Table 4. Alarm time of smoke and CO detectors at station $2 relative to first arrival of smoke

Detector alarm lag time, s

Experiment co A 8 ¢ D E E
4 7 : 39 18 7 25 NA
2 42 8 54 30 45 NA NA
3 40 23 66 26 Erd 22 NA
4 48 32 101 39 NA 30 NA
5 87 32 NA 43 55 NA NA
6 46 47 >4 min 34 46 20 NA
z 6 NA -10 8 70 -39 NA
8 102 108 >4 min 107 >4 min 51 NA
9 162 74 >4 min 125 >4 min 70 34

NA = Not Available

detector alarm, as the mean ambient temperature plus ten stan-
dard deviations of the signal noise, then a comparison can be
made of a thermal and CO alarm time. For experiment § the ther-
mal detection lags the CO alarm by 14.7 min and 14.2 min at sta-
tions S1 and S4, and for experiment 6 the lag times are 5.2 and
10.3 min. For the less intense 30 kW fires, there was no signiti-
cant temperature rise 30 m from the fire. At a distance of 15m
from the 30 kW fire in experiment 7, there was a significantly
smaller temperature rise than there was 30 m from the 296 and
264 kW fire in experiments 5 and 6. During the fire of experiment
7, the thermocouple 15 m from the fire at S2 measured a tempera-
ture increase from an ambient value of 6°C to a maximum tem-
perature of 17°C, although the measured CO exceeded 45 ppm
at the same location.

Smoke Sensors

A relative comparison was made of the response of the smoke
detectors listed in Table 1 with the first detection of smoke by the
light obscuration device at station S2. The minimum measured
light transmission at S2 was between 5 and 15 pct for the more
intense fires of experiments 1-6, whereas for the relatively lower
intensity fires of experiments 7-9, the minimum optical transmis-
sion varied between 53 and 58 pct. The relationship between vis-
ibility and optical density (Rasbach, 1971), based upon visibility
studies using placards, implies that the minimum visibility for ex-
periments 1-6 varied between 0.6 and 1.0 m, and for experiments
7-9 the minimum visibility varied between 2.9 and 3.4m. Each of
these values is less than the 4 m visibility required for escape from
a building for someone familiar with the surroundings (Jin, 1981).

Ifonly the results in the experiments for those smoke detectors
and the CO detector at S2 which alarmed within 4 min of the first
detection of smoke as measured by the light obscuration device
are considered, then the lag or lead time of the detector relative
to the first detection of smoke are within 92 s of each other, as
shown in Table 4. Table 4 shows that smoke detectors A, C, and
E and the CO detector are consistent indicators of the diesel fuel
fire within a 92 s time span of each other for the nine experiments
conducted

A comparison was made of the alarm time of each smoke de-

tector and the diffusion CO detector 5 ppm alarm times.

The comparison of alarm times at S2 can be made with smoke
detectors by type - ionization and optical. With regard to the ion-
ization type, E alarmed an average 36 s before the CO detector
for seven of the nine experiments. For experiments 2 and 5, E
was not operational. Detector C alarmed an average 16 s prior to
the CO detector, except in experiment 8 in which it alarmed 5 s
after the CO detector. For experiment 9 the alarm time of detec-
tor F occurred 2.1 min before the CO detector.

For the high intensity fire experiments 1-3 and 5-6, smoke de-
tector D alarm time occurred within 20 s of the CO detector. It
‘was not operational for experiment 4. For the low fire intensity,
experiment 7, detector D's alarm was 1.1 min after the CO alarm,
and it was 9.9 and 9.0 min after the CO alarm for low fire inten-
sity experiments 8 and 9. The difference between experiments 7,
8, and 9 is attributable to the 15 m spacing between the fire and
S2 for experiment 7, and 30 m spacing for experiments 8 and 9.
Although detector D has a manufacturer defined alarm, as does
detectors E and F, it operates in a diffusion mode, which, as ex-
pected in a zero or low airflow condition, would be at a disadvan-
tage with respect to a pump mode detector. Smoke detector C,
which operates in a diffusion mode, was more effective than de-
tector D for the less intense fires of experiments 7-9 because of
its continuous analog output signal with a user specified alarm.

