Threshold powers and delays for igniting propane and butane-air mixtures 

by cw laser-heated small particles 

Thomas H. Dubaniewicz, ~r.~) 

National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, 626 Cochrarzs Mill 

Road, Pittsburglz, Penrzsylvania 15236 

(Received 26 September 2005; accepted for publication 3 May 2006) 

The National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory 

conducted a study of laser safety in potentially flammable environments. Researchers measured 

threshold igniting powers as a function of beam diameter for butane and propane-air mixtures by 

laser-heating targets placed on optical fiber tips using a 1064 nm laser. The minimum igniting 

powers for propane-air and butane-air were about 250 and 300 mW, respectively. Threshold igniting 

powers for propane-air were approximately proportional to beam diameter for beam diameters from 

0.1 to 2.0 mm. Results suggest that relatively powerful beams may be used in these atmospheres 

without causing ignition, provided the beam diameter is controlled. Threshold ignition delay times 

using 91125 and 62.51 125 pm fibers were approximately proportional to the inverse square of the 

laser power. Ignition delays were about 45 ms or longer for laser powers up to 800 mW. 

Preproduction samples of a commercial optical fuse prevented ignitions under selected test 

conditions. A self-healing effect was observed for one sample. Comparisons are made with the 

results of other researchers. O 2006 Laser Institt~te of America. 

Key words: propane ignition, butane ignition, laser ignition, nonbeam hazards, fiber optic fuse 

I, INTRODUCTION 

The National Institute for Occupational Safety and 

Health, Pittsburgh Research Laboratory (NIOSH, PRL) conducted a study of laser safety in potentially flammable environments (The findings and conclusions in this report are 

those of the author and do not necessarily represent the views 

of the National Institute for Occupational Safety and Health.) 

Fiber optic systems are being deployed in locations where 

explosive gas atmospheres are normally present or are 

present under fault conditions. For example, fiber deployment through sewer systems and natural gas lines is occurring to overcome the "last mile" hurdle for metro fiber optic 

2 systems.' Montgomery suggests large core optical fibers up 

to 1 mm in diameter may be more suitable than small core 

fibers in many hazardous (classified) industrial environments 

due to enhanced ruggedness, ease of use, and maintenance 

issues. Research of high power fiber optic sensors for hostile 

environments such as fuel storage tanks continue^.^ 

Propane and butane are found in natural gas and found 

in, or produced from, petroleum. Propane and butane are 

included with the Class I Group D or Group IIA flammable 

materials? Propane is typically used for purposes of testing 

and approval of electrical equipment for use in Class I Group 

D or Group IIA hazardous (classified) location^.^ Propane 

generally ignites more easily than methane, the primary component of natural gas. Natural gas consists mostly of methane gas (typically greater than 85%), with variable and much 

smaller amounts of ethane, propane, other hydrocarbons, and 

hydrogen. Testing with propane may be preferable to natural 

gas for safety evaluations in situations where natural gas 

contains variable or significant amounts of hydrocarbons 

heavier than methane. Propane may not be a conservative 

test gas for process gas containing significant amounts of 

hydrogen.4 

The optical radiation guided by fiber optic systems can, 

under certain conditions, be considered a potential ignition 

source for explosive gas atmospheres. For wavelengths in the 

visible through mid infrared spectrum, optical ignitions occur with the lowest powers when a target contacts the beam 

and converts the optical energy to thermal energy.677 The best 

targets for causing ignition absorb most of the optical radiation and do not disintegrate before they become hot enough 

to ignite the gas. An optical fiber tip that becomes dirty (ob-* 

structed) can form a target approximating these conditions. 

Experiments reported here used obstructed fiber tips to simulate a worst case situation where a near infrared continuous 

wave (cw) beam is exposed to the flammable mixtures and 

potential targets are unknown. 

Fiber optic power limiters can prevent excessive powers 

from propagating through the fiber and into hazardous environments. Where the beam is normally exposed to a flammable atmosphere, such a device must activate before ignition can occur under overpower fault conditions. Knowledge 

of ignition delay times would therefore be helpful to properly 

use the fiber optic power limiters as ignition prevention devices in these situations. The NIOSH Research to Practice 

(r2p) initiative emphasizes translation of research findings to 

effective products, and the need to evaluate and demonstrate 

the effectiveness of these products.8 A commercial fiber optic 

fuse was identified with activation powers and delays sufficient to prevent ignitions based on the experimental findings 
TABLE I. Threshold laser igniting powers for propane-air and butane-air mixtures. 

