UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

°C

4B/km

g/gmol

degree Celsius
centimeter

decibel

decibel per kilometer
decibels above 1 milliwatt
gram per gram-mole
hour

Kelvin

kilometer

pm
uw
mm
om

nW

vol pet

micrometer
microwatt
millimeter
nanometer
nanowatt
percent

volt

volume percent

watt



This can be modeled as
Pray * OP = NEP,
at the detection limit.

Therefore, A = (Pyay- NEP)/Prnay-

50.0

20.0

10.0

a
oO

CONCENTRATION, vol pct
- YN
fo] oO

MINIMUM DETECTABLE CHq
a

1 L L

po

2,0 3040506070 100

FIBER LENGTH, km

Figure 5.—Minimum detectable CH, concentration as function
of fiber length.

01 02 03 10

Prax

a

dP= NEP

DETECTED POWER LEVELS

Again, this calculated A is used to find the minimum de-
tectable concentration from a graph similar to figure 4.
This effect is responsible for the sharp upturn in figure 5
after 2 km.

Modifying different components of the sensor affects
the performance curve of figure 5. To increase the sensi-
tivity in the short range, one can increase the path length
of the gas cell or use a narrower filter. This would shift
the horizontal portion of the curve downward. These
changes also would tend to decrease the optical efficiency
of the sensor, shifting the vertical portion of the graph to
the left. Alternatively, increasing the light power or using
a detector with a lower NEP would greatly improve per-
formance in the long range while providing moderate im-
provement in the short range.

TEMPERATURE EFFECTS

During a series of in-mine tests, changes in environ-
mental temperature caused a noticeable drift in the output
of the monitor. For example, an 8° C change in ambient
temperature caused the output to drift by 3 pct CH,. Oth-
er researchers have noted these effects and have attributed
them to temperature-sensitive optical filters (4). Based on
these results, the system will require temperature stabiliza-
tion. A parallel fiber with a reference gas cell can be add-
ed to the system and monitored to compensate for subse-
quent temperature-induced errors.

REFRACTIVE INDEX EFFECTS

Other hydrocarbons similar in molecular structure to
CH, were introduced into the gas cell to investigate their
effect on the monitor. The response to ethane (C,H,) was
about one-third of the response to CH,. The introduction

Prnax

Prax ~ dP= NEP
NEP

DETECTED POWER LEVELS

equivalent

power when
CH, |Is Introduced Into gas cell; P,,., = power received at detector
for a given length of fiber with no CH, present.)

Figure 7.—Long-range limitation. (NEP = nolse equivalent
power of detector; dP = fractional drop In recelved power when
CH, Is Introduced Into gas cell; P,,,, = power recelved at detector
for a given length of fiber with no CH, present)



INTRODUCTION

One of the greatest hazards faced by coal miners is the
ignition of methane (CH,) gas. CH, is odorless and explo-
sive when mixed with air to a concentration between 5 and
15 vol pct. Many coal seams naturally liberate CH, into
the ventilation air, where it can accumulate to hazardous
levels because of ventilation inadequacies or fluctuations.
Conventional remote CH, monitors can reliably detect
these accumulations. However, these monitors must rely
on power supplies that are not intrinsically safe. In case
of an emergency, mine electrical power must be shut
down. The monitors must then rely on temporary backup
batteries. Once these batteries run out, the monitors are
disabled, creating a potentially dangerous situation. Mod-
ern mining practices have caused special emphasis to be
placed on the reliability of CH, monitoring. The depletion
of more economical and easily accessible seams has led to
the mining of deeper, gassier seams, High-recovery min-
ing methods liberate CH, into the atmosphere more quick-
ly than traditional methods do. These are just a few of the
factors that have led to a reported increase in the annual
frequency of CH, ignitions by friction in American coal
mines (1). A recent survey of 19 reports of investigations
on noninjury CH, ignitions involving longwall machines
revealed that 4 of the incidents involved improperly op-
erating or calibrated CH, monitors. One way to enhance
the reliability of CH, monitoring while eliminating the
need for an electrical power supply involves a totally
passive, intrinsically safe system that uses fiber optics (FO)
to exploit the light-absorbing properties of the gas.

A literature search identified several researchers in
other countries such as Japan, England, Norway, and

Australia (2-5) that have made some progress on such a
system. Although some researchers reported working
models, attempts to obtain units for independent testing
were unsuccessful. Since no commercial units were avail-
able, an engineering system was developed for research as
part of the Bureau’s program to improve mine safety.
This report focuses on the status of the Bureau’s develop-
ment of a FO CH, monitor.