With regard to the optical detectors, detector A's alarm oc-
curred an average 27 s prior to the CO detector alarm. Detector
B showed more variation in its alarm time with respect to the CO
alarm time. B's alarm time occurred on the average 42 s after the
CO detector alarm for the high intensity fire experiments 1-4 and
6 for which data were available. For the low intensity fire experi-
ment, experiment 7 with a 15 m detector-fire separation, the de-
tector B’s alarm time was comparable to the CO alarm time. At
the 30 m detector-fire separation in the low intensity fire experi-
ments 8 and 9, detector B's alarm time occurred 2.6 and 2.2 min
after the CO detector alarm. In this regard, Detector B’s response
characteristic was similar to that of smoke detector D.

At station S3, detector C’s alarm occurred an average 17 s af-
ter the diffusion mode CO detector alarm for experiments 1-7.
Detector C's alarm time was earlier than the CO aiarm time for
two of the experiments, and later than the CO alarm time for the
other five experiments. For experiment 8, detector C alarmed 4.8
min earlier than the CO detector. This greater alarm time differ-


Tabie 3. POC Spread Rates

Average Fire intensity, kW

POC average spread rate, m/s at

30m 60m 90m
330. 0.22 + 0.03 0.13 + 0.05 0.06 + 0.01
30 0.08 + 0.02 0.04 + 0.006 0.04 + 0.02

less than 0.1 ppm. This intrinsic detector error, in addition to the
difference in the detector’s sampling mode and chemical sensor
cell will produce expected differences in detector alarm times.

50 r T

4oF

° 09 18 ar 36
TIME, 1035

Figure 3. Measured CO at airway root and midheight at station $1 for
experiment 9.

The less intense fire presents an opportunity to clearly see the
vertical mixing. Figure 3 shows the measured CO at the roof and
entry midheight at station S1 for experiment 9, and figure 4 shows
the measured CO at the roof and entry midheight at $2. A com-
Parison of these figures shows the stratification of the POC which
existed at S1 has all but disappeared at S2 above the entry
midheight.

50 T T T

Root 4
A= Mianaignt

° 09 18 27 36
TIME, 105s

Figure 4, Measured CO at airway root and miaheight at station $2 for experiment 3

Figure 2 showed a composite of average POC velocities at the
sensor stations for the nine experiments. From the reference
point of sensor spacing, it is useful to consider the CO alarm time
at the various stations. Sensor spacing is defined to be twice the
distance of the measurement station from the fire, since the POC.
can spread in two directions. The data in experiments 1-6 for an
average fire intensity of 330 kW, and that in experiments 7-9 for
a 30 KW fire are shown in figure 5 for the corresponding sensor
spacings. Interpolation of the data in figure 5 for the 330 kW and
30 kW fires with the 14.25 min time required for a developing coal
fire to ignite a conveyor belt implies minimum sensor spacing of
183 m and 105 m, respectively, would be necessary for fire detec-
tion at very low airflow conditions. Even though these results
would not be strictly invariant with respect to airway dimensions
the results have wide applicability, since the SRCM is not atypi-
cal of coal mines.

,
2 a
. .
SS

SENSOR SPACING, m

Figure $. CO alarm time dependence upon detector spacing for average 330 kW
and 30 KW fires.

