UNITED STATES BUREAU OF MINES

UNITED STATES DEPARTMENT OF THE INTERIOR

RI 9586 REPORT OF INVESTIGATIONS/1995

Evaluation of Smoke Detectors

for Mining Use

By John C. Edwards and Gerald S. Morrow
U.S. Department of the Interior

Mission Statement

As the Nation’s principal conservation agency, the Department of the

Interior has responsibility for most of our nationally-owned public

lands and natural resources. This includes fostering sound use of our

land and water resources; protecting our fish, wildlife, and biological

diversity; preserving the environmental and cultural values of our

national parks and historical places; and providing for the enjoyment

of life through outdoor recreation. The Department assesses our

energy and mineral resources and works to ensure that their

development is in the best interests of all our people by encouraging

stewardship and citizen participation in their care. The Department

also has a major responsibility for American Indian reservation

communities and for people who live in island territories under U.S.

administration.
Report of Investigations 9586

Evaluation of Smoke Detectors

for Mining Use

By John C. Edwards and Gerald S. Morrow 

UNITED STATES DEPARTMENT OF THE INTERIOR

Bruce Babbitt, Secretary

BUREAU OF MINES

Rhea Lydia Graham, Director
International Standard Serial Number

ISSN 1066-5552
CONTENTS

Page

Abstract ...................................................................................... 1

Introduction .................................................................................... 2

Experimental procedures ........................................................................... 2

Experimental results .............................................................................. 4

Smoke detector response ......................................................................... 4

Detector A ................................................................................ 4

Detector B ................................................................................ 5

Detector C ................................................................................ 5

Detector D ................................................................................ 5

Detector E ................................................................................ 5

Detector F ................................................................................. 6

CO measurement .............................................................................. 7

Odor measurement ............................................................................. 7

Conclusions and recommendations .................................................................... 8

References ..................................................................................... 9

ILLUSTRATIONS

1. Schematic of the smoke chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2. Schematic of the sample chamber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

3. Optical density and response of detectors for smoldering coal combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4. Optical density and response of detectors for flaming coal combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

5. Response of detector A with respect to optical density for smoldering and flaming coal combustion . . . . . . . . . . . . . . . . . 13

6. Response of detector D with respect to optical density for smoldering and flaming coal combustion . . . . . . . . . . . . . . . . . 14

7. Response of MIC with respect to d n determined from measurements with detector E for smoldering and flaming coal combustion g o

15

8. Measured CO concentration dependence on optical density for smoldering and flaming coal combustion . . . . . . . . . . . . . . 16

9. Correlation of odor monitor alarm time with time for CO concentration to reach 5 ppm above background for smoldering and 

flaming coal combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

10. Correlation of odor monitor alarm time with smoke detector C alarm for smoldering and flaming coal combustion . . . . . . . 17

TABLES

1. Smoke detector type and sampling mode ............................................................ 3

2. Smoke detector noise and background signal for detectors A to D .......................................... 6

3. Smoke detector response at optical densities of 0.011, 0.022, 0.033, and 0.044 m for detectors A-D ................ 6 -1
UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

cm centimeter mV millivolt

g gram pct percent

m meter ppm part per million

m inverse meter s second -1

m square meter V volt 2

m @V square meter volt V@m volt meter 2

mm millimeter μm micrometer

OTHER ABBREVIATIONS AND SYMBOLS USED IN THIS REPORT

a coefficient, V@m O percent obscuration u

 per meter, percent/m

b coefficient, V

S output signal, V

CO carbon monoxide

T

transmission, 1

D optical density, m-1

T transmission in clear air, 1 o

d number mean particulate g

 diameter, m

V charged particle collection c

H S hydrogen sulfide 2

 electrode voltage,V

6 extinction coefficient, m V voltage in charging region, V -1

E

R optical path, m Y MIC measurable

 quantity, 1

n particulate number concen- o

 

 tration, particles/m3

Reference to specific products does not imply endorsement by the U.S. Bureau of Mines.
Research physicist. 1

Electronics technician. 2

 Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh PA.

Evaluation of Smoke Detectors for Mining Use

By John C. Edwards and Gerald S. Morrow 1 2

ABSTRACT

The U.S. Bureau of Mines has constructed a smoke chamber and developed sensitivity tests for smoke detectors.