Over the last several years government and private in-
dustry have researched FO sensing, but the mining in-
dustry has enjoyed few benefits of that research. Part of
the reason is the small return on investment in the limited
mining market, but perhaps a bigger reason is the percep-
tion that optical fiber is unable to withstand the hostile
mine environment. Thousands of miles of fiber placed on
the ocean floor for communication systems attest to the
strength of FO cable. Fiber is becoming increasingly
easier to maintain. For instance, manufacturers recently
introduced connectors that require no epoxy or polishing.
Cost is becoming another attractive feature as production
technology is perfected. The abundance of silica insures
the availability of fiber well into the future. Other advan-
tages of fiber include high data capacity and light weight
compared with copper cable. Most important, however,
are the potential safety benefits of this technology. A well-
known characteristic of optical fiber is intrinsic safety.
Conventional fiber is made with silica or plastic, or
sometimes both (fig. 1). Since these materials are not
conductors, sparking is not a problem.

SYSTEM COMPONENTS

Figure 2 shows the components that comprise the FO
CH, monitor assembled by Bureau researchers. A light-
beam chopper forms pulses from a tungsten halogen lamp,
and a condensing lens focuses the light pulses onto an op-
tical fiber. The fiber then guides the light to the remote
site to be monitored. The light exits the fiber, passes
through a 50-cm cell where CH, gas may be present, and
then enters another fiber that guides the light away from
the remote site. A coupler directs the light to two band-
pass filters. Detectors behind each filter convert the light
into electrical signals that are processed to calculate the
concentration of CH, present in the gas cell.

The CH, monitor takes advantage of the optical proper-
ties of the gas. Infrared light is absorbed by CH, at two

3italic numbers in parentheses refer to items in the list of references
at the end of this report.

spectral regions within the optical "window of convention-
al silica fiber. One absorption band lies near 1,666 nm
and the other lies near 1,330 nm (fig. 3) (2, 6). By observ-
ing the amount of light returning to the detector in one of
these absorption bands, the concentration of CH, can be
determined. The system was designed to monitor light
energy near the 1,666-nm peak, primarily because CH,

Light
roy

Figure 1.—Profile of fiber-optic cable.


REMOTE FIBER-OPTIC METHANE MONITOR

By T. H. Dubaniewicz, Jr.’ and J. E. Chilton?

ABSTRACT

Fiber-optic technology is progressing rapidly, including the development of fiber-optic sensors for
many applications, These sensors have the advantages of high sensitivity, light weight, small size, high
bandwidth, and freedom from electromagnetic influences. The U.S. Bureau of Mines is investigating
the use of fiber-optic technology to monitor mine atmospheres. This report describes a methane
monitor based on differential absorption of infrared light. The monitor can detect methane concen-
trations as low as 0.2 vol pet as far away as 2 km via fiber-optic cable. The upper range is 100 vol pet
methane. Since the system requires no electrical power within the mine, it is intrinsically safe.

TBlectrical engineer.
?Research chemist.
Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA.


wavelength of 1,666 nm by tilting. This tilting technique is
useful in compensating for dielectric constructed filters
that may be slightly out of specification. An optical power
meter using a cooled germanium detector with a noise

equivalent power (NEP) of -80 dBm converted the light to
an electrical signal. This signal was processed by a lock-in
amplifier and other electronics to indicate the actual CH,
concentration,

RESULTS

This assembly of components successfully measured
CH, below the lower explosive limit. Figure 4 shows the
percent absorption for various concentrations of CH,. The
exponential nature of the graph agrees with the 1 - ef</)
response predicted by theory (4), where c is the concentra-
tion of the gas and a is the constant. The deviation from
theoretical values at higher concentrations arises from
changes in the refractive index of the analytical system (7).

Figure 5 shows the minimum detectable concentration
of CH, as a function of fiber length. Two primary factors
limit the sensitivity of the monitor. For short lengths of fi-
ber, the sensitivity limit is reached as the fractional change
in power caused by CH, absorption approaches the NEP
of the detector (fig..6). The horizontal line of figure 5
approximates this limit, calculated as follows:

dP = (A)Pmax)s

dP = fractional drop in received power
when CH, is introduced into the
cell,

where

A = fraction of light absorbed by CH, in
the cell,

and Prax = power received at the detector for a
given length of fiber with no CH,
present,

Also, dP = NEP at the limit of detection.

Therefore, A = NEP/Pusy

A graph similar to figure 4 can then be used to find the
minimum detectable concentration using the calculated
value of A. The actual minimum concentrations measured
(about 0.2 vol pet) were slightly higher than the calculated
values because drifting of the output signal made it
difficult to resolve these small concentrations,

An additional limitation on the sensitivity of the moni-
tor occurs in long lengths of fiber as the maximum power
received at the detector approaches the NEP (fig. 7).

