10

Figure 12 shows the responses of two smoke detectors of type E
located at Stations 1 and 6 for experiment 4. The two curves are
similar in shape. There is not the significant attenuation in the
detector’s maximum response at Station 6 as was shown for the
CO response at Station 6 in figure 8. The data in table 4 show
that the measured time difference between peak values of
detector E's response to smoke is approximately equal to t,,
regardless of whether the crosscuts are accessible for
experiments 3-9. This could be possibly caused by the micron
size dimension of the smoke particulates compared to the
molecular dimension of the CO. This size difference could result
in less entrainment of smoke than of CO into the crosscuts. This
is a conjecture that requires verification. For reduced airflow
experiments 10-12, smoke detector E's measured smoke travel
times based upon peak signal were significantly greater than T,.,
as shown in table 4. These reduced-flow cases resulted in sig-
nificant attenuation of the peak signal of detector E at Station 6
compared to Station 1, unlike experiments 3-9, which had
comparable peak signal responses at both stations. Table 4

Key
—— Stetion 1
Md = Station 6

DETECTOR OUTPUT, V
©
@

1 n 1 1 n 1
© 200 400 600 800 1,000 1,200 1,400 1,600
TIME, &

12—Comparison of response of smoke detector E to smoke
at the beginning and end of F Butt for experiment 4.

further shows that for normal airflow experiments 3-9, a
measurement of smoke travel time based upon an alarm signal
for smoke detector E as defined earlier, is not appreciably
different than travel time based upon the maximum signal.

CONCLUSIONS

From the results of this study, several conclusions can be
drawn.

1. Pump-mode, optical-type smoke detector A demon-
strated increased signal response for less intense diesel-fuel fires
as characterized by smoke intensity, when compared with the CO
alarm of a diffusion-mode CO detector, CO-1. For the less
intense fires, smoke detector A alarmed prior to CO-1, and for
the more intense fires the opposite alarm sequence occurred.

2. — Acomparison of pump-mode ionization-type smoke
detector B, and a diffusion-mode CO detector, CO-1, for diesel
fuel fires, showed that the smoke detector consistently alarmed
before the CO detector. The CO alarm occurred at an average
optical density of 0.085 m", whereas the smoke detector alarm
occurred at an average optical density of 0.021 m”.

3. The diffusion-mode, ionization-type smoke detector,
detector C, alarmed for those experiments for which the optical
density was greater than 0.15 m'. Its alarm occurred prior to the
alarm of the CO-1 CO detector.

4. The diffusion mode, optical-type smoke detector,
detector D, alarmed in all cases earlier than the CO detector, CO-
1. It also alarmed in two experiments for which the CO con-
centration was less than its alarm value.

5. The diffusion-mode, ionization-type smoke detector,
detector E, alarmed for each experiment earlier than the CO
detector, CO-1, and alarmed for one experiment for which the
CO concentration was less than its alarm value.

6. A correlation made of the transport time of 5 ppm
above background CO between three pairs of detectors separated
by 26, 71, and 91 m for six diesel fuel fire experiments, showed
that the travel time can be estimated by +, (t. /t) . The

exponent, 0.67, is only slightly greater than the exponent of 0.62
determined from CO release experiments (4).

7. The transport time of smoke over 91 m, as measured
by smoke detector E, did not show a dependence on crosscut
volume under normal airflow conditions.

8. From an evaluation of the buming rate of diesel fuel
for mine diese! fuel fire experiments with convective airspeed of
2m¥/s and one experiment with lower airspeed, about 0.4 m/s, the
fuel burning rates were lower than the values of the rate reported
elsewhere (5) for zero airflow conditions.

9. Visual observation of flame tilt indicated a tilt of 47°
to 59° from the normal to the fuel surface, whereas previous
experimental correlations (6-7) predicted 76° and 70°.

10. An estimation of the coagulation of smoke particles
demonstrated that this effect could reduce the smoke particulates
number concentration by 35 pct and correspondingly increase
the smoke particulate's average diameter by 16 pct, for a 450-8
smoke travel time. This could affect the response characteristic
of a smoke detector.

Based upon this set of diesel-fuel fire experiments, it is
recommended that smoke detectors that have a continuous
analog output signal be used whenever possible as part of a mine
atmospheric monitoring system. These sensors would give
‘greater flexibility for setting alarm values for fire detection at low
smoke levels. Smoke detectors that require relatively low main-
tenance, such as diffusion-mode detectors, have a reasonable
expectation of being at least as effective as CO detectors. Based
upon results from experiments with one pair of ionization-type,
diffusion-mode smoke detectors, the effect of crosscuts on
smoke travel time would be minimal for a particular smoke


COMPARATIVE IN-MINE EVALUATION OF CARBON MONOXIDE AND
SMOKE DETECTORS

By John C. Edwards’ and Gene F. Friel?

ABSTRACT

A series of liquid fuel fire experiments evaluated the comparative responses of five types of commercially
available smoke detectors and a diffusion-mode carbon monoxide (CO) detector under normal and reduced
airflow conditions based upon the alarm times of the detectors. These experiments were conducted in the Safety
Research Coal Mine at the U.S. Bureau of Mines Pittsburgh Research Center. Two of the smoke detectors had
manufacturer set alarms. For the other three smoke detectors, an alarm point was defined in terms of their
background analog signal and the detector's electrical output noise under ambient conditions. A correlation was
developed of the travel time of 5 ppm CO between pairs of CO detectors with the travel time calculated from
entry and crosscut volumes and measured airflow. Based upon the relative performance of smoke detectors in
this limited study, smoke detectors can be as effective as CO detectors for mine fire detection once identifiable
alarm values are defined. Implementation of smoke detectors as part of an atmospheric mine monitoring system
will improve mine safety.

