A SECOND-GENERATION REMOTE OPTICAL METHANOMETER

By Robert A. Franks,’ John J. Opferman,’ Gene F. Friel, and John C. Edwards, Ph.D.*

ABSTRACT

As the use of deep-cut mining increases, there is expected to be a greater demand for methane (CH,)
measurement devices capable of scanning working faces al depths in excess of 10 m to alert mine personnel
of hazardous concentrations of CH, and to satisfy regulatory requirements, Remote optical devices offer the
most promise lo satisfy these objectives while the operator remains safely under supported rool, A second-
generation remote optical methanomcter (ROM) has been designed and is undergoing laboratory evaluation
al the National Institute for Occupational Safety and Health." It operates on the principle of differential
absorption of the "v,” fundamental absorption band of CH, centered at 3.31 ,2m in the infrared region of the
spectrum. ‘The new instrument diflers from the previous prototypes by the replacement of the two-band pass
differential absorption technique of measurement with a single- filter gas correlation technique and a
refinement of the amplifier and gating circuitry used within the unit. This resulls in a more accurate isolation
of the CH, spectral absorption band centered at 3.31 ym. A specially designed 0.4-m-diam variable-length
gas mixing (unnel was constructed and instrumented with CH,-sampling capability to calibrate the ROM.
Experiments have demonstrated the ability of the detector (o respond to various conditions of CH, (peter) at
distances as great as 12 m. Thermat radiation generated within the instrument is the controlling factor in the
drift of the output signal and, therefore, the reliability of the instrament. Several approaches to reduce this
extrancous radiation interfering with the infrared detector are described, such as the installation of a fan in the
instrument and thermoelectric coolers on the detector’s heat sink. Implementation of this technology will
significantly improve the sufety of miners

Blectranie engineer
Hee
‘Chemical engineer.

“Research physicist

Piutsburgh Research Center, National tnstitute for Occupational Safety and Health, Pittsborgh, PA

“This reseatch originated under the former U.S. Bureau of Mines prior to transferring to the National Institute for Occupational Salty and Healdh ity 1996.

nies technician.



6

Data Display and Alarm

Signal flow now moves to the display and alarm hoard. ‘This
circuit board is the heart ef the instrument. It wkes lhe
accumulated channel signals and uses them te determine if an
excess of CH, is apparent in the optical path or not. ‘Yo do this.
the incoming signals are fed into an Analog Devices ADS34
Internally Trimmed Precision IC Multiplier configured to act as
a divider circuit. The signal channel (Vy) is (cd into the
numerator input, and the reference channel (V,) is fed into the
denominator input of the AD534. ‘The output equation lor this
divider is

a)

‘This output signal is now inverted before being used to provide
a final signal formula of

0 of

As can he seen from the instrument's output equation, the unit
will read zero volts when properly balanced, V, = V,,, in clear
air. As CH, enters the optical path, V, will tend to be reduced
to a greater extent than V,, thereby causing the output voltage,
Vo. to rise to increasing positive values.

This board also generates the instrament's Iront pancl alarms,
which also control the AGC circuit in AUTOMATIC mode.
The reference signal [rom the seference channel integrator is
measured by a comparator circuit for a high or low out-of-range
signal. ‘This signal is displayed as the instrument's SIGNAL OK
indicator and in AUTOMATIC mode also conuols the AGC
circuit. When the SIGNAL OK indicator is off, the AGC is in
constant gain mode: when the SIGNAL OK indicator again
returns to on, the AGC ix returned to variable gain mode after
an appropriate delay to allow the instrument's integrators to
stabilize. This prevents the AGC from going ino de saturation
for very low reference channel signal values. ‘Phe SIGNAL OK
indicator also controls another DG201 analog switch that leeds
the resultant Vy output signal from the data display and alarm
board to the display. When this indicator signal is ON, the
‘output signal is ted to the display and when it is OFF the output
signal is blocked.
instrument's output signal. V,, is finally sent to the
Modutee BT.176301 digital panel meter on the front panel for
display. ‘The display is a 3-1/2 digit LCD display setup to read
from -20,00 V de to +20.00 Vide, The maximum range of the
output signal, V,,. is +15.00 ¥ de lw -15.00 V de. The in-
strament is maintained at slightly less than 0.0 V when balanced
in clear air and would reach values of +0.50 to +1.0 V during
tests in the calibration tunnel,

(2)