Thermal Sensor

‘One type of fire detector used as part of an atmospheric mine
monitoring gystem is a thermal detector. It is considered to be in
alarm when the temperature reaches 57°C (Custer and Bright,
1974). In order to determine the POC temperature which devel-
oped in the SRCM diesel fire experiments at the CO measure-
ment location nearest to the fire, a thermocouple was installed
near the entrance inlet to the roof CO detector at stations S1 and
S4, Experiments 5 and 6 are representative of the high intensity
fire experiments. The maximum temperature during experiment
5 at S1 and S4 was 36 and 34°C, respectively. The ambient air
temperature was 15°C. For experiment 6, the maximum values
were 30 and 26°C, and the ambient temperature was 10°C.
These values are considerably less than the 57°C alarm tempera-
ture of a thermal detector. If an alarm is defined for the thermal
detection in an analogous manner to the definition of a smoke



ence for experiment 8 at S3 in comparison to the measurements
at S2 is possibly a result of the slower increase in POC farther
from the less intense 30 kW fires. A similar effect was measured
at S6 for experiment 9 for which detector C's alarm time occurred
4.6 min earlier than the diffusion mode CO detector alarm time.

At S4, for experiment 9, a comparison was made of smoke de-
tector C’s and F's and the CO alarm times. Smoke detectors C
and F alarmed 1.0 and 2.0 min earlier than the CO detector re-
spectively.

This comparison of smoke detector's alarm time with CO alarm
time shows that detectors A, C, E, and F are more consistent early
mine fire warning smoke detectors than are B and D.

CONCLUSIONS

The results of the diesel fuel fire experiments conducted under
zero imposed air flow conditions resulted in the following conclu-
sions.

1, Average POC spread rates of 0.22, 0.13, and 0.06 m/s were
determined for 330 kW average intensity fires at 30, 60, and 90
m distances from the fire. For 30 kW average intensity fires
these average POC spread rates were reduced to 0.08, 0.04,
and 0.04 m/s.

. Thermal detectors with an alarm value of 57_ C would not be
adequate for fire detection even at 30 m from an average 330
KW fire.

3. A diffusion mode and a pump mode ionization type smoke de-

tector, and a pump mode optical type smoke detector, and a

diffusion mode CO detector, were consistent indicators of a fire

within 4 min after the first optical detection of smoke. These
detectors alarmed within 92 s of each other.

A less restrictive comparison of the smoke detectors’ alarm

time with that of a diffusion mode CO detector within 30 m dis-

tance of the fire demonstrated that a smoke detector can be
more effective for mine fire detection if the continuous analog

‘output signal from the smoke detector is used to identify an

alarm.

The results of this research can be used to make recommen-

dations for sensor spacings in a zero or low air low section of a

mine. Interpolation of fire sizes between 30 and 330 kW could be

used to estimate detector spacing as part of an atmospheric mine

x

bal

monitoring system in a mine section in which low airflow is ex-
pected. The recommended detectors would be CO, or smoke de-
tectors with alarm values identifiable from a continuous analog
output signal. Implementation of these measures can be ex-
pected to improve miners’ safety.

ACKNOWLEDGEMENT

The authors acknowledge the miners who maintain the Safety
Research Coal Mine at the Pittsburgh Research Center for their
assistance in preparation of the experiments.

REFERENCES

Alpert, R.L., 1972, “Calculation of Response Time of Ceiling-
Mounted Fire Detectors,” Fire Technology, Vol. 8, No. 3, AU-
gust, pp 181-195.

Custer, R.L.P., and Bright, R.G., 1974, “Fire Detection: The
State-of-the-Art,” U. S. Dept. of Commerce NBS Technical
Note 839, p 13.

Edwards, J.C., and Friel, G.F., 1996, “Comparative In-Mine Evalu-
ation of Carbon Monoxide and Smoke Detection,” U.S. Bureau
of Mines Report of Investigations 9622, 11 pp.

Edwards, J.C., and Morrow, G.S., 1995, “Evaluation of Smoke
Detectors for Mining Use,” U.S. Bureau of Mines Report of In-
vestigations 9586, 17 pp.