"Research physicist,
*Chemical engineer.
Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA.


above background at an optical density of 0.022 m. However,
as pointed out above, the diffusion-mode response time of the
CO sensor does not permit a simultaneous measurement of CO
concentration and optical density.

g° —— 1 5
>
g “or Detector co-1 ]*
Detector A
gr Pa
& aot 428
g 5
§ 10- 75
8 44... . TM o
0 100 200 300 400 600 600 700 800 900

TIME, &

Figure 2.—Comparison of measured CO concentration and
response of smoke detector A at Station 2 for experiment 4.

ao T T r 6
>
3 90r ‘ 45
2 at | My HE
! ‘ 3
Evol | 43 8
Fe H ‘ £
a Key 42
2 1 —— Detector Co-1 a
Oo 6p ~~~ Detector B ==! G
8 fl 1 1. 1 f lo
° Too 200 300 400 600 600
TIME, 6

Figure 3.—Comparison of measured CO concentration and
response of smoke detector B at Station 3 for experiment 4.

n a J n n
© 100 200 300 400 500 600 700 800

TIME, s

Figure 4.—Response of light obsuration meter at Station 3 for
experiment 4.

Smoke detector C was located at Station 4 adjacent to a CO-1
CO detector. The responses of the CO and smoke detectors are
shown in figure 5 for experiment 4. Table 3 shows a comparison
of the alarm time for CO-1 detector with smoke detector C
through an evaluation of At(CO, C). For experiments 1, 2, 10,
and 11, smoke detector C did not alarm. In each of these
experiments, the minimum optical transmission over a 1-m
distance was greater than 89 pct (optical density less than 0.051
m"). For those experiments for which smoke detector C
alarmed, it alarmed prior to the CO alarm and the optical density
‘was greater than 0.15 m*, When a comparison is made between
the alarm time of smoke detector C relative to the alarm time of
its adjacent CO detector, and the alarm time of CO smoke
detector B relative to the alarm time of its adjacent CO detector,
quite dissimilar results are observed. For experiments 1, 2, and
10 with higher minimum optical transmission, detector B
responds much earlier than the diffusion-mode CO detector.
However, for these same experiments, smoke detector C does not
alarm. This could be a consequence of the pump-mode
characteristic of smoke detector B in comparison with the
diffusion-mode characteristic of smoke detector C.

For experiments with nearly the same minimum optical
transmissions, the effect of airflow on At(CO, B) and At(CO, C)
was assessed. Table 3 shows that At(CO, B) and At(CO, C) are
somewhat, but not significantly, higher for reduced airflow
experiment 12 in comparison with the normal airflow
experiments 3-8.

At Station 5, a type CO-1 CO detector and smoke detector D
were positioned near the roof. Figure 6 shows the responses of
the CO-1 detector and smoke detector D for experiment 4. Both
detectors show a néarly coincidental response time to the prod-
ucts of combustion. The maximum response of smoke detector
D occurred prior to the maximum response of the CO-1 detector.
Since both smoke detector D and the CO-1 detector are
diffusion-mode detectors, the difference in response time at
maximum signal is primarily due to the instantaneous response
of the optical smoke detector compared to the response time
characteristic of the chemical cell in the CO detector. An alarm
was defined for smoke detector D for each experiment in the
same manner as the definition of the alarm for detector A, the
average background signal plus 10 standard deviations of the
detector's noise under ambient conditions. Detector D alarmed
for each experiment based upon this criterion. For normal
airflow experiments 3-9, smoke detector D alarmed an average
of 63 s prior to the adjacent CO-1 detector. The CO detector did
not alarm for experiments 1 and 11; and for experiments 2, 10,
and 12, the CO alarm occurred 175, 156, and 93 s, respectively,
after the alarm of smoke detector D. These results are nearly the
opposite of those for detector A, which was more responsive to
less intense fires than the CO detector. Detector D is more re-
sponsive to fires with higher smoke concentrations than the CO-
1 detector.



©, Sat t

4 Brattice "

<— Airfiow direction 10

Figure 1.—Plan view of the mine section used for the
experiments.

CO detectors of type CO-1 were located in crosscuts (rooms 11,
13, and 16). Also, pairs of the CO-1 CO detector were sus-
pended at Stations 1 through 6.

To measure the optical obscuration in the smoke, an optical
obscuration meter was constructed. This consisted of a halogen
lamp source and a photovoltaic cell separated by a distance of
1m. The lamp and photo cell were supported in a rigid frame
constructed from 10-cm-diameter plastic pipe with diametrically
opposed openings spaced along the length of the pipe to permit
smoke-laden air to cross the optical path. The frame was mount-
ed at mid-height such that the optical path was transverse to the
airflow along the airway. Complete mixing over the entry cross-
section is expected at this location. The entrance port to each de-
tector was about 0.4 m from the roof. For the diffusion CO sen-
sors, the tilt of the diffusion tube was approximately 20° upwind.
This orientation provides for optimal sensor operation, as re-
ported earlier (4).

The analog signals from each CO and smoke detector, as well
as from the light obscuration meter, were converted to digital
signals and collected by a computer monitoring system. Data
sampling occurred at 2-s intervals.

Air quantity in the mine section was controlled by adjustment
of the fan pitch and installation of brattices in the SRCM. To
ensure an even distribution of air quantity at the entrance to F
Butt, a brattice was positioned oblique to the entrance of F Butt
as shown in figure 1.

Smoke detectors were positioned along the centerline of F
Butt. Adjacent to each smoke detector was a diffusion-mode CO
detector, CO-1. For several of the experiments, a pump-mode
CO detector, CO-2, was located at the entrance of F Butt, and at
the far downwind position of F Butt.

