140 T T T

2oF * 1
aN

® Xx Recovery time
o
# 100 x 1
=
u x
6 3
a
i Rise time
60 4
40 1 1 1
ie} fe) 20 30 40

TEMPERATURE, °C

Figure 11Effect of temperature on response times with
40 ppm CO test gas.

100 T . T

80
°
oO
£
& 60 |
ui
ao
z
2 40 1
ra
rd KEY
°PT-1
20 aPr2 |
= PT3
) 20 «40~«~OSC«SC«*O

CONCENTRATION, ppm CO

For both the rise time and the recovery time tests, the
response times were well under the 2 min recommended
in reference 11,

LINEARITY TESTS

The three monitors were tested for linearity of response
to gases from 0 to 94 ppm CO. Test gases were supplied
through the calibration fixtures at a flow rate of 100 mL/
min. Figure 12 shows a plot of the data (response versus
gas concentration) used to calculate the linear regression
equation. In the linear equation Y = A + BX, Y is the
monitor response, X is the gas concentration, A is the
Y-axis intercept, and B is the slope. Table 3 gives the es-
timates for A, B, S,, (the standard deviation of the errors
of response from the universal line of regression), and r*
(the coefficient of correlation). The coefficient of cor-
relation is a measure of the amount of variation in the
dependent value (monitor response) that results from
variation in the independent value (gas concentration).

The responses in ppm CO for PT-1, PT-2, and PT-3
were determined from the monitor output voltages correct-
ed for the 0.1-V offset. PT-2’ is the response of PT-2 in
ppm CO, obtained from the display on the Rx. The Rx
was connected to the CO monitor and Tx by a 1.5-m
length of fiber-optic cable. The intercepts, A, were ap-
proximately 1 ppm CO, and the slopes approached unity.

Figure 12.—Linearity of CO monitor response to gas inlet with
calibration feature,

The 1? values indicated that more than 99.9% of the vari-
ance in all of the monitor responses resulted from changes
in the test gas concentrations.

Table 3.—Regression statistics

Y A B Sx Pe

PT-1.... 1.0959 0.9926 0.4476 0.9998
Pr2. 1.1101 9938-7058 9996
PT-2’1.. 9382 9989 6694-9907
PT-3.... 1.1257 9967 8245 9995
‘A Intercept.
B Slope.

? Coefficient of correlation.

S,, Estimate of standard deviation of
the error of response from the universal line of
regression.

Y Monitor label.

1PT-2" is response of PT-2 in ppm CO ob-
tained from the display on the receiver.

These linearity tests using the calibration fixture were
tun with the case doors open, using electrical leads to
connect to the voltmeter to measure response. Other tests
were run in the environmental chamber without calibration
fixtures with the monitor case doors both closed and open.
Figures 13 and 14 give the responses of monitors PT-1 and
PT-3, With the case doors closed, as they would be in
mine use, the responses are linear. A referee CO monitor
inside the chamber measured the test gas concentrations.


UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT

°C degree Celsius BA microampere

dB decibel BA/ppm microampere per part per million
dB/km —_—_decibel per kilometer min minute

dBm decibel referenced to 1 milliwatt mL/min milliliter per minute
fpm foot per minute mm millimeter

ft foot um micrometer

h hour mm Hg millimeter of mercury
Hz hertz ms millisecond

in inch om nanometer

kHz kilohertz ppm part per million

kn kilometer s second

m meter v volt

mA milliampere



SUMMARY

© The Bureau has designed, fabricated, and tested a
fiber-optic data telemetry system that can transmit a 0- to
1-V analog signal from a mine sensor via fiber-optic cable
to a remote receiver,

¢ A battery-powered CO monitor with range of 0 to
100 ppm CO has been integrated with the fiber-optic
telemetry system for use as an intrinsically safe CO moni-
toring system.

© Characteristics of the Giner, Inc., CO monitor have
been measured, including response times, linearity, and the
effects of temperature and pressure on monitor response.

The results of these tests demonstrate that the CO moni-
tor would perform as well as or better than other commer-
cial membrane or capillary CO sensors.

¢ The Bureav’s fiber-optic telemetry system will oper-
ate with any type of sensor that has an output signal of 0
to 1 V (or preferably 0.1 to 1.1 V). Such a system would
be useful for measurement of many parameters, such
as temperature, pressure, airflow, or the concentration of
other gases. The system also could be used with binary
output devices, such as conveyor belt or electrical power
on-off indicators.