Auxiliary Circuit Boards

Three auxiliary circuit boards are necessary to complete the
circuitry used to operate this instrument, The first is the gating
and timing bourd. This board has as its input two sets of pulses
associated with the filter wheel. The first pulse input comes
from an index notch detector mounted directly on the filter
wheel/detector mounting assembly. The [ilter wheel has a smali
index notch cut in the aluminum rim of the wheel. The wheel
itself is a solid aluminurn disk, except for the wwo window
orifices wilh their respective absorption cells mounted over
them and the small notch in the rim of the wheel. Mounted
down over the rim is a TRW Optron OPB8&48 Slotted Optical
Switch that makes optical contact each time the wheel rotates
the notch past this switch, This results in transmitting a reset
pulse through the gating and timing board, ‘The second pulse
input comes from the optical encoder mounted on the back of
the MicroMo 1624T0I2S02A 12 V dc micromotor. ‘The cn-
coder generates 15 quasi-sinusoidal pulses for cach revolution
ol the motor shaft coincident with particular shaft azimuth
locations. These pulses are fed to the gating and timing board
as clock pulses, The gating and timing board has a Texas
Instruments SN74HC193N Binary Synchronous 4 Bit Up/Down
Counter that is reset by the index pulse and then counts the
encoder pulses generating lour-bit binary words indicating filter
wheel position. The gating and timing board also has a bank of
lwo TI SN7411C85N 4 Bil Magnitude Compacitors, one for
cach channel, that compares this position word to a DIP switch-
selectable preset word. When the two words match for a par-
ticular channel, the respective four-hit magnitude comparator
triggers the associated adjustable-width Harris CD74HC221E,
One Shot Multi vibrator, also in a bank of two, The One Shot
Multi vibrator signal is used to open (hat channel's DG201 an-
alog switch on the main analog processor board for its proper
duration to pass the associated signal pulse. This constitutes the
(wo output signals for the gating and liming hoard mentioned
earlier.

The next cssential auxiliary circuit board is the ther
moclectric cooler controller beard. This circuit maintains the
-40 °C to -60 °C detector temperature by driving the detector's
internal owo-stage thermoelectric cooler. ‘lhe board detives its
input signal from an internally mounted thermistor within the
detector case. This thermistor is connected as a component in
a voltage divider spanning between common and +5 V de pow-
er. This divider generates a signal (hat is inversely proportional
to the temperature of the detector: the lower the temperature, the
higher the signal. This is algebraically differenced against a
preset adjustable set point, When the detector is at the set tem-
perature, the difference signal is vero. This difference signal is
then integrated, inverted, and fed to the cooler driver circuit as
the activation signal for the cooler. When the instrument ini-
tially comes on, the detector temperature is high and the therm-
istor signal is low. The integrator accumulates a large positive
signal driving the cooler, which sends the detector temperature



PRINCIPLE OF OPERATION

The ROM is an open-path optical CH, detector that uses
light reflected from mine surfaces (o measure the CH, con-
centration over the path length, The target range required of an
ROM at this time is 10m, Figure | shows the second:
generation ROM, which has dimensions of 18 cm by 43 cm by
54 cm and weighs tess than 17 kg, The unit has three principal
components: (1}a light source, (2) a collecting system with an
electronic signal-processing unit. and (3) a battery module
Each component is described in detail below.

GENERAL THEORY

‘The ROM's operation is based on infrared light absorption.
As light passes through the atmosphere. certain frequencies of
the light are absorbed by the gases in the atmosphere, These
missing frequencies are specific lor a given gas and correlate lo
particular rotational and vibrational states of the absorbing gas.
These missing frequencies are called spectral absorption lines.
A gas det
called differential absorption.

All of the prototype ROM’s sponsored hy the National Insti-
ute for Occupational Safety and Health (NIOSH) prior to the cur-
rent one have heen based on differential gas absorption. In this
technique, infrared radiation is measured at wo difierent fre-
quency bands—one band coincident with a specific gas absorp-
tion line. the other very close but not coincident with the absorp-
tion line. When the measured gas is introduced into the optical
path of the instrument beuween the light source and the detection
‘optics, the measured band coincident wilh the absorption line will
have reduced amounts of radiation pass from the source to the de
tector, while the other band, not coincident with the absorption
line, will be unaffected. Therefore, a calculation of the ra
the measured signal from the absorption band to the measured
signal from the nonabsorpiion hand will be inversely proportional
1 the concentration of the gas in the optical path measured in
om.
he current model is hased on a revision to this technique
called gas correlation. In this technique, the infrared radiation
is measured in only one band, but it is teusuced in two different
ways. The first way in which it ix measured is after the radia-
tion bay passed through a sample of the gas under test al a spe-
cific concentration, The second way is after the radiation has
passed through an incrl sample of gas contained in the same
manner as the gas under test, The radiation signal passing
through the gas under test will have much of the light in the
hand of interest removed by passing through the test sample.
‘This significantly blocks the capability for further tight ab-
sorption for the associated measurement. When the measured
gas 18 introduced into the optical path between the fight source
and the detector, the measurement made via the inert sample
will be reduced more significantly than that made via the gas-
under-lest sample because of the reduction in the wbsorption
capability for the latter measurement. Therefore, the calculation

tion method that is based on this phenomenon is

of

Figure t.—Second-generation remote optical methanometer
(ROM).
of the ratio of the measured signals from both measurement
conditions will again be inverscly proportional to the concen-
tration of gas in the optical path measured in percent meters.
‘The gas correlation method has (wo major advantages over
the differential absorption method of gas detection. First, it is
very specific to the particular gas under test. There is no cross
measurement from other gases of similar characteristics. Tt was
noted that ethane, a minute component of natural gas, created
a significant interference with CH, measurements when Lesting
Previous prototypes. Second, the gas correlation methed iso-
lates a particular radiation band of interest in which all of the
measurements are made, ‘This compensates for any changes in
reflectivity or radiation transmission strength that may occur
between different radiation bands.

