentation. 
 Table A-1 displays the temperature- 
 resistance relationship for 9090-100 
 series cable. It should be noted that 
 the sensor will detect a high temperature 
 on a short length of the cable as well as 
 a lesser temperature over a longer length 
 of the cable. 
 
 TABLE A-1. - Temperature versus 
 resistance for thermistor 
 strip shaft fire detector 
 
 Temperature , °F Resistance, Q 
 
 50 1,000,599,928 
 
 100 69,098,544 
 
 150 7,397,155 
 
 200 1,111,113 
 
 250 218,002 
 
 300 52,998 
 
 350 15,343 
 
 400 5,131 
 
 450 1,935 
 
 500 808 
 
 550 368 
 
 600 180 
 
 650 94 
 
 700 52 
 
 750 30 
 
 The 40-ft sensor sections are connected 
 in series to form two sensor circuits 
 each 600 ft (15 sections) in length. 
 Each 600-ft circuit is monitored sep- 
 arately. The two circuits meet at an 
 elevation of 580 ft where they are ter- 
 minated in a stainless steel junction 
 box. Stainless steel junction boxes are 
 also provided at the -1,180- and +20-ft 
 elevations to terminate the other ends of 
 each sensor circuit. The entire sensor 
 length and all three junction boxes are 
 coated with a heavy polymer jacket for 
 further protection from the corrosive 
 atmosphere . 
 
20 
 
 The 30 detector cable sections were 
 provided by the vendor in ten 120-ft 
 lengths. Three 40-ft sections were fac- 
 tory spliced to produce each 120-ft 
 length. The factory splices were sealed 
 using a heavy duty heat shrink jacket. 
 
 The center conductor at both ends of 
 each sensor circuit is connected to the 
 control unit by single conductor shielded 
 cable. The sensor case is connected to 
 the control unit via the shield. 
 
 Each sensor circuit is monitored by a 
 detection panel and a hotspot panel. The 
 upper and lower circuits share common 
 prealarm, alarm, and hotspot indicators, 
 making the two circuits appear as one. 
 
 The hotspot panel provides a 5.6 V dc 
 voltage clamp that determines the sensor 
 center-conductor-to-case voltage at nor- 
 mal ambient temperatures. Each hot-spot 
 panel generates a linear to 10 V dc 
 analog voltage that is indicative of the 
 hotspot location for its associated 
 sensor circuit. A special combining cir- 
 cuit selects the output from the circuit 
 that is in alarm and converts it to the 
 proper analog voltage (-18 V dc to +2 V 
 dc) to drive the hotspot location meter 
 at the annunciator. If both circuits are 
 simultaneously in alarm, the lower cir- 
 cuit overrides the upper circuit. 
 
 If a large section of sensor is heated, 
 the hotspot circuitry tends to indicate 
 the edge of the hotspot closest to the 
 excited end of the sensor (-600-ft eleva- 
 tion) . The hotspot indication for a 
 growing fire will drift towards the -600- 
 ft level. 
 
 If a section of sensor is heated to the 
 point where the center-conductor-to-case 
 resistance of the sensor falls to the 
 prealarm setting, the amber prealarm in- 
 dicator is illuminated at the control 
 unit and annunciator. The auxiliary pre- 
 alarm relay is energized, transferring 
 three customer available form C relay 
 contacts. The hotspot location indicator 
 at the annunciator is also energized. 
 All of these response indicators reset 
 automatically when the sensor resistance 
 rises above the prealarm threshold. 
 
 If the sensor is heated to a point 
 where the sensor center-conductor-to-case 
 resistance falls to the alarm setting. 
 
 the red alarm indicator is illuminated at 
 the control unit and annunciator. The 
 alarm responses reset automatically when 
 the sensor resistance rises above the 
 alarm threshold. 
 
 An abnormal condition such as an open 
 or short in the detection cable, an open 
 or short between the detection cable and 
 the control panel, or the loss of ac or 
 dc power is indicated by audible and vis- 
 ual alarms at the control panel and the 
 annunciator. 
 
 SUBMICROMETER PARTICULATE 
 (SMOKE) DETECTOR 
 
 The submlcrometer particulate detector 
 selected for the underground copper mine 
 and underground trona mine fire detection 
 systems was the Anglo American Electron- 
 ics Laboratory/Wormald Electronics Becon 
 MK IV ionization type combustion particle 
 detector. 
 
