IN-MINE EVALUATION OF UNDERGROUND FIRE 

AND SMOKE DETECTORS 

Russell riff in' 

Twin Cities Mining Research Center, Bureau of Mines, 

U.S. Department of the Interior, Twin Cities, Minn. 55111 

ABSTRACT 

The current state of the art of fire and smoke detection technology is reviewed from the standpoint of suitability for use in underground metal and nonmetal mines. Detection modes, fire signatures, and 

environmental considerations are included. Preliminary results of longterm, in-mine tests are presented. 

INTRODUCTION 

Classification of the various smoke and fire sensors into three 

categories--optical view field, direct contact, and products of combustion (P0C)--requires some explanation since most off-the-shelf devices (systems) have more than one principle of response and may respond to more than one parameter. Hybrid sensors may include heat detectors as well as photoelectric- and/or ionization-type product of 

combustion (POC) detectors, thus classification is according to the 

primary intended use. For the nonconventional fire detectors that have 

been developed for gas analysis and detection, the classification may 

be less obvious. Most of the nonconventional detectors are described 

in this report under POC types of sensors, even though they may depend 

on optical or filament-type principles. A further distinction of detector types divides them into point (or local) source, as opposed to 

the extended-area-type sensor. 

For more detailed information on the types of fire detectors 

available, there are good survey articles (5: and National Fire 

Protection Association (NFPA)reference books (fi), as well as manufacturers' sales literature. 

FIRE SIGNATURES 

Any product of a fire that changes the ambient conditions is 

called a fire signature (5) and is potentially useful for detection. 

To be practical for detection, a fire signature must cause a measurable change in some ambient condition. With other factors being equal, 

such as hardware costs and detection times versus hazard level, the 

preferred fire signature is one that generates the highest signal-to- 

'~lectronics engineer. 

2~nderlined numbers in parentheses refer to items in the list of 

references at the end of this report. 
noise ratio in the earliest period of fire development. The principal 

fire signatures used in the detectors discussed herein are aerosol, 

energy release, and gas signatures. 

Aerosols 

Aerosols are particles suspended in air. The process of combustion releases large quantities of solid and liquid particles into the 

atmosphere ranging in size from 5 x to 50 p. 

Aerosols resulting from a fire represent two different fire signatures. 

Particles less than 0.3 pm do not scatter light well and are classified 

as invisible. Those larger than 0.3 pm do scatter light and are classified as visible. The invisible aerosol signature is usually referred 

to as "products of combustion," and the visible aerosol signatures, as 

"smoke." Invisible aerosol is the earliest appearing fire signature 

in most cases. (Coal can evolve CO before particulates--PVC materials 

evolve HC1 quite easily). 

Energy 

Fire also constantly releases energy into the environment and this 

energy release provides some useful fire signatures. The earliest 

energy signatures detectable with available hardware are the infrared 

(IR) and ultraviolet (W) portions of the electromagnetic spectrum. 

With the exception of highly unsaturated hydrocarbons such as acetylene, infrared emissions from hydrocarbons are particularly strong in 

the 4.6-pm region owing to carbon dioxide and in the 2.7 p region 

owing to water vapor. This radiation signature can be used effectively 

for detection but there is the possibility of noise from manmade IR 

sources. UV fire signatures appear in flames as emissions from OH, CQ, 

and CO in the 0.27- to 0.29-IJ~ region. They have been used effectively 

for flame detection, especially in cases where magnesium or its alloys 

may be involved. 

Gases 

Many gases are added to the atmosphere during a fire and these 

changes are called evolved gas signatures. A related change is the 

reduction of oxygen content (the oxygen depletion signature). These 

gases may include CO, COa, HC1, HCN, HI?, Has, NH3, and NOx depending on 

the type of material burning. The most useful gas for detection is CO, 

since it is present in almost all fire situations. Slow burning and 

smoldering fires in particular produce large quantities of CO. 

OPTICAL VIEW FIELD FLAME DETECTORS 

Fire detection devices of the optical view type respond to radiant 

energy portions of the electromagnetic spectrum generated during flaming 

combustion of materials. The principal sensing elements used include 

solid-state detectors--junction type and bulk effects type, vacuum or 

gas-filled tubes, and thermocouples and thermistors for special applications. The photodetector family type fills a large part of the solidstate category. 
Ultraviolet Sensing Detectors 

The variety of W detectors (wavelengths less than 0.4pm approximately) is small compared with those in the IR region because of the 

basic problems associated with W detection. Ordinary glass windows 

cut off radiation below 0.3 pm and quartz and W grade sapphire become 

opaque below 0.18 pm. Below 0.1 p there is essentially no suitable 

window material. 