Prior to ignition of the diesel fuel source for each experiment,
a ventilation survey was made of the mine section. The ventila-
tion was generally measured 1.8 m upwind of the sensors in the
entry. Since most velocity measurements contained wind and
fan fluctuations, 5- and 9-point time-averaged flow measure-
ments were made during the ventilation survey. Smoke-tube
measurements of the airflow were made in selected crosscuts.

‘The CO sensors were calibrated with a zero and span gas. It
‘was occasionally noted during the course of the experiments that,
even though a chemical cell for a sensor would hold its cali-
bration, a deterioration in the performance of the cell was as-
sociated with a lag in the sensor response. This was indicated by
an unusually long decay back to the ambient concentration from
the maximum concentration. This was verified by the inter-
change of two sensors, located adjacent to each other at the same
downwind position in the entry, and a repetition of the exper-
iment. If the concentration measurements of the interchanged
sensors were reproducible, then a deterioration in the chemical
cell of the CO sensor was verified, and the cell was replaced.
The normal expected useful lifetime of a chemical cell would
decrease through repetitive use in an environment with CO and
other product-of-combustion gases associated with fire tests.

Each smoke detector was adjusted according to the manuf-
acturer's specifications. The amplifier circuit on the photocell
used in the light obscuration meter was adjusted prior to each
experiment so that the output of 1 V corresponded to 100 pet
optical transmission through clear air.

After calibration of the sensors and completion of the
ventilation survey, the fuel tray was placed in the mine portal.
Square tray sizes of 0.46 m side length and 0.76 m side length
were used. It was determined that inadequate CO was generated
from a diesel fuel fire in a 0.46-m side-length tray. For this
reason, conveyor belt strips were mounted on the rim of the tray
above the diesel fuel to increase the CO production through their
combustion. Although this measure increased the CO, it was not
adequate to assure that 5 ppm of CO above background was less
than the inflection point on the CO concentration versus time
profile at a sensor location. This is important for application of
the analysis of previous work (4) to transport time between
sensors based upon entry and crosscut volume. For this reason,
a tray with a 0.76 m side length was used for the remainder of the
experiments in which the fire was located in the mine portal. It
was found by trial and error that the addition of 500 mL of
gasoline to the diesel fuel assured rapid, uniform burning of the
diesel fuel.

In order to observe the effect of ventilation on flame tilt, a
pole was mounted vertically about 1 m downwind of the fire for
several experiments. Visual observations were made of the
flame tilt at 15-cm intervals marked on the pole. From the
horizontal distance between the pole and the fire, and the vertical
position on the pole intersected by a straight line from the
extended flame, the angle of tilt of the flame was determined for
the experimental conditions of ventilation and fuel surface area.

A summary of the fire experiments conducted in the SRCM
is presented in table 2. The experiments are characterized by fuel
tray size, fuel loading, average measured airflow quantity in F
Butt, and the crosscut accessibility for the airflow in F Butt.

Experiments 1-9 were conducted under normal mine ven-
tilation conditions. Experiments 10-12 were conducted to in-
vestigate the responses of smoke and CO detectors under re-
duced airflow conditions. For experiments 1-9, the air quantity
at the entrance to F Butt was 8 to 11 m’/s. Leakage along the



INTRODUCTION

Currently, the CO detector is the more common detector
used for fire detection as part of atmospheric monitoring system
in underground mines. In recent years, smoke detectors have
been evaluated in several mines (1). It also was shown that the
performance of smoke detectors can be evaluated in the labora-
tory with the use of a smoke chamber (2) for smoldering and
flaming coal combustion. Subsequently, a relative evaluation of
six smoke detectors was made with the smoke chamber (3).
Those studies showed, based upon analysis of the smoke
detector's analog response signal, that it is possible to detect the
occurrence of smoke from coal combustion when the optical
density is less than 0.022 m". It was further demonstrated that
the optical density of 0.022 m" corresponded to a measurable
CO concentration of 5 ppm above ambient for smoldering and
flaming coal combustion. This U.S. Bureau of Mines (USBM)
research was undertaken to enhance the safety of mine workers
through the utilization of an improved atmospheric mine mon-
itoring system.

In order to make a comparison of the responses of CO and
smoke detectors to an in-mine fire, a series of experiments was
conducted by the USBM in the Safety Research Coal Mine
(SRCM) located at the Pittsburgh Research Center. A previous
USBMexperimental study (4) showed that dead-end crosscuts

EXPERIMENTAL

A plan view of the SRCM section used for the experiments
is shown in figure 1; the airflow direction is also shown. F Butt
was instrumented with CO and smoke detectors for each
experiment. The total separation distance from the first to the
last sensor in F Butt was 91 m. The source fire was located in the
mine portal, approximately 360 m from Station No. 1. Smoke
detectors from five different manufacturers and CO detectors
from two different manufacturers were used for the experiments.
Table 1 lists the primary detectors used for most of the ex-
periments at the stations indicated, the operational type as either
ionization or optical for the smoke detectors, and the sampling
mode as either diffusion or pump. The maximum measured
output signal of smoke detector A for the fire experiments was
approximately 2.5 V, with a background of 0.13 V in clear air.
The alarm of smoke detector B is 2.5 V on a 1- to 5-V range.
Unlike the other smoke detectors, which have an accessible
analog output signal, smoke detector C has two discrete output
signals. For clear air the signal is 4 mA; for smoke-laden air the
signal is 20 mA. Smoke detector D is calibrated to read 0.4 Vin
clear air and 2.0 V when the smoke obscuration is equivalent to
an optical density of 0.046 m. The output signal range of
smoke detector E is -0.9 to -0.2 mA, which was converted
electronically to 0.9 V for clear air to 0.2 V for air saturated

“iualic numbers in parentheses refer to items in the list of references at the
‘end of this report.