DETECTED POWER LOSS, pct

02 03 O05 Lo 20

50 100

CHg CONCENTRATION, vol pct

Figure 4.Percent power loss for varlous concentrations of CH,.


RI} 9407 REPORT OF INVESTIGATIONS/1992

Remote Fiber-Optic Methane Monitor

By T. H. Dubaniewicz, Jr., and J. E. Chilton

UNITED STATES DEPARTMENT OF THE INTERIOR

BUREAU OF MINES



800

600

400

200

REFRACTIVE INDEX FUNCTION [(n-

ANALOG OUTPUT, V

Figure 9.—Analog response to varlous gases.

MACROBENDING‘ EFFECTS

Light losses from macrobends in the fiber are signifi-
cant in the 1,666-nm region (6). Macrobend losses in sili-
ca fiber become apparent around 1,500 nm, and the losses
increase with wavelength. As a result, the output of the
monitor drifted as the fiber was bent excessively. A sig-
nificant portion of the total power also was lost as the
fiber was bent. Light losses of 20 pct occurred when
100-ym-core-diameter fiber contained in a 3-mm-diameter

100.0 —.—.—.—_.——+

10.0F ° 7

POWER LOSS, pct

1
any i

Or
N

1 1 1 1
23 4 5
LOOP DIAMETER, cm

Figure 10.—Bending losses In fiber at 1,666 nm.

jacket was looped around a 1-cm dowel. Losses decreased
to less than 0.1 pet for loop diameters greater than 6 cm
(fig. 10). Again, choosing a reference wavelength within
a few nanometers of the absorption line will minimize the
errors caused by wavelength-dependent losses such as this.
However, excessive bending must be avoided to maintain
an acceptable signal-to-noise ratio.

SUMMARY

A FO-based remote CH, monitor was assembled and
tested by Bureau researchers. The system detects CH, by
measuring the absorption of near-infrared light as it passes
through the gas. The FO cable used to direct the light to
and from the remote site is intrinsically safe and therefore,
eliminates the sparking hazard found in the power supply
of electrically based remote monitors. This monitor can
detect CH, concentrations as low as 0.2 vol pet as far away
as 2km. The upper range is 100 vol pct CH,.

These results are encouraging; however, this system
requires more work before it is ready for mine usage.

4Macrobending refers to bending the fiber into loops with a diameter
much larger than the fiber diameter.

Temperature-, bending-, and refractive-index-induced er-
rors must be decreased. Temperature stabilization will be
required to reduce the temperature-induced error to an
acceptable level. Choosing a reference wavelength close
to the absorption line will help to minimize the errors
associated with wavelength-dependent errors seen with the
bending and refractive index effects. Furthermore, some
method of monitoring multiple sites is needed to make the
system economical. The tungsten lamp provided enough
power for a single-point sensor; however, a more powerful
light source is required to overcome the losses of multiple
gas cells.


of propane (C;H,) and butane (C,H,,) into the gas cell
caused unexpected negative readings. The index of refrac-
tion of the gases is the likely cause of these errors.

Whenever light passes through any system of glass
lenses (refractive index of 1.53) to air (refractive index of
1.000292), part of the light reflects and the rest passes
through. For a single air-glass interface, Fresnel’s expres-
sion gives the reflected light R as follows:

R = -n)*/(o + ay,

where b is the refractive index of glass (a constant), and n
is the refractive index of the gas. The value of n varies
from 1.000292 for air to 1.001410 for C,H». By substi-
tuting the factor g for n - 1, the equation can be rewritten
as follows:

R= (b-1-g)/(+1+ 8)
By differentiating R with respect to the variable g,
AR = -db(b - 1- g)/(b + 1+ g)° de,
which for g < 1 approximates to
dR = -0.20 dg.

The reflected light R = (I, - 1)/Iy, where I, is the initial
intensity and I is the transmitted intensity of light.

aR = -dI/I,,
df = 0.201, dg.

This expression states that the increase in transmitted
light is proportional to the increase in gas refractive index.
This calculation is for a single surface, but similar ex-
pressions hold for multiple lens-air interfaces as well. The
function f = (n - 1) x 10°, where n is the refractive index
of the gas, is plotted in figure 8 versus molecular weight.
The data for H, and hydrocarbons from CH, through pen-
tane (C,H,,) fall on a straight line when plotted against the
molecular weights. Reference 8 provided the refractive in-
dex values taken at the Na D line wavelength. The refrac-
tive indexes for C,H, and C,H,) were estimated from this
graph from the intersection of their molecular weights and
the hydrocarbon function line.