Edwards, J.C., and Morrow, G.S., 1994, "Evaluation of the Re-
sponse of Diffusion-Type Carbon Monoxide Sensors,” U. S.
Bureau of Mines Report of Investigations 9520, 9 pp.

Friel, G.F., Edwards, J.C., and Morrow, G.S., 1994, “Effect of
Dead-End Crosscuts on Contaminant Travel Times in Mine
Entries,” U.S. Bureau of Mines Report of Investigations 9517,
18 pp.

Jin, T., 1981, “Studies of Emotional Instability in Smoke From
Fires," J. of Fire and Flammability, Vol. 12, April, pp 130-142.

Litton, C.D., Lazzara, C.P., and Perzak, F.J., 1991, “Fire Detec-
tion for Conveyor Belt Entries,” U.S. Bureau of Mines Report of
Investigations 9380, 23 pp.

Rasbash, D.u., 1975, “Sensitivity Criteria for Detectors Used to
Protect Life,” Fire International, Vol. 5, No. 40, pp 30-49.



MINE FIRE DETECTION
UNDER ZERO AIRFLOW CONDITIONS

John C. Edwards, Gene F. Friel, Robert A. Franks, and John J. Opferman
Pittsburgh Research Center, NIOSH, Pittsburgh, Pennsyivania, USA

ABSTRACT

A series of diesel fuel fire experiments were conducted in the
Pittsburgh Research Center's Safety Research Coal Mine
(SRCM) to determine products-of-combustion (POC) spread rates
along a single entry under zero imposed airflow conditions. Six
experiments with an average fire intensity of 330 KW and three
experiments with an average fire intensity of 30 kW were con-
ducted in a 180 m long entry which had an average 2 m height
and 4 m width. POC spread rates were measured by the re-
sponse time of diffusion type CO detectors, positioned at 30 m
intervals, to CO concentrations 5 ppm above ambient. For the
330 kW fires, average POC spread rates of 0.22, 0.13, and 0.06
m/s were determined at 30, 60, and 90 m distances from the fire.
For the 30 kW fires these average spread rates were reduced to
0.08, 0.04, and 0.04 m/s. The measured maximum roof layer
temperature 30 m from two of the 330 kW fire was 30 and 36°C,
which is less than the 57°C alarm point of a typical mine thermal
sensor, It was determined that smoke detectors can be more ef-
fective for mine fire detection than CO detectors. The experimen-
tally determined POC spread rates can be used to provide guid-
ance for specification of sensor spacing to improve early fire de-
tection at zero or very low air flows.

INTRODUCTION

Previously reported (Litton et al., 1991) recommendations for
detector alarm thresholds for two detector spacings in a conveyor
belt entry were based upon the assumption of forced convective
transport of the fire products-of-combustion (POC) under positive
ventilation conditions and an increasing fire intensity with associ-
ated POC production levels. Detector alarm thresholds were de-
termined from the average time required, 14.25 min, for a small
flaming coal fire to ignite a conveyor belt. In that work, the buoy-
ancy induced flow of the POC was not significant, since forced
convective airflow was dominant. This assumption was also used
in previous studies (Friel et al., 1994, Edwards and Friel, 1996)
which compared the effect of crosscuts on POC spread times
along a single entry under positive ventilation. In a mine section
with low airflow, the fire generated buoyancy forces become im-
portant, In the limiting case of zero airflow prior to the fire gener-
ated airflow, the POC are transported solely by the fire’s buoyant
forces.

To determine the buoyancy generated POC spread rates as a
function of fire intensity, a series of nine experiments were con-
ducted in the Pittsburgh Research Center's Safety Research Coal
Mine (SRCM). Under zero airflow conditions in a horizontal air-
way, the time for POC spread was determined from the arrival of
a CO concentration which was 5 ppm above ambient, at CO de-

tectors positioned near the roof and spaced in either direction
along an entry from the fire. The results of these experiments can
be used to quantify the POC convective transport velocity in low
airflow applications of the methodology previously developed
(Litton et al., 1991) for specification of POC alarm levels. To pro-
vide a fire source of near uniform intensity and burning rate, the
fuel source selected was diesel fuel. It has been demonstrated in
laboratory smoldering and flaming coal combustion experiments
at the Pittsburgh Research Center (Edwards and Morrow, 1995)
that smoke detectors can be more effective for fire detection than
CO detectors. These iarge scale mine experiments also provided
the opportunity to make a relative comparison of six smoke detec-
tors to each other based upon their alarm time, and the alarm time
of a CO detector.