 Externally the detector is cylindrical 
 in shape, 10 in. in total height, and 6- 
 1/8 in. in diameter (refer to figures 1 
 and 2 in the main text) . A cylindrical 
 cap having a height of 3-1/2 in and a 
 diameter of 7-1/4 in, which houses the 
 power terminal connectors and test sock- 
 et, is mounted at the top of the detec- 
 tor, A supension eye ring is affixed to 
 the top of the cap to facilitate the 
 hanging of the detector in the fire haz- 
 ard area. Because of the highly humid 
 and corrosive underground mine environ- 
 ment , the cylindrical outer casing of the 
 Becon detector is made from nylon-dipped 
 stainless steel to ensure detector 
 longevity and to provide a radiation 
 shield. 
 
 Towards the lower end of the Becon de- 
 tector are vertical rectangular ports, 
 which allow the mine air to enter the 
 ionization chamber. The ports in the 
 stainless steel shield are internally 
 overlapped by a nylon-dipped stainless 
 steel baffle plate, which shields the 
 areas outside the detector from direct 
 radiation, reduces effects of high veloc- 
 ity airflow in the ionization chamber, 
 and causes a mixing of the mine air in- 
 side the ionization chamber. 
 
21 
 
 Internally the Becon MK IV particle de- 
 tector is comprised of a shielded single 
 ionization chamber, a radioactive source, 
 an ion collecting electrode (grid) , and a 
 current amplifier. Because of the inher- 
 ent corrosive nature of the underground 
 mine atmosphere, all internal components 
 of the detector are made of plastic or 
 are hermetically sealed. 
 
 The radiation source, which ionizes the 
 air within the ionization chamber, is a 
 sealed glass vial containing 5 mCi of 
 krypton 85 gas. The vial is connected to 
 the grid inside the ionization chamber by 
 two cable ties. 
 
 The ionization chamber (conducting 
 plastic chamber case) is constructed from 
 conducting plastic and completely encir- 
 cles the grid, also made of conducting 
 plastic. The plastic chamber case is 
 cylindrical in shape, but the circumfer- 
 ence of its walls is not continuous. In- 
 stead, the wall is constructed from a 
 number of overlapping curved rectangular 
 plates of conducting plastic. These 
 plastic plates are affixed to the disk- 
 shaped base of the chamber case at two 
 alternating radii about the mean circum- 
 ference of the chamber. The longer edges 
 of the rectangular plates run parallel to 
 the axis of the chamber case. This stag- 
 gering of the sides of the chamber case 
 wall allows mine air to enter the chamber 
 and causes further baffling of the mine 
 air velocity. The plastic chamber case 
 of the ionization chamber acts as the 
 ground electrode with respect to the 
 grid, which is the negative electrode. 
 Because the plastic chamber case along 
 with the conducting plastic upper case 
 are at ground potential they electrically 
 shield the ionization chamber and all in- 
 ternal electronics from electromagnetic 
 radiation external to the detector. 
 
 The hermetically sealed amplifier 
 electronics and the grid are electrical- 
 ly isolated from the conducting plas- 
 tic cases, by a deep annular grooved 
 insulator and conductive plastic guard 
 ring. The annular grooves are present to 
 create the longest possible leakage path 
 between the grid and the case. Electri- 
 cal leakage could occur if high humidity 
 saturates the inside of the detector 
 with moisture or if a conductive dust is 
 
 present in the mine atmosphere and even- 
 tually settles within the detector. The 
 annular grooved insulator also serves the 
 purpose of supporting the grid and ampli- 
 fier electronics. The guard ring pre- 
 vents leakage, by its connection to the 
 non-inverting terminal of the operational 
 amplifier. The inverting terminal of the 
 operational amplifier, which is connected 
 to the grid, is maintained at the same 
 potential as the non-inverting terminal, 
 therefore no potential difference can 
 exist between the guard ring and the 
 grid, which results in no current flow. 
 
 The Becon MK IV detector is a single 
 ionization chamber analog output particle 
 detector. The conducting plastic chamber 
 case and the grid are separated by a po- 
 tential difference of approximately 10 V. 
 This potential difference, with ionized 
 air as the medium, produces an ionization 
 chamber base current of approximately 0.5 
 nA. The 5-mCi krypton 85 beta radiation 
 source (half-life of 10.8 yr) is used to 
 ionize inflow air. The ionization cur- 
 rent across the ionization chamber is 
 adjusted by varying the potential across 
 the case and the grid to yield an ioniza- 
 tion current level proportional to a 
 -0.9-mA output current. This base ion- 
 ization current level corresponds to the 
 particle concentration in normal ambient 
 mine air. 
 