Infrared Sens inn Detectors 

Infrared detectors have the problem of background radiation at 

0 ambient temperatures (25 C) being entirely in the IR region (wavelengths greater than 0.8 vm). A common method of discriminating 

against this background is that of chopping the incident radiant flux 

so that the detector receives only a fixed radiant frequency, typically 4 to 30 Hz. 

Infrared detectors are classified according to their method of 

response to heat and photon flux: thermal detectors and quantum 

detectors. 

Thermal detectors respond to energy absorbed by a temperature-senitive material or an absorbing film in contact with the temperatureensitive material. Included in this category are such devices as 

thermocouples, metals or semiconducting layers with resistance a function of temperature (bolometers and thermistors), pyroelectric detectors whose polarization is temperature dependent, and gases with pressure being temperature dependent. 

Quantum, or energy, detectors respond to photon flux falling on a 

sensing element and exciting electrons in a bound state to a free or 

conducting state. 

Within these broad classifications the IR spectrum is divided 

into three divisions: near infrared (0.8-1.4 pn), intermediate 

infrared (1.4-7 ~m), and far infrared (>7 pm). Near infrared requires 

conventional silicon and germanium detectors; intermediate infrared 

uses special IR detectors; and far infrared calls for thermal detectors such as thermocouples and thermistors. In some cases there is an 

overlapping spectral area of response. PbS is one of the most versatile photoconductors and responds to IR from approximately 1 to 4 pm. 

Doping Ge with Hg and Cu extends its response to 8 to 14 p. Sensitive 

2 2 

areas range from 0.Olmrn to 10 m , with time constants shorter than 

1 ns. (InSb may be more useful than PbS since it covers the CO and CP2 

bands ) . 

Pyrodectric-type detectors are a more recent development (1-2). 

These consist of a slice of ferroelectric material sandwiched between 

electrodes to provide a sensing cell. Typical materials include triglycine sulfate, tourmaline, Rochelle sal t, barium titanate, and polyvinyl f 1 our ide. Electrically , the material behaves similarly to a 

capacitor dielectric with a strong temperature-dependent polarization 
of the magnetic domains. One of the electrodes is transparent to IR 

radiation which causes a small temperature increase producing a polarization charge on the pyroelectric material and a corresponding voltage 

change across the two electrodes of the cell. 

HEAT SENSORS (DIRECT CONTACT DETECTORS) 

There are two general types of heat sensors; those that employ the 

fixed temperature principle, and those that employ the rate-of-rise 

principle (6). With the fixed temperature approach an appropriate temperature level setting is selected. When the active element is heated 

to its operating temperature the active element will bend, expand rapidly or maximally, fuse (change its electrical conductivity and physical state), or produce an electromotive force (emf) which can be amplified to actuate an alarm. Commercially available devices include 

temperature-sensitive ampoules or pellets, bimetallic elements, eutectic solders and salts, snap discs, thermocouples, and thermistors as 

point sensors. There is a subcategory of continuous line or wire types 

using thermistor material, eutectic sal t, pressurized gas, or twisted 

insulated wires under tension. Excluding specially prepared rapid response thermocouples, a major disadvantage of these fixed-temperature 

types of sensors is slow response time because of thermal inertia. 

Another disadvantage is their inabil ity to detect low level incipient 

combustion in its early stages. Commercial fire detector thermocouples 

have response times on the order of a few tenths of a second. Advantages of direct contact detectors are low cost, reliability, freedom 

from maintenance, and insensitivity to vibration and dust-laden atmosphere. Continuous line heat sensors have been tried in underground 

mines . PRODUCTS OF COMBUSTION DETECTORS 

Products of combus tion in fires include sol id particulates and 

liquid mists (including invisible and visible particle sizes), ionized 

species gases, and radiant energy. Combustion product detectors sense 

one or more oE these constituents including heat or flame. 

Ionization-Type Detectors 

Ionization-type detectors constitute one general class of POC detectors. The most common source of ionization is analpha emitter such 

as R~,""'so,, or ~rn~~'. Beta emitters are sometimes used but not as 

often since stronger sources are needed to create ionization currents 

similar to those produced in.alpha detectors. Krs5 is one beta emitter 

that is used. The radioactive emitter ionizes the air in a chamber 

between two electrodes. Current measurable in picoamperes is produced 

from production and transport of positive and negative ions to the 

opposite poles of the plates. A decrease in current, relative to clean 

air, is obtain'ed when combustion products enter the chamber because the 

combustion particles become ionized and, being larger and heavier than 

the air molecules, move much more slowly toward the end of the chamber. 