EXPERIMENTAL PROCEDURE

once 5] Stren rs
Arm i

Figure 1. Plan view of mine section.

Figure 1 shows a plan view of the mine entry, F Butt, and ad-
joining mine rooms, 10 and 18, which were used as the isolated
airway for the zero airflow mine experiments conducted in the
SRCM. The elevation change in the entry floor was less than one
percent. Aside from undulations in the mine roof, this entry pro-
vided a region which was nearly free of elevation changes which
would affect the buoyancy induced natural ventilation, F Butt has
an average height of 2.0 m and width of 4.6 m. Room 10 has an
average height of 2.2 m and width of 2.8 m. Room 18 has an
average height of 1.7 m and width of 4.0 m. All crosscuts in the
entry were isolated with brattices, and a brattice stopping was
positioned in Room 18 for each of the nine experiments. For ex-
periments 4-9, Room 10 was isolated by a brattice stopping. For
experiment 1, the mine's exhaust fan moved air through the sec-
tion of the mine used for the experiments, which resulted in a flow
of 0.53 ms in F Butt. For experiments 2-9, a door at the mine


portal was opened to shunt air directly to the exhaust fan, and
bypass the remainder of the mine. As a result, no detectable air-
flow was measured in F Butt. For each of the nine experiments,
except 7, the fire was located as shown in figure 1 midway be-
tween stations $1 and S4. For experiment 7 the fire was located
midway between stations S1 and S2. The measurement station
separation between adjacent stations other than S1 and S4,
which are 60 m apart, was 30 m. To measure the POC advance
from the fire, a pair of diffusion mode CO detectors were sus-
pended from the roof at each of the stations, S1-S6. One detec-
tor of each pair was positioned with its inlet diffusion tube approxi-
mately 0.4 m from the airway roof, and the other detector was
positioned midheight between the roof and floor. As a result of
previous research (Edwards and Morrow, 1994), the detector's
diffusion tube was mounted approximately 20° from the normal to
the roof towards the expected airstream direction. A pump mode
CO detector was also located at the stations most distant from the
fire, S3 and S6.

To measure the smoke intensity, a light obscuration device
was installed at station S2. The device consisted of a 6 volt lamp
and a photoelectric cell separated by an optical path, L, of one
meter. This device provided a direct measurement of the normal-
ized optical transmission, T, through the smoke, and thereby of
the smoke optical density OD, which is defined as,

0D =110g,,7. 1
109 a)

To compare the alarm times of smoke detectors to a CO detec-
tor, smoke detectors specified in Table 1 were positioned at S2 for
each experiment. For experiment 9, C detector was also posi-
tioned at station S4 and S6, and F detector was positioned at S2
and $4. The smoke detectors are divided into two type classiti-
cations - optical and ionization, and two sampling mode classifi-
cations - pump and diffusion. Optical type refers to the absorption
or scattering of light by smoke particles as the operating principle
of the detector, and ionization type refers to the adsorption of ion-
ized-air molecules on the surface of the smoke particles and the
reduction in the ionization current through electron-ion recombina-
tion. Pump sampling mode refers to point sampling of smoke
laden air through a tube connected to a pump or a blower fan.
Diffusion sampling refers to access to the detector through a com-
bination of turbulent diffusion and forced convective air move-
ment.