affect the transport time between CO detectors by providing an
entrainment mechanism for dilution of the CO concentration in
the entry. The complex geometric characteristics of smoke par-
ticulates, such as size distribution and concentration, and
physical properties, such as dielectric constant, could result in a
variable response time of smoke detectors compared to CO
detectors. This is in addition to expected differences in meas-
urable responses based upon whether the detector operates in a
diffusion mode or a pump mode. The effect of ventilation flow
and entrainment at dead-end crosscuts on the responses of CO
and smoke detectors at neatly identical locations will provide a
basis for future recommendations for the implementation of
improved atmospheric monitoring systems in mines. For this
study, a mine combustible, diesel fuel, provided the smoke and
CO source. The practical advantage of diesel fuel as a com-
bustion source is its relatively uniform production of smoke and
0. For 2 of the 12 experiments, conveyor belt strips were used
to increase CO production. For the other 10 experiments, a
small quantity of gasoline was added to the diesel fuel to assure
uniform ignition of the diesel fuel. Normal airflow conditions
in excess of 1.0 m/s and reduced airflow conditions with air
velocities less than 0.4 m/s were considered.

PROCEDURES

with smoke. Smoke detectors A and B operate in the mechanical
pump mode, whereas smoke detectors C, D, and E operate in the
diffusion mode. A and D are based upon optical detection prin-
ciples, whereas B, C, and E are based upon ionization principles.

Table 1.—Primary product of combustion detectors
used in mine fire experiments.

Station Detectors
Carbon monoxide Smoke (type, sampling mode)

C0-1, CO-2 E(.D
cO-1 A (0,P)
cO-1 BP)
con cD
0-1 DD)
0-1, CO-2 Eo

‘Two different types of CO detectors were used. One type of
CO detector, CO-1, operates in a diffusion mode, and the other
type, CO-2, operates in a mechanical pump mode. Both CO-1
and CO-2 operate on the principle of electro-oxidation of ox-
ygen, although with different types of chemical cells. Additonal


Table 3.—Minimum optical transmission, optical density at alarm time
‘of amoke detector B and comparison of alarm times.
of smoke detectors B and C with CO detector

Experiment ‘Minimum optical D,mTM ‘Ax(CO, B), = Br(CO, Cy, s
transmission, pet
a1 ° 132 NA
92 ° 147 NA
53 0.033 52 2
60 0.044 36 18
57 0.026 42 18
69 0.025 53 a
59 0.021 53 8
70 0.017 40 15
NA NA NA 18
eo 0.0070 80 NA
95 0.0035 NA NA
57 0.054 60 33.

At Station 2, smoke detector A was positioned next to a type
CO-1 CO detector. Figure 2 shows the responses of smoke
detector A and CO detector CO-1 for experiment 4. An alarm
was defined for smoke detector A for each experiment as the av-
erage background signal plus 10 standard deviations of the
detector's noise under ambient conditions. For normal airflow
experiments 3-8, smoke detector A's alarm occurred an average
of 17 s after the alarm of the adjacent CO detector CO-1. Data
were not available for smoke detector A for experiment 9. For
experiments 1, 2, 10, and 11, characterized by dilute CO con-
centrations and higher minimum optical transmissions, smoke
detector A's alarm occurred prior to the CO alarm. The alarm
times of smoke detector A and CO alarm CO-1 were nearly si-
multaneous for experiment 12. For experiments 1, 2, 10, and 11,
smoke detector A's output voltage had increased by more than
300 mV when the adjacent CO sensor alarmed. The fires for
these particular experiments were characterized by minimum op-
tical transmissions over a 1-m distance that were greater than 89
pet. For fire experiments 3-8 and 12 with the more intense fires
and associated minimum optical transmissions of less than 70 pet
over 1 m, smoke detector A's output voltage increased by 30 mV
when the CO sensor reached alarm stage. Although the min-
imum optical-density is used to refer to fire intensity, this is not
a true measure of fire intensity since air dilution controls the
smoke intensity as measured by the optical transmission. The
earlier alarm time of smoke detector A for less intense fires,
compared with when the CO-1 detector reached alarm, implies
that, at least for the fire size and growth rate developed, smoke
detector A could be more sensitive for fire detection of low in-
tensity fires than the CO detector, dependent on the CO alarm
level.

At Station 3, smoke detector B was positioned adjacent to the
CO-1 CO detector. Figure 3 shows the comparative responses
of smoke detector B and CO detector CO-1 for experiment 4.
The initial responses of both the CO-1 detector and smoke
detector B are almost coincidental even though the CO detector
is a diffusion mode, and the smoke detector operates in the pump
mode. Table 3 provides an evaluation of the optical density at
which smoke detector B alarmed. The average optical density at
smoke detector B's alarm was 0.021 mTM, and the standard

deviation was 0.018 m', The CO concentration at Station 3 was
less than 1 ppm above background when smoke detector B
alarmed. For experiments 3-8 and 10-12, which had only diesel
fuel as the combustion source, the optical density at which the
smoke detector B alarmed ranged from 0.0035 to 0.054 m’, with
an average value of 0.026 m” and a standard deviation of 0.016
m’. For the normal airflow experiments 3-8, the average optical
density at which smoke detector B alarmed was 0.028 m", while
for the reduced airflow experiments 10-12, the average optical
density at alarm was 0.022 m". For experiments 3-8, 10, and 12,
the average optical density when the diffusion-mode CO-1
detector reached 5 ppm above ambient was 0.085 m" with a
standard deviation of 0.046 m’. A comparison was made of the
alarm times of CO detector CO-1 and smoke detector B. In each
case, smoke detector B's alarm time occurred prior to that of the
©O detector. This is listed in table 3 as At(CO, B), where At(X,
Y) is defined as the differential time by which detector X's alarm
time follows detector Y's alarm time. In addition to having an
earlier alarm time than the CO detector, smoke detector B gave
a significantly earlier alarm time for those experiments with less
intense fire sources, as characterized by the minimum optical
transmissions listed in table 3. In particular, experiments 1, 2,
and 10 with minimum optical transmissions over a 1-m distance
greater than 89 pet, had values of At (CO, B) greater than 80 s.
For experiment 9, smoke detector B was not operational and for
experiment 11, the CO detector at Station 3 was not reliable.