 The potential difference between the 
 case and the grid remains essentially 
 constant, however, the ionization current 
 will vary depending upon the size and 
 concentration of particles carried by the 
 inflow air into the ionization chamber. 
 
 A charged smoke particle is much heav- 
 ier than an air molecule, therefore its 
 drift velocity due to the potential be- 
 tween the ionization chamber wall (case) 
 and the grid is very small compared to 
 the convective airflow velocity. The 
 smoke particle also has a much larger 
 surface area than an air molecule, which 
 reduces the mean free path between colli- 
 sions of the ions responsible for current 
 flow, allowing for greater numbers of 
 positive ion and electron recombinations. 
 Because of recombination, the electron 
 and ion mobility are reduced, which re- 
 sults in a detectable decrease in the 
 ionization current. 
 
22 
 
 A reduction in the ionization current 
 corresponds to a similar reduction in the 
 output of current from the current ampli- 
 fier. The ionization current and there- 
 fore the output current are varied by 
 adjusting a potentiometer that is acces- 
 sible through a hole in the outer casing 
 of the detector. A capacitor ensures the 
 stability of the current output and sup- 
 plies the necessary feedback to maintain 
 correct circuit operation when connected 
 to any high output load capacitance. 
 
 In order to prevent internal leakage 
 between the grid and the chamber case be- 
 cause of dust accumulations, a circular 
 guard ring is installed between the case 
 and the grid. Any electrical leakage 
 between the grid and the casing would 
 result in a reduced current flow from the 
 grid to the current amplifier and cause 
 an erroneous output current, A potenti- 
 ometer is adjusted so as to maintain an 
 input offset voltage of approximately 
 V. 
 
 Because there is no potential differ- 
 ence between the guard ring and the grid, 
 no current can leak between the two, 
 thereby the grid is electrically isolated 
 from the chamber case. 
 
 Humidity, which causes precipitation of 
 moisture on the insulation material sur- 
 rounding the hermetically sealed elec- 
 tronics, can also cause electrical leak- 
 age between the grid and the chamber 
 case. However, because of the strategic 
 location of the current amplifier within 
 the component box, the heat generated by 
 the amplifier keeps the insulator essen- 
 tially dry around the grid area. Though 
 other components of the detector may be 
 covered with moisture, no current can 
 leak across the insulator to the grid. 
 
 The location for mounting the Becon MK 
 IV detector should be near or on the 
 downwind side of a potential fire hazard 
 area, however the ventilation air veloc- 
 ity in the chosen area should not exceed 
 6 m/s . 
 
 Input power requirements are -15 V dc, 
 -5.5 mA dc. A four-conductor shielded 
 cable should be used to provide for the 
 input power and output signal. 
 
 Because the Becon MK IV detector has no 
 moving parts, very little maintenance is 
 necessary. Periodic examination of the 
 
 electrical cable for breaks or frays and 
 calibration are all that is required. 
 
 Because of the natural decay of the 
 krypton 85 radiation source (half-life 
 10.8 yr) , the output current will drop as 
 the source decays with age. Therefore, 
 calibration according to the previous 
 section should be carried out annually to 
 ensure proper and consistent operation. 
 This calibration adjustment can compen- 
 sate for an approximate 50 pet reduction 
 in the strength of the source. Because 
 of this reduction in source strength, the 
 source should be replaced at some time 
 approaching the half-life of the krypton 
 85. 
 
 CARBON DIOXIDE DETECTOR FOR SPONTANEOUS 
 COMBUSTION FIRE WARNING 
 
 The Anglo American Electronics Labora- 
 tory Spanair analyzer was selected for 
 CO2 detection in the spontaneous combus- 
 tion fire warning system. The Spanair 
 nondispersive infrared analyzer detects 
 the attenuation of radiation due to 
 molecular absorption by the sample gas. 
 Variations of the basic cell will show 
 gas concentrations of CO, CO2, CH2, NO2, 
 or SO2. A nichrome filament pulsed at a 
 specified frequency radiates broad-band 
 energy. This energy passes through the 
 sample gas in a reflective optical cham- 
 ber, through a spectral filter, and is 
 measured by a pyroelectric cell photo- 
 detector. The electrical signal output 
 is inversely proportional to the gas con- 
 centration. Selectivity to the sample 
 gas is determined by the band-pass spec- 
 tral filter. 
 