Detector F was available only for experiment 9. Its alarm re-
sponse time was about 5 s. The response time of the detector E
was about 15 s. For experiment nos. 1-8, the sampling line inte-
rior diameter for detector A was 6.35 mm, and resulted in an av-
erage 70 s sampling response time. For experiment 9 a sampling
fine with an interior diameter of 25 mm was used, and the aver-

Table 1. Smoke Detectors

Smoke Detector Type _|_ Sampling Mode
A optical pump
B optical diffusion
c ionization diffusion
D ionization diffusion
E ionization pump
F ionization pump

age sample time was reduced to about 5 s. The measured alarm
times for detector A for experiments 1-8 were compensated for
the 65 s difference in sampling time from experiment 9. These
sampling times were based upon striking a match near the sam-
pling tube inlet and measuring alarm time.

Each of the smoke detectors, except for D, has a continuous
analog output voltage which is a measure of the smoke intensity.
Detectors D, E, and F have manufacturer defined alarms. For de-
tectors A and B, the manufacturer provided a relationship be-
tween the analog voltage output and the optical obscuration per
meter. The latter quantity can be related to the smoke optical
density. A ten standard deviation change in the average back-
ground signal was used to define an alarm signal for smoke de-
tectors A, B, and C. For the smoke detectors D, E, and F, the
manufacturer set alarm was used. First detection of smoke by the
light obscuration device was defined as a reduction in the average
background signal by ten standard deviations of the background
noise. CO detector alarm was defined as 5 ppm above back-
ground.

For experiment 5-9 a thermocouple was installed adjacent to
the inlet of the CO detector near the roof at stations S1, S2, and
$4. A thermocouple temperature transmitter was used to supply
to the data acquisition system a 4-20 ma signal which is linear
with the temperature measurement.

The detector analog output was acquired with a data acquisi-
tion system with a 2 s polling time interval between samples.
Each detector was interfaced with an accessor card to convert the
analog output signal to a digital signal.

Four pan sizes were used to contain the diesel fuel fires.
Square pans of side lengths 0.46 m, 0.61 m, and 0.76 m were
used, as well as a cylindrical pail with a diameter at the liquid sur-
face of approximately 0.24 m. The pan depths were approxi-
mately 15 cm, and the pail depth was 28 cm. The quantity of die-
sel fuel used for these experiments ranged from 2 to 24 liters. For
uniform ignition of the fuel with a propane torch, approximately
100 mi of gasoline was added to the fuel. A thermocouple was
mounted above the pan to monitor the fuel burn time for experi-
ments 2-5 and 7.

RESULTS

‘A compilation of the fuel quantity, fuel exposed surface area,
fuel burn time, and heat release rate is listed in Table 2. The heat
release rate is based upon a heat of combustion for diesel fuel of
42.3 kJ/g, and a combustion efficiency of 0.84 associated with
high molecular weight hydrocarbons.

The fuel burning time is approximately proportional to the liq-
uid fuel depth in the pan. This proportionality was used to esti-
mate the fuel bum time for experiment 1 and experiment 2. For
experiment 6, the roof temperature profile at stations S1 and S4
was used to estimate the fuel burn time. The burn time for experi-
ment 8 was assumed to be the same as the measured bur time
for experiment 7, and for experiment 9 the proportionality of burn
time to liquid fuel depth was used based upon the measured burn
time for experiment 7.

POC Spread Rate

The primary monitoring of POC spread in the mine entry was
accomplished with the diffusion mode CO detectors.