Response of ionization- and optical-type commercially available smoke detectors have been investigated. Six smoke

detectors were measured with respect to visually obscuring smoke characterized by a corresponding optical density

for smoldering and flaming coal combustion in the smoke chamber. It was determined that for one type of

ionization smoke detector the alarm time was nearly equivalent to that of an odor monitor's alarm for smoldering

coal combustion experiments and earlier for flaming coal combustion experiments. The experiments showed that

an average CO concentration of 5 ppm corresponded to an optical density of 0.022 m for smoldering and flaming -1

coal combustion. Two of the commercially available ionization-type smoke detectors were more responsive to

flaming than smoldering coal combustion at an optical density of 0.022 m , whereas the optical smoke detectors -1

showed the opposite trend. The responsive characteristics of the detectors evaluated with respect to known smoke

conditions in the smoke chamber shows their potential for use as mine fire sensors or part of a mine atmospheric

monitoring system to improve mine safety.
2

INTRODUCTION

In order to evaluate potential mine fire detection systems, it is a current between two charged electrodes. Diffusion of smoke

important to consider not only CO detectors that are currently particulates into the path of the ion current reduces the ion current

used in some coal mine conveyor belt entries, but other candidate through attachment of the ions to the smoke particles. This

fire signature detectors. A commonly used fire detector for process slows the movement of the ions and thereby increases the

commercial and residential property is the smoke detector. Its ion's probability of recombination. The current reduction is

potential for in-mine use has been examined in mines (1). In that amplified as a measurable signal. 1

in-mine evaluation, the detectors occasionally identified conveyor Optical smoke detectors operate on the principle of scattering

belt heating. Also, for a mine using diesel-powered equipment or absorption of light over an optical path through which the

in which a comparison could be made with CO levels at the time smoke particulates can migrate. For optical scattering, the

of smoke detector alarm, the smoke detector had a lower detector is located to the side of the optical path to measure the

frequency of false alarms than CO sensors. More recently, the amount of light scattered by any smoke particulate present. For

U.S. Bureau of Mines (USBM) constructed a smoke chamber and optical absorption, the detector is located along the path from the

developed sensitivity tests for smoke detectors using smoldering light source and measures the amount of light transmitted without

and flaming coal combustion (2). scattering or absorption. Black smoke particulates are more

The objective of this work is to investigate response susceptible to optical absorption than scattering.

to smoldering and flaming coal combustion of several ionization- The standard smoldering and flaming coal sensitivity test

and optical-type commercially available smoke detectors intended procedure for smoke detectors enables the measurement of

for industrial and in-mine use and to make recommendations for additional combustion gas products generated within the smoke

the development of an evaluation procedure for the smoke chamber. In addition to the response of the smoke chamber's

detectors. The response of the detectors as measured by their photo cell to optical transmission through the smoke and of the

analog output signal or manufacturer alarm will be determined measuring ionization chamber (MIC) to smoke particulate size

with respect to the measurable smoke optical density. and concentration as described in (2), CO and combustion

Smoke detectors can be classified into two types based on product gas odors were monitored. This provides a benchmark

their operational principle—optical or ionization. Their sampling of smoke detector response against CO detection and the

method will be either diffusion- or pump-controlled. The smoke previously evaluated odor monitor (3) as an early warning fire dedetectors examined in this report are representative of both types tector. It was determined from the previous study that an odor

and both sampling methods. monitor alarm time associated with a 1 ppm increase in H S above

Ionization smoke detectors contain a radioactive source that ambient was comparable to the alarm level for one ionization-type

ionizes the air. The oppositely charged air ions form smoke detector for smoldering combustion of sulfur-containing

2

coal.

EXPERIMENTAL PROCEDURES

The USBM smoke chamber shown in figure 1 was used in !9.4 %6.7 mm mesh. CO was monitored with two Ecolyzer

accordance with the procedure established previously (2) for a Model 4000 CO detectors that sampled combustion product gases

comparative evaluation of smoke detectors. As shown in figure from the bottom and top of the smoke chamber. Each Ecolyzer

2, the coal is placed on the heater disc in the sample chamber has an internal pump that draws the sample from a line connected

external to the smoke chamber. Access of smoke to the smoke to a smoke chamber. A comparison of their values showed the

chamber is limited by the iris setting. The iris settings for combustion gases in the smoke chamber were well mixed. The

smoldering and flaming coal combustion are described in (2). CO values were validated against samples drawn into an

Both smoldering and flaming coal combustion sensitivity evacuated glass container from the chamber and subsequently

experiments were conducted. The fuel source for the experiments analyzed by gas chromatography.

is 80 g of ground Pittsburgh Seam coal sized to The six smoke detectors used in the experiments are listed in

Italic numbers in parentheses refer to items in the list of references at the end 1

of this report.

table 1. Their intended use has been industrial and mining. The

type (ionization or optical) and sampling method characteristics
' & R

3

Table 1.—Smoke detector type and sampling mode

Detector Sampling mode

Optical type:

 A ........................................ Pump.