An electric circuit detects the drop in current and initiates the 

alarm. 
which is a function of gas concentration. The adsorption of a gas 

molecule on the surface of a semiconductor generally results in the 

transfer of electrons due to the differing energy levels of the gas 

molecule and the semiconductor surface. Oxygen, which can accept 

electrons is adsorbed on the surface of n-type semiconductor. The 

transfer of electrons from the donor level of the semiconductor to the 

layer of adsorbed gas results in decreased conductivity of the semiconductor material. 

When a TGS that has adsorbed oxygen in this manner comes into contact with reduction or combustible gases such as CO, hydrocarbons, etc. 

the molecules of these gases are adsorbed and the transfer of electrons 

is in the opposite direction to the oxygen reaction, releasing the 

electrons to the semiconductor space charge layer and causing a large 

increase in the conductivity of the sensor. The sensor output is sufficiently large to allow gas detectors to be designed using a minimum 

number of components, thus allowing the production of low-cost 

detectors. 

Significant advantages of these solid-state electrolytic cells are 

the very long lifetimes expected, usually several years, the low cost 

and simplicity inherent in its design, and the relatively simple electronics needed to utilize the sensor output. Other features include 

not being permanently poisoned by the toxic gases, resistance to vibration and shock, and having no loss of sensitivity even at gas concentrations so high that air (oxygen) is displaced. 

A severe disadvantage is its ease of alarming in areas containing 

engine combustion products, and its sensitivity to a wide variety of 

easily oxidized gases not necessarily associated with products of 

combus tion. 

For fire detection use the TGS responds readily to CO, which is 

one of the principal gases given off in the early stages of fire. However, because of the sensitiv.ity of TGS to gases other than POC and exhaust gases, its practical use may be limited to a hybrid system involving ionization or photoelectric detectors. TGS detectors can be 

calibrated within the range of 200 to 1,000 ppm CO and to a limited 

extent can be made selective to CO. 

Electrochemical Detectors 

Electrochemical (fuel cell ) techniques for CO sensing depend on 

the electrochemical reaction of the detected gas, oxidized or reduced 

(depending on the sensor) at an appropriate electrode potential, 

producing an electric current that, because of membrane diffusion controlled conditions, is directly proportional to the gas concentration. 

These devices are subject to interference from gases other than the 

desired one for detection, but are capable of precision calibration, 

making them useful as an analytical device. They tend to be expensive 

and not too rugged for the mine environment but are useful for estab1 is hing a1 arm 1 eve1 s and personnel monitoring . 
Photoelectric-Type Detectors 

Photoelectric fire and smoke detectors constitute another general 

type POC detector. The requirements are a source of light and a 

detector of that light to measure its radiant power. Four different 

modes of operation are possible depending on the amount of light transmitted or absorbed by the medium, the amount reflected, scattered, or 

refracted. Most, if not all, comercially available units possibly 

suitable for mine use are of the light scattering type. A beam of 

light from a source travels across a light-tight chamber to a light 

trap or collector opposite the source. A photocell positioned at right 

angles to the beam senses no light as long as the air inside the 

chamber is clean. The chamber is open to its surrounding atmosphere 

through baffling, and if smoke (POC) enters the chamber, light from the 

beam is reflected or scattered i n a l l directions. Some of this scattered light reaches the photocell, changing its resistance and via 

suitable electronics initiates an alarm. The alarm level is adjusted 

to actuate at a given smoke concentration level (normally 3.3 to 6.6 

pct -m'l' or 1 to 2 pct per foot). In a few models the electronics are 

also designed to give an alarm if the rate of rise of smoke obscuration 

exceeds a given value (commonly 0.33 pct -m'l-min'l or 0.1 pct per foot 

per minute). Auxiliary or backup heat detection is often provided with 

these detectors. 

Photoelectric detectors, like ionization detectors, may respond to 

particulate matter from sources other than fire. A dusty mine environment might cause some false alarms and would also require periodic 

cleaning of the sensor chamber internal surfaces. In a wet mine, aerosol forms of fog and mists might cause some false alarms. These detectors are also somewhat dependent on airflow velocity (good chamber design) and location because of the tendency for smoke to stratify in 

airstreams. They are also less sensitive to black smoke compared with 

white or grayish smoke. 

Solid-State Detectors 

Solid-state devices for detecting and sometimes sampling the 

gaseous components of products of combustion are a third general type of 

detector that can function as a fire detector. 