 Both analyzer head and power supply are 
 mounted in a 14- by 18-in fiberglass en- 
 closure designed for underground instal- 
 lation. A dust filter is fitted to the 
 sample plenum. No pump or thermal con- 
 trols of major importance are required. 
 Electrical connection is made in a junc- 
 tion box partitioned from the analyzer. 
 Input power is 110 V ac , 50 to 60 c/s; 
 output is to 1 V dc analog, with system 
 failure indicated by a 0-V output signal. 
 The output signal decreases logarithmic- 
 ally with increasing gas concentration. 
 Although determination of actual gas con- 
 centrations require conversion of the 
 
23 
 
 voltage via a calibration curve (provided 
 with each unit) , unusual excursions from 
 normal levels are readily apparent on 
 strip charts and can trigger alarms. 
 
 In operation, the Spanair analyzer CO2 
 signal shows a constant level of about 
 330 ppm, which is the concentration of 
 CO2 in normal atmospheric air. As incip- 
 ient heating occurs in combustible mate- 
 rial, large volumes of CO2 will be given 
 off well before pyrolysis begins. The 
 system will report these changes as a 
 gradual increase in CO2. 
 
 A receiver (surface unit) processes the 
 analyzer output into alarm levels. The 
 chart records input, voltmeter, and sys- 
 tem failure signals. For long distance 
 data transmission, a frequency-division 
 multiplex telemetry system is utilized. 
 Several remote analyzer heads can com- 
 municate over one balanced transmisison 
 line (two wires and suitable ground). 
 
 The only difficulties anticipated were 
 a lack of published performance specifi- 
 cations and a lack of repair parts or 
 maintenance service available from the 
 Republic of South Africa. However, a 
 competent technician can maintain the 
 electronic circuitry and analyzer head 
 with use of the furnished manual. The 
 only anticipated maintenance consists 
 of periodic cleaning of the particulate 
 filter if the environment is dusty. Mir- 
 rors should be cleaned every few years to 
 maintain a strong signal. Calibration 
 requires an output adjustment to 1.0 V 
 during nitrogen purge. 
 
 Mine fire detection systems are expect- 
 ed to operate under conditions that would 
 normally disable laboratory instruments. 
 Thus, performance data obtained under 
 stable laboratory conditions do not fully 
 predict performance expected for a mine 
 where conditions are harsh and unstable. 
 Laboratory tests were conducted to deter- 
 mine the degree to which the instrument 
 is immune to such harsh and unstable con- 
 ditions. Conditions that are expected 
 underground and that are reproducible to 
 a certain degree in the laboratory in- 
 clude the following: 
 
 1. Line voltage variation between 90 
 and 140 V ac. 
 
 2. Blackouts for long time periods. 
 
 3. Changes in temperature between 10° 
 and 40° C. 
 
 4. Changes in ambient moisture level 
 between 20 and 95 pet relative humidity. 
 
 The analyzer displayed a slight in- 
 crease in sensitivity to CO2 concentra- 
 tions at temperature extremes, however, 
 the problem is considered to be minor. 
 The analyzer is not sensitive to changes 
 in relative humidity or line voltages. 
 Following power interruption, the instru- 
 ment restabilizes within 1 minute after 
 power is restored. 
 
 OVERHEAT DETECTION FOR CONVEYOR DRIVE 
 
 Overheat detection for the conveyor 
 drives at the trona mine was provided 
 with Edison Electronics mpdel 377 control 
 and model B fire detection (thermistor) 
 cable. The 377 control is 2-1/2 by 1-3/4 
 in. in size; is wired through an eight- 
 pin connector to power (24 V dc); con- 
 tains a detection cable, audio alarm, and 
 lights; and is mounted to electrical ter- 
 minals inside a 6- by 8-in corrosion re- 
 sistant box. The model B fire detection 
 cable is a thermistor; that is, a temper- 
 ature sensitive resistor. The model B 
 cable is tubular, 0.070 in. in total 
 diameter, with a 0.020-in-diam iron wire 
 center conductor imbedded in a 0.010-in- 
 thick layer of metal oxide semiconducting 
 material. It is 20 ft long and operates 
 within the temperature range of -40° to 
 2,000° F. 
 