 B ........................................ Diffusion.

Ionization type:

 C........................................ Pump.

 D........................................ Diffusion.

 E ........................................ Pump.

 F ........................................ Diffusion.

(pump or diffusion mode) are listed. All of the smoke detectors combustion experiments with 5-cm-diam coal particles as well as

are commercially available, except for detector E. mine-size coal particles. The odor monitor responds primarily to

The diffusion mode smoke detectors were mounted on the acquisition system and recorded on a personal computer. The

interior shelf adjacent to the air straightener of the smoke chamber time interval between data samples was 10 s. The error of the

in figure 1, and the pump-mode detectors were mounted external data acquisition system was 2.4 mV for a 5-V signal, or 0.048

to the chamber and sampled from port H. The sample chamber pct.

that holds the smoldering or flaming combustion coal sample is A primary instrument for smoke measurement is the optical

shown in figure 2. transmission of visible light through the smoke. Human visibility

Twenty-two combustion experiments were conducted with studies (4) have led to the proposed visibility criterion (5) that a

Pittsburgh Seam coal. Each of the detectors in table 1 were used visibility of 10 m corresponds to an optical density of 0.08 m .

for a minimum of two smoldering and two flaming coal Optical density D is defined in terms of the reduction of light

combustion experiments. Some experiments evaluated more than transmission from a transmission T in clear air over path R to

one detector. Detector C, which has a manufacturer-specified transmission T.

alarm, is a pump type that is expected to assure a prompt response

to smoke, and also produces a measurable analog signal in

response to smoke concentration. For these reasons, it was used

for 16 of the reported experiments to provide a database for

comparison with the response of the odor monitor. Because of

the experimental configuration of the other detector, it could not

be used in every experiment.

The MIC was used in each experiment to provide an

ionization-type smoke detection measurement independent of the

detector selected. It measures the relative increase of the particles

of combustion during each experiment. The MIC measurement

and measured optical transmission provide, as discussed in (2), a

basis for standardization of smoke chamber smoke particulates.

The factors that define a cloud of smoke particulates are

particulate diameter, mass or number concentration, and index of

refraction.

In order to acquire additional information regarding the

comparative detection capability by identification of odor, CO, or

smoke, an odor monitor manufactured by Sensidyne (P/N

7016019) was used to sample product of combustion gases from

the smoke chamber. The odor meter was used in previously

reported research (3) for coal combustion

aromatic hydrocarbons and, to a lesser extent, to odorless gases

and gases classified as simple asphyxiants. Its operating principle

is based on the absorption of gas molecules on the surface of a

metal oxide semiconductor and the measurable changes in the

sensor's electrical conductivity. A component of the product

gases of the sulfur-containing Pittsburgh Seam coal is H S, which 2

is detectable by the odor monitor in the concentration range from

0.1 to 10 ppm. It is expected that other component product gases

will contribute to the odor monitor response. The odor monitor

has an internal pump that draws a sample through a line

connected to the smoke chamber.

Experimental data were acquired with an analog-to-digital data

-1

o

(1)

The optical path length R for these experiments was 1.483 m.

A mine fire smoke sensor must detect smoke as early as possible

with a maximum exclusion of nuisance alarms. Smoke detector

alarm can be characterized by the smoke optical density.

Deployment spacing of smoke detectors has been evaluated for

smoke optical density as low as 0.011 m (6) for various linear -1

airflows. It was proposed (6) that smoke detectors for

underground mines be divided into two classes according to

optical density. Accordingly, detectors that alarmed at an optical

density less than 0.022 m would be designated Class I detectors, -1

and those that alarm at an optical density greater than 0.022 m-1

but less than 0.044 m would be designated as Class II detectors. -1

These values correspond to human visibilities of 40 m and 20 m,

respectively, based on other research (5).
' &6R

' %

' 6 %

4

EXPERIMENTAL RESULTS

SMOKE DETECTOR RESPONSE The mathematically smoothed voltage signals of detector A for

A total of 12 smoldering coal combustion experiments and 10

flaming coal combustion experiments were conducted.

Figure 3 shows the time-dependent measured optical density

and corresponding response for each of the six smoke detectors

at least once, for smoldering coal combustion. The advantage of

recording the voltage analog signal output for the detectors is the

capability to discern early response of the detector to smoke and

not be constrained by the manufacturer's designated alarm value.

