The chamber was sealed and evacuated after inserting the prepared fiber end. The explosive gas-air
mixture was then introduced into the chamber by partial pressures. The flammability of the explosive
gas-air mixture was periodically verified using electric matches when experiments resulted in
nonignitions. The pressure transducer and photo diode flame sensor detected ignitions in conjunction
with the high-speed video camera. A personal computer-based high-speed data acquisition system
recorded the pressure and flame sensor data along with an electrical signal triggered when the laser was
switched on.

RESULTS

Results of tests using the modified NT transmitter are shown in Figures 2 and 3. The minimum power
that produced an ignition was 250 mW. This occurred with a 150- to 212-zm diameter coal particle in a
15% H,-sir mixture. The 15% H,-air mixture also produced ignitions with inert rock dust particles at
approximately 500 mW. Both the coal and the rock dust particles would be heated to a high temperature
by the laser energy. An additional amount of energy is produced by the combustion reaction of the coal
with oxygen in air. The combustion reaction would contribute to the coal particle igniting the flammable
gas mixture at a lower laser power than that for the inert rock dust particle. No ignitions were observed
in 10% CH,-air mixtures with coal or aluminum particles using up to approximately 500 mW.

DISCUSSION

Although this study represents a small fraction of the effort required to adequately study this
phenomenon, some general observations can be made. Experiments were based in large part on customer
input from a standards-making subcommittee of the ISA. The CRADA effort represents the first known
tests of a commercial system employing intense fiber-coupled laser energy intended for use in potentially
explosive atmospheres. The NT transmitter, modified to simulate several worst case conditions, ignited
an explosive mixture of hydrogen (15% H,-air) at a minimum power of 250 mW. The NT transmitter
produced no ignitions in explosive methane mixtures (10% CH,-air). Observed ignitions verify the need
for safety standards for commercial fiber-optic instruments intended for use in hazardous (classified)
locations.

Minimum igniting power levels found in this study are significantly higher than the recommended safe
power in reference 4 because of differences in test conditions and procedures. The use of different
atmospheres such as carbon disulfide, methane, or hydrogen; absorbers such as rock dust, coal, or
aluminum; and many other factors significantly affect ignition levels. A standard based on carbon
disulfide testing would not be representative of conditions found in coal mines. The challenge for the
various international standard-making bodies is to arrive at a consensus on testing procedures that
Provide a conservative level of safety without being unreasonably restrictive.


Figure 1. Horizontal cross section of 20-L test chamber.


relatively independent of gas concentration from somewhat less than stoichiometric to stoichiometric
(22). The autoignition temperatures were measured in closed volume chambers (20, 22-23) with all walls
uniformly heated so that there was a large area of contact with the gas and the entire gas volume was
heated to the test temperature. The minimum ignition temperatures are much higher if the heated area is
much smaller and only part of the gas volume is heated. For example, the ignition temperature of H,-air
was 750 °C and that for CH,-air was 1,220 °C when they were heated by a 1-mm diameter nichrome
wire (20). These latter values may be more appropriate when considering a flammable gas mixture that is
ignited by a small particle heated by a laser.

TEST SETUP

NT International modified the fiber-optic powered transmitter in two ways to simulate worst-case
operating conditions. First, the laser output was altered from its normal pulse mode operation (which
also disabled an automatic shutoff feature) to continuous wave operation. Second, an output power
controller was added to the transmitter. The laser diode within the transmitter is normally operated at a
fraction of its rated output power to extend the life of the diode. The controller allowed researchers to
operate the laser diode above 150 mW, the peak optical power designed for the unmodified instrument.
Maximum power output of the modified NT transmitter with 200-;.m fiber attached, was about 500 mW.
The laser wavelength was approximately 810 nm.

Ignition experiments were conducted in a 20-liter test chamber (Figure 1) designed for explosion testing
of dusts, gases, and their mixtures (24). Tt can be used to measure lean and rich limits of flammability,
explosion pressures and rates of pressure rise, minimum ignition energies, minimum oxygen
concentrations for flammability, and amounts of inhibitor necessary to prevent explosions. 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. For these tests,
chamber instrumentation included a pressure transducer, a flame sensor, and a high-speed video camera.
The fiber-optic cable was inserted into the chamber through a feed-through in the wall.