A promising new development in POC gas detection is the semiconductor cell detector known as the Taguchi gas sensor (TGS). This 

device uses a selection of bulk n-type metal oxides such as SnOz, ZnO, 

and Fez03 impregnated in a solid-state matrix material supported by 

conductive filaments. Variation of the chemical formulation allows the 

cell to be made sensitive to a particular gas or group of gases, and 

ins ens i t ive to o ther s . A heater electrode imbedded in the cell provides sufficient increase of cell temperature above ambient to allow the cell to operate 

by diffusion, eliminating the need for a sampling pump arrangement. A 

collector electrode provides for measurement of cell conductivity, 
Optical Gas Detectors 

Optical gas sensors are generally confined to laboratory use, although recent prototype models have been developed for personnel hazard 

monitoring. These use the strong adsorption band of CO at 4.65 pm (infrared). Radiation from an infrared source is passed through a cell 

through which the sample gas flows and is adsorbed by the CO molecule. 

A filtered infrared detector responds to this change in radiation and 

its output is compared with a reference cell, conditioned by suitable 

electronics and read out on an appropriately marked meter. 

A technique useful with different POC detectors is the use of tube 

bundles (L). With these, air samples can be drawn from different sections of the mine and sequentially monitored at a central location. 

This procedure provides protection and stability for the detector systems and can add to the flexibility of the mine fire protection system. 

Slow response times and wall-diffusion losses for submicrometer particles provide inhibitions to its use. 

ENVIRONMENTAL CONSIDERATIONS 

There are reaily two environmental problems to be faced in designing a reliable fire detection system: Will it continue to operate, 

and if it does, can it discriminate against the normal mine ambient 

parameters? 

The latter problem can possibly be dealt with by the use of combinations of detector types in the system to provide detection of cellulos ic and hydrocarbon fires, spontaneous combus tion, and electrical 

fires. Ambient parameters will differ in various parts of the mine. 

Occasionally air ventilation networks can improve a POC detector' s 

sensitivity and effectiveness . Sampl ing tube networks or remote detectors with telemetry can monitor several locations individually. A 

detector type useful in one location may be quite unsuitable for use 

in another location; therefoce, if conditions change with time, the 

sensor system should be reevaluated. 

Ability to discriminate against normal mine ambient parameters is 

an important consideration in the design of reliable fire detection systems. Mine Safety Appliances Research Corporation (MSAR) under a Bureau 

of Mines contract (2) provided CO, C02, NOx, and NO concentration data 

for shot firing and diesel operations in selected mines and sampling 

sites, within the mine. Table 1, taken from their report, summarizes 

much of their data in the form of ratios of contaminants measured in 

connection with various mine operations. The report suggests that a 

duel sensing system involving contaminant ratios may provide an approach to minimizing or eliminating false alarms. The CO/CO, ratio of 

diesel exhaust is quite comparable to that of the combustion products 

of a metal mine fire. Similarly, the CO/NOx ratios for shot firing and 

a leaking bulkhead, where a hot spot was thought to exist, are also 

quite similar. The C02/NOx ratio is significantly different for shot 

firing and diesel operations compared with the ratio for a fire and/or 

the leaky bulkhead with a hot spot. This much higher latter ratio may 
TABLE 1. - AVERAGE CONTAMINANT RATIOS FOR 

VARIOUS UNDERGROUND ACTIVITIES 

provide a reliable means of fire detection. The report also suggests 

that studies of the time-distance behavior of the NOINOX ratio be made 

if the C02/NOx ratio were to be employed as a means of incipient fire 

detection. 

Activity 

Shot Firing........ 

Diesel............. 

Fire............... 

Leaking Bulkhead. .. (4,2001 ) 

Leaking Bulkhead. .. (3,6001) 

In an early Bureau contract study by the Gillette Research Institute, two different types of battery powered ionization detectors were 

taken underground for a short-term performance evaluation. One, taken 

into the Bunker Hill mine, Kellogg, Idaho, performed poorly after a 

few hours of operation in areas contaminated with high concentrations 

of aerosols and in regions of high ventilation airflows. It did operate satisfactorily in regions supplied by low-velocity intake air. 

The other detector, a dual-gate reference type, performed reliably 

at Central Rock Company, Lexington, Ky. The latter device was not necessarily superior to the former because the Bunker Hill mine presented 

far greater extremes of environment. What the experiment tended to demonstrate was that state-of-the-art ionization sensors, developed for 

normal industrial or residential use, military or connnercial aviation 

fire protection, have questionable reliability or, in this case, are 

inappropriate for use in a mine environment. 