Although the smoke detector response is unique for each detector,

figure 3 shows that each of the detectors, with the exception of

detector F, shows a measurable response at the inception of a

change in the optical density corresponding to a reduction in

optical transmission due to absorption and scattering of light by

the smoke particulates. Detector F is unique insofar as the output

signal is not a continuous rise in response to smoke, but indicates

the manufacturer's alarm with a jump in the analog voltage. For

each of the smoldering coal combustion experiments shown, the

first response to smoke is about 400 s after the heating element in

the sample chamber was energized.

Figure 4 shows the time-dependent optical density and

response of the smoke detectors for flaming coal combustion

experiments. Again, each smoke detector is represented by at

least one test result. The response of the detectors, except for

detector F, is coincident with a measurable change in optical

density, which occurs simultaneously with the opening of the iris

between the sample and smoke chambers. As discussed in (2),

for flaming combustion tests the iris is opened only after flaming

combustion has occurred, by which time smoke production from

the heated coal sample has evolved from the smoldering stage to

the flaming stage.

Detector A

Detector A is an optical, pump-mode smoke detector with an

output signal from 0.1 to 2.8 V. The principle of operation is

based on optical scattering of light. The average output voltage

that detector A indicates for an optical density of 0.022 m is 2.34 -1

V (95 pct confidence interval from 1.96 to 2.72 V) for the eight

smoldering coal combustion experiments. The value is 1.33 V

(95 pct confidence interval from 0.91 to 1.75 V) for eight flaming

coal combustion experiments conducted.

smoldering coal combustion experiments versus optical density,

are shown in figure 5A. The smoothed data correspond to the

curves in figure 3A, B, and C. The average slope of the curves is

89.6 V@m with a standard deviation of 6.1 in a linear

approximation. Figure 5B shows the smoothed data for the

flaming coal combustion experiments reported in figures 4A, B,

and C. The average slope of the curves in a linear approximation

is 54.7 V@m with a standard deviation of 4.3. These results

indicate detector A is more responsive to smoldering than to

flaming coal combustion. For a given smoke optical density

produced by smoldering and flaming coal combustion, the

detector would have a greater response signal for smoldering than

for flaming coal combustion.

The optical transmission through the smoke particulates will

be attenuated according to Bouguer's law (7):

(2)

where 6 is the extinction coefficient. The linear relationship

between the output signal S for detector A and the optical

density, D, can be written

(3)

where a and b are constant coefficients. A combination of

equations 1, 2, and 3 yields

(4)

Equation 4 shows linear proportionality between S and 6.

The extinction coefficient, 6, is proportional to the ratio of the

smoke particulate mass concentration to particle diameter or,

equivalently, to the product of the smoke particulate number

concentration and particle cross-sectional area. Previous research

(8) has shown the smoke particle diameter is smaller for flaming

than for smoldering combustion. This indicates, based on an

increased value of S for smoldering than for flaming combustion

for a fixed value of D (or equivalently of 6), that the smoke

particulate number concentration is greater for flaming than for

smoldering combustion. The larger particulate diameter for

smoldering combustion than for flaming combustion could be a

contributing factor to the increased detector response to

smoldering combustion.
' & &

5

Detector B Detector B is an optical-type, diffusion-mode responsiveness of detector C to flaming than to smoldering coal

smoke detector. Extinction of infrared radiation from the optical

path reduces the signal at the receiver. This decrease in

transmission corresponds to an increase in pct obscuration per

meter, O . The obscuration per meter is related to the optical u

density by

(5)

Detector B has a range from 0 to 10 pct obscuration per meter.

A 10 pct obscuration per meter corresponds to an optical density

of 0.046 m . The analog signal range for detector B is 0.4 to 2.0 -1

V.

The calibration procedure for detector B required adjustments

for a zero, corresponding to clear air transmission, and a span,

corresponding to 10 pct obscuration per meter. Errors as large as

15 pct occurred in the calibration of the span. A comparison was

made of the optical density based on the optical obscuration

measurement in the chamber, and the optical density that

corresponds to the expected obscuration per meter based on the

analog voltage signal of detector B. For a measured optical density of 0.022 m , detector B indicated an average obscuration per -1

meter of 0.78 ±0.015 pct for two smoldering experiments. This

is equivalent to an optical density of 0.0034 m . For the two -1

flaming combustion experiments, detector B indicated an average

obscuration per meter of 0.62 pct when the optical obscuration smoldering coal combustion experiments, the output signal is

meter indicated 0.022 m . This corresponds to an optical density -1

of 0.0027 m . Detector B did not achieve the maximum output -1

of 2 V corresponding to a 10 pct obscuration per meter for any of

the two smoldering or two flaming coal combustion experiments

even though for each of the experiments an optical density of

0.08 m was achieved. Although the data in figures 3 and 4 -1

show an incipient rise in the detector response coincident with the

onset of measurable obscuration, its use as a quantitative

instrument is limited.