Optical energy from the fiber-coupled transmitter was converted to thermal energy by placing various
particles on the output end of the fiber. An MSHA-rated fiber-optic cable with a 200-m core diameter
and a 230-m-cladding-diameter fiber was used for all NT transmitter tests. A fiber optic cleaver (York
model FBK 11C) provided a flat, perpendicular, optical surface for each test. The output power from the
cleaved end of the fiber was measured using a laser power meter (Scientec Model D200PC) with attached
calorimeter (Scientec Model AC2501) prior to each test. The laser was then turned off and a particle
attached to the fiber end. First attempts using static electricity to attach the particle were unsuccessful.
Better results were obtained by dipping the fiber end into optical coupling gel (Cargille code 608), then
wiping off the residue until only a thin film remained. The particle was carefully attached to the cleaved
fiber end while viewed under a microscope. Three different types of particles were used in the
experiments: coal, aluminum, and limestone rock dust. Coal and rock dust represent material likely to be
encountered in a coal mine. Aluminum particles were included because of their incendiary characteristics.
Experiments included various particle sizes ranging trom 90 to 600 um diameters.


REFERENCES
1. Huber, J. C., Industrial Fiber Optic Networks, Instrument Society of America, 1995, 221 pp.

2. Cucci, G. R., Sensors and Transmitters Powered by Fiber Optics. Sensors Expo Proceedings, May
16-18, 1995, Boston, MA, pp. 233-237.

3. Dubaniewicz Jr., T. H., J. E. Chilton, Optically Powered Remote Gas Monitor, U.S. BuMines
RI9558, 1995, 9 pp.

4. Wesoff, Eric, Optical Power and Data Delivery For Remote Sensing Applications, Sensors
Magazine, Vol. 12, No. 9, September 1995, pp. 57-59.

5. McGeehin, P., Optical Techniques in Industrial Measurement: Safety in Hazardous Environments,
European Commission Report EUR 16011 EN, 1994, 15 pp.

6. Hills, P.C. , D. K. Zhang, P. J. Sampson, and T. F. Wall. Laser Ignition of Combustible Gases by
Radiative Heating of Small Particles, Combustion and Flame, 91:399-412, 1992, pp. 399-412.

7. Zhang, Dong-Ke. Laser Induced Ignition of Pulverized Fuel Particles, Combustion and Flame,
90:134-142, 1992, pp. 134-142.

8. McGeehin, P. Thermal Ignition in Hazardous Environments Due to Stray Light from Optical Fibres.
Fiber Optic and Laser Sensors X, Boston, MA, USA, Sept. 8, 1992, Proceedings of SPIE—The
International Society for Optical Engineering, v. 1795, 1993, pp. 286-295.

9. Adler, J., F. B. Carleton, and F. J. Weinberg. Ignition of Flammable Atmospheres by Radiation-
heated Fibrous Agglomerates, Proc. R. Soc. Lond. A (1993) 440, 1993, pp. 443-460.

10. McGeehin, P. Are Optical Fibre Sensors Intrinsically, Inherently, or Relatively Safe?, Proceedings of
SPIE—The International Society for Optical Engineering, v. 1504, 1991, pp. 75-79.

11. Tortoishell, G. The Safety of Optical Systems in Hazardous Areas, Paper 14, The Hague,
Netherlands, “In-Process Optical Measurements and Industrial Methods," SPIE—The International
Society for Optical Engineering, v. 1266, March 14-15, 1990, pp. 115-124.

12. Feng, H. T. Theoretical Studies of Ignition in Flammable Atmospheres by Optical Methods, Health
and Safety Executive Report, RPG2524, United Kingdom, 1/10/89-12/3 1/89, 15 pp.

13. Carleton, F. B., and F. J. Weinberg. Radiative Ignition of Loose Agglomerates of Fine Fibres, Fiber
Optics ‘90, Proceedings of SPIE—The International Society for Optical Engineering, v. 1314, April 1990,
pp. 298-306.


a = Ignition, coal

4 No ignition, coal

@ Ignition, rock dust

© No ignition, rock dust

0.6 i q T T
05+ s40 ageo rvy 4
= Le 4
ott 4 4000 a
Wi a a 4
5 O3- a amc AAA a 4
L AAA J
o2F 4 AOA AMA -

o.t b 1 L l i

90-106 150-212 250-300 350-425
PARTICLE SIZE, pm
Figure 2. Data for ignition tests in 15% H, in air, using the NT laser with 200 zm core fiber,
using coal or rock dust particles of various sizes.
Key
© No ignition, aluminum
4 No ignition, coal

525 1 . , t
E
E s00k o 4
oe 4400 4 4

x) 440 4
F3 475 rn
a

it | L 1

150-212 250-300 350-425 500-600
PARTICLE SIZE, pm

Figure 3. Data for ignition tests in 10% CH, in air, using the NT laser with 200 m core fiber,
using coal or aluminum particles of various sizes.