Optical fiber Threshold 

Fuel-air core diameter igniting power 

Study (% by vol) Target material (w) 

- - 

Hills et aLa 5% propane Coal particle 50 0.24 

Welzel et aLb 5% propane (FeMn) 203 62.5 0.25 

Hawksworthc 4% propane (FeMn),03-Resbond 400 0.69 

Welzel et slab 5% propane (FeMn)z03 400 0.842 

HawksworthC 4% propane (FeMn)203-Resbond 600 1.2 

HawksworthC 4% propane (FeMn)203-Resbond 1500 3.6 

Hawksworthc 4.6 % butane (FeMn)203-Resbond 400 0.68 

aReference 11. 

b~eference 12. 

"Reference 10. 

of this study.g Samples of the fiber optic fuses were tested to 

evaluate their potential for ignition prevention purposes, in 

accordance with the NIOSH r2p initiative. 

11. LITERATURE REVIEW 

Table I lists threshold igniting powers as a function of 

beam diameter for propane-air and butane-air observed by 

~awksworth," Hills et al.," and Welzel et al.12 Propane 

threshold igniting powers generally increase with fiber core 

diameter for diameters >O.l mm (Table I). Dubaniewicz 

et a1.13 observed an approximately linear relationship between igniting power and beam diameter for methane-air 

mixtures using beam diameters from 200 to 800 pm. In the 

current study, fiber optic core diameters ranging from 9 pm 

to 2.0 rn were used to measure threshold igniting powers 

for butane-air and propane-air mixtures as a function of 

beam diameter. These diameters and gases were selected to 

see if the linear relationship between igniting power and 

beam diameter applied over a larger range of beam diameters 

for several straight chain hydrocarbons. The resulting ignition curves could potentially be used as a guide for using 

relatively powerful beams guided by large core fibers in 

Class I Group D or Group IIA hazardous (classified) 

 location^.^ As suggested by Montgomery: large core fibers 

may have practical application for hazardous (classified) locations for several others reasons as well. 

Optical equipment may be installed in locations where 

potential beam targets are not well known or controlled. For 

these situations, researchers have taken a worst-case approach in selecting targets for experimental research. Using 

cw lasers with small beam diameters, strongly absorbing and 

thermally stable targets appear to ignite gases most easily. 

Moore and weinberg14 were unable to ignite methane-air 

mixtures using laser-heated cotton wool particles, which 

turned to ash and disintegrated. They were able to ignite 

methane-air mixtures using thermally stable refractory insulating wool as the laser target. The insulating wool absorbed 

the selected laser wavelength well. Welzel et al.12715 found 

black iron manganese oxide (FeMn)203 targets produced the 

lowest gas igniting powers of a number of targets tested under most experimental conditions using cw lasers. Dubaniewicz et nl.13 found Fe304 targets produced slightly lower 

igniting powers than coal targets in methane-air mixtures, 

using a nonflammable grease to attach the targets to large 

core optical fibers. Also, larger coal particles required higher 

laser powers to ignite the methane-air, suggesting the heat of 

combustion from the coal was not a significant contributor to 

ignition compared to the heat generated by the laser. 

~awksworth'~ observed combustible targets burn away without igniting selected gases at power levels that produced ignitions with (FeMn)203. Also, the (FeMn)20, produced 

lower igniting powers for selected atmospheres when it was 

mixed with a ceramic adhesive and applied to large core 

optical fiber tips as a paste. One exception was Hills et aL ,I1 

who reported that Appin coal particles ignited selected atmospheres at the lowest powers. ~ubaniewicz'~ suggests some 

aspects of Hills results are more indicative of pulsed ignition, 

possibly due to laser overshoot. ~ubaniewicz'~ found iron 

oxide-ceramic adhesive targets on single mode fiber tips ignited selected mixtures more readily than other selected targets. Iron oxide-ceramic adhesive mixtures were used as targets in the current study as a conservative worst-case 

approach to targets that may obstruct optical fibers, for situations where potential targets are unknown or not controlled. 