Detector C

Detector C is a pump-mode ionization detector with a range

from 1 to 5 V. The manufacturer-suggested alarm is 2.5 V. A

summary of eight smoldering combustion experiments showed

the manufacturer-suggested alarm point corresponded to an

average optical density of 0.011 m and an average CO -1

concentration of 2 ppm above background. For eight flaming

coal combustion experiments, the alarm occurred at an average

optical density of 0.0072 m and an average CO concentration -1

less than 1 ppm above background. This indicates a greater

combustion based on the optical density and CO concentration at

which the detector alarms.

Detector D

Detector D is an ionization-type smoke detector that samples

smoke through a diffusion mode. The measurable output voltage

is shown in figure 3 for smoldering coal combustion, and in

figure 4 for flaming coal combustion. There was not a

manufacturer-recommended alarm for detector D. Figures 6A and

B show a comparison of the output voltage and the optical

density for the smoldering and flaming coal combustion,

respectively. The data in figures 6A and B are smoothed. The

data in figure 6A are for the two experiments reported in figures

3B and D as well as two additional experiments, and the data in

figure 6B are for the two experiments reported in figures 4B and

D as well as two additional experiments. The initial voltage for

detector D in clear air is &0.9 V. The maximum available signal

response of the detector to smoke is &0.2 V. Based on four

smoldering experiments in figure 6A, the average signal output

at an optical density of 0.022 m is &0.77 V, and &0.61 V for -1

four flaming combustion experiments in figure 6B. This

represents an increase of 19 pct and 41 pct, respectively, over the

detector's full-scale range. Figure 6A shows that for the

nearly linear with respect to optical density, whereas for the

flaming coal combustion experiments shown in figure 6B, the

response is nonlinear with respect to optical density. This is in

contrast to the optical-type detector, detector A, which showed a

linear response for both smoldering and flaming combustion.

Also, in contrast to detector A, is the greater responsiveness of

detector D, an ionization-type detector, to flaming rather than to

smoldering combustion. This is associated with detector D's

nonlinear response to flaming coal combustion.

Detector E

Detector E is a prototype ionization-type, pump-mode detector

developed by the USBM (9). There are two measurable output

voltages associated with the detector, V E and V . V is the voltage c E

in the charging region, and V is the charged particle collection c

electrode voltage. In clear air, V is about 1.5 V and V is about E c

0.0 V. During the response to smoke, V decreases and V E c

increases. Figures 3D and 4D show the response of the detector's

output voltages for smoldering 
6

Table 2.—Smoke detector background signal and noise

for detectors A to D, V

Detector Background Noise 

A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.1 ±0.02

B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.4 ±0.006

C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.0 ±0.002

D.............................. -0.9 ±0.003

and flaming combustion. A theoretical evaluation of the output The measured results reported above can be used to quantify

voltage based on theory, results in a computation of the product

of the smoke particulate number concentration, n , and the num- o

ber mean particle diameter, d . A comparison was made of the g

quantity, Y, calculated from the MIC output voltage with the

product, d n . Figure 7 g o A shows the comparison corresponding to

the smoldering coal combustion, and figure 7B shows the

comparison for the flaming coal combustion experiments. There

is a near linear relationship between Y and d n . The slope for g o

smoldering combustion is 0.26 × 10 m per particle; for flaming -4 2

combustion it is 0.15 × 10 m per particle. As previously noted -4 2

(2), Y is proportional to the number concentration n .o

The smoke number average particulate diameter can be

calculated from the model equations for detector E. A calculated

smoke particulate average diameter for the data points for the

smoldering coal combustion case shown in figure 7A is 0.45 μm;

for the flaming coal combustion data points shown in figure 7B,

the average calculated diameter is 0.38 μm.