Ignition Tests With a Fiber-Optic Powered Instrument

Thomas H. Dubaniewicz, Jr. Kenneth L. Cashdollar Gerald R. Cucci
Electrical Engineer Supervisory Research President
Electrical and Electronic Physicist NT International, Inc.
Systems, Gregory M. Green Minneapolis, MN 55421
Pittsburgh Research Center Physical Science Technician
U.S. Bureau of Mines Fires and Explosions and
Pittsburgh, PA 15236 Explosives,

Pittsburgh Research Center

U.S. Bureau of Mines
Pittsburgh, PA 15236

KEYWORDS
Fiber Optics
Lasers
Hazardous Locations
ABSTRACT

New types of industrial instruments use fiber-coupled laser energy to power remote sensors. Fiber-optic
based instruments are useful in classified (hazardous) locations found in many industrial plants because
the fiber provides a natural barrier against accidental electrical discharge into potentially explosive
atmospheres. However, current safety standards do not cover the use of laser-powered fiber-optic
instruments in potentially explosive environments such as found in coal mines. The U.S. Bureau of Mines
began an investigation of laser-powered fiber-optic instruments in explosive atmospheres in support of
the standards-making process. A commercial fiber-optic interface system, modified to simulate several
worst-case operating conditions, ignited explosive hydrogen-air mixtures. Worst-case simulations
consisted of disabling a safety shutoff feature, operating the laser at significantly higher powers than
intended for normal operations, and cleaving fiber-optic connections to direct laser energy onto selected
absorptive materials. At the highest power level available, the fiber-optic interface system did not ignite
selected methane-air mixtures. The obtained data should prove useful in establishing appropriate safety
standards.


14. Hills, P. C., P. J. Samson, I, Webster. Optical Fibres Are Intrinsically Safe: Reviewing The Myth.
Journal of Electrical and Electronic Engineering, Australia, v. 10, No. 3, Sept. 1990, pp. 207-220.

15. Ludlum, N. P. Safety of Optical Systems in Flammable Atmospheres— Phase III. Sira Report No.
R/1008/00/A (prepared for Optical Sensors Collaborative Assoc. (OSCA), contract 55/sss, Document
No. 89/67, by Sira Safety Services Ltd, Saighton Lane, Saighton, Chester CH3 6EG, England,
Telephone: Chester (0244) 332200), May 1989, 38 pp.

16. Miller, J. Fiber Optics for Use in Hazardous Locations. Proceedings of the Instrument Society of
America, Advances in Instrumentation and Control, ISA/94 International Conference, Exhibition, and
Training Program, May 3-5, 1994, Philadelphia, PA, USA, v. 49, Part 1, 1994, pp. 99-106.

17. Earley, M. W., RH. Murray, J. M. Caloggero (eds.). National Electric Code, Sixth Edition,
Chapter 5, Article 500-3. Published by the National Fire Protection Association, 1992, pp. 552-563.

18. Cashdollar, K. L., M. Hertzberg, I. A. Zlochower, C. E. Lucci, G. M. Green, and R. A. Thomas.
Laboratory Flammability Studies of Mixtures of Hydrogen, Nitrous Oxide, and Air. Final Report to DOE
and Westinghouse Hanford, WHC-SD-WM-ES-219, June 26, 1992, 71 pp.

19. Zabetakis, M. G. Flammability Characteristics of Combustible Gases and Vapors. BuMines B. 627,
1965, 121 pp.

20. Kuchta, J. M. Investigation of Fire and Explosion Accidents in the Chemical, Mining, and Fuel-
Related Industries - A Manual. U.S. BuMines B. 680, 1985, 84 pp.

21. Lewis, B. and G. von Elbe, Combustion, Flames, and Explosions of Gases. Academic Press, 1951,
p. 414.

22. Conti, R. S., and M. Hertzberg. Thermal Autoignition Temperatures for Hydrogen-Air and
Methane-Air Mixtures. J. of Fire Sciences, v. 6, 1988, pp. 348-355.

23. De Soete, G. G. The Flammability of Hydrogen-Oxygen-Nitrogen Mixtures at High Temperatures.
La Rivista dei Combustibili, v. 29, 1975, pp. 166-172.

24. Cashdollar, K. L., and M. Hertzberg. 20-Liter Explosibility Test Chamber for Dusts and Gases.
Rev. of Sci. Instrum., v. 56, 1985, pp. 596-602.


INTRODUCTION

In the past 15 years, fiber optics has become the medium of choice in telephony and data
communications. More recently, fiber optics has become the wiring medium of choice in high
performance industrial applications (1). Industrial fiber-optic systems are weil suited to the underground
mining environment where long cable runs and high-voltage distribution systems and machinery present
safety and reliability concerns. Optical fiber is virtually immune to electromagnetic interference from
prevent accidental electrocutions where miners must work in close proximity to power lines often in wet

New instruments can take electrical isolation one step further by combining both power and
communications over fiber-optic cable (2,3). In addition to the benefits already mentioned, these systems
can provide a natural barrier against accidental electrical discharge into potentially explosive
environments such as found in coal mines. Therefore, an all fiber-optic interface would not be subject to
some of the special requirements for wire installations in explosive environments.