Knowledge of ignition delay times would be helpful for 

selecting power limiters that activate quickly enough to prevent ignition under overpower fault conditions. Although 

Moore and weinberg14 did not use optical fibers with their 

laser beam, they found ignition delays generally decreased 

hyperbolically as radiant power flux increased for several 

hydrocarbons including propane, methane, pentane, and ethylene. Methane produced the longest ignition delays while 

ethylene, a Class I Group C or Group IIB material: produced the shortest ignition delays. Moore and weinberg14 

also found ignition delays near the rninimum radiant power 

flux were on the order of seconds. Schenk et a1.17 observed 

high volatile coal targets produced longer ignition delays 

than low volatile coal targets for selected flammable atmospheres. Also, the ignition delays near the minimum igniting 

power of selected gases were on the order of seconds. 

~ubaniewicz'~ found igniting delays for methane were approximately proportional to the inverse square of laser powers using iron oxide targets on single mode fibers. Ignition 

delays were about 90 ms or longer for laser powers up to 

800 mW. In the current study, ignition delays were measured 

as a function of beam power using targets attached to 
SMF-28 and 62.5 pm diameter fibers. The resulting ignition Sapphire window 

curves were used to select power limiters for the ignition 

prevention tests. 

Ill. EQUIPMENT AND MATERIALS 

An IPG Photonics PYL-1OM ytterbium fiber laser system was used for ignition experiments. The fiber laser wavelength was centered at 1064 nm and the output power was 

adjustable up to 10 W cw. IPG indicated the overshoot was 

less than twice the set power, settling to the set power within 

10 ps. The laser controller provided a laser-on electrical output signal used for ignition delay measurements. The system 

combines a red aiming laser with the high power beam 

within a single mode fiber optic pigtail. The pigtail was fusion spliced to a Corning single mode 1 by 2 coupler to 

protect the laser from fiber fuses. Optical fibers used for 

ignition experiments were fusion spliced or connectorized to 

one of the coupler's output ports. The unused port was terminated with a "black hole" connector to minimize back 

reflections. Power levels were also kept below 1 W for tests 

involving single mode fiber to avoid possible fiber fuses. 

Optical fibers used for experiments included 91125 (core1 

clad) pm Corning SMF-28 silica, 62.51125 pm silica, 

1001 140 pm silica, 2001 220 pm FiberGuide AFS silica, 

4001430 pm Spectran HCS silica, 8001880 pm FiberGuide 

AFS silica, and 200012150 FiberGuide APC plastic clad 

silica fibers. The SMF-28 single mode fiber core diameter is 

the mode field diameter at the 1300 nm wavelength. More 

than one mode propagates through the SMF-28 optical fiber 

at the 1064 nm wavelength. Fibers with core diameters up to 

100 pm were fusion spliced to the couplers output port. 

SMA connectors were used to couple the laser beam to fibers 

with core diameters from 200 to 2000 pm. A fiber optic 

cleaver (York model FBK 11C) provided a flat, perpendicular, optical surface on the 9-400 pm core fiber tips. A 

manual cleaving tool was used for 800 and 2000 pm core 

fibers. 

Two KiloLambda optical fuses were used for ignition 

prevention tests. The two samples were preproduction versions with specified activation threshold powers of 

20.02 1.0 dB m (100-1- 1.3 mW). The fuses were constructed 

with Corning SMF-28 fiber pigtails on both ends. The 

samples were directional, i.e., there was a specified input end 

and output end. 

A Scientec power meter (model D200PC) with attached 

calorimeter (model AC2500) measured the laser powers. Scientec checked the meter accuracy annually, traceable to National Institute of Standards and Technology standards. 

Power measurements are reported to be accurate to within 

k5%. 

Ignition experiments were conducted in a 20 L test 

chamber (Fig. 1) designed for explosion testing of dusts, 

gases, and hybrid mixtures.18 The chamber can be used at 

initial pressures that are below, at, or above atmospheric as 

long as the maximum explosion pressure is less than 21 bar, 

which is the rated pressure of the chamber. Sapphire windows allowed viewing inside the chamber. The pressure 

transducer was a Viatran model 218-28. A personal computer 

Scale. crn 0 5 10 

FIG. 1. 20 L chamber laser ignition test setup. 

(PC) based data acquisition program recorded the pressure 

transducer signal and the laser-on signal from the laser controller. A high speed video camera (NAC model 400) recorded selected tests at 200 frames per second. Time measurement uncertainty due to the frame speed was -t-5 ms. 