OPTICAL

The ROM is cquipped with a 90-W axial filament quartz-
halogen incandescent lamp as the light source. The lamp is
mounted in its own housing, which is attached to the front top
of the unit, The lump tousing contains a highly reflective para-
bolic mitror (manufactured by Melles Griot) to concentrate the
light into a parallel beam with a divergence of Jess than 0.50 m
per 10.67 m of path length. A sapphire window is used on the
tront of the lamp housing to protect the bulb and prevent spec-
tral loss through the window. The source beam is projected for-
ward from dhe instrument toward a mine surface within an entry
under test.

The reflected light from the mine surface is collected by a
Fresnel lens with both a focal length and diameter of lens of
18 cm that focuses the light onto the detector, The reflected
light passes through two absorption cells that are mounted on a
filter wheel attached to the detector housing 3 to 5 em in lront
of the detector. The filter wheel, rotating at 4,400 rpm, is driv-
enby a 12 V de micromotor and alternately passes the reflected



we

INTRODUCTION

Methane gas (CH,) is a mining hazard that requires constant
measurement and control. A natural byproduct of coal that is
released during mining. CH, is explosive when mixed wilh air
at concentrations from 5 to 15 vol pet; it is most explosive
when occurring at the stoichiometric concentration, 9.8 pet.
CH,-air explosions can initiate more devastating enal dust ex-
plosions as the thermal ignition and aerodynamic source.
Cognizant of this, Federal regulations [30 CER® 75.362 (1996)]
require on-shift examination at 20-min intervals. or more fre-
quently. during operation of equipment in the workplace. As
an added precaution against mine explosions, CH, monitors are
required to be installed on mining machines used to extract or
load coal, and the monitor must give a warning at | pet CH,
and deenergize the machine on which it is mounted when the
CH, concentration reaches 2 pel [30 CIR 75.342 (1996)]
Because the molecular weight of CH, is only slightly greater
than one-half that of air, it can form a roof layer in a mine and,
through transport by the entry airflow, quickly extend its zone
of influence to mine sections that are relatively free of CH,
sources. Ji may, in this mode, uct as a path for igniuen propa-
gation to a region containing coal dust that is not properly
protected by rock dusting.

Two measures exist to deal with CH, in a mine, One is 10
deplete the coal seam of as much CIL as possible through CH,
drainage techniques, However, this method is not in itself ade-
quate to contro! CH, ina mine, This method is dependent
upon the coal having an adequately high permeability for dif-
ferential pressure methods to overcome the coal’s aerodynamic
resistance (0 CH, outgassing, CH, drainage applications repre-
sent a major prevention elfort. However, the elficiency of this
technique is never adequate to guarantee that CH, concentra-
ions will be below the explosive limit,

A second measure is to dilute the CHL-air concentrations
and simultancously remove the CH, from the mine airways us-
ing adequate ventilation airflow, ‘This approach can be im-
proved through the application of a predictive ventilation com-
puter program [Edwards and Li 1984] to the mine network,
“Phis program determines the expected CH, concentration based
on changes in ventilation with known measured CH, concen-
trations for a specified ventilation plan. Even though sufficient
ventilating air may be present to reduce the average CH, Wo
concentrations well below the explosive fevel, CH, could still
accumulate within certain areas, such as dead-ended Face areas.
If the latter should occur, line bratlices or other ventilation
devices would be required.

XCde uf Federuh Regutations. Sco CFR in soferonves

A significant factor in both of these measures for the control
of CH, is detection through frequent, and often continuous,
monitoring of mine air. The carlicst CH, detector was the
flame safety lamp. The flame salety lamp was a gas (carbide)
Tamp with a flame calibration seale on the shield and was used
by a fire boss while making preshift inspections. The lamp
used calcium carbide immersed in water to produce acetylene
gas thal was used for fucl for the flame. An increase in the
flame size indicated (he presence of a combustible gas; a re-
duction indicated oxygen depletion (blackdamp). The flame
salety lamp was reliable and durable, but was only an indicator
of CH, and not 2 quantitative measurement of CH, concen-
tration. Modern CH, sensors are based on thermal, clectro-
chemical, or optical technologies [McPherson 1992|. The
standard usage of each method is for a point measurement ol
CH, With the increase in deep-cut mining, it is expected that
there will bo a greater need for sulely monitoring face arcas in
excess of 10 m as they are being develuped. One obvious can-
didale for safe, mobile detection is a remote optical instrument
that analyzes the reflected light from coal mine surfaces while
the operator remains under permanently supported roof.

