The use of light scattering and ion chamber

responses for the detection of fires in diesel

contaminated atmospheres

Charles D. Litton*

National Institute for Occupational Safety and Health, Pittsburgh Research Center, Cochrans Mill Road,

Pittsburgh, PA 15236, USA

Received 20 February 2001; received in revised form 4 June 2001; accepted 27 August 2001

Abstract

Experiments were conducted to determine the optical scattering properties of diesel

particulate matter (DPM) and various combustion aerosols from both flaming and smoldering

combustion sources at discrete angles of 151 and 301 in the forward direction and at a light

source wavelength of 635 nm using a simple light scattering module. In addition to the

scattering data, simultaneous measurements were made of the total aerosol mass concentration; light extinction at an average wavelength of 546 nm; and the response of a common

bipolar ion chamber typical of residential smoke detectors modified to allow the aerosols to

flow through the chamber. The results of these experiments indicate, for DPM and

combustion aerosols, the intensities per unit mass concentration depend not only upon

whether the aerosol is DPM or combustion aerosol but also upon the type of combustion

aerosol. The results also indicate that the ion chamber responses are greatest for DPM,

followed by the response to flaming combustion aerosols (FCA) and lowest for smoldering

combustion aerosols (SCA). For light scattering, the greatest intensities are found for SCA,

followed by the intensities from FCA, and lowest for DPM. This report describes the

experiments, their results, and the use of these results to develop design criteria for early

warning fire sensors capable of the rapid and reliable detection of fires in atmospheres that

may or may not be contaminated by the products produced from diesel engines. r 2002

Elsevier Science Ltd. All rights reserved.

*Tel.: +1-412-386-6752; fax: +1-412-386-6561.

E-mail address: chl3@cdc.gov (C.D. Litton).

0379-7112/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 3 7 9 - 7112(01)00059-5
1. Introduction

1.1. Background

Fire sensors that detect the smoke and gases produced during the early stages of

developing fires are often compromised by the presence of background levels of

aerosols or gases that mimic the signatures of the developing fires, often resulting in

frequent false, or nuisance, sensor alarms. When this frequency is high, the tendency

is to either ignore sensor alarms, or to de-energize the sensors, with the potentially

catastrophic consequence that an actual fire is not detected. For instance, previous

surveys of installed residential smoke detectors [1], indicated that nearly 20% of the

detectors did not have functioning power sources, and of these, about one-third were

intentionally disconnected because of nuisance alarms. In another study [2], 273

smoke detectors were examined by fire departments subsequent to the extinguishment of residential fires that went undetected. Of these, 159 (59%) were found to be

disconnected from the power source.

Nuisance alarms can occur in industrial settings, as well, with similar actions and

consequences, real alarms that may be ignored or sensors that are disconnected from

their power source, resulting in fires that destroy both life and property. A recent

workshop [3] highlighted the problems associated with nuisance alarms in aircraft

cargo areas and critical telecommunications systems and stressed the need to develop

improved fire sensing systems and test procedures for installed fire detection systems.

Fire detection in underground mines and tunnels is often compromised by exhaust

products from diesel engines or other vehicles, or by routine procedures, such as

welding or cutting. In mines, and to a somewhat lesser degree in tunnels, dust is an

ever-present problem.

A significant level of research is being done to resolve some of these problems.

For smoke, efforts continue to more accurately and completely define the

properties of smoke produced from different sources [4–6] and to develop

improved techniques for smoke measurement [7]. Characterizing the signatures

of interfering sources using multi-sensor arrays coupled with neural networks

or other multi-signature alarm algorithms [8–10] offer significant promise in

many applications. But the use of these multi-sensor approaches are

generally application-specific in that different applications may require different

sensors and the necessary algorithms can vary significantly from one application

to the next. In some of these approaches, it is not only the relative signals

from different sensors, but also the manner in which these signals vary with

time, that allow for the discrimination, but incorporating time into the process

can also be detrimental to the early warning capability of the system. In

underground mines, multi-sensor approaches and simpler gas ratio techniques

[11,12] have also been used with varying degrees of success. In the mine application,

and others, the use of multi-sensor packages and software to process the signals

and make decisions increases not only the complexity, but also the cost of the

system, not only in terms of base, initial expense, but also in terms of

system maintenance and sensor replacement.

410 C.D. Litton / Fire Safety Journal 37 (2002) 409–425
One alternative to these approaches is the development of simple, stand-alone fire

sensors that capitalize on the differences between common, interfering aerosol and/

or gas backgrounds and those that result from developing fires so that the

discrimination occurs via the sensor and its associated electronics rather than from a

potentially more complex processing algorithm. This paper describes such a sensor

as it applies to the detection of fires in underground mines, where a major

background source is due to the emissions from diesel engines that are used routinely

in day-to-day mining operations.