Microsoft Excel 2002 created the graphs with threshold 

curve fits and coefficients of determination (R'). 

Black iron oxide is a combination of ferrous (FeO) and 

ferric (Fe203) oxide having a theoretical formula of Fe304 

and average particle size of 0.4 pm. Labsphere Inc. measured the absorptance of the Fe304 to be 92% at the 1064 nm 

wavelength. The Fe304 was mixed with a zirconia based 

ceramic adhesive (Cotronics Resbond 904), 10% iron oxide 

to 90% adhesive by weight. The white adhesive mixed with 

the iron oxides produced a grey colored paste, likely absorbing the laser less efficiently than the iron oxides alone. The 

Resbond 904 provides adhesion up to 2200 "C. 

IV. METHODS 

An adequate length of fiber was pulled through a feedthrough in the 20 L chamber (Fig. 1) to allow preparation of 

the fiber tip. The power emanating from the cleaved end of 

the fiber was measured before and after the test. A thin layer 

of the Fe304-Resbond 904 target paste sufficient to block the 

red aiming laser was applied to the fiber tip and air dried 

with the aid of the mixing fan. Except for the 2 mm core 

fiber, the fiber was placed in the chamber, pointing upward. 

The 2 mm fiber had to be inserted straight into the chamber 

horizontally because of its stiffness. The chamber was evacuated, and flammable gas-air concentrations were set using 

partial pressures. The chamber pressure was set to 1 bar at 

room temperature. The mixture was allowed to settle for 

about 1 min after fan mixing. Non ignitions were generally 

repeated ten times before ending a test series. 
rro 

000 .. 0 

0 \\ e e \ 

'\ * . ... :* + 

'F! - - - - - - - - - - - 

@se 

- - - - - a**-----&$ - - @ @@@ 

e lgnition using SMF 28 fiber 

0 No ignition using SMF 28 fiber 

I 

0 

3 4 5 6 7 8 1 2 3 4 5 6 7 2 Butane-air concentration (% vol.) 

Propane-air concentration (% vol.) 

FIG. 3. Most easily ignited butane-air mixtures using laser-heated FIG. 2. Most easily ignited propane-air mixtures using laser heated Fe304-Resbond 904 targeu on single made fiber tips. Fe,04-Resbond 904 targets on single mode fiber tips. 

The ignition delays were measured from the PC data 

plots of the laser-on signals, pressure traces, and rate of pressure rise (dPldt) traces versus time. High speed video recordings of selected experiments showed the flame front 

emanating from the fiber tip before an appreciable rate of 

pressure rise in this chamber. Ignition delay times reported 

here based on the pressure transducer measurements were 

reduced accordingly to account for the difference. , 

For an ignition prevention test, the fiber pigtail to the 

input end of the optical fuse was fusion spliced to the output 

port of the coupler. A low power beam was directed through 

the fuse during the fusion procedure to help align the fibers. 

The power through the fuse was monitored during and after 

the fusion procedure to ensure the device was not activated 

before the ignition prevention test. The red aiming laser was 

also visible at the output end of the fuse after the fusion 

splice procedure, indicating the devices were not activated 

prior to the ignition prevention tests. The fiber pigtail to the 

output end of the fuse was pulled into the 20 L chamber 

(Fig. I), and Fe304-Resbond 904 target paste was applied to 

this end sufficient to block the red aiming laser. The laser 

power was set to 800 mW for both tests. After the tests, the 

gas-air mixtures were ignited with another ignition source ,to 

verify flammability. 

6 

V. RESULTS 

5 

Figures 2 and 3 show results of tests to determine the E most easily ignited gas-air mixtures. Data points were ad- 4 

justed to avoid overlap. Propane-air mixtures ranging from H 3 4% to 6% and butane-air mixtures ranging from 3% to 5% a 

produced the lowest igniting powers. 5% propane-air and 3 

4.5% butane-air mixtures were selected for subsequent tests 

to measure threshold igniting power as a function of beam 1 

diameter. These concentrations were also selected for testing 

ing irradiances were calculated by dividing the threshold igniting power by the surface area of the fiber-optic core. 

Additional tests were conducted at power levels above 

the threshold values to provide the ignition delay data shown 

in Figs. 8-11. Ignition delays became more variable at the 

lower igniting powers. In Fig. 8 for example, delays ranged 

0 

0 50 100 150 200 

beam diameter (vm) 

a 

. 4. 