‘Westinghouse Electric Corp. developed under contract with
the former U.S. Burcau of Mines a remote optical metha-
nometer (ROM), which was previously ovaluated {Franks et al.
1986]. This first-generation ROM measured the differential
absorption of CH, with two band-pass filters, one centered at
3.31 ym (an absorption band for CH,) and the other centered
at 3.45 jim (slightly outside the CH, absorption band). ‘The in-
strument responded inadequately to CH, in controlled experi-
ments. Based on these results. a second-gencration ROM was
developed by Westinghouse that incorporated two major
changes: (1) the use of a continuous light source instead of
pulsed flashlamp and (2) the use of a single infrared detector
instead of two detectors. Both improved design features were
incorporated into this new prototype. Recently, « major change
was incorporated into the instrament that cnables the detector
to evaluate the reflected light at the isolated 3.31-4m line. This
involyes the replacement of the two band-pass filters with a
single band-pass filter and (wo gas correlation cells. This tech-
nique provides a more accurate determination of the light
attenuation due to CH, absorption along the optical path be-
tween the detector and the reflective coal surface. The two gas
correlativa cells also make the measurement specific to CH,

‘This report deserihes the theory of operation of this second-
generation ROM and initial testing of the new prototype.



Notes
1 Battery power feeds source lar dect'y
2. Bottary pack is malecna Yo the instrument
‘ond not shown the chogram

a

Front ponel display
‘ond controls

sone soon

eens Hagen
pared

Lamp
Rousing,

Fstote

romics cord cage

Wain Tipiay aia Baas ~~ |

==]

EGO 6 udson

PAS

1

i

H
i

Bekce enw ain ompttier board

|
|
|
|
|
|

“Trermosiecwic cooler
‘commie board

Power supey boord

Figure 2.—Block diagram of remote optical methanometer (ROM).

above, In the AUTOMATIC mode, the AGC switches to a
fixed denominator level from the AGC feedback circuit when
the integrator level gocs outside a predetermined signal OK
level (front panel indicator). Ttshould be noted that the AGC
has a fixed dynamic range thal cannot compensate for gross loss
of signal.

“The oulput of the AGC is fed through another de clansping
cireuit to ensure that zero reference is maintained and into the
analog switch bank, The analog switch is constructed of a
DG201 Quad CMOS single-poll-single-throw (SPST) analog
switch module (a solid-state relay module), Two of the four
relay switches «re used. The sigaal from the de clamping cir
cuit at the output of the AGC is routed Lo the throw arm input
(usually the source input to an KET) of the two relays used
The contact side of the switch serves us the output. creating
the (wo channels of the insirument as described above, the ref-
crence channel and the signal channel, The relay actuator in-
put to the two switches (the FET gate input) is fed by signals
from the gating and timing board, which is described later.
The galing and timing board tukes triggers from the filter
wheel and the filler wheel motor and generates the gating
pulses for the analog switch bank at the correct time and
duration to permit the complete absorption cell signal pulse to
pass into the relevant channel integrator.

The Iwo individual signals pass to the balance amplifier
bank. The reference channel signal passes through a unity gain
verting amplificr, while the signa) channel signal passes
through a variable gain inverting amplifier with a maximum
gain of two, This amplifier’s gain is controlled with a front pan-
el balance knob. This permits the instrument to be balanced for
a vero CH, reading in @ clean atmosphere.

Both signals from the two channels now pass into the in-
tegrator bank. This stage is made up of two identical integra-
fors, one for each channel. The integrators are composed of
practical op amp integrator circuits that use a paralle] RC feed-
back circuit to climinale offset voltage buildup on the integra
tor, The cutover frequency of the practical integrator circuit,
f, + I/2mR,Cp. is used to himit the integration period, The in-
tegrator circuit will not function as an integrator for frequencies
less than this cutover frequency or, inversely, for time periods
greater than the time constant, t, RC. of the feedback circuit.
“‘{he integration period is controlled by a front panel control and
sets 7, ° 0.3, 1.0, or 3.0's. ‘The output of each of these two inte-
grators is ade signal that is the accumulation of the gated detee-
tor returns (only as the respective absorption cell passes the
detector) for the respective channel for a period of t,. These
two signals are the main outputs for the main analog processor
board and are two of the four signals acquired «luring the
libration experiments.