Figure 4 shows the response of the light obscuration meter
for experiment 4. A comparison of figures 3 and 4 shows that
the CO maximum concentration is approximately 50 s after the
minimum optical transmission. This is a characteristic indicative
of the response time of the chemical cell in the CO detector.

A significant difference between average optical densities of
the CO alarm was determined based upon airflow. For normal
airflow experiments 3-8, the average optical density at which the
CO-1 detector was 5 ppm above ambient, was 0.11 m", whereas
for reduced airflow experiments 10 and 12, the average optical
density at alarm was 0.017 m?. When the measured optical
density was 0.022 m’, the average measurable CO concentration.
was less than 1 ppm above background for experiments 3-8. For
experiments 10-12, the CO concentration was less than 4 ppm


PORT OF INVESTIGATIONS/ 1996

Comparative In-Mine Evaluation of Carbon
Monoxide and Smoke Detectors

By John C. Edwards and Gene F. Friel



Additional measurements are required with regard to smoke
particulate diameter, such as with a three-wavelength optical de-
tector as described in previous work (10), before a more definite
conclusion can be drawn regarding particulate coagulation.

Equation 3 implies that, for a smoke cloud with a constant
mass concentration, the quantity, d In (T)/Q, is constant. The
ratio of Q to d is constant over a small range of particulate di-
ameter. This is based upon calculated values of Q which show
amonotonic decrease with respect to decreases in d. Since over
a small range of diameters the quantity d/Q is nearly constant, the
implication is that the optical transmission, T,, is nearly constant.
This implies that coagulation could affect an ionization-type
more than an optical-type smoke detector.

‘Measurement of the arrival time of specific CO concentrations
at the beginning (Station 1) and end of F Butt (Station 6)
provides information on the effect of crosscuts on CO transport
time. Table 4 shows the measured travel time of the CO con-
centration, which is 5 ppm above background from Station 1 to
Station 6 for experiments 3-12. A CO concentration 5 ppm
above background is considered to be the alarm value for this
study. Experiments 1 and 2 were excluded because the CO con-
centration was too low. For experiments 10 and 12, the 5 ppm
CO concentration above background was nearly at the maximum
measured CO concentration in those tests. For experiment 11,
the maximum CO concentration at Station 6 was less than 5 ppm.
Figure 8 shows typical CO concentration profiles at Station 1 and
Station 6, which are 91 m apart, as measured with a diffusion-
mode CO-1 detector for experiment 4. The flat portion in the
concentration profile at Station 1 between 150 and 200 s is
because the maximum detector range of 50 ppm was exceeded.
‘The reduction in the maximum CO concentration at Station 6 is
caused by the entrainment of CO at crosscuts.

‘Table 4.—Measured CO alarm and smoke response
travel times and predicted travel times, seconds:

Experiment Measured Predicted
“CO Smoke’ Smoke” __F, 1
Ww 100 96 101 101
122 104 101 6 153
110 94 94 100 100
122 6 85 7 124
V27 110 100 98 127
136 104 107 106 166
130 102 108 9 9
548 532 1,390 267 448
NA 1,110 428 299 472
777 1,032 616 355, 561

NA Not available.
‘Based upon maximum signal from smoke detector E.
*Based upon detector E's average background signal less ten
standard deviations of detector's noise under ambient conditions.

An analysis was made of the CO travel times between alarms
at seventeen pairs of CO-1 sensors in F Butt. This included the
path from Station 1 to Station 6, a 91-m separation, for
experiments 3-8; the path from Station 3 to Station 6, a 71-m
separation, for experiments 3-8; and the path from Station 3 to
Station 4, a 26-m separation, for experiments 3-8, exclusive of
experiment 4, which yielded an outlier. A correlation of the

measured travel time, t,, between alarms at the ends of the path
with t, and t,, yielded

Ta PM, )

where ¢, is the predicted travel time based upon measured entry
airflow and entry volume, and ¢,, is the predicted transport time
based upon measured entry airflow and total volume, entry and
crosscuts. Figure 9 shows a plot of t,, /t, with respect to t,. /t.,
‘as well as the correlation equation (equation 5). The standard
deviation of the regression was 0.1. The exponent 0.67 in
equation 1 is in close agreement with the exponent 0.62
determined from the analysis of CO release experiments and
reported by Friel and others (4) as equation 2. The data for
experiment 9 were treated as an outlier. This is consistent with
the experimental observation that the detector cell for the CO
detector at Station 6 had deteriorated for experiment 9. The
analysis developed in (4) applies to the measured transport time
of CO concentration less than the half-height of the maximum
CO concentration. Experiments 10-12 were for reduced airflow
and resulted in CO concentration at Station 6 for which half the
maximum CO concentration was less than 5 ppm above back-
ground and, therefore, these experiments were excluded from the
correlation.