0. 

. 0. 

* 

€Em 

e 0 0 

- 0. 0. 0. 

0333 Om m 

833 

e Ignition, 5% propane-air 

I 

flG. 4. 5% propane-air igniting powers for beam diameters from 9 to 

200 pm using laser heated Fe304-Resbond 904 targets. 

e 

a Ignition, 5% propane-air 

8 

o No ignition, 5% propane-air 

33 mrmrrn 

-& 1 I I I I I 

- 

by other researcher^.^ o 

Figures 4-7 show results of tests to measure igniting 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 

Beam diameter (mm) powers vs. beam diameters for 5% propane-air or 4.5% 

butane-air mixtures. Data points were adjusted to avoid over- nG. 5. 5% propane-air igniting powers for beam diamers from 0.4 to 

lap. Threshold values are listed in Table 11. Threshold ignit- 2 mm using laser heated Fe,04-Resbond 904 targets. 
0 Ignition using SMF-28 fiber 

-Threshold fit 

0 20 40 60 80 100 120 140 160 180 200 220 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 

Beam diameter (pm) Laser power (W) 

FIG. 6. 4.5% butane-air igniting powers for beam diameters from 9 to FIG. 8. Ignition delays vs laser powers (p) for 5% propane-air mixtures 

200 pm using laser heated Fe30,-Resbond 904 targets. using Fe30,-Resbond 904 targets on single mode fiber tips. Fit to threshold 

delays (t) for powers 3350.mW shown. 

from about 0.3 to 1.2 s @ 350 rnW, compared to delays 

ranging from about 45 to 75 ms @ 800 mW. The shortest 

delay measured at a particular power was taken as a threshold value. Threshold values are listed in Table 111. Figures 

8-11 include curve fits to threshold delays (t) versus igniting 

power 01). In Figs. 8 and 9, the threshold delays at the minimum igniting powers were unusually long and were not included for the curve fit. The unusually long delays near the 

threshold power are attributed to convection or variable target thickness (see Sec. VI). 

The SMF-28 constlucted optical fuses prevented ignitions in two tests using a set power of 800 mW, 

Fe304-Resbond 904 targets, and 5% propane-air. No target 

incandescence was apparent from the high speed video recordings of the two tests. This was in contrast to significant 

target incandescence leading to ignition for all observed ignitions in prior tests. The fiber tips were cleaved after the 

tests and the red aiming laser was not visible, indicating 

activation. The power throughput of one activated fuse was 

about 10 mW, monitored for 15 rnin following the test. The 

power throughput of the second activated fuse was about 

10 mW after the test, but rose to 180 mW after 10 min of 

continuous irradiation. The input side of the fuse was cleaved 

after each test and input powers to the fuses were measured 

at 780 and 790 mW. The difference between these values 

'and the 800 mW set power is attributed to the fusion splice 

loss. 

I . 1 1 I I I 

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 

I 

VI. DISCUSSION 

Beam diameter (mm) Rich propane and butane-air mixtures ignited most easFIG. 7. 4.5% butane-air igniting powers for beam diameters from 0.4 to ily (StOichiometric for and 

2 mm using laser heated Fe30,-Resbond 904 targets. 

TABLE 11. Threshold igniting powers and irradiances for 5% propane-air and 4.5% butane-air using 

Fe304-Resbond 904 laser-heated targets. 

5% propane-air 4.5% butane-air 

Beam Igniting Ignif ng Nonigniting Igniting Igniting Nonigniting 

diameter power irradiance power, ten tests power irradiance power, ten tests 

(~m) (W) (W/mm2) (W) (W) (w/m') (W) 
0 

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 

Laser power (W) 

FIG. 9. Ignition delays vs laser powers (p) for 5% propane-air mixtures 

using Fe304-Resbond 904 targets on 62.51125 pm fiber tips. Fit to threshold delays (t) for powers 2300 mW shown. 

U 

butane-air are 4.02% and 3.12%, respectively.) This is in 

contrast to methane-air mixtures that ignite most easily with 

lean mixtures. ~ertzber~' explains lean methane-air mixtures ignite most easily because of selective diffusion. The 

Ignition using 62.5 micron fiber 

-Threshold fit 

0.0 

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 

Laser power (W) 

0 a CI 

. I I I 

FIG. 10. Ignition delays vs laser powers (p) for butane-air mixtures using 

Fe304-Resbond 904 targets on SMF-28 fiber tips. Fit to threshold delays (t) 

shown. 