When pure gases were fed into the gas cell, channel "a"
of the optical power meter measured light energy from the
cell at the 1,666-nm wavelength and channel "b" measured
light energy at the reference wavelength. Both channels
were input to a lock-in amplifier. The ratio of the two

Then

And finally,

1,800 T
CHi2

1600 KEY CsHios
_" © Hydrocarbons
5 © Estimated hydrocarbons
S 400k  * Carbon oxides
3° 4 Noble gases Catto
1,200
z
2
5 1,000 1
2
2
800
i
3
2
w 600
2
é
8
= 400
i
o
&

200

° 60 80

MOLECULAR WEIGHT, 9/gmol

Figure 8.—Refractive index as function of molecular weight.

amplified signals was then displayed as a voltage. This
displayed voltage was initially set to 10.00 V with N, gas in
the cell. Several gases caused a change in the voltage, and
figure 9 shows the function of (10 - V) versus the re-
fractive index of the gases. This figure shows that CH,
and C,H, are positive and to the right of the general curve,
because both gases absorb light. Both C,H, and C,Hiy
have negative values, indicating an increase of light energy.
This increase can only occur when the refractive index
change allows more light to pass through the gas cell. In
this plot, even He acts as if there were an absorption of
energy at 1,666 nm. Since He lacks absorption lines in this
portion of the spectrum, again the refractive index effect
is indicated. The result is that gases with significantly dif-
ferent indexes of refraction than air have a wavelength-
dependent effect on the transmission of light through a
system of lenses, and such gases interfere with the meas-
urement of CH, in the gas cell. For instance, the moni-
tor’s response to equal quantities of C,H, and CH, would
almost cancel each other, resulting in a null reading.
Modifying the system to monitor a reference wavelength
very close to the absorption band will minimize this
wavelength-dependent error. Any changes associated with
this refractive index effect will be nearly the same in both
the reference wavelength and the absorption wavelength,
and will cancel when the ratio is determined.


Optical
fiber
i Gas
cit, Q ' cell 4
ial }-
Light
source
Optical!

fiber

Filters
Display

amplifier with
ratio module _
Figure 2.-Fiber-optic CH, monitor.

Transmission

H spectra of
CHa
‘Absorption spectra

\ of fiber-optic cable

ARBITRARY OPTICAL UNITS
=

XO Nn
N /

\
we

O7 10 13 16 19 2.2
WAVELENGTH, pm

Figure 3.—Flber-optic window and CH, absorption peak.

absorbs much more infrared energy there than at the
1,330-nm peak. A differential detection technique com-
pensates for wavelength-independent fluctuations caused
by, for example, lamp dimming or lens dust.

Several types of light sources emit in the 1,666-nm
region. One light-emitting diode (LED) met the spectral
requirements of the system, but the intensity was marginal.
This device had a peak emission wavelength of 1,650 nm,
a minimum output power of 100 »W, and a maximum
spectral width of 190 nm. Although this LED was accept-
able for short lengths of fiber, transmission over longer
distances required more power. No diode lasers operating
at the proper wavelength were commercially available to

test for suitability to the mine environment. Another type
of laser that shows promise is the rare-earth-doped fiber
laser. These fibers lase when pumped with conventional
lasers, Trimming the fiber tunes the output to different
wavelengths. These lasers are still in the experimental
stage; until more work is done with them, an alternate
source must be used.

Tungsten halogen lamps are an inexpensive alterna-
tive that emit appreciable amounts of energy in the near-
infrared. One of several tungsten halogen lamps evaluated
met the requirements of a single-point monitoring system.
This was a high-output, miniature lamp with a color tem-
perature of 2,800 K. (The sensor requires infrared energy
at a wavelength of 1,666 nm; this wavelength corresponds
to a color temperature of approximately 2,200 K.) The
lamp coupled more than 400 nW of power in the spectral
band of interest through a lens and into a 100-ym-dia-
meter fiber core. The life of the 8-W lamp is rated at
3,000 bh, equivalent to more than 4 months of continuous
use.
Large-diameter silica fibers maximize the amount of
light coupled from the lamp filament. Fiber core diam-
eters of up to 1 mm are available, but these are mostly
expensive specialty fibers that are generally not available
in the lengths required in a mine. The communications in-
dustry, which typically uses fiber cores up to 100 ym in
diameter, strongly influences the cost of fiber. Attenuation
also affects the choice of fiber, Although larger core fi-
bers capture more light, they typically exhibit greater at-
tenuation per unit length. Fiber with a core diameter of
100 ym and attenuation of 2.5 dB/km (at 1,300 nm) was
chosen, considering cost, attenuation, and availability.