1.2. Overview

In the initial stages of this research, the objective was to study, both theoretically

and experimentally, the angular distribution of scattered light from a polydisperse

system of particles produced from diesel engines (diesel particulate matterFDPM)

and from both flaming and smoldering combustion as part of an effort to develop

fire sensors that can reliably detect a developing fire in atmospheres contaminated by

the exhaust products from diesel engines, since these products represent signatures

similar to those from fires and can cause conventional products of combustion fire

sensors to produce frequent false, or nuisance, alarms that degrade the early warning

capability and reliability of the fire sensor. Previous research in this area [13] had

partially quantified the angular scattering signatures of DPM and various

combustion aerosols, with the result that DPM and combustion aerosols do, indeed,

produce different characteristic light scattering signatures, yet additional research

was necessary before final conclusions could be reached relative to the effectiveness

and feasibility of this approach. This report addresses and assesses the potential of

light scattering and, as it evolved, ion chamber response, and/or combinations of the

two, as they apply to the detection of developing fires and to the discrimination

between aerosols produced from fires and those produced from diesel engines.

1.3. Light scattering

The use of light scattering as a technique for detection of developing fires is not

new. Light scattering is attractive primarily because it is simple, generally

inexpensive to implement, and highly sensitive. Currently, there exist many detectors

that are available commercially that utilize this technique, generally referred to as

photo-electric type smoke detectors, although significant differences may exist

among the various detectors, such as angle(s) for detection, light source wavelength,

etc. For DPM and combustion aerosols, previous studies [13] had shown that the

Mie theory is not adequate to describe their angular scattering properties, and that

treatment of these particles as fractal-like aggregates is more appropriate, yielding

much better agreement with the experimental angular scattering data. In addition,

the angular intensities per unit mass, especially at forward angles less than about 301,

are greater by a factor of 2–3 for smoldering combustion aerosols (SCA) compared

to flaming combustion aerosols (FCA). This difference in sensitivity results in a

detector that is more sensitive to smoldering fires than to flaming fires, in direct

C.D. Litton / Fire Safety Journal 37 (2002) 409–425 411
contrast to the sensitivity of ionization-type smoke detectors discussed below, where

the highest sensitivity is expected for DPM and the lowest for SCA. The previous

data also indicated that the angular intensities per unit mass are significantly lower

for DPM than for aerosols from flaming combustion. These results are partially due

to differences in size of the different aerosols and also to the volatile content of the

different aerosols, where aerosols with a high volatile fraction, i.e., aerosols from

smoldering combustion, tend to scatter light more efficiently.

1.4. Ion chambers

Ion chambers in the form of ionization-type smoke detectors have also been used

routinely as early warning fire sensors since the mid 1970s. Typically, the source of

ions in these types of ion chambers is a very low-level radioactive source of 241Am.

241Am decays via the emission of alpha particles (He atoms) and as these particles

traverse the air space between two electrodes, both positive and negative ions are

created. The positive ions drift to the negative electrode while the negative ions drift

to the positive electrode. This separation of ions, coupled with the geometry of the

chamber, creates a space charge that distorts the electric field and electric potential

within the ion chamber. A third, floating electrode is generally located between the

positive and negative electrode at a position where the electric potential reaches its

maximum distortion, and the potential at this electrode is continuously measured.

When aerosols enter the air space between the electrodes, the positive and negative

ions rapidly attach to the aerosol particles, depleting the ion concentrations, which,

in turn, reduces the distortion of the electric potential causing the potential at the

floating electrode to increase.

Very early in the development of these chambers it was found that such detectors

are more sensitive to aerosols produced from flaming combustion than they are to

aerosols produced from smoldering combustion. The reason for this difference in

sensitivity is the result of two factors. First, the response of an ionization chamber to

the presence of an aerosol at low concentrations varies directly with the product of

number mean particle diameter, dG; and number concentration, nO; or dGnO:

Consequently, for aerosol sources that produce large number of very small particles,

the sensitivity will be high. Second, when small particles form aggregates of much

larger extent, high sensitivity will also result for low to moderate concentrations of

these larger aggregates. Previous studies [13] indicate that the primary particle

diameters for FCA and SCA are around 30 and 70 nm, respectively. FCA form

aggregate particles containing 500–1000 of the primary particles, an average mass of

around 1.5  1014 g, and with an average radius of gyration, RG; in the range of

350–400 nm. SCA tend to form aggregates containing roughly 100 primary particles,

an average mass of around 2.5  1014 g, and with an average RG in the range of

200–250 nm. At an average mass concentration of 2  109 g/cm3 (2 mg/m3

) the

number of flaming aggregates per cm3

, nO; is around 1.3  105 while the number of

smoldering aggregates is about 8  104

. If dG is set equal to 2RG; then for flaming

aggregates, dGnO is in the range of 4.5–5, while for smoldering aggregates the value is

in the range of 1.6–2.0. These simple calculations indicate that the ion chamber is 2–3

412 C.D. Litton / Fire Safety Journal 37 (2002) 409–425
times more sensitive to FCA than to SCA. If it were assumed that no aggregation

occurred, then the difference in sensitivities would be even greater.