Detector F

Detector F is an ionization-type diffusion mode smoke

detector. The response of the detector for smoldering and flaming

coal combustion is shown in figures 3 and 4. As the figures

indicate, the detector alarm results in an electrical contact without

any earlier indication of smoke detection, as would be indicated

by a detector with a continuous analog output signal. For the two

smoldering coal combustion experiments, the alarm occurred at

an average optical density of 0.12 ±0.003 m . Alarm occurred at -1

an average optical density of 0.077 ±0.029 m for the two -1

flaming coal combustion experiments. This would indicate the

detector is more responsive to flaming than smoldering

combustion.

the concept of alarm based on optical obscuration for the

commercially available detectors. From the perspective of

establishing a reliable smoke detector alarm based on detector

output voltage, the signal change must be some factor times the

peak-to-peak noise of the detector. Table 2 shows the measured

noise in the background signal in clear air for the detectors with

an analog voltage output, detectors A to D. If the factor was 10,

for example, then reliable signal values for detectors A, B, C, and

D would be 0.5, 0.52, 1.04, and -0.84. These projected values

can be compared with measured values for specific optical

densities.

Table 3 lists the average signal and standard deviation for

smoldering and flaming experiments for optical density values of

0.011, 0.022, 0.033, and 0.044 m for detectors A to D. Based -1

on the above consideration of a projected alarm value associated

with a signal change of at least ten times the peak-to-peak noise

values, a minimum reliable optical density can be determined for

each detector alarm using the average background and noise

values in table 2. In this example, based on the average values

reported in table 3, detectors A, C, and D would be in alarm at an

optical density of 0.011 m ; detector B would be in alarm at an -1

optical density of 0.033 m . The choice of ten for the factor -1

represents a reasonable expectation of a selected alarm value that

is not affected by detector background noise.

tors A-D

Table 3.—Smoke detector response at optical densities of 0.011, 0.022, 0.033 and 0.044 m for detec -1

Detector 0.011 0.022 0.033 0.044

Av SD Av SD Av SD Av SD 

A:

 Smoldering ....................... 1.02 0.25 2.34 0.38 2.73 0.03 2.75 0.02

 Flaming .......................... 0.70 0.30 1.33 0.42 1.88 0.58 2.37 0.53

B:

 Smoldering ....................... 0.47 0.01 0.52 0.01 0.59 0.0 0.64 0.0

 Flaming .......................... 0.45 0.0 0.50 0.0 0.53 0.0 0.56 0.0

C:

 Smoldering ....................... 2.52 0.23 4.27 0.48 NAp NAp NAp NAp 

 Flaming . . . . . . . . . . . . . . . . . . . . . . . . . . NAp NAp NAp NAp NAp NAp NAp NAp 

D:

 Smoldering ....................... -0.83 0.0 -0.77 0.01 -0.72 0.01 -0.67 0.02

 Flaming .......................... -0.74 0.04 -0.61 0.04 -0.52 0.03 -0.46 0.02

Av Average.

SD Standard deviation.

NAp Not applicable.
7

The results in tables 2 and 3 can be used to make a ±2,000 units. The analog signal corresponding to one unit is 0.1

comparison of the signal's change above background at an optical mV. The instrument response is linear on a log-log scale with

density of 0.022 m for smoldering and flaming coal combustion. respect to a pure gas component. A product gas component asso- -1

For detectors A and B, the ratio of the change in signal above ciated with a sulfur-containing coal such as Pittsburgh Seam coal

background for smoldering to flaming coal combustion was 1.8 is H S. The odor monitor responds to H S concentrations as great

and 1.2. For detector D, the ratio in signal change of flaming to as 10 ppm. The threshold human odor response to H S is 0.1

smoldering coal combustion was 2.2. For detector C, the ppm. According to the manufacturer's response chart, an H S

maximum analog signal was achieved in the flaming combustion concentration of 1 ppm corresponds to a monitor reading of 210

case prior to an optical density of 0.022 m , and a numerical ratio arbitrary units. Since a manufacturer's value was available, a -1

cannot be assigned. reading of 210 above background was selected as a criterion for

CO MEASUREMENT tectors. A comparison of the response of the CO detector and

For each of the experiments, the CO in the smoke chamber experiments showed their initial response to products of

was continuously measured with two Ecolyzer CO sensors. combustion was within 130 s of each other. A comparison was

Figure 8A shows the CO increase with respect to optical density made of the odor monitor alarm time with the CO detector alarm

for five smoldering coal combustion experiments. Figure 8B time, which is the time for the CO concentration to reach 5 ppm

shows the CO increase with respect to optical density for five above background. Figure 9 shows a comparison of the times for

flaming coal combustion experiments. Four of the five the eight smoldering and seven flaming coal combustion

experiments reported in figure 8A correspond to the four experiments. The data were not available for the other seven

experiments reported in figure 3; similarly, four of the five experiments. The line for perfect correlation is also shown in

experiments reported in figure 9. Figure 9 shows that in seven of the eight smoldering

figure 8B correspond to the four experiments reported in figure coal combustion experiments, the identified odor monitor alarm