REPORT OF INVESTIGATIONS/1997

RI 9640

A Second-Generation Remote Optical Methanometer

Cn
U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
Public Health Service
Centers for Disease Control and Prevention
National Institute for Occupational Safety and Health on TOR RCASE G 4
STEN Fon Dieta omer

%
count
a

Je BEALI
of é
&


CALIBRATION RESULTS

A series of 12 experiments (A through L} was conducted in
the calibration tunnel. Each experiment used on the average
four different CH, concentrations. Table 1 fists the detector-to-
reflector length lor cach experiment, as well as the maximum
CH, pets for the experiment.

Table 1.—Experlmental conditions

Tunnel Maxinum

Experiment length m CH, peiom
A. 3.0 0.79
B.. 3.0 0.80
Cc. 57 142
Do. 61 1.21
e. 61 144
FL 6.4 141
G. 84 2.37
H 8S 1.98
lee BS 223
J. 88 2.32
K 116 2.87
L 16 4.9%

The measurable quantity of interes! is the ratio as expressed
by equation 2, However, to account for different initial values
of the integrated signal and reference chanocl voltages, Vg and
Vos their values are normalized by the ROM famp-on and lamp-
oll condition. ‘The new definition of RATIO is given by

RALIO- 10} 1

where

and

on

We7 Ve) |:

ON yy OF
(Yr 7 Veo)

An explicit applivation of equation 3 is shown in figure 4 for
experiment F as the calculated ROM ratie. All measured data
were mathematically smoothed to elimimate detector noise, The
sharp peaks in CH, as measured by the infrared analyzer during
injection of the bottled gas correspond to sampling al the end of
the tunnel where CH, is injected. Following the CH, injection,

the flat profile of the CH, concentration corresponds to mixing
of CH, in the tunnel. It is measured after the mixing fan has
creaicd a homogeneous mixture after the injection procedure,
As figure 4 shows, the value of RATIO is a series of plateaus
corresponding to the time increments over which the CH, is
well mixed. The valuc of RATIO increases with increasing CH,
concentration for a fixed tunnel length or, equivalently, for
increasing CH, petsm. Also shown in figure 4 is the measured
unnormalized ratio, which is a direct output from the ROM.
The reduction in the initial peak value of the RATIO after the
CH, is mixed is attributed to dilution of the CH, in the optical
path during mixing. Because the CH, is injected into the optical
path, higher readings were observed until the mixture became
homogencously mixed. This implies that once mixing is con
plete, CH, petem values obtained from the optical path will be
reduced because the CH, was distributed over the enure volume
of the tunnel, reducing the actual concentration in the optical
path, The accumulated data from the 12 experiments are shown
in figure 5. As expected. the RATIO increases as the CH,
pelem increases.

30 T 1 T 0.5
25+
04

20b

2 0.3 5
i 1.6b &
= =
Zrof 025
&

o5r — Normalized ROM ratic O14

=== Unnormalized ROM ratio
1A analyzer outpLt (pet CHat

2

1 L L n
1,800 2.400 3,000 3,600 4,200 800 B00”
TIME, s

Figure 4.-Measured CH, concentration, ROM output signal
RATIO, and calculated ROM RATIO.

12 T T T T T T T

Key .
10r + Experimental al
——Predicted .
De . . 4
2
2 6b 4
= 6
@
ab J
2 4

L L L 1 L
© 05 10 15 20 25 30 35 40 46
CHg pete

Figure 5.—-RATIO versus pet CH,em for 12 experiments.


downward toward the sel point. When the set point is reached,
the thermistor signal rises and balances the negative set point.
‘The integrator accumulation drops 10 7ero, and the cooler current
drops back to a low maintenance value. ‘The detector temperature
will float up and down around the sct point as heat is gained by
the detector enclosure from the surrounding environment and the
controller intermittently produces more drive signal to the cooler
to compensate. It should be noted that auxiliary thermoelectric
cooling experiments are underway to provide active cooling to the
heat sink assembly in which the dotecior enclosure is mounted, as
this wea is experiencing a heat buildup that is interfering, with the
detcetor operation after prolonged use of the instrument,

‘The final essential auxiliary circuit ts the power supply board
and associated batlery assembly. ‘The instrument power is de~
rived Irom two 6-V Eagle-Picher CEM6V10 10-AsH¢e hattcries
comprising the unit's 12-V battery bank. This 12-V supply is

used (o directly operate the 100-W quartz halogen lamp source,
power the 12-V micromotor after passing through a noise filter
1o eliminate feedback motor noise, and as input to the power
supply board, Each hattery is also switch-selecable to drive the
detector's two-stage thermoclectric cooler driver circuit. On the
power supply board, the 12-V power is connected as the source
power for an Analog Devices AD-953 DC to DC Converter.
The output of this module is +15 V, -15 V, and common to
power the instrument's clectronics, This power is again filtered
al the power supply board prior to its use for the electronics.
There are also taps taken for regulated +12 V, -12 V, and +5 V
by using National Semiconductor LM78L12AC, LM79L12AC.
and LM78L05AC regulators off of the two 15-V leads. The
lwo regulated 12-V taps are used to power the Judson PA-5S
preamp. The regulated +5-V tap is used to operate the logic
ciscuits on the display and alarm board