Similarly, these research results indicated that the primary particle diameter for

DPM is in the range of 25 nm and that these primary particles aggregate to form

filaments of irregular shape with a typical radius of gyration, RG; in the range of

450–500 nm. In addition, these aggregates are typically composed of about 1500

individual primary particles. If it is assumed that each primary particle has a density

of 1.5 g/cm3

, then a simple calculation indicates that the mass of one aggregate

particle containing 1500 primary particles is of the order of 2  1014 g, and that the

number of aggregates per cm3 necessary to achieve a mass concentration of

2  109 g/cm3 (2 mg/m3

) is 1  105. If the number mean particle diameter is taken to

be twice the radius of gyration, then the product, dGnO; is in the range of 9–10. This

simple calculation implies that the ion chamber is about 2–3 times more sensitive to

DPM than to aerosols from flaming combustion and about 5–6 times more sensitive

to DPM than to aerosols from smoldering combustion.

Regardless of the exact relative values, these numbers indicate that an ion chamber

should be significantly more sensitive to DPM than to aerosols produced from either

flaming or smoldering combustion while light scattering sensitivities are in the

reverse order. These relative sensitivities would mean that if the ratio of ion chamber

response to light scattering response were obtained for the different aerosols, then a

high value would result for DPM, a lower value for FCA, and a much lower value

for SCA, and that this ratio could possibly be used to provide for a high degree of

discrimination between the different aerosol types. To test this hypothesis, the

response of a typical ion chamber and the angular intensities were measured for

DPM, FCA, SCA and mixtures of DPM/FCA and DPM/SCA. The experiments

conducted and their results follow.

2. Experimental

Experiments were conducted in the configuration shown in Fig. 1 and described in

more detail in Ref. [14], where aerosols from either flaming or smoldering

combustion are generated in a cubical enclosure measuring 0.30 m along each edge

and then diverted into a standard smoke box through a variable-orifice iris that

controlled the rate of aerosol accumulation within the smoke box. For DPM, the

exhaust from a diesel generator was flowed into the cubical chamber via a 10.2 cm

i.d. flexible hose and then diverted into the smoke box. Within the smoke box, two

small fans provided for continuous mixing of the aerosol. The optical density of the

aerosol was measured over a 1.483 m optical path using an incandescent lamp and a

standard photocell with a peak response at a wavelength of 546 nm and a spectral

response matching the spectral response of the human eye. During the experiments,

aerosol samples were continuously extracted from a metal tube inserted into the top

of the smoke box and flowed to the various measuring devices. In this configuration,

data were obtained for flaming No. 2 diesel fuel (a small pool flame), flaming coal,

flaming wood, flaming paper, flaming styrene–butadiene rubber (SBR), smoldering

C.D. Litton / Fire Safety Journal 37 (2002) 409–425 413
coal, smoldering wood, smoldering SBR, and DPM from the exhaust of the diesel

generator.

2.1. Measuring devices

During the experiments, a tapered element oscillating microbalance (TEOM) [15]

was used to continuously measure the mass concentrations of aerosol. In the TEOM,

Fig. 1. Schematic of the smoke box and sample chamber in which the experiments were conducted.

414 C.D. Litton / Fire Safety Journal 37 (2002) 409–425
a small filter is mounted onto what amounts to a hollow tuning fork vibrating at a

fixed frequency. Particles in the flow through this filter are deposited on the filter

increasing the filter mass. As the filter mass increases, the frequency of vibration

decreases proportionally.

For light scattering, several prototype scattering cells were fabricated and

evaluated via some initial, exploratory experiments to arrive at the final scattering

module, shown schematically in Fig. 2. The scattering module is a right circular disk

with a diameter of 7.62 cm and a height of 2.54 cm. Two PIN-125DP/L photodiodes,

with a square active detecting surface 1.27  1.27 mm2

, and with responsivities of

0.39 A/W at l 1⁄4 635 nm, were positioned, with their active surfaces located 3.2 cm

from the disk center, to measure the angular intensities at 151 and 301. The output

signals from the two photodiodes were amplified and converted from current to

voltage using a simple amplifier with a negative gain of 5  108

. The air space

between the photodiode and central scattering volume is cylindrical with a diameter

of 0.20 cm. For this module, a small laser diode operating at a wavelength of 635 nm

and an output power level of 5–6 mW, was used as the light source and the beam

focused at the center of the scattering volume with a spot diameter of about 2 mm. In

addition, for this module, the aerosols were flowed through the scattering volume at

right angles to the direction of the laser diode beam.