4. The values in figure 8 are the result of mathematically occurs prior to the CO alarm time. The average background CO

smoothing the measured values. This was done because of the was less than 1 ppm. For the flaming coal combustion

fluctuations in the optical density. The measured response of CO experiments, the sequence of alarm occurrence was almost equally

was nearly linear with respect to optical density for smoldering divided between the odor monitor and the CO detector. This is

and flaming coal combustion. consistent with previous research (3) in the USBM intermediate

The average CO concentration in the smoke chamber was scale tunnel that showed for heating of larger-size coal particles,

evaluated at an optical density of 0.022 m for 12 smoldering and the odor monitor alarm occurred after the CO reached 5 ppm -1

10 flaming coal combustion experiments. For the smoldering above background; for mine-size coal, the odor monitor alarm ocexperiments, the sample average CO concentration is 4.9 ppm curred prior to the CO level reaching 5 ppm above background.

above ambient, with a standard deviation of 1.1 ppm, and for the A comparison was also made of the odor monitor alarm response

flaming experiments, the sample average CO concentration is 4.6 with the identified alarm time of sensor C for six smoldering coal

ppm above ambient, with a standard deviation of 1.8 ppm. It can combustion experiments and five flaming coal combustion

be stated that an optical density of 0.022 m corresponds to an experiments. The results are shown in figure 10. Also shown is -1

average CO concentration of 5 ppm above ambient for both the line for perfect correlation. The alarm time for the odor monismoldering and flaming combustion under these experimental tor and smoke detector C is equally divided about the line for

conditions. perfect correlation for the smoldering coal combustion

ODOR MEASUREMENT smoke detector C registers an alarm prior to the odor monitor in

The Sensidyne odor monitor was observed in both the substantial agreement with a comparison of odor monitor

smoldering and flaming coal combustion experiments to respond response and smoke detector response for 10 experiments in the

early relative to smoke and CO detection. Although the odor USBM intermediate scale tunnel (3).

monitor responds to many gaseous compounds, the response

sensitivity is greater for aromatic hydrocarbons. The instrument

response is read as arbitrary units, with a range of 

2 2

2

2

comparison purposes for an odor monitor alarm and other deodor monitor for eight smoldering experiments and seven flaming

experiments. For the flaming coal combustion experiments,

four of the five flaming coal combustion experiments. This is in
8

For the tests conducted, a comparison could be made of the indicated an alarm in 11 of the smoldering and 3 of the flaming

odor alarm time with respect to the time for the optical density to coal combustion experiments prior to an optical density of 0.022

reach 0.022 m . It was found that for 12 smoldering and 7 m . -1

flaming coal combustion experiments that the odor monitor

-1

CONCLUSIONS AND RECOMMENDATIONS

1. For each of the smoke detectors evaluated for which a 7. Detector D, an ionization, diffusion-mode smoke detector,

continuous analog signal was the measurable output, the smoke showed a linear response of the analog output signal with respect

detectors showed a nearly coincidental initial response with an to the optical density for smoldering coal combustion, and

increase in the optical obscuration. nonlinear for flaming coal combustion. Based on an evaluation

2. An average CO concentration of 5 ppm above ambient with respect to optical density, detector D showed a greater

was determined to correspond to an optical density of 0.022 m responsiveness to flaming than to smoldering coal combustion. -1

for the smoldering and flaming coal combustion experiments. At an optical density of 0.022 m , the ratio of the change in the

This is significant for establishing the equivalence of CO and detector's analog output above background of flaming to

smoke detection for mine-wide early warning fire detection smoldering coal combustion was 2.2.

systems. 8. The analog output signal from the MIC expressed as a

3. For the smoldering coal combustion experiments, the measurable quantity was compared with a quantity derived from

identified odor monitor alarm showed an advantage with respect the output voltages of the pump-mode, ionization-type detector,

to CO detection (5 ppm alarm), but did not demonstrate an detector E. The derived quantity is the product of the smoke

improvement with respect to CO detection for the flaming particulate number concentration and the number mean smoke

combustion experiments. particulate diameter. For both smoldering and flaming

4. Detector A, an optical, diffusion-mode smoke detector combustion the dependence was linear. Further analysis showed

based on light scattering, showed a significant difference between the smoke particulate diameter is larger for smoldering than for

smoldering and flaming coal combustion when the detector's flaming combustion.