- CALIBRATION

CALIBRATION TUNNEL

The ROM does not measure a specific pel CH, at a point, but
rather the integrated average of the CI, concentration over the
path length. This value can be related to conditions thal favor a
CH,-air ignition that can result in a transition to a coal dust. cxplo-
sion, There is a minimum volume of CH, required for this event

This idea of CH, petem over an optical path led to the de-
velopment of a calibration method for evaluating the insirument’s
reference and signal channel ratio for a specific CH, petem u
al least two different path lengths and CH, concentrations. To
accomplish this, a calibration tunnel was constructed. With the
use of 3-m-long yegments af 0.4-m-diam mine ventilation tubing,
the length of the tunnel could he varied by adding or removing
scpments of the tubing. Atone end of the tunnel. there was a gas
mixing fan and at the opposite a target painted with black paint to
simulate a coal surlace. The light from the ROM, located near the
fan, was directed at the target. Leakuge of the calibration gas
mixture at the segment joints was prevented by wrapping
houschold-grade plastic wrap around the tubing. A controlled
volume of 4.5 pet CH, in nitrogen was passed from u cylinder
through a plastic wube into the tunnel, Nitrogen was used for
salety considerations. After the appropriale volume of CH, meas-
ured by a gas volume meter was led into the tunnel, a solenoid
check valve, which was open during the CH, introduction 0
maintain one atmosphere pressure. was closed, At this point, the
mixing fan was slurted, To promote rapid mixing, a flexible 15-
cm-diam bypass tube was connected to the tunnel behind the fan.
and at the opposite end near the target. Sample tubing was
connected to cach end of the tunnel and, through a sequence of
valves, gas from the tunnel was fed through an inirured CH,
analyzer. When the same CH, concentration was measured al
cach ond of the tunnel. mixing was considered to be complete and
the blower fan was shut off, The ROM lamp was turned on, and

the ROM reference, signal, and ratio data, as well as the infrared
CH, analyzer dala, were acquired with an analog-to-digital data
acquisition system. Afier sufficient data were acquired, the above
procedure for adding CH, to the tannel was repeated and the
measurements made for a new CH, pets value. When sufficient
data had been acquired for a fixed tunnel length, which generally
was three CH, concentration values. the tunnel was purged of CH,
and another 3-m segment was added to the tunnel, The entire
calibration process was then repeated, The maximum CH, con-
centration in the tunnel never reached | pet of actual gas con-
cenlration as measured by the pump-driven infrared CH,
analyzer. The petem value, which is a calculated product of con-
centration and path length and is the instrument's ineasured value,
will he large for small pet CH, concentrations over long path
lengths. Figure 3 shows adjustments being made to the ROM
prior to un experiment,

Figure 3—Adjustments being made to ROM in calibration
tunnel.


light through these two 2.54-cm-diam by 1-cm-long absorption
cells onto the detector (a secondary double convex lens is lo-
cated behind the filter wheel to augment the collection of
reflected light). One cell is filled with pure nitrogen (N,), the
other with pure CH,, Between the filter wheel and the detector
is also mounted a dielectric optical notch filter centered at ap-
proximately 3.31 wm, the center of the principal CH, absorption
line, and with a transmission width equal to that of the absorp-
tion line, These two small metal cylinders with quartz windows
(absorption cells), in conjunction with the optical notch filter,
are the basis of the gas correlation technique described previ-
ously. The detector used is an EG&G Judson 312 series indium
arsenide 2-mm-diam infrared detector.

‘This particular Judson J12 detector is cooled with an internal
two-stage thermoelectric cooler to maintain the surface temper-
ature of the detector between -40 °C and -60 °C. The precise
temperature depends on how well the heat is removed from the
detector enclosure base. The detector has a natural spectral
bandwidth from [.0 to 3.8 um, a responsivity of 1.5 AAV peak-
ing at 3.3 to 3.5 yam, and a D* detcetivity’ of 10 cmeHz!"9W |
The detector acts as « photodiode.

SIGNAL PROCESSING

A block diagram of the ROM is shown in figure 2, The elec
tronies has boards grouped into six functional units. ‘These
electronics boards are: (1) power, (2) thermoelectric cooler,
(3) data display and alarm, (4) gating and timing, (5) analog
processing. and (6) main amplifier.

‘The detected signal first enters the instrument's electronic
subsystem for processing and analysis as an output current from
the Judson J12 detector. The output current from the detector
is then collected by a PA-5 (ransimpedance (current-mode)
preamplifier (also built by G&G Judson) with  transim-
pedance gain of 10° and converted into a voltage signal.