CO/CO, 

0.38(0.44) 

0.04 

.025 

.03 

.001 

It is fair to say that there is no universally applicable fire and 

smoke detector--one that responds equally well and consistently to all 

types of fires. The various detector types have been discussed along 

with some of their advantages and disadvantages. Long-term evaluation 

of the various detectors l sensitivity and reliability under the extremes 

of ambient conditions just described are underway and may lead to viable, 

cost-effective detection and suppression systems for underground mine 

fires. Until such systems have been developed and environmentally and 

operationally reliable detectors of various types have been found through 

long-term testing, it is best that unreliable fire detectors not be installed in mines. 

CO/NOx 

6.35(4.58) 

6.9 

100 

-- 

12 

C% /NO, 

11.6(12.4) 

97 

4,000 

-- 

8,000 
Another important consideration involving mine environment is the 

spacing and positioning of fire and smoke detectors in the mine. 

Little is known about optimum detector positioning and spacing in 

mines. NFPA installation standards do not offer any criteria directly 

useful in a mine because their codes deal with residential and commercial situations. Mines vary so widely in distribution of combustibles, 

ventilation patterns, and roof support systems that an installation 

code may not be feasible or even desirable. Complicated ventilation 

patterns, random ceiling obstructions, and a variety of tunnel shapes 

would not allow general applicability of such a code. 

Because of the fuel-to-air ratio influence, a knowledge of the 

relationship between fire and mine ventilation is essential for the 

proper selection and location of sensors and detectors and also for the 

correct interpretation of information provided by them. Mine fires can 

be categorized into two distinct types--oxygen rich (overventilated) or 

fuel rich (underventilated). The fuel-to-air ratio determines which 

type occurs; the oxygen-rich type can evolve into the fuel-rich type by 

spontaneous growth. Timber fires (fuel rich) represents an extreme 

toxicity hazard because of low 0, concentrations and high CO and COz 

concentrations. Fuel-rich conditions al!:o produce more smoke, increasing toxic species exposure and obscured vision. 

The greatest hazards of mine fires are caused by toxic and sometimes explosive products of combustion being carried through the mine 

by the ventilation system and unexpected airflow reversals carrying 

toxic fumes to intake air ventilation areas, such as fire escaperoutes, 

hoist areas, and other areas usually thought of as safe in the event of 

a fire. The analytical prediction of airflow reversal in a mine, even 

under ideal conditions, is a very difficult task and beyond the scope 

of this report. However, Greuer (4) has compiled a large number of 

references and provided a detailed accounting of the influence of mine 

fires on the ventilation of underground mines for those who wish to 

pursue the problem in detail. 

Greuer (4) makes a few quantitative predictions of ventilation 

disturbances that may be useful in interpreting the indications seen 

when monitoring a fire detection system. These include the following: 

Throttling and natural draft changes can cause changes in the quantity of ventilating air currents and sometimes a reversal of their direction. Similar changes can occur in neighboring connected airways as 

well as that of the fire scene. 

Air (oxygen) quantity reductiom (Oz pct) in non-gassy mines can be 

neglected. In gassy mines air reductions can lead to explosive mixtures which may be carried back through the fire zone. 

Ventilation disturbances can also cause smoke layering, which may 

affect the ability of a detector to operate effectively. 
BUREAU OF MINES EXPERIENCE 

Shaft Installations 

To work toward an answer to the major problem of fire hazard and 

contaminated air resulting from fire, the Department of the Interior, 

U.S. Bureau of Mines, contracted with the F'MC Corporation, San Jose, 

Califr(Contract H0252016 "Mine Shaft Fire and Smoke Protection System"), to evaluate mine shaft fires and their hazards @). The contract also called for the development and demonstration of a low-cost, 

reliable mine shaft fire protection system that could be adapted to a 

majority of metal and nonmetal mine shafts, raises, and winzes, especially in deep mines. Results of this contract have been published 

elsewhere (10) and so need not be extensively detailed here. However, 

as an example of a proven system, sensor and detector components that 

performed successfully in underground tests with controlled fires will 

be highlighted here. 

As part of the background preparation for designing the systems 

produced, it was found that approximately 67 pct of shafts in metal and 

nonmetal mines are of wooden construction, and contaminated air was 

responsible for about 77 pct of fatalities from fires. Causes of fires 

in mines were predominantly electrical (35 pct) and welding (18 pct). 

Most fire-connected fatalities occurred in or near shaft stations, the 

gathering point to normally leave the mine. 