analog output signal was compared with respect to the optical 9. Ionization-type smoke detectors C and D responded with

density. Although the response was linear for both smoldering greater signal intensity to flaming than to smoldering coal

and flaming combustion, the rate of increase was greater for combustion. The optical-type smoke detectors, detector A and

smoldering than for flaming coal combustion. At an optical detector B, responded with greater signal intensity to smoldering

density of 0.022 m , the ratio of the change in the detector's than to flaming coal combustion. The comparison was based on -1

analog output above background of smoldering to flaming coal smoke optical density of 0.022 m . The alarm for detector F, an

combustion was 1.8. ionization-type smoke detector, occurred at a lower optical

5. At an optical density of 0.022 m , the ratio of the change density for flaming than for smoldering coal combustion. -1

in detector B's analog output above background of smoldering to 10. A comparison of the measured signal of the commercially

flaming coal combustion was 1.2. The experiments conducted available analog-output-type smoke detectors, A to D, showed

showed that detector B would not indicate an alarm for an optical that for detectors A, C, and D, an alarm could be established at an

density less than 0.022 m based on a criterion for detector alarm optical density of 0.011 m without expecting a false alarm due -1

that defined the alarm point as average signal plus ten times the to detector electrical noise, whereas for detector B, an alarm could

peak-to-peak noise. be identified at a smoke optical density of 0.033 m .

6. For smoke detector C, which had an identifiable alarm, the Important implications for evaluation of smoke detector

smoke detector alarm and identified odor monitor alarm were at approval for in-mine use as part of a mine fire detection strategy

nearly equivalent times for the smoldering coal combustion is, first, determining the optical density at which a smoke detector

experiments. For flaming coal combustion, the smoke detector is required to respond and, second, determination of the reliability

alarm occurred prior to the odor monitor alarm. Smoke detector of the smoke detector. The optical density at which the detector

C alarmed at a lower optical density for flaming than for should respond is reasonably expected to correspond to a CO

smoldering coal combustion. At an optical density of 0.022 m , concentration no greater than the current reliability of CO sensors. -1

the output signal for detector C for flaming coal combustion

had reached the instrument's maximum value, whereas the signal

was less than maximum for smoldering coal combustion.

-1

-1

-1

-1
9

It is recommended for the evaluation of a smoke detector with at which the smoke detector alarms should be determined for

a measurable analog output, the detector alarm should be selected smoldering and flaming combustion of expected fuel sources in

such that the alarm signal is the background signal in clear air a mine. The CO concentration at the smoke detector alarm

plus some factor times the peak-to-peak noise in clear air. Smoke should be evaluated for smoldering and flaming fuel combustion.

detectors with either continuous analog output signals or a This provides a relative comparison of CO and smoke detector

manufacturer-set alarm should be evaluated for reliability with response to the combustion products for a particular smoke

repetitive testing in a smoke chamber under environmental detector. Such a comparison assists in planning the most efficient

conditions of dust, humidity, and temperature expected in mining mine fire detection system.

operations. The minimum optical density 

REFERENCES

1. Morrow, G. S., and C. D. Litton. In-Mine Evaluation of Smoke Detectors. USBM IC 6. Litton, C. D., C. P. Lazzara, and F. J. Perzak. Fire Detection for Conveyor Belt

9311, 1992, 13 pp.

2. Edwards, J. C., and G. S. Morrow. Development of Coal Combustion Sensitivity Tests

for Smoke Detectors. USBM RI 9551, 1995, 12 pp.

3. Edwards, J. C. Detection of Smoldering Combustion of Coal with an Odor Meter.

Proceedings of 12th WVU International Mining Electrotechnology Conference, Dept. of

Electrical and Computer Engineering, College of Engineering, West Virginia Univ.,

Morgantown, WV, July 27-29, 1994, pp. 150-154.

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Flammability, v. 12, April 1981, pp. 131-142.

5. Rasbash, J. D. Sensitivity Criteria for Detectors Used To Protect Life. Fire

International, v. 5, No. 49, Sept. 1975, pp. 30-44.

Entries. USBM RI 9380, 1991, 23 pp.

7. Hottel, H. C., and A. F. Sarofim. Radiative Transfer. McGraw Hill Book Co., NY,

1967, pp. 201-203.

8. Edwards, J. C., M. R. Egan, and J. Corry. CO and Smoke Signatures for Smoldering

Coal Combustion. 1993 Annual Conference on Fire Research: Book of Abstracts, NIST, U.S.

Dept. of Commerce, NIST IR 5280, Oct. 1993, pp. 125-126.

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