Main Amplifier

The output signal from the PA-5 preamplitier is fed through
the main amplifier board. This board provides a maximum
voltage gain of 2.5%10" and amplifies the instrument's retrieved
signal to the 10-V range used for processing. The main am
plifier board contains a four-stage operational amplifier circuit
with low-pass filtering, offset adjustment, and variable gain

“D-Star (D¥) detectivity 36 a relative sensitivity purayneter used (0 compare
performance of different detecior types. D* is the sigal-lenoise rab il
particular electrical frequency and ina |-Hz bundwidih when |W uf radiant
power is meideat on a |-cm" active area detector. The higher the D* value, the
better the detector

[ActiveArea,,,,,]

NEP win “5

Ds,

ran Ue Ws

The low-pass filter reduces wide band random noise. The cir-
cuit uses a Burr Brown OPA-121 op amp in the inverting mode
for each stage. Kiltering is provided by capacitance in the
feedback circuil of the first stage of the amplificr. The main
amplificr board has a direct-current (dc) clamping circuit at the
input to provide zero reference fer the subsequent amplification
process and an optical coupler circuit at the eutput to provide
maximum isolation between this noise-sensitive, high-gain am-
plificr and the subsequent processing circuitry. 7

Main Analog Processor

At the output of the main amplifier, the signal is composed
of a train of 10-V pulses at a frequency of 147 pps that are gen-
erated from the filter wheel, spinning at 4,400 rpm, rapidly
placing cach of the two absorption ceils in front of the detector
{a chopping effect), This train of pulses is then divided into
separate components, one pulse train for each of these two cells,
made up of the pulses associated with that cell. A signal is then
generated for each pulse train that is proportional to the running
amplitude average of that pulse train for a certain sliding time
window (time constant). This is accomplished by the main
processor board. The circuit elements that separate out the
pulses from the N,-lilled absorption cclt are designated the
signal channel; the circuit clements that separate out the pulses
from the CH, filled absorption celll arc designated the reference
channel. Each produces an integrated de output signal that was
measured during calibration experiments that were conducted
in a tunnel described later in this report.

‘The main processor board is a combination of four separate
circuit sections: an automatic gain control (AGC) circuit, an
analog solid-state switch bank, a balancing amplifier bank, and
an integrator bank. The three latter sections are called banks
because they are made up of two components cach, one for each
channci.

The AGC circuit is composed of a Burr Brown DIV 100
divider module that acts to amplify or attenuate the incoming
signal to maintain a constant signal level at the integrators de-
spite Nuctuations in reflected light due to hand motion with the
instrument. This is accomplished by monitoring the amplitude
of the pulse train from the reference channel at the input to its
integrator stage and then generating an AGC signal of proper
polarity and amplitude to compensate for any signal fluctu-
alions. To produce the AGC signal, the pulse train is converted
into a peak-detected de signal, scaled. filtred to reduce ripple,
and inverted to the proper polarity. This signal is then used as
the driver signal for the DIV 100 denominator input. The in-
coming signal channel from the main amplifier board is fed into
the numerator input. The AGC is switch-selectable between
constant OH, constant ON, and AUTOMATIC. In the constant
OFF mode, the denominator input is sot at a fixed de level gen-
erated by a voltage divider, In the constant ON mode, the
denominator is locked to the AGC feedhack circuit described



UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

Adir — ampere-hour petm percent meter
AW ampere per watt pps pulse pet second
om centimeter rpm. revolution per minute
em square centimeter 5 second

t frequency in hertz v volt

He hertz Vde volt, direct current
ky kilogram vol pet volume percent

m meter Ww watt

min minute pnt micrometer

mm millimeter °C degree Celsius

pet percent

OTHER ABBREVIATIONS AND ACRONYMS USED IN THIS REPORT

AGC automatic gain control opamp — operational amplifier

Cc concentration Quad quadruple

CH, methane RC resistive-capacilive

CMOS complementary metal- ROM remote optical methanometer

oxide semiconductor
SPST _ single-poll single-throw

de direct current
a absorplion coefficient
DIP Dual In-Line Pack
7 constant pi (3.14159)
FET ficld cffect transistor
v time constant
Te integrated circuit
wy Spectroscopic name for the
IR infrared fundamental absorption band
caused by the vibrational-
LCD liquid crystal display rolational transition of the
methane molecule
N, molecular nitrogen
t length
NEP noise effective power

Mention of any company name or product docs not constitute endorsement by the National Institute for Occupational Safety and
Health.

|-NIOSH
tral

Lo receive additional information about mining issues or other occupational safety and health problems, call 1-80-35
(1-800-356-4674), or visit the NIOSH Home Page on the World Wide Web at hup://www.cde.gov/niosh/nomopag!



CONTENTS

Page

Abstract.
Tatroduction .
Principle of operation .
General theory ... .
Optical
Signal processing . .
Main amplifier ......
Main analog processor .
Data display and alarm .
Auxiliary circuit boards
Calibration
Calibration tunnel
Calibration results .