Four major needs were brought out for providing effective mine 

fire and smoke protection: 

1. Reliable sensors are necessary for early fire and smoke 

warning. 

2. Isolation barriers (vent doors) would stop the flow of contamina ted air . 

3. Available water supplies could assist with fire extinguishing. 

4. Recording sensor systems and control ventilation doors and 

extinguishing systems from the surface would be preferable to 

sending fire-fighting teams underground. 

The possibility of a fire starting somewhere in the mine is always 

present because combustibles and ignition sources are usually close together. In addition to constant fire prevention efforts, provisions 

should be made to detect and extinguish fires as soon as possible. Sensors and detectors are needed because fires can occur spontaneously in 

abandoned workings or.accidently in unattended areas. Furthermore, depending on detection by persons working in or passing through an area is 

not realistic. 

Besides simply detecting a fire, systems should be made to alert 

an attended area that fire exists, and where it exists; to alert people underground that a fire has been observed or reported; to take some 

action to isolate the fire area; and to extinguish the fire if possible 

The contract showed that meeting these requirements was possible with 
existing equipment integrated into a functional protective system consisting of sensors, extinguishing agents, isolation barriers, and appropriate controls. 

The harsh environmental conditions that exist underground have already been mentioned. Because labor and maintenance efforts are expensive underground, all components used in this detection-alarm system 

must be extremely rugged to limit repair needs or they may be out of 

service at just the wrong times, and reliable to instill confidence in 

their ability to perform and alarm only in a fire situation. 

The first prototype mine shaft fire and smoke protection system 

was tested at an inactive silver mine in the Coeur dtAlene district of 

Idaho. The system was installed 3,000 feet below the shaft collar to 

protect 50 feet of shaft, the shaft station, and 100 feet of drift in 

two directions. 

Environmental conditions in the mine were 100' F and essentially 

100 pct humidity. Because the mine was inactive, there was little or 

no dust present during the 2-week test period. Ventilation was not 

typical of that found in a working mine because the mine was used exclusively for ventilation and as an alternate escape route for an adjoining working mine. Airflow was temporarily modified for the test. 

The sensorsinstalled included thermal wire, CO gas, and ionization 

types. The thermal wire provided backup reliability for the other two 

early warning types. CO gas and ionization sensors were placed in sections of the shaft and shaft station areas. A pair of each was placed 

near the shaft crown plate to sense air entering the area, which was 

downcast intake air. Others were located near the ventilation barriers 

towards the mine workings (away from the shaft station area). 

Sensors were selected to respond to health hazard levels of fire 

or contamination by fire (gaslsmoke), while being available off-theshelf, reasonably priced, and compatible with metal/nonmetal mine environments as previously described. Sensors consist of smoke (ionization) detectors, Becon Mk 11, manufactured by Anglo American Electronics, Republic of South Africa; two carbon monoxide (gas) sensors, Model 

CO-181 manufactured by Dynamation Enterprises, Inc., Ann Arbor, Mich., 

and Model MSD-1 manufactured by Emrnet Co., Ann Arbor, Mich.; and thermal 

wire, type WPP, 155O F manufactured by the Protectowire Company. 

The ionization detectors are referred to as smoke detectors in the 

system even though they primarily sense invisible submicrometer particles (as distinguished from the second detector type, labeled CO, which 

responds to CO and other gaseous components of smoke). 

During extensive factory tests, the ionization detectors performed 

well. They consistently detected wood fire smoke within habitable levels. Test data indicste the detectors alarmed at smoke density levels 

of 2 to 6 pct obscuration per foot. This compares favorably with industry standard set points for commercial and residential smoke detectors. A poor response to burning plastic produced smokes appears to be 

characteristic of ionization-type detectors, and the Becon unit is no 

exception. On the other hand, the Becon unit is designed for mine 
environments and does not readily exhibit problems from corrosive environments, dust, humidity, and very high air velocities. 

The CO detectors in the system used the Taguchi gas sensor (TGS). 

They were used at factory settings of 200 pprn CO on the Enmet unit and 

75 pprn CO on the Dynamation units. These levels were felt to be realistic for the conditions expected in the test shaft. There was no 

blasting or diesel equipment operating to increase the CO level at the 

demonstration site. On the other hand, a CO concentration of 150 pprn 

may be encountered with some regularity in active mines where blasting 

and diesel equipment are present. A level of 150 pprn is tolerable 

without hazard for up to 3 hours so setting alarm points of CO monitors 

above 150 pprn would represent a realistic alarm level within the limits 

of safe habitation for short periods, but an extended 200 pprn concentration could be a valid indication of a fire, especially if combined 

with other indications. 