In the Bureau’s monitor, the CH, interacts with the
light in a gas cell. The gas cell, constructed in-house, uses
20-power microscope objectives to couple light between
the emitting and receiving fiber. This cell had less than
5-dB loss over its 50-cm optical path length. A custom-
made field version of the cell based on these results had
an attenuation of about 6 dB. This custom-made cell uses
a lens tilting alignment technique, eliminating the need for
high-precision micropositioners. The simple design greatly
eased light coupling while providing ruggedness for the
harsh mine environment.

Once the light returns from the remote site, several
different approaches can be used to separate the absorp-
tion and reference wavelengths from the broad-band beam.
One alternative is to split the light and send each beam
through a dielectric interference filter. This type of filter
is inexpensive, and it passes significantly more light than
slit-type spectrophotometers, which is helpful when work-
ing with low light levels. A filter with a center wavelength
of 1,670 nm and bandwidth of approximately 3 nm was
placed in the light path, then tuned to the absorption


CONTENTS

Abstract
Introduction ..
System components .
Results ........4
Temperature effects .
Refractive index effects
Macrobending effects .
Summary ..
References .

ILLUSTRATIONS

Profile of fiber-optic cable . .
Fiber-optic CH, monitor ...
Fiber-optic window and CH, absorption peak ......
Percent power loss for various concentrations of CH,
Minimum detectable CH, concentration as function of fiber length .
Short-range limitation ...
Long-range limitation .
Refractive index as function of moles
Analog response to various gases ...
). Bending losses in fiber at 1,666 nm .

Syrmravayyp

Page

BIAINUUANNHE

YI AUNUUNALYD


Library of Congress Cataloging in Publication Data:

Dubaniewicz, T. H, (Thomas H.)
Remote fiber-optic tiethane monitor / by T-H. Dubsniewicz, Jr, and J.E. Chilton

P. cm. — (Report of investigations; 9407)
Includes bibliographical references (p. 8).
Supt, of Docs. no.: I 28.23:9407.
1. Fire-damp—Measurement, 2. Optical detectore—Testing. 3, Fiber optics,

L Chilton, J. E, II, Title. Ill. Series: Report of investigations (United States.
Bureau of Mines); 9407.

TIN23.U43 [TN305] 622s-=de20 [62282] 91-4299 CIP



Report of Investigations 9407

Remote Fiber-Optic Methane Monitor

By T..H. Dubaniewicz, Jr., and J. E. Chilton

UNITED STATES DEPARTMENT OF THE INTERIOR
Manuel Lujan; Jr., Secretary

BUREAU OF MINES
TS Ary, Director


REFERENCES

1, Watson, R. W. Prevention of Friction Ignition on Longwall Sec-
tions. J, Mines, Met., and Fuels, June-July 1989, pp. 261-265.

2. Chan, K., H. Ito, and H, Inaba, All-Optical-Fiber-Based Remote
‘Sensing System for Near Infrared Absorption of Low Level CH, Gas.
J, Lightwave Technol, v. LT-S, No, 12, Dee. 1987, pp. 1706-1711.

3, Hordvik, A., A. Berg, and D, Thingbo. A Fiber Optic Gas De-
tection System. Paper in Proceedings of 9th European Conference on
Optical Communication (Geneva, Oct. 23-26, 1983). North-Holland,
Amsterdam, 1983, pp. 317-320.

4, Dakin, J. P., C. A. Wade, D. Pinchbeck, and J. S. Wykes. A Novel
Optical Fibre Methane Sensor. Proc, Soc. Photo-Opt. Instrum. Eng,
v. 734, 1987, pp. 254-260.

5. Hopkins, J. C,, P. J. Samson, and A. D. Stuart. A Fibre Optic
Based Continuous Mine Monitoring System. Paper in Proceedings of
the 7th International Conference oa Optical Fibre Sensors (Sydney,
‘Aust., Dec. 2-6, 1990). Inst. Radio wad Electron. Eng, 1990, pp. 207-210.

6. Kalish, D., and L. G. Cohea. Single Mode Fiber, From Research
and Development to Manufacturing AT&T Tech. J., v. 66, No.1,
Jan-Feb, 1987, pp. 19-32.

7, Willard, H. H., L. L. Merrit, Jr, J. A. Dean, and F. A. Settle, J
Instrumental Methods of Analysis. Wadsworth, 7th ed., 1988, pp. 159-
163.

8 Weast, RC. (¢d.). Handbook of Chemistry and Physics. Chem.
Rubber Publ. Co., 54th ed., 1974, p. E-224,

INT-BU.OF MINES,PGH.PA 29525,