Conclusion ..
Acknowledgment .
References

COSC WMIYNADAARHRAWWUNH

ILLUSTRATIONS

Sccond-generation remote optical methanometer (ROM)
Block diagram of remote optical methanometer (ROM)
Adjustments being made to ROM in calibration tunnel
Measured CH, concentration, ROM output signal RATIO. and calculated ROM RATIO
RATIO versus pet CH,*m for 12 experiments ..--.. 0.06.5. sees

ww

exer

ws
00 oc “I

1. Experimental conditions ... bcveveeees bob eet eee eee ees ve bbe eet ee treet eseensenene 8


Report of Investigations 9640

A Second-Generation Remote Optical Methanometer

Robert A. Franks, John J. Opferman, Gene F. Friel, and John C. Edwards, Ph.D.

U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES
Public Health Service
Centers for Disease Control and Prevention
National Institute for Occupational Safety and Health
Pittsburgh Research Center
Pittsburgh, PA

May 1997


The ratio of the integrated signal to reference voltage can be
delined in terms of the path length, ¢, and the average CH, con-
centration, C, in the path using an absorption coefficicnt, a, (a
specific characteristic of the detection system) for the CH,

Sas gone, (4)

<

The factor 2 accounts for the absorption ol the infrared
radiation over the distance, £, [tom light source to larget, which
is traversed twice by the light. A regression analysis of the data
in figure 5 yields an absorption coefficient of 0.158 (CH,
petem)", (If experiments [and L are excluded, due to higher
values of the RATIO, or alternatively, lower values of ViVi
the absorption coefficient determined by regression analysis is,
0,082 (CH, petem) (indicating low response).) Pigure 5 also
shows the values of RATIO (solid line) calculaicd from equa-
tions 3 and 4 using @ = 0.158 (CH, peiem)"'.

Several factors account for Ure scatter in (he data in figure 5.
These include (1) alignment of the optics in the ROM,
(2) extraneous radiation that interferes with the infrared
detector, ic., the truc zero of the instrument is changing with

time duc to heating within the field of view of the detector (gray
body emissions), (3) electrical noise [rom circuit components.
(4) inability to maintain adequate cooling of the infrared
detector, and (5) variances in the aligament of the ROM within
the calibration tunnel from test to test. The optical alignment
has some dependence upon distance [rom detector to reflective
surface and must be further investigated. ‘The source of the
extraneous radiation is the detector's case where the internal
thermoelectric cooler deposits its heat. Efforts are being made
to reduce the thermal energy storage in the case back plate and
associated heat sink with the introduction of a conductive plate.
with two external thermoelectric coolers atlached, to the de-
teclor's heat sink. The use of a small fan on the heat sink of
each thermoelectric cooler produced electromagnetic noise that
interfered with the detector signal. Variances in the ROM
alignment within the calibration tunnel are caused by variances
in the angle of reflectance of the famp light. This increases or
decreases the returned light intensity, Decreased light intensity
requires higher electronic gain in the amplifier stages to get an
adcquate signal for the processing electronics. and higher gains
increase the noise interference to the instrument. Even if the
thermal noise source is constant, increasing the detector gain
also influences the rate at which the zero levels increase duc to
the gray body cmissions.

CONCLUSION

A second-generation ROM was developed that is based on the
replacement of the two-Blter differential absorption technique
with a single-filler gas correlation technique. Refinements were
made to the amplifier and gating circuitry within the unit, Cali-
bration tests in a specially constructed tunnel have shown that the
ROM responds to increased CH, petem at distances as great as
11.6 m. The ROM is undergoing modifications to eliminate

signal drift. These modifications are primarily in the design of a
more efficient system for the dissipation of thermal cnergy
generated by cooling of the infrared detector. It is anticipated that
in the {inal version, the ROM will greatly improve miner safety
by detecting hazardous CH, levels in deep cuts while the operator
is under permanently supported rol

ACKNOWLEDGMENT

The authors wish to acknowledge the helpful discussions
held with C.D. Litton of the NIOSH Piusburgh Research

Center with regard to the operational principles and optical
design of the ROM.

REFERENCES

CER. Code of Federal regulations, Washington. DC: US. Goverament
Printing Office. Office of the Federal Register.

Edwards JC. Li IS [1984]. Computer simulation of ventilation in rm
mines. Proceedings of the Third International Mine Ventilation Congress. Har
rugate, Englani, U.K.: Institulion of Mining and Metallusgy/Instiqian of
Mining Engineers, pp 47 51

Franks RA, Sapko MJ, Denes LI. Ryan FM | 1986]. Optical remote meth-
anumeter. Proceedings of the Bighth WVU Mining Flectruiechnology Com-
ference, Morgantown, WY: West Virginia University, pp. 114-18

McPherson MJ [1992]. Subsurface ventilation and enviconmental en
invering. New York, NY: Chapman and Half, pp. 390-397.