The significance of using CO detectors with the same op-
erating characteristics to evaluate the transport time of a specific
CO concentration between stations, is demonstrated through a
comparison of the responses of types CO-1 and CO-2 detectors,
which have different operating characteristics, at the same
station. A comparison was made at Station 6 of the CO levels
measured by the CO-1 and CO-2 detectors. Figure 10 shows the
measurable responses for experiment 4 of each CO detector. The
measured values for CO-2 were mathematically smoothed.
There are three major observations with regard to figure 10.
First, the diffusion-mode CO detector, CO-1, responds earlier
than the pump-mode detector, CO-2. Second, the maximum CO
concentration is lower for the diffusion-mode detector. Third,
based upon a S-ppm alarm setting, both detectors yield an alam.
at the same time. These observations were repeatable for ex-
periments 5, 6, and 7, for which measurements of CO responses
with CO-1 and CO-2 detectors were both available. The CO
detectors differed not only in the diffusion versus pump mode,
but in the chemical cell used to detect the CO. Both factors
account for the different responses of the detectors. Based upon
the faster delivery rate of CO to the detector with a pump than
through diffusion, it is expected that the real-time CO
concentration is more closely represented by the results from
CO-2. Because differences in arrival times are considered in the
crosscut analysis presented above, it is necessary to use CO
detectors of the same measurement and response characteristics
for that analysis.

A simple mathematical model can be constructed to explain
the response of the diffusion-mode detector. Let the true CO
concentration be denoted by C,, and the measured CO con-
centrations by C. The measured CO concentration in the
diffusion-mode detector lags C, due to diffusion in the entry tube
of the detector, diffusion through the membrane covering the



At Stations 1 and 6, a smoke detector of type E was po-
sitioned near the roof adjacent to a CO-1 CO detector. For each
experiment, an alarm was defined for detector E as the average
background signal less 10 standard deviations of the detector's
noise under ambient conditions. For normal airflow experiments
3-9, smoke detector E alarmed an average of 49 s before the CO-
1 detector at Station 1, and 73 s earlier at Station 6. The longer
average lag time at Station 6 compared to Station 1 is a result of
the dilution of the CO as it traverses from Station 1 to Station 6.
For reduced airflow experiments 10-12, the lag time was longer
than for normal airflow conditions. At Station 1, the average lag
time was 110 s, and at Station 6, the average lag time was 262s
for experiments 10-12. For experiment 11, smoke detector E
alarmed at Station 6 while the CO concentration did not reach its
alarm value. Figure 7 shows a comparison of the responses of
smoke detector E and the type CO-1 CO detector at Station 6 for
experiment 4. Smoke detector E and CO detector CO-1 have
nearly simultaneous initial responses. The time lag observed in
the peak response of the CO detector with respect to smoke
detector E in figure 7, is due to the finite chemical reaction rate
of the CO detector's chemical cell. This was also the case for a
CO detector's response compared to smoke detector D's
response.

Two factors that can affect fire detection in a mine are the
growth in the smoke-particle diameter d and decrease in the
smoke particulate number concentration, n, as the smoke
traverses the mine entries. This effect has been described as a
rate process (8). The proposed model equation is

Lig d, o
a a,
where t is the elapsed time the particles are in suspension, n, is
the initial smoke particulate concentration, and the coagulation
coefficient, k, is approximated as 0.6 x 107 m’/(particle « s) (8).
The number concentration is related to the mass concentration,

Ca» bY

Cq= Fan, Q)

where p is the smoke mass density. C,, can be determined from
a measurement of the optical transmission, T, over a path length,
4, using the Bouguer-Lambert law (9), which can be rewritten as

reo 3
Cc, 3Q0 | 100)” @)
where Q is the extinction coefficient. Equations 2 and 3 yield

ts)
. 4)

TM Qed?

Equations 1 and 4 can be used to evaluate the reduction in
particulate number concentration, For example, for experiment
4, the minimum measured optical transmission at the light
obscuration meter was 60 pct, which occurred 450 s after
ignition of the fuel. With an approximate extinction coefficient,
Q =2, and an assumed smoke particle diameter of 0.5 yan when

416

Detector CO-1
== Detector C

g 8 8

3
T
'
'
u

CO CONCENTRATION, ppm
x
3
T

fi
° 100

200

1 1
300-400

L
‘500

°
600

TIME, 8

Figure 5 —Comparison of measured CO concentration and re-
sponse of smoke detector C at Station 4 for experiment 4.

i 25 or t0
>

5 20F \ Key Ob Ee

3 — Detector CO-1

E isk \ === Detectoro Jog

§ ake oa

8

g S

8 5 4025

8 a

ts) aan

fg gy n °
100 200 300 400 600 600 700 800 8001,000
TIME, #

Figure 6 —Comparison of measured CO concentration and re-
sponse of smoke detector D at Station 5 for experiment 4

°

i

x
z
T

DETECTOR OUTPUT, V

CO CONCENTRATION, ppm
a
T

1. 1 1 °
300 400 600 600
TIME, ¢
Figure 7—Comparison of measured CO concentration and re-
‘sponse of smoke detector E at Station 6 for experlement 4.

° 100 200

the smoke reaches the light obscuration meter, the calculated
reduction in the expected number concentration of smoke
particles is 35 pct as the smoke traverses the distance from the
fire to the light obscuration meter, when the optical transmission
is 60 pet. A reduction in particle number concentration is
associated with an increased particle diameter based upon the
assumption of constant mass concentration, The implication of
this calculation is that for a 35 pct reduction in particle
concentration to that associated with a particle diameter of 0.5
yum, the smoke particle diameter at the fire would be 0.43 pan.
This represents a 16 pet increase in smoke particle diameter.



‘Table 2.—Experimental conditions

Tray size, m Diesel fuel, L F Butt air quantity, m/s Crosscuts
Station 1 ‘Station 6
0.46 by 0.46 20 98 a4 Closed,
0.46 by 0.48 20 98 at Closed.
0.76 by 0.76 40 91 74 Closed,
0.76 by 0.76 40 89 74 Open.
0.76 by 0.76 35 10.2 78 Closed.
0.76 by 0.76 35 93 79 G Butt side open;
E Butt side closed.
Teseseee 0.76 by 0.76 35 9.0 a3 G Butt side closed;
E Butt side open.
8. 0.76 by 0.76 30 89 72 Open.
9. 0.76 by 0.76 3.0 95 73 Closed.
10 0.76 by 0.76 40 18 3.0 Open.
" 0.76 by 0.76 40 3.0 3.0 Open.
2 0.46 by 0.46 20 25 24 Open.