For the scattering module, the actual scattering volumes vary inversely with the

sine of the scattering angle, so that as the angle increases, the scattering volume

decreases. Assuming the scattering volumes to be defined by the light source beam

diameters dBEAM; and the diameters of the cylindrical light paths from the beam to

the detectors, dDET; then the volumes at each angle may be calculated approximately

from the expression

VðyÞ1⁄4ðp=4Þd2

DETdBEAM=sinðyÞ: ð1Þ

For the dimensions defined above, the following approximate scattering volumes

result: Vð15Þ 1⁄4 0:02546 cm3 and Vð30Þ 1⁄4 0:01318 cm3

. The effect of a smaller

scattering volume at 301 is to reduce the actual intensity by an additional factor of

Fig. 2. Schematic of the light scattering module using a laser diode at l 1⁄4 635 nm as the light source. In

this chamber, aerosols are flowed through the scattering module normal to the direction of the diode laser

beam.

C.D. Litton / Fire Safety Journal 37 (2002) 409–425 415
1.93, assuming that the number of aerosol particles per unit volume are uniformly

distributed.

The bipolar ion chamber used in these experiments, typical of ion chambers

found in ionization-type smoke detectors, is shown schematically in Fig. 3. In

this chamber, a 2.0 mm diameter source of 241Am with an activity of

0.80 mCi and face coated with a 2.0 mm thick Pd foil, is placed in the center of

the base of a small cylindrical cavity with a diameter of 0.95 cm and height

of 0.45 cm. At the distance of 0.45 cm from the source surface, the chamber

abruptly expands to form a larger cylinder with a diameter of 3.25 cm. At the

top of this larger cylinder is the negative electrode with its surface located a

total distance of 1.90 cm from the surface of the source, which also serves

as the positive electrode. Within these two volumes, both positive and negative

air ions are created so that the chamber is referred to as a bipolar chamber. At

the top of the smaller cylinder (0.45 cm away from the source) is a floating

conductor used to measure the electric potential at this plane in the chamber.

The position of this floating conductor corresponds approximately to the

plane of maximum distortion of the potential within the chamber.

When operated at the standard voltage of 9.0 V, the potential at this plane

is generally in the range of 3.4–3.9 V, a result of distortion of the electric field

and potential due to space charge effects. If there were no source in the chamber, a

simple electrostatic analysis indicates that the potential at this plane would be

B6.85 V. The difference, 2.95–3.45 V, represents the dynamic range for the potential

at this position. When aerosols enter the chamber, they deplete the ions via

attachment, and cause the potential to increase. Normally, aerosols enter the

chamber via diffusion or via convective air flows, but for these experiments, the

chamber was modified so that aerosols could be flowed through the chamber,

entering and exiting at two opposite points halfway between the floating conductor

and the top, negative electrode.

Fig. 3. Schematic of the bipolar ion chamber utilizing a floating electrode for detection and measurement

of aerosols.

416 C.D. Litton / Fire Safety Journal 37 (2002) 409–425
3. Results and analysis

3.1. DPM and combustion aerosols

Because of their simplicity, experiments could be conducted quickly resulting in a

large number of experiments so that several tests under identical conditions could be

used to assess the reproducibility of the data and the variations that occurred from

one test to the next. In general, DPM and combustion aerosol mass concentrations

varied over a broad range, from roughly zero to more than 50 or 60 mg/m3

, although

in the data and analysis that follow, particular attention was paid to mass

concentrations o 10 mg/m3

, since it is in this region that early warning fire detection

is generally achieved. The basic data acquired are summarized as averages in the

following Tables, where the averages represent measurements from typically 2–4

tests for each aerosol source. It is also worth noting that, although differences for

one source, from one test to the next, did occur, these differences were generally

within 715–20% of the average reported. Also, the sensitivities, presented in terms

of the signal per unit mass concentration, were determined from linear regressions of

the signals as functions of the mass concentrations. This approach is preferable to

simple signals divided by the mass concentrations, because it better reflects the

changes in signal per unit change in mass concentrations.