A surface control unit was installed in the hoist room and connected to an underground control unit in the shaft station at the3,OOOfoot level. The latter was the junction point of the sensors and related components and provided the interface between them and the surface control unit. The two control units were connected by a single 

pair of wires with a second pair for redundancy. Multiplexing the signals allowed the surface control unit to handle 16 inputs from the 

underground control and process 8 outputs for transmittal back to the 

underground unit. 

A fire fueled by wood cribbing was built in a steel pan 3 feet 

square, about 18 inches deep, with a lid that was raised and lowered 

to adjust smoke density and products of combustion. 

Smoke and CO sensors responded to contaminants in the fire area, 

all system commands were remotely operated from the surface control unit 

and performed as designed. The fire was extinguished by the installed 

sprinkler system during a 6-minute discharge. The thermal wire was 

located too high above the controlled pan fire to reach its alarm temperature and did not respond. 

The first prototype system was installed for only 2 weeks in the 

Idaho mine, which was not sufficient to test the long-term reliability 

of the system and components. A second mine was found where the system 

could be installed and given a longer test. This mine was the Union 

Carbide Corporation's Pine Creek mine in Bishop, Calif. 

Several modifications were made to adapt the system to the particular mine location and it was installed and continuous monitoring began on December 19, 1975. A second prototype system, based on recommendations derived from developing, installing, and monitoring the first 

prototype, was built and installed at a second level in the second mine 

as an add-on unit, to the first prototype using the same interconnecting wires, and monitoring began during April 1976. The latter system 

included a new micro processor controlled surface unit, replacing the 

original prototype surface unit. System testing was concluded in July 

after controlled fire test demonstrations were performed in each of the 

two underground areas. Final results include 3 months operation of the 
first prototype system, providing 87 days of 24-hour-per-day monitoring. The second prototype system was installed 3$ months, providing 

a combined total of 105 additional days of monitoring for both levels. 

The surface control unit was installed on the surface at a guardhouse where it was monitored 24 hours per day. The fire and smoke 

protection systems were placed at two locations deep within the mine-- 

one in a large compressor station 14,000 feet in the mine at the 9,400- 

ft level, and the second 14,000 feet in the mine at a shaft and shaft 

station area at the 11,271-ft level. Because the mine is inverted, 

deeper operations are at higher elevations. 

Three types of sensors were used at each of the two test sites as 

in the initial test setup. Thermal wire was strung throughout the drift 

and machinery space of the compressor station and around and down the 

shaft at the 11,271-ft level. 

Ionization sensors and CO sensors were used in pairs except none 

of the CO sensors were used in the compressor room. An ionization sensor was located above the compressor machinery, one located on each 

side away from the compressor station, in intake air, beyond remotely 

controlled vent doors, and one in the upcast exhaust airflow from the 

shaft at the 11,271-ft level. An additional ionization sensor was 

placed in the diesel maintenance area, three fans were monitored, one 

fan was controlled, and air lock doors were monitored. 

During the monitoring period, the smoke (ionization) detector in 

the diesel maintenance bay regularly alarmed during vehicle activity, 

requiring an adjustment of its sensitivity. Also the smoke detector in 

the compressor station was readjusted because of alarms. The sensors 

responded to various known mine activities--diesel exhaust, blasting, 

and gas and electric welding and cutting vapors. 

After the monitoring period, each location was subjected to a test 

fire using a pan and cribs identical to those used in the Silver Summit 

test. kt the 11,271-ft level it was allowed to burn for nearly 12 

minutes before the surface control unit was used to turn on the installed sprinkler system. During the fire the sensors detected over 

100 ppm gas, and smoke, and alerted surface personnel that they were in 

alarm. All alarming and control functions performed satisfactorily 

through the surface control unit operated by surface personnel. 

At the 9,400-ft level a similar fire was allowed to burn for 13 

minutes before the surface control unit turned on the sprinkler system there. Again, alarm and control functions operated satisfactorily 

through the surface control unit, responding to the buildup of smoke. 

Other Underground Tests 

Among the recommendations coming from this contract testing was 

that continued laboratory and in-mine tests should be conducted to 

learn more about gas and smoke detectors. Responses to fires from 

different fuels, nonfire stimuli, effects of air velocity variations, 

dust buildup, and effects of mine environments would be of interest 

over long-term testing. 
Based on the recommendations, the Bureau's Twin Cities Mining Research Center entered into a purchase contract with the FMC Corporation 

to obtain a sensor package suitable for long-term, in-mine testing. A 

cooperative agreement for locating and testing the package was also 

negotiated with the Hecla Mining Company, Lakeshore mine near Casa 

Grande, Ariz. 