0 

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 

Laser power ON) 

RG. 11. Ignition delays vs laser powers (p) for butane-air mixtures using 

Fe304-Resbond 904 targets on 62.5 pm fiber tips. Fit to threshold delays (t) 

shown. 

TABLE IU. Threshold ignition delays for propane-air and butane-air using 

Fe304-Resbond 904 laser-heated targets placed on 91125 ,urn SMF-28 and 

62.51 125 pm fiber tips. 

9 pm diameter beam 62.5 pm diameter beam 

5% propane-air 4.5% butane-air 5% propane-air 4.5% butane-air 

Power ignition delay ignition delay ignition delay ignition delay 

(W) (s) (s) (4 (4 

lighter methane molecules enter the combustion zone faster 

than oxygen molecules, creating a fuel rich mixture in the 

ignition volume close to the ignition source. Lean methaneair mixtures should therefore create most easily ignited stoichiometric mixtures (9.48% by vol) in the ignition volume. 

Similar arguments were used to explain why rich mixtures 

ignite more easily for the heavier-than-oxygen propane and 

butane. The lighter oxygen molecules enter the combustion 

zone faster than the heavier hydrocarbons, creating an oxygen rich mixture in the ignition volume close to the ignition 

source. Rich propane or butane-air mixtures should therefore 

create most easily ignited stoichiometric mixtures in the ignition volume. 

Figures 4 and 7 show threshold igniting powers leveled 

off for beam diameters < 100 pm and increased with beam 

diameters for beam diameters > 100 pm. Figures 12 and 13 

shows threshold powers found here combined with those 

listed in Table 11. The curve fits in Figs. 12 and 13 indicate 

the threshold igniting power was approximately proportional 

to beam diameter for beam diameters a100 pm. Figure 13 

shows threshold irradiances calculated from the butane-air 

threshold power data. There is an inverse relationship between threshold powers and irradiances as a function of 

0 

0 , 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 

Beam diameter (mm) 

FIG. 12. Propane-air threshold igniting power (p) vs beam diameter (d) 

using laser heated targets on optical fiber tips. Fit to power for diameters 

20.1 mm and coefficient of determination (R') shown. 
0 0 

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 

Beam diameter (mm) 

4.5% butane-air, single mode fiber, 

this study 

A 5% propane-air, single mode fiber, 

t = 0.0471 p-2'24 

I f I t I I 0 

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 

Laser power 

FIG. 13. Butane-air threshold igniting powers (p) and irradiances vs beam FIG. 15. Ignition delays vs laser power for several hydrocarbons using 

diameters (d) using laser-heated Fe304-Resbond 904 targets. Fit to power Fe304-Resbond 904 targets on single mode fiber tips. 

for diameters 20.1 mm shown. 

beam diameters. These trends are in agreement with observations of Adler and carleton2' and Carleton and 

~einber~,~' who concluded that a minimum power is the 

limiting ignition criteria for small cw beam diameters, and a 

minimum irradiance is the limiting ignition criteria for large 

cw beam diameters. Figure 13 shows that beams of intermediate dimensions that exceeded both the minimum igniting 

power for small diameters and minimum igniting irradiance 

for large diameters were needed to cause ignition. These results suggest that relatively powerful beams can be used in 

these gas-air mixtures without causing ignition, provided the 

beam power and irradiance remain conservatively below an 

appropriate ignition curve such as shown in Fig. 13. Figure 

14 shows similar threshold powers for straight chain hydrocarbons butane-air and propane-air, and somewhat higher 

threshold powers for methane-air. The data for methane-air 

in Fig. 14 was obtained from Dubaniewicz et a~.,'~.'~ Hills 

et al.,ll Hawlcs~orth,~~ and Welzel et al.12 

Other potential ignition mechanisms should be considered as appropriate. For example, ~awlcsworth~~ reported 

300 mW caused smoldering in a 15 mm deep coal dust layer, 

which could possibly lead to large scale burning. Power 

0.5 

I 

0 

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 

Beam diameter (mm) 

0 

0 

0 Threshold power, butane-air 

(Tables 1 & 2) 

(Tables 1 & 2 ) 

! 