Data for the ion chamber and the dual angle light scattering module are shown in

Table 1. Inspection of the data in Table 1 indicates that the ion chamber, CEV=M; is

most sensitive to DPM, less sensitive to aerosols from flaming combustion, and even

less sensitive to aerosols from smoldering combustion. For the angular intensity

sensitivities, DIð15Þ=M and DIð30Þ=M; just the reverse is true. These data confirm the

hypothesis previously discussed relevant to the detector sensitivities to different types

of aerosols. These results indicate that the ratios of change in ion chamber CEV to

change in angular intensities, DCEV=DIð15Þ; DCEV=DIð30Þ; DCEV=ðDIð15Þ2DI 

Table 1

Measured sensitivities, V/(mg/m3

), for diesel exhaust particles and various combustion aerosols using the

bipolar ion chamber with floating collection electrode and the dual angle scattering module

Aerosol source DCEV=M DIð15Þ=M DIð30Þ=M DIð15Þ=M2DIð30Þ=M

Diesel exhaust 0.2083 0.0122 0.007 0.0052

Flaming

No. 2 Diesel fuel 0.0529 0.0884 0.0372 0.0512

Pgh seam coal 0.0457 0.0733 0.0360 0.0373

SBR 0.0684 0.0977 0.0399 0.0578

Douglas fir 0.1082 0.0755 0.0403 0.0352

Bond paper 0.1255 0.0659 0.0263 0.0396

Smoldering

Pgh Seam coal 0.0280 0.1450 0.0760 0.0690

SBR 0.0252 0.2140 0.0888 0.1252

Douglas fir 0.0212 0.1645 0.0722 0.0923

C.D. Litton / Fire Safety Journal 37 (2002) 409–425 417
ð30ÞÞ; or even the intensity difference DIð15Þ2DIð30Þ may be sufficient to discriminate

between particles produced from fires and particles produced from diesel exhausts.

To determine which of these possibilities present the best discrimination capabilities,

the values from Table 1 were computed as averages for the three generic sources,

diesel exhaust, flaming combustion, and smoldering combustion and are shown in

Table 2.

The values shown in Table 2 indicate that the ratio of ion chamber sensitivity to

angular intensity difference, DCEV=ðDIð15Þ DIð30ÞÞ; provides for the greatest

discrimination between fire combustion aerosols and DPM, although the ratio,

DCEV=DIð15Þ; is comparable. For these two ratios, more than two orders of

magnitude separate DPM and aerosols from smoldering fires. More than an order of

magnitude separates DPM from aerosols from flaming fires, and even for firegenerated aerosols, the ratios for smoldering are a factor of 7 lower than the ratios

for flaming, indicating that there exists a distinct possibility that, not only can DPM

be discriminated from fire-produced aerosols, but also, that the stage of combustion

may also be determined. From a practical viewpoint, the simple ratios, DCEV=

DIð15Þ or DCEV=DIð30Þ; are simpler to implement and may be more reliable than

the ratio, DCEV=ðDIð15 DIð30ÞÞ; since the latter ratio would require 3 measurements rather than 2 and additional electronics would be needed to subtract the

intensities before determining the ratio. Alternatively, the intensity difference, DI 

ð15Þ DIð30Þ; also requiring only two measurements, could possibly be used. In this

case, there would also be no need for the ion chamber, resulting in a simpler device,

but for this alternative, the condition(s) for alarm become dependent upon the

aerosol mass concentrations and no true ability to discriminate exists. The ratios,

however, are independent of aerosol mass concentration, and as long as the signals

are measurable with reasonable signal-to-noise ratios, high discrimination capabilities do exist.

To better understand how these ratios and discrimination capabilities would work

in actual practice, it is instructive to look at the optical densities and specific

extinction coefficients measured during these experiments. The optical density, D;

expressed in inverse meters (m1

) measured at an average wavelength of 546 nm, as

previously discussed, is calculated from the transmission, T; of light through the

system of aerosol particles via the expression

D 1⁄4 ð1=LÞlogð1=TÞ; ð2Þ

where the path length, L; is constant at 1.483 m. The optical density per unit mass

Table 2

Sensitivity ratios and angular intensity differences for the three types of aerosols

Aerosol source DCEV=DIð15Þ DCEV=DIð30Þ DCEV=ðDIð15Þ2DIð30ÞÞ DIð15Þ=M2DIð30Þ=M

Diesel exhaust 17.1 29.8 42.5 0.0052

Flaming combustion 1.05 2.37 1.94 0.0442

Smolder combustion 0.147 0.315 0.279 0.0955

418 C.D. Litton / Fire Safety Journal 37 (2002) 409–425
concentration is related to the specific extinction, QEXT; in units of m2