REFERENCES

1. US. Code of Federal Regulations. Title 30—Mineral Resources;
Chapter I—Mine Safety and Health Administration, Department of La-
bor; Subchapter O~Coal Mine Safety and Health; Part 75—Mandatory
Safety Standards—Underground Coal Mines; Section 75.11034—
Automatic Fire Safety and Warning Device Systems; Installation; Mini-
mum Requirements; July 1, 1989,

2. ___. Section 75.326~Aircourses and Belt Haulage Entries; July
1, 1989,

3. Mine Regulation Reporter. Hearing Set for Ventilation Regula-
tions for Underground Coal Mines. Mar. 30, 1990, p. 136.

4. US. Code of Federal Regulations, Title 30—Mineral Resources;
Chapter I—Mine Safety and Health Administration, Department of La-
bor; Subchapter O—Coal Mine Safety and Health; Part 75—Mandatory
Safety Standards—Underground Coal Mines; Section 75.301-2—Harmful
Quantities of Noxious Gases; July 1, 1989.

5. Stricklin, K.G, Current Usage of Continuous Monitoring Systems
in Coal. Min, Eng, v. 39, Sept. 1987, pp. 859-863.

6. Holtzberg, J.T. Carbon Monoxide Detectors Revolutionize Fire
Detection, Coal Min, and Process., v. 18, Apr. 1981, pp. 118-120.

7. Oswin, H. G., and K. F. Blurton, Electrochemical Detection Cell.
US. Pat. 3,776,832, Dec. 4, 1973.

8 Tantram, A. D. S. Gas Sensor Electro Chemical. U.S. Pat.
4,324,632, Apr. 13, 1982.

9. Kosek, J. A, J. P. Giordano, and A.B, LaConti, Development of
SPE-Diffusion Head Instrumentation. Paper in Proceedings of National
Symposium on Recent Advances in Pollution Monitoring of Ambient Air
and Stationary Sources. EPA 600/9-85-007, 1983, p. 333.

10. Giner, Inc. (Waltham, MA). Operation and Maintenance Instruc-
tions, Carbon Monoxide Monitor Model WMCO100, 1991, 10 pp.

11. Chilton, J. B,, and A. F. Cohen. Interim Performance Speci-
fications for Transducer Modules Used With the Bureau of Mines In-
trinsically Safe Monitoring System. Carbon Monoxide, Methane, and
Air Velocity. BuMines IC 8943, 1983, 20 pp.

12. Carpenter, C. R,, J. E. Chilton, and G. H. Schnakenberg, Jr. A
Dynamic Gas-Mixing System. BuMines IC 8934, 1983, 30 pp.

INT.BU.OF MINES,PGH.,PA 29589


115 Vac

60 Hz - A Receiver _.
- oo - -—
= = Surface
FRA DPON AD
‘Was Fiber- y°/S,

soot

cable

co
monitor-
transmitter

Mine

i—optic <<

Figure 3.-CO mine monitor telemetry system using fiber-optic cable.

Gate
generator
50 ms

ST
nn
ile i Opto- ~~, Gate
receiver Butter generator
Fiber- ~~ 90s
optic
cable cae
generator Gate
100 ms
vt
5V
oD Power LI Pulse width
set
supply
15 Vac i 7
60 Hz - isplay
v ppm CO

Figure 4.—Fiber-optic recelver diagram.

Alarm


INTRODUCTION

Carbon monoxide (CO), a toxic and odorless gas, is
present in many underground mines, Sources include die-
sel exhaust; explosives fumes; fires involving coal, wood,
brattice cloth, or conveyor belting; and air oxidation of
pyrophoric coals. The ambient concentrations in coal
mines in Pennsylvania and West Virginia are 0 to 4 ppm
CO and for some coal mines in Illinois and Colorado are
12 ppm CO.

Often, CO monitors are used in underground coal
mines for fire detection on conveyor belt entries (1)?
More than 112 coal mines have installed CO monitoring
systems to test the air traveling in belt haulage entries to
ensure that it can be safely used to ventilate the active
workings (2). In these instances Mine Safety and Health
Administration (MSHA) guidelines require these systems
to have CO monitors at the beginning and end of each
belt flight and allow a maximum of 2,000 ft between
additional monitors, The monitors are set for alert alarm
at 10 ppm CO above ambient and for full alarm at 15 ppm
CO above ambient. MSHA has now proposed new venti-
lation standards for underground coal mines. These