However, under certain circumstances, the intense optical energy characteristic of fiber-optic-powered
instruments raises concern about current usage in potentially explosive atmospheres (4). Currently, there
are no standards or guidelines for the safe use of fiber-optic-powered systems in hazardous (classified)
locations in the United States. The Intemational Society for Measurement and Control (ISA) has formed
the SP12.21 Fiber Optics subcommittee to establish such standards. In an effort to support the
standards-making progress, the U.S. Bureau of Mines (USBM) began an investigation into potential
hazards of laser-coupled fiber-optic instruments that may be intended for use in hazardous (classified)
environments.

Active members of the SP12.21 subcommittee made several suggestions in regard to the USBM
investigation. First, testing should be directed toward materials defined in the hazardous materials
classification scheme used in the United States. Subcommittee members also felt that the USBM study
should encompass practical aspects of fiber-optic usage in the field. One suggestion was to conduct tests
using large-diameter fiber. Large-diameter fibers are popular in industrial applications where potentially
explosive atmospheres are most likely to be encountered. For instance, a survey of fiber-optic cable
manufacturers identified one type of cable that had been rated by the Mine Safety and Health
Administration (MSHA) that was a large diameter fiber (200 rm core diameter). Another suggestion
was to include commercially available fiber-optic systems in planned experiments. In this regard, the
subcommittee helped to locate a vendor whose product contained a fiber-coupled laser source. The
vendor, NT International,’ entered into a Cooperative Research and Development Agreement (CRADA)
with the USBM to test its recently introduced fiber-optic-powered process control transmitter in

‘Reference to products of specific manufacturers is for identification purposes only and does not
imply endorsement by the USBM.


LITERATURE REVIEW

Most of the research to date involving the safety of fiber-optic systems in potentially explosive
atmospheres has been conducted in Europe (including the United Kingdom (UK)) and Australia (5-16).
The reported ignition mechanism that requires the least amount of optical energy occurs when absorbing
material converts the optical energy to thermal energy. For example, a particle on the end of a broken
fiber-optic cable could be heated by laser energy. The most comprehensive study to date was recently
completed by a collaboration of researchers from the UK, France, and Germany. This effort began in
December 1990, and conchuded at the end of May 1994. The final recommendation of this effort,
contained in a European Commission (EC) report (5), is as follows:

"It is concluded broadly, that continuous wave devices radiating in the visible and near

a) the radiated power is less than 35 mW, or

b) the peak radiation flux is less than 5 mW/mm?.
For pulsed or intermittent light sources, specific atmospheres, sources of area smaller than
0.001 mm? and where sensitive materials are present, different values of power and flux may be
appropriate."

This recommendation is based on the lowest observed ignition levels for carbon disulfide-air mixtures,
plus an additional safety factor of about 2. The recommendation was passed along to the SP12.21
subcommittee for consideration as a U.S. standard. However, the subcommittee is hesitant to support
the recommendation as a standard for several reasons. One important reason is the use of carbon
disulfide as the model explosive gas. Carbon disulfide is not represented in the hazardous materials
classifications established in the United States because it requires safeguards beyond those required for
any of the atmospheric group classifications (17). The subcommittee would prefer that safety standards
conform with established classification schemes. For this reason, explosive mixtures of hydrogen in air
and methane in air were used in the USBM study. In the United States, hydrogen is classified as a Class I
group B hazardous material, and methane is classified as a Class I group D hazardous material.

FLAMMABILITY AND IGNITIBILITY OF HYDROGEN (H,), METHANE (CH,),
AND CARBON DISULFIDE (CS,) IN AIR

Hydrogen in air is flammable over the range of 4 to 75% H, and the stoichiometric value is 29.5% H,
(18-20). Methane in air is flammable over the range of 5 to 15% CH, and the stoichiometric value is
9.4% CH, (19-20). Carbon disulfide in air is flammable over the range of 1.3 to 50% CS, and the
Stoichiometric value is 6.5% CS,. Near-stoichiometric mixtures of H,-air, CH,-air, or CS,-air are very
easy to ignite with an electric spark. The minimum spark ignition energies (20) are: 0.017 mJ for H,-air,
0.30 mJ for CH,-air, and 0.015 mJ for CS,. The minimum spark energies are found at concentrations
slightly less than stoichiometric (20-21). The autoignition temperature of H,-air is 500 to 520 °C and the
autoignition temperature of CH,-air is 600 to 630 °C (20, 22-23). The autoignition temperature of
CS,-air is 100 °C, much lower than for H, or CH,. The autoignition temperatures of H, and CH, are