/g, by the

simple expression

QEXT 1⁄4 2303  D=M ð3Þ

with the mass concentration, M; in mg/m3

. The optical density is important because

Federal Regulations [16] require that when smoke detectors are used for early

warning fire detection in underground coal mines, they shall provide alarm at optical

densities less than 0.022 m1

. In addition, for a detector to function reliably, the

signals from both the ion chamber and the scattering module must be significantly

greater than their respective noise levels. For the ion chamber CEV; the noise level,

expressed as the standard deviation in the steady-state signal was determined to be

70.0157 V. For the Ið15Þ and Ið30Þ signals, the noise levels (standard deviations)

were similarly determined and found to have values of 70.0115 and 70.005 V,

respectively. To increase the reliability of the detection process, it is assumed that

both the CEV and Ið15Þ voltage signals must increase by a voltage equal to 10 times

their respective standard deviations. For the CEV; Ið15Þ; and Ið30Þ voltages, the

respective increases are 0.157, 0.115, and 0.050 V. The data for optical density are

shown in column 2 of Table 3, column 3 is the mass concentration at which

D 1⁄4 0:022 m1

, column 4 is the mass concentration at which the CEV voltage

increase equals 10 standard deviations (calculated using the data of column 2 of

Table 1), and column 5 is the mass concentration at which the Ið15Þ voltage increase

equals 10 standard deviations (calculated using the data of column 3 of Table 1).

If it is assumed that both signals must satisfy the condition of increase equal to 10

standard deviations, then from Table 3, two points are worth noting. First, the mass

concentrations in columns 4 and 5 are always less than the values in column 3,

meaning that reliable signals occur before the specified alarm value of optical density

is reached. Second, for combustion aerosols, with the exception of flaming Douglas

Table 3

Measurements of optical density per unit mass concentration for diesel exhaust particles and combustion

aerosols, along with the expected mass concentration at D 1⁄4 0:022 m1

, the mass concentrations at an

increase of 10 standard deviations of the ion chamber CEV; and the mass concentrations at an increase of

10 standard deviations of the 151 intensity

Aerosol source D=M M @ D 1⁄4 0:022 m1 M @ 10 CEVSDs M @ 10Ið15ÞSDs

Diesel exhaust 0.00147 15.0 0.755 9.43

Flaming

No. 2 diesel fuel 0.00417 5.3 2.97 1.30

Pgh seam coal 0.00347 6.3 3.44 1.57

SBR 0.00395 5.6 2.30 1.18

Douglas fir 0.00327 6.7 1.45 1.52

Bond paper 0.00195 11.3 1.25 1.75

Smoldering

Pgh seam coal 0.00315 7.0 5.61 0.79

SBR 0.00253 8.7 6.24 0.54

douglas fir 0.00205 10.7 7.42 0.70

C.D. Litton / Fire Safety Journal 37 (2002) 409–425 419
fir and flaming bond paper, the mass concentrations at 10 standard deviation

increases are greater for the ion chamber than for the light scattering module, and

even for these two exceptions, the difference between the two is small.

Finally, if these voltage increases are satisfied as the initial step of the detection

process, the second and final step leading to alarm is to form the ratio, DCEV=DI 

ð15Þ; and if this ratio is less than some pre-set value, then the detector will issue an

alarm. From column 2 of Table 3, the average ratio is in the range of 1.05 for FCA,

although, for flaming bond paper, the calculated ratio is 1.904, and for flaming

Douglas fir, the calculated ratio is 1.433. It follows from these values, that a

reasonable ratio for discrimination may be in the range of 2.0. This ratio would be

approximately 9 times lower than the ratio for DPM, indicating good to excellent

discrimination capabilities. It should be noted, and stressed, that in mines that do

not use diesel equipment, either the ion chamber or the light scattering module could

stand alone, but, considering the values in columns 4 and 5 of Table 3, the light

scattering module sensitivity is generally equivalent to or better than the ion chamber

sensitivity to FCA, while for SCA, the light scattering module sensitivity is greater by

a factor of 10. It should be noted that combustion aerosol mass concentrations

calculated at increases in Ið30Þ equal to 10 standard deviations, or 0.050 V, are

essentially identical to those in column 5 of Table 3. In addition, the average ratios,

DCEV=DIð30Þ; for combustion aerosols relative to DPM, are in about the same

proportions as the DCEV=DIð15Þ ratios relative to DPM. This would mean that

either 151 or 301, and most probably any scattering angle lying between these two,

could be used with equivalent effectiveness.