"Three strips of conveyor belt 0.53 m by 5.1 cm by 1.1 cm were mounted over top of tray.

airway through the crosscuts reduced this to 6 to 9 m’/s at the
exit of F Butt. For experiment 10, a brattice was positioned as an
airflow regulator at entrance room 10 to reduce the airflow in F
Butt. A reduction in air quantity was achieved in F Butt.
However, a positive differential pressure was created from a
parallel entry into F Butt. This resulted in air leakage from the
parallel entry into F Butt through the brattices separating the
connecting rooms of the airways. This resulted in dilution of the
CO along F Butt. To correct this condition, a brattice was
positioned as a regulator at the junction of F Butt and room 18

for experiments 11-12. This increased the pressure of F Butt
relative to the parallel airway, and prevented dilution of the CO
in F Butt due to air leakage from the parallel entry.

The linear airflow in F Butt varied from 0.8 to 1.2 m/s for
experiments 1-9, whereas for experiments 10-12, the linear
airflow was less than 0.4 m/s. For experiment 12, the fire source
was moved from the portal to Room 10, about 45 m from the
junction of room 10 and F Butt, to increase the smoke
‘concentration in F Butt.

RESULTS

Experiments 3-12 resulted in nearly uniform ignition and
combustion of diesel fuel because of the addition of a small
quantity of gasoline to the diesel fuel. Experiments 3-11 were
conducted with a 0,76-m side-length tray, and experiment 12 was
conducted with a 0.46-m side-length tray. For experiments 3-11,
the average linear burning rate of the fuel was between 0.07 and
0.17 cmmin, with an average rate of 0.12 cm/min. The burning
rate for experiment 12 was 0.19 cm/min. These burning rates are
lower than the values of 0.2 cm/min and 0.3 cm/min for 0.46-
and 0.76-m-diameter pans, respectively, as reported in previous
work in zero airflow conditions (5). This could be an effect of
the ventilation. Experiments 3-11 differed from experiment 12
not only in tray size (side length of 0.76 rh for experiments 3-11
and 0.46 m for experiment 12), but in the ventilation flow at the
fire location. The fire was located in the mine portal for ex-
periments 3-11, where the average ventilation linear speed was
approximately 2 m/s. For experiment 12, the fire source was
located in a reduced airspeed (about 0.4 m/s) section of the mine.

A visual observation of flame tilt was made relative to a
vertical pole 1.07 m downwind from the fire with 15-cm markers
on the pole, Thomas (6) reported correlations of the convective
wind-induced tilt of a flame based upon data from wood crib
fires. A study by the American Gas Association (AGA) (7) also
reported a correlation of flame tilt with wind velocity. For a
diesel-fuel experiment with a 0.46-m side-length tray, and a wind

speed of 1.86 m/s, flame tilt of 47° to 59° from the normal line
to the horizontal fuel surface was observed. Thomas’ model pre-
dicts a tilt angle of 76°, whereas the AGA model predicts a tilt of
70°. These predictions are significant overestimates of the meas-
ured tilt. The comparison of measured flame tilt with model
predictions is somewhat closer for the AGA model with a rel-
ative error between 16 and 33 pet. Flame tilt dependence upon
ventilation is important for determination of flame propagation
over a fuel surface.

Table 3 identifies the minimum optical transmission over a 1-
m path length that occurred for each experiment. Experiments
1 and 2 were conducted with a sufficiently small fuel tray size
such that the optical transmission did not decrease below 90 pet
for the ventilation used. ‘This optical transmission corresponds
to an optical density of 0.046 m*. Experiments 3-8 were con-
ducted with a larger tray size than experiments 1 and 2, and had
significantly lower values for minimum optical transmission.
Optical transmission data were not available for experiment 9.
Experiments 10 and 11 had high optical transmissions because
the airflow primarily bypassed F Butt, where the optical ob-
souration meter was located, and passed through a parallel entry.
Experiment 12 had a relatively low optical transmission because
the fire was located in reduced airflow in Room 10 connected to
F Butt, even though the tray size for experiment 12 was the same
as for experiments 1 and 2.


UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

om centimeter m®/(particles) cubic meter per particle per second
cm/min _ centimeter per minute mA milliampere

kg kilogram mL milliliter

kg/m? _ kilogram per cubic meter mV millivolt

L liter pet percent

m* inverse meter ppm part per million

m meter s second

m? square meter v volt

m/s meter per second ym micron

m/s cubic meter per second

Reference to specific products is for identification only and does not imply endorsement by the U.S. Bureau of Mines.



detector alarm level under normal airflow conditions. This
aspect would be incorporated in a mine fire location strate-
gy. Smoke detectors, when incorporated into a mine

11

atmospheric monitoring system, will complement CO detectors
and thereby improve mine safety.

ACKNOWLEDGMENT

The authors wish to acknowledge Gerald S. Morrow,
Electronics Technician, USBM Pittsburgh Research Center,

for providing technical assistance in preparation for the mine
fire experiments.

REFERENCES

1. Morrow, G. S,, and C. D. Litton. In-Mine Evaluation of Smoke
Detectors. USBM IC 9311, 1992, 13 pp.

2. Bawards, J. C., and G. S. Morrow. Development of Coal Combustion
Sensitivity Tests for Smoke Detectors. USBM RI 9551, 1995, 12 pp.

3. __. Evaluation of Smoke Detectors for Mining Use. USBM RI 9586,
1995, 17 pp.