A three-sensor package containing a Becon Mark I1 smoke sensor, 

and Enmet ISA-33 CO gas sensor and two Pyrotector 30-2013-9 flame detectors were assembled An additional CO gas sensor (Dynamation CO 181) was supplied for additional testing at a different location in the Lakeshore mine. 

The three-sensor package was installed in the underground diesel 

fuel storage area located at the end of the 500 East Exhaust Crosscut. 

The single CO gas sensor was installed in the south decline near the 

mine's existing Ecolyzer gas sensor. Each sensor is connected by 

telephone wire pairs to a 9-channel recorder on the surface. These 

packages were installed in late March 1976 and have been undergoing 

long-term testing. 

Periodic visits to the mine to observe the sensors have shown some 

problems. The interface electronics and the CO sensors are packaged in 

NEMA IV electrical enclosures and well protected from the mine environment. The TGS sensors that are part of the CO monitors are not stable 

in that their base level output drifts over time and with temperature 

variations. They are also quite sensitive to diesel exhaust emissions 

and blasting residue gases in the mine. 

The flame detectors have remained operational but they do become 

dust covered with time since they are in an exhaust crosscut where the 

air velocity is quite high and dusty. 

The smoke sensor has remained operational but after several months 

was not alarming as it should. Blowing accumulated dust away from its 

baffle entrances caused it to go, into alarm, and we were able after some 

trials, to get it to alarm by checking its trigger level adjustment. 

Dust does build up in the baffle area in this location to the extent it 

needs to be cleaned out every few months. The alarm level was adjusted 

later so it would not respond to the exhaust fumes from standing, idling 

load-haul-dumps in the passageway to the fueling area. Additional longterm testing continued in this mine as well as others with differing 

environmental conditions. 
REFERENCES 

Astheimer, R. W., and J. Schwartz. Thermal Imaging Using Pyroelectric Detectors. Applied Optics, v. 7, No. 9, September 1968, 

pp. 1687-1695. 

Cohen, J., S. Edelman, and C. F. Vezzetti. Pyroelectric Effect in 

Polyvinylfluoride. Nature (London) Physical Science, v. 233, 

No. 36, September 6, 1971, p. 12a. 

Drinker and Hatch. Industrial Dust, McGraw-Hill, New York, N.Y., 

1936, p. 7. 

Greuer, R. E. Influence of Mine Fires on the Ventilation of Underground Mines. Michigan Technological University, BuMines Open 

File Report 74-73, July 6, 1973, 173 pp; available for consultation at the Bureau of Mines libraries in Pittsburgh, Pa., Minneapolis, Minn., Denver, Colo., and Spokane, Wash.; at the National 

Library of Natural Resources, U.S. Department of the Interior, 

Washington, D.C., and from National Technical Information Service, 

Springfield, Va. ; PB 225 834lAS. 

Fire Detection, the State of the Art. NBS Technical Note 839, 

June 1974, National Bureau of Standards, 56 pp. 

Fire Protection Handbook, National Fire Protection Association, 

13th ed., 1968, 1946 pp. 

Hertzberg, Martin, and Charles D. Litton. Multipoint Detection of 

Products of Combustion With Tube Bundles. BuMines RI 8171, 1976, 

40 PP* 

Litton, Charles D., and Martin Hertzberg. Principles of Ionization 

Smoke Detection. Development of a New Sensor for CombustionGenerated Submicrometer Particulates. BuMines RI 8242, 1977, 

21 PP* 

Rodgers, Sheridan J. Analysis of Noncoal Mine Atmospheres. Final 

Report, MSA Research Corp., BuMines Open File Report 78-77, May 

1976, 118 pp; available for consultation at the Bureau of Mines 

libraries in Pittsburgh, Pa., and Denver, Colo.; at the National 

Library of Natural Resources, U.S. Department of the Interior, 

Washington, D.C.; and from National Technical Information Service, 

Springfield, Va., PB 266 764lAS. 

Stevens, R. B. Mine Shaft Fire and Smoke Protection System. 

BuMines Open File Report 24-77, July 1975, 407 pp.; BuMines Open 

File Report 43(1)-(3)-77, 54 pp., FMC Corp.; available for consultation at the Bureau of Mines libraries in Pittsburgh, Pa., 

Minneapolis, Minn., Denver, Colo., and Spokane,Wash., and at the 

National Library of Natural Resources, U.S. Department of the 

Interior, Washington, D.C.