3.2. Mixtures of DPM and combustion aerosols

The data just presented were for experiments that contained only one aerosol

component. Just as there is concern about measurements in DPM/dust mixtures, the

typical use of the envisaged smoke detector would be one in which the sensor is

continuously exposed to some level of DPM contamination and the aerosols

produced from a fire would have to be detected in the presence of this contamination

as the combustion aerosols mix with DPM in the mine atmosphere. To examine this

scenario, tests were conducted during which DPM was flowed into the smoke box at

a fixed rate resulting in a steady-state or near steady-state concentration of DPM and

the establishment of constant signals from the ion chamber and the scattering

module. Combustion aerosols, both from flaming combustion (a small pool flame of

No. 2 diesel fuel) and smoldering combustion (from Pgh seam coal) were then

diverted into the smoke box simultaneously with the DPM and the resultant changes

in aerosol mass concentration, CEV; Ið15Þ; Ið30Þ; and optical density measured.

Now, in keeping with the prior description of the operation of the detector, four

conditions must be satisfied before the detector will issue an alarm.

1. The change in CEV from its base value @ zero aerosol concentration must be

greater than 10 standard deviations, or 0.157 V;

420 C.D. Litton / Fire Safety Journal 37 (2002) 409–425
2. The change in Ið15Þ from its base value @ zero aerosol concentration must be

greater than 10 standard deviations, or 0.115 V;

3. The ratio, DCEV=DIð15Þ; for these two signals must be o2.0; and

4. The aerosol optical density should be less than 0.022 m1

.

The data from three tests are shown in Figs. 4–6. Fig. 4 is for a test for a FCA

where the pre-existing level of DPM was 2.5 mg/m3 with an optical density of

0.0037 m1

; Fig. 5 for a SCA where the pre-existing level of DPM was 2.8 mg/m3

with an optical density of 0.0041 m1; and Fig. 6 for a SCA where the pre-existing

level of DPM was 7.55 mg/m3 with an optical density of 0.0094 m1

. In general, all

three experiments show the same trends. As the combustion aerosol begins to enter

the smoke chamber, all three signals begin to increase, with the Ið15Þ and Ið30Þ

signals increasing more rapidly than the CEV: For all tests, the CEV from DPM is

sufficient to satisfy condition 1 above. For the FCA test (Fig. 4), condition 2 is

satisfied 24 s after this aerosol begins to enter the smoke chamber; condition 3 is

satisfied at 36 s where the optical density is 0.020, satisfying condition 4. For the first

SCA test, condition 2 is satisfied 34 s after the aerosol initially enters the smoke

chamber and condition 3 is satisfied at 56 s at an optical density of 0.0135 m1

,

satisfying condition 4. For the second SCA test, condition 2 is satisfied 32 s after the

aerosol initially enters the smoke chamber and condition 3 is satisfied at 54 s at an

optical density of 0.020 m1

.

Fig. 4. Responses of the bipolar ion chamber and the scattering module to FCA generated in the presence

of a pre-existing background of DPM, where the background concentration of DPM is 2.5 mg/m3

.

C.D. Litton / Fire Safety Journal 37 (2002) 409–425 421
Fig. 5. Responses of the bipolar ion chamber and the scattering module to SCA generated in the presence

of a pre-existing background of DPM where the background cncentration of DPM is 2.8 mg/m3.

Fig. 6. Responses of the bipolar ion chamber and the scattering module to SCA generated in the presence

of a pre-existing background of DPM where the background concentration of DPM is 7.55 mg/m3

.

422 C.D. Litton / Fire Safety Journal 37 (2002) 409–425
For all three experiments, the ratio, DCEV=DIð15Þ; in the presence of DPM only,

was in the range of 15–20. In the flaming combustion test, the ratio reached a

minimum of 1.17 at a total aerosol concentration of 10.7 mg/m3

; in the first

smoldering combustion test, the minimum ratio of 0.544 occurred at a total aerosol

concentration of 11.3 mg/m3

; and for the second smoldering combustion test, the

minimum ratio of 1.379 was reached at a total aerosol concentration of 11.2 mg/m3

even though roughly 67% of the total mass was due to DPM. These results are

extremely encouraging.

4. Conclusions

The major results of these studies, minus the analyses and embellishments, are

summarized in Figs. 7 and 8, where, in Fig. 7, the sensitivities per unit mass for both

the ion chamber and the second light scattering module are plotted as a function of

the type of particle. The results are clear and dramatic–the ion chamber sensitivity

decreases with the type of particle while the light scattering sensitivity increases.

These two opposite responses, one decreasing the other two increasing, result in the

ratios, DCEV=DIð15Þ and DCEV=DIð30Þ; shown in Fig. 8, that vary by more than 2

orders of magnitude from DPM to SCA. Also shown in these figures are the

measured angular intensity sensitivities for respirable coal dust, a contaminant that

Fig. 7. The average sensitivities of the bipolar ion chamber and the scattering module for the different

types of particles studied in this research.