A 

A g 

thresholds derived from tests of targets obscuring fiber optic 

tips suspended in flammable gas atmospheres such as studied 

here may not be conservative for all conceivable ignition 

mechanisms. 

Ignition delays became more variable at the lower laser 

powers (Figs. 8-11). The variability may have been caused 

in part by air currents generated from the target heating. The 

longer heating times required for ignition using the lower 

powers would allow more time for stronger air currents to 

develop past the fiber tip, dissipating thermal energy at the 

fiber tip. Another possible reason for the variability is a variable thickness of target material applied to the fiber tip. 

Thicker targets would take longer to heat to attain the needed 

ignition temperature. These possible effects may explain why 

ignition delays at the lowest igniting powers tended to occur 

significantly above the threshold curve fits. 

Figures 15 and 16 show ignition delays vs igniting powers for several hydrocarbons grouped by optical fiber diameter. The curve fit exponents averaged 2.4 and 1.87 in Figs. 

15 and 16, respectively. The curve fits suggest the ignition 

delays were approximately proportional to the inverse square 

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 

Laser power (W) 

FIG. 14. Threshold igniting powers vs beam diameters for three FIG. 16. Ignition delays vs laser power for propane and butane-air using 

hydrocarbon-air mixtures. Fe30,-Resbond 904 targets on 62.5 pm fiber tips. 
of the laser powers using the SMF 28 and 62.5 pm fibers. 

The curve fits are similar in shape to the hyperbolic relationships observed by Moore and weinberg.14 

Figures 15 and 16 were used to select fiber optic power 

limiters that should activate quickly enough for the ignition 

prevention tests. The preproduction versions of the Kilolambda optical fuses prevented ignitions using about 

800 mW under selected test conditions. Effects of the IPG 

laser overshoot on fuse activation were not measured. Target 

heating was not sigeficant based on the video recordings 

showing no apparent incandescence. The activation time for 

the optical fuses was reported to be about 5 ps for input 

powers near the activation threshold power. Activation time 

varies with power. The 5 ps activation time is significantly 

less than the ignition delays found in this study (Figs. 15 and 

16) and reported by Hills et nl. l1 One of the activated fuses 

showed a self-healing effect, allowing a throughput power 

above the specified threshold power several minutes after 

activation under continuous irradiation. 

VII. CONCLUSIONS 

Rich mixtures of propane-air and butane-air ignited most 

easily. Most easily ignited mixtures for propane-air ranged 

from 4% to 6%, and for butane-air ranged from 3% to 5%. 

Threshold igniting powers for these straight chain hydrocarbons were approximately proportional to beam diameter 

for beam diameters ranging from 0.1 to 2 mm. Threshold 

igniting irradiances decreased as beam diameters increased. 

Relatively powerful beams of intermediate dimensions were 

typically needed to ignite these hydrocarbon-air mixtures under selected test conditions. Other potential ignition mechanisms should be considered as appropriate. 

Igniting powers tended to level off for beam diameters 

<I00 pm, in agreement with observations of several other 

researchers. Minimum igniting powers for the propane-air 

and butane-air were about 250 and 300 mW, respectively. 

Threshold ignition delays for these hydrocarbons were 

approximately proportional to the inverse square of the igniting power using targets attached to SMF-28 and 62.5 pm 

diameter fibers. Ignition delays became more variable at the 

lower laser powers, possibly due to thermally induced air 

currents or variable target thickness. Ignition delays were 

about 45 ms or longer using laser powers up to 800 mW 

under selected test conditions. 

The ignition curves were used to select fiber optic power 

limiters that activated quickly enough to prevent ignitions 

under selected test conditions. A self-healing effect was observed for one sample. 

ACKNOWLEDGMENT 

The author would like to thank Kenneth L. Cashdollar, 

NIOSH, PRL, for guidance through all phases of this work. 

Mention of company names or products does not constitute 

endorsement by the National Institute for Occupational 

Safety and Health. 

'J. K. Jeyapalan, "Optical fiber cables in existing sewers and natural gas 

pipes" (Dr. Jeyapalan & Associates LLC., New Milford, CT, 2002); http:// 

home.earthlink.net~-jkjeyapalan/intro/index.html. 

2~. Montgomery, "Avoid fiascoes with fiber optics," InTech 50 (6), 56-57 

(2003). 

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