 The model B fire detection cable is 
 similar to the cable used in the salt 
 mine shaft. It is constructed with a 
 metallic outer sheath and a metal wire as 
 the center conductor. They are elec- 
 trically isolated from each other by 
 a cylindrical semiconductor layer. The 
 thermistor's resistance between conduc- 
 tors is depicted by a negative tempera- 
 ture coefficient with a drop in resist- 
 ance that is nearly exponential with a 
 linear increase in temperature. The rate 
 at which the resistance drops and the 
 temperature at which it drops can be al- 
 tered by varying the type and quality of 
 the semiconductor material. 
 
 The semiconductor material used in the 
 Edison model B fire detection cable has a 
 
24 
 
 conductivity between that of a metal and 
 that of an insulator. Within a semicon- 
 ductor there exists three discrete energy 
 levels that electrons may cross or occu- 
 py; the valence band, the energy gap, and 
 the conduction band. If there are unoc- 
 cupied high energy levels within the 
 valence band or if the valence band flows 
 smoothly into the conduction band, addi- 
 tional kinetic energy can be given to the 
 valence electrons by an applied electric 
 field, resulting in conduction such as in 
 a metal. However, if the valence band of 
 the material is completely full and there 
 exists a large energy gap, such as 6 eV, 
 between the valence band and the conduc- 
 tion band, the material acts as an 
 insulator. 
 
 The semiconductor material has a full 
 valance band, an essentially empty con- 
 duction band, an energy gap of approxi- 
 mately 1 eV, and behaves as an insulator 
 at room temperature. Unlike an insula- 
 tor, the semiconductor, when heated, can 
 gain enough thermal energy from, its sur- 
 roundings to allocate electrons from the 
 valence band to the conduction band. The 
 semiconductor's conductivity increases 
 with temperature as more electrons are 
 elevated to the conduction band. Elec- 
 trons in the conduction band and hole, 
 vacant spots in the crystal lattice of 
 the valence band, are free to move under 
 the influence of an electric field. 
 
 The resistance is measured from the 
 center conductor through the temperature- 
 sensitive semiconductor material to the 
 outer conductor. The resistance is 
 equivalent to that of an infinite number 
 of resistors connected in parallel. The 
 Edison 377 control senses the drop in re- 
 sistance when the model B fire detection 
 cable is heated, by a proportional drop 
 in the potential difference across two 
 conductors of the cable. If the total 
 resistance of the cable is between 38 and 
 315 Qy the control senses the voltage 
 proportional to the resistance and acti- 
 vates the fire alarm via two transistors 
 that are turned on by an operational am- 
 plifier. If on the other hand the re- 
 sistance between the two conductors of 
 the thermistor is less than 38 Q, the 
 
 control senses a lower porportional volt- 
 age through a second operational ampli- 
 fier. This operational amplifier turns 
 on several transistors that activate the 
 cable fault light and lock out the alarm 
 circuit. 
 
 ULTRAVIOLET FLAME DETECTION 
 
 The DetTronics U7602 ultraviolet (UV) 
 flame detector was selected for fire de- 
 tection in the grease niches at the trona 
 mine. This detector responds to the 
 wavelengths of light given off by a fire 
 in the UV range of 1,850 to 2,450 A. The 
 electronics are housed in an explosion- 
 proof enclosure 8.91 in. in total length, 
 constructed of two screw-together coaxial 
 cylinders — one of 4.84-in length and 2.3- 
 in diam, and the other of 4.07-in length 
 and 3.25-in diam. The UV viewing area is 
 a 90° cone. Input voltage is 120 V ac 
 with a maximum power consumption of 3.0 
 W. Two digital alarm modes are provided: 
 one closed relay for fire alarm and one 
 open relay for dirty lens alert. 
 
 The DetTronics UV detector utilizes a 
 Geiger-Mueller type tube to sense UV 
 radiation emitted from a fire. A typical 
 Geiger-Mueller tube is constructed with a 
 wire anode, which operates between 160 
 and 250 V above the cathode. The tube is 
 sealed from the air and filled with an 
 inert gas such as argon or helium. Light 
 can enter through only one end of the 
 tube; all other sides are optically 
 isolated. 
 