C.D. Litton / Fire Safety Journal 37 (2002) 409–425 423
may also affect the reliable operation of fire detectors. For coal dust, no change in

CEV occurred at concentrations up to about 15 mg/m3

, so to provide some measure

of the CEV sensitivity, the measured noise level was divided by an average coal dust

concentration equal to 5.0 mg/m3

.

The combination of ion chamber and light scattering module was shown to

provide excellent discrimination capabilities as a fire sensor for the detection of

developing fires in atmospheres contaminated by DPM, both in underground mines

or in any other application where similar problems exist. The use of an alarm based

upon satisfying 3 conditions for the sensor signals (the 4th condition is imposed

externally) was shown to have merit in situations that are typical of normal usage.

For applications where DPM contamination may not be a problem, either

component could be used independently, although the light scattering approach

appears to offer greater sensitivity to both types of combustion aerosols. Even in

applications where DPM contamination is not a problem, the use of a combined

sensor allows for the determination of the stage of the combustion process, should

the need to make this determination exist. While there may be other approaches to

solving this problem, the simplicity, low-cost, high sensitivity, and high discrimination capabilities of this technique offer distinct advantages. When one can purchase

an ion chamber, two photodiodes, a diode laser, and some electronics for less than

$80, the investigation of alternative techniques rapidly becomes a moot point.

Fig. 8. The average ratios, DCEV=DIð15Þ and DCEV=DIð30Þ; for the different types of particles studied in

this research.

424 C.D. Litton / Fire Safety Journal 37 (2002) 409–425
Prototypes of these fire sensors will be fabricated for subsequent test and evaluation

in underground mines and in other applications where similar needs exist.

References

[1] Smith CL. Smoke detector operability survey. Report on Findings. US Consumer Product Safety

Commission, October 1994. 98pp.

[2] Smith LE. Fire incident study. National Smoke Detector Project. US Consumer Product Safety

Commission, January 1995. 61pp.

[3] Grosshandler WL editor. Nuisance alarms in aircarft cargo areas and critical telecommunications

systems. Proceedings of the Third NIST Fire Detector Workshop, NISTIR 6146, BFRL, NIST,

March 1998. 34pp.

[4] Mulholland GW, Croarkin C. Specific extinction coefficient of flame generated smoke. Fire Mater

2000;24(5):227–30.

[5] Choi MY, Mulholland GW, Hamins A, Kashiwagi T. Experimental study of the optical properties of

soot and smoke. NISTIT 5499, NIST, Annual Conference on Fire Research: Book of Abstracts,

October 1994. p. 123–4.

[6] Sorensen CM, Feke GD. Post-flame soot. Proceedings of the International Conference on Fire

Research and Engineering (ICFRE), September 1995. p. 281–5.

[7] Mulholland GW, Johnsson EL, Shear DA, Fernandez MG. Design and testing of a new smoke

concentration meter. Proceedings of the Fall Conference. Fire Retardant Chemicals Association

(FRCA), October 1998. p. 41–9.

[8] Gottuk DT, Peatross MJ, Roby RJ, Beyler CL. Advanced fire detection using multi-signature alarm

algorithms. Proceedings of the International Conference on Automatic Fire Detection AUBE 99,

March 1999. p. 237–46.

[9] Milke JA. Application of neural networks for discriminating fire detectors. Proceedings of the

International Conference on Automatic Fire Detection AUBE 95, April 1995. p. 213–22.

[10] Grosshandler WL. Review of measurements. Candidate Signatures for Early Fire Detection. NISTIR

5555, BFRL, NIST, January 1995. 36pp.

[11] Brinn M, Bott B. A fresh approach to mine fire detection. Trans Inst Min Eng 1994; 71–4.

[12] Litton CD, Conti RS, Tabacchi JG, Grace R. Evaluation of a nitric oxide-compensated carbon

monoxide fire sensor. BuMines IC 9339, 1993. 10pp.

[13] Litton CD. Fractal properties of smoke produced from smoldering and flaming fires. Proc ASME Int

Mech EngCongr Exposition, Dallas, TX, HTD 1997;352:119–34.

[14] Edwards JC, Morrow GS. Development of coal combustion sensitivity tests for smoke detectors. US

Bureau of Mines Report of Investigations 9551, 1995. 12pp.

[15] Patashnick H, Rupprecht G. Microweighing goes on-line in real time. Res Dev 1986;28(6):74–8.

[16] Title 30, US Code of Federal Regulations, Part 75.351, July 1, 1995. p. 479.

C.D. Litton / Fire Safety Journal 37 (2002) 409–425 425