The currently used mine fire alert value of 5 ppm above back-
ground was used to measure the POC advancement. This value
is more definite than first arrival time of CO because of possible


Tabie 2. Diesel Fuel Fires

Fuel
Experiment Fuel, L Area, sq.m Burn Time,s Heat Release
Rate, kW
1 4 0.58 250 495
2 6 0.37 589 315
3 12 0.37 1,222 303
4 18 0.37 1,910 291
5 24 0.37 2,506 296
6 24 0.21 2,816 264
Zz 2 0.047 2,024 30
8 2 0.047 2,024 30.
9 3 0.047 3,036 30

ambient fluctuations and CO sensor cell accuracy of +1 ppm. For
each experiment the POC advanced to stations S1, S2, and $3.
Because the fire had extinguished before the POC reached S3 for
experiments 1 and 2, their values were not included in the POC.
spread rate evaluation at S3. With the exception of experiment 7
in which the fire source was located midway between stations S1
and $2, the POC advanced to station $4. Station S5 was reached
by the POC for experiments 4-6 and 8-9. For experiments 4-6
and 8-9, the POC advanced to station S6. However, for experi-
ments 4 and 5 the POC reached Sé6 after the fire had extin-
guished, and their values were not included in the POC spread
rate evaluation. The capability of experiments 6, 8, and 9 to pro-
duce POC which reached S6, in view of the considerably less in-
tense fire for experiments 8 and 9 is probably due to the seasonal
temperature effect. Experiments 6-9 were conducted in the win-
ter months, October through February, with lower outside air tem-
peratures than the months of May through July in which experi-
ments 1-5 were conducted. The ambient air temperature near S2
in F Butt varied from 14 to 21°C for experiments 1-5, and from 10
to 14°C for experiments 6-9. Under fan driven ventilation condi-
tions, the normal ventilation proceeds from S6 to S3. During the

one.
028: ge ry
. - Experiment
¢ . . oa}
. * 920. . 3
a : tS
S : ons ts
3 : -_
Es ° oo
210:
Bg} _ rn rs
ro, 008. “8
: °

oo 60 60 40 2 0 2 40 60 80 100
3s DISTANCE, m a6

Figure 2. POC spread rata dependence upon distance from fire.

‘summer months the heat exchange with the strata will cool the air
as it proceeds from S6 to S3, while in the winter months the op-
posite thermal effect occurs. The thermally induced air density
gradient will favor one direction in the airway once the fire, the
primary air pump, is ignited under zero airflow conditions.

The POC spread rate was calculated from the sensor spacing
and the measured POC travel time between adjacent CO sen-
sors. This value represents an average over the distance be-
tween the two sensors, and because of the 15 and 30 m sensor
spacings considered, is only an approximation.

Figure 2 shows the POC spread rates towards S3 and S6 for
each experiment. It is apparent from Figure 2 that, as expected,
the more intense fires of experiments 1-6 have a higher associ-
ated POC spread rate than the less intense fires of experiments
7-9. A comparison of the POC spread rate of the more intense
fires and less fires can be made at 30 m, 60 m, and 90 m. Experi-
ments 1-6 are in one group characterized by an average fire in-
tensity of 330 kW, and experiments 7-9 are in a second group
characterized by a fire intensity of 30 KW. Table 3 lists the aver-
age POC spread rate for these two average fire intensities at the
three locations.

Table 3 shows that within the first 60 m from the fire the smoke
average spread rate is about three times greater for the fire which
has the greater intensity. The results reported (Alpert, 1972) for
a fire plume in a room show the maximum gas velocity outside the
plume is proportional to the fire intensity to the 0.33 power. The
results in Table 3 for the average spread rates at 30 m from the
fire imply the smoke spread rate is proportional to the fire inten-
sity to the 0.42 power. Alpert's result applies to a plume from
which the gas can spread radially and relates to actual velocity.
The experiments in the SRCM have ribs along the entry which
confine the smoke spread.

At S3 the pump mode CO detector alarm occurred an average
5 s later than the diffusion mode CO detector alarm for experi-
ments 1-8. At S6 the pump mode CO detector alarm occurred an
average 82 s earlier than the diffusion mode CO detector alarm
for experiments 4-6 and 9. The average standard deviation of the
ambient background measured by the pump mode CO detector
was 1.3 ppm, whereas the standard deviation of the ambient
background measured by the diffusion mode CO detector was