 When UV light passes into the tube, 
 electrons are knocked off the gas atoms 
 and ions are created. The electrons are 
 accelerated to the anode because of the 
 applied electric field and in turn knock 
 off more electrons from the gas atoms, 
 resulting in an avalanche effect. The 
 ions move to the cathode and electrons to 
 the anode causing the tube to conduct. 
 However, when all the possible ions and 
 electrons have been attracted to their 
 respective electrodes, conduction ceases. 
 Therefore a quenching circuit is neces- 
 sary to allow the positive ions and elec- 
 trons to recombine and reactivate the 
 tube. 
 
25 
 
 When the tube conducts it draws down 
 the voltage across a capacitor. The ex- 
 tinguish voltage on the detector is in 
 the region of 160 V, a level at which the 
 ionization processes that support the 
 discharge can no longer be maintained. 
 At this point, the tube will stop con- 
 ducting and the capacitor will recharge 
 through a resistor that is a current lim- 
 iting resistor. As the capacitor re- 
 charges, it will reach a voltage level in 
 the vicinity of 250 V, which is the nor- 
 mal striking or starting voltage of the 
 tube. If UV radiation of sufficient in- 
 tensity is present at this moment, the 
 tube will fire again, and this process 
 will be repeated over and over as long as 
 radiation is present. The more intense 
 the radiation the more frequent the dis- 
 charge rate of the detector. 
 
 The fire warning relay is closed when 
 25 or more discharges occur per second. 
 
 The DetTronics U7602 detector is also 
 equipped with an UV test lamp that mon- 
 itors the integrity of the optical lens 
 and deenergizes a relay when the surfaces 
 become obstructed with oil, dirt, or 
 dust. The UV test lamp emits UV radia- 
 tion that passes through the lens, re- 
 flects off a beveled reflecting ring mir- 
 ror, passes back through the lens and 
 into the tube. 
 
 CARBON MONOXIDE DETECTION 
 
 Carbon monoxide detection for the spon- 
 taneous combustion fire warning system 
 and the trona mine fire detection system 
 was provided by the Energetic Sciences 
 Ecolyzer 4000 and the MSA 571. 
 
 Both the MSA 571 and the Ecolyzer 4000 
 are CO detectors that utilize the elec- 
 trochemical properties of a fuel cell to 
 sense CO. Input power to both of these 
 detectors is 120 V ac. The electrochemi- 
 cal sensor is constructed of three 
 
 electrodes — the sensing electrode, the 
 reference electrode, and the counter 
 electrode — all suspended in an acid solu- 
 tion. The materials to be chemically 
 reacted are CO and oxygen gases from the 
 mine's ambient air. These gases diffuse 
 into the acid (or in the case of the 
 Ecolyzer 4000 are pumped into the fuel 
 cell by an air pump) solution and ionize. 
 Refer to the following half reactions: 
 
 2 (CO + H2O) = 2CO2 + ^^'^ + ^e- 
 
 O2 + 4H"^ + 4e- = 2H2O, 
 2C0 +02= 2CO2 
 
 The CO is electrochemically oxidized 
 at the sensing electrode while oxygen 
 reduction occurs at the counter elec- 
 trode. The ion concentration in the acid 
 solution because of the dissolved gases 
 is proportional to the concentration of 
 CO in the air; likewise, the current 
 flow through the cell is proportional to 
 the ion concentration in the solution. 
 Therefore, the current flow through the 
 cell is proportional to the CO content of 
 the air. This current flow is then am- 
 plified and compensated for temperature 
 before it is sent to the sensor control. 
 
 The MSA 571 and Ecolyzer 4000 CO detec- 
 tors are very similar in their function. 
 Their input amplifiers generate a 1-V 
 full-scale analog signal output from the 
 signal received from the sensor cells. 
 The input amplifier drives a meter on the 
 detector's front panel and also provides 
 a 0- to 1-V output proportional to the CO 
 concentration. Operational amplifiers 
 used as voltage comparators monitor the 
 output voltage of the input amplifier. 
 When this output voltage reaches a level 
 proportional to 20 ppm, the warning relay 
 activates; at a level proportional to 50 
 ppm, the alarm relay activates. 
 
 i-U.S. CPO. 1985-50SO19/20,073 
 
 INT.-BU.OF MINES, PGH., PA. 28029 
 
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