4. Friel, G. F., J. C. Edwards, and G. S. Morrow. Effect of Dead-End
Crosscuts on Contaminant Travel Times in Mine Entries. USBM RI 9517,
1994, 18 pp.

5. Burgess, D. S.,J.Grumer, and H.G. Wolfhard. Burning Rates of Liquid
Fuels in Large and Small Open Trays. Ch. in The Use of Models in Fire
Research. Nat. Acad. Sci.-Nat. Res. Council, Pub. 786, 1961, pp. 68-75.

6. Thomas, P. H. The Size of Flames from Natural Fires. Paper in
Proceedings of the Ninth Symposium (International) on Combustion (Ithaca,

NY, Aug. 27-Sept. 1, 1962). The Combustion Inst. (Academic Press,
New York), 1963, pp. 844-859,

7. National Fire Protection Association. SFPE Handbook on Fire
Protection Engineering. National Fire Protection Assn., Quincy, MA, 1988, pp.
2-52.

8. McPherson, M. J. Subsurface Ventilation and Environmental En-
ineering. Chapman & Hall, New York, 1993, pp. 778-781.

9. Hotel, H. C., and A. F. Sarofin. Radiative Transfer. McGraw-Hill
Book Co., New York, 1967, p. 201.

10. —Cashdollar, KL, C. V. Lee, and J. M. Singer. Three-Wavelength Light
‘Transmission Technique To Measure Smoke Particle Size and Concentration.
Appl. Optics, v. 18, No. 11, June 1, 1979, pp. 1763-1769.


CONTENTS

Abstract .
Introduction . -
Experimental procedure
Results
Conclusions .
Acknowledgment
References ....

ILLUSTRATIONS

1, Plan view of mine section used for experiments .

2. Comparison of measured CO concentration and response of smoke detector A
3. Comparison of measured CO concentration and response of smoke detector B
4. Response of light obscuration meter .

5. Comparison of measured CO concentration and response of smoke detector C
6.

7.

8.

9.

Comparison of measured CO concentration and response of smoke detector D
Comparison of measured CO concentration and response of smake detector E.
Comparison of CO concentration at beginning and end of F Butt
Correlation of t,, /7, with t,,./t, for CO detector separations of 26, 71, and 91m .

10. Comparison of responses of CO detectors C0-1 and CO-2 ......6...0e08

11. Comparison of model prediction of CO concentration with CO measurement .

12. Comparison of responses of smoke detector E to smoke at beginning and end of F Butt

TABLES

Primary product of combustion detectors used in mine fire experiments .
Experimental conditions .

Minimum optical transmission, optical density at alarm of detector B, and comparison of alarm times of smoke
detectors B and C with CO detector alarm time

4. Measured CO alarm and smoke response travel times and predicted travel times .

Pre

Seoceeivniaaay


Report of Investigations 9622

Comparative In-Mine Evaluation of Carbon
Monoxide and Smoke Detectors

By John C. Edwards and Gene F. Friel

UNITED STATES DEPARTMENT OF THE INTERIOR
Bruce Babbitt, Secretary

BUREAU OF MINES
Rhea Lydia Graham, Director


detector's chemical cell, and chemical reaction in the cell, These
three processes can be described by an effective rate constant, k,
which is the reciprocal of an effective time constant associated
with these processes. The assumption of a first-order linear rate
process results in the model equation,

ac

X- -KC-c).

a (C-C). (6)
a
40] 4
Fo
& 30 4
E 20) . 4
2 .
8 10 4
2 ’
8 Fersaan 1 nl

° 00200. 300 400 + +600 600
TIME, 8

Figure 8.—Comparison of CO concentration at the beginning
and end of F Butt for experiment 4.

1 1. 1 L
os 10 11 12 13 14 18 1.6 17

Tec/Te
Figure 9.—Correlation of 1,/r, with 1, /1, for CO detector

separations of 26, 71, and 91 m.

e a
zor Key 4
g — Detector C0-1
ae =--Detectorco-2
% 20

3

810

8

8

°

100 200 300 400 500 600
TIME, 5

700800 800 1,000

10.—Comparison of response of CO detectors CO-1 and
CO-2 at Station 6 for experiment 4. Data for detector CO-2 have
been smoothed.

Equation 6 was applied to an assumed triangular distribution of
CO, C,, shown in figure 11 as an approximation to the values
measured with detector CO-2. The functional form of the
driving term, C,, was

C, = Cit, Octet,

Cy1-C4-4 My), ttt,

=0 t>2t,. 10)

Based upon the observed value from detector CO-2 in fig-
ure 10, the model application was made with C, = 45 ppm and t,
= 146 s. The time axis was shifted so that the rise in the CO
concentration coincides with the first measurable response of the
CO-1 detector. An analytic solution was developed as a solution
to the model, equation 6, and the driving term C,, in equation 7.
A time constant equal to 90 s was used to define the rate
constant, k = 1/90s". The model equation solution, C, is shown
in figure 11 along with the driving term, C,, and the meas-
urement from detector CO-1, The model yields very good
agreement with the measured CO from the initial detection to the
inflection point of the curve on the decreasing side of the CO
concentration curve. Discrepancy beyond the inflection point is
possibly due to the assumed symmetrical distribution of the
actual CO concentration, C,. In reality, the processes of con-
vection and dispersion along the airway and CO entering and
leaving crosscuts are expected to produce an asymmetrical
distribution of CO. The model shows the linear rate assumption
is realistic.

50 T T T

Key
—— Detector CO-1 4

b
i)

o
°

CO CONCENTRATION, ppm
x
i]

°

800
TIME, s

Figure 11.—Comparison of model prediction of CO concen-
‘tration at Station 6 with CO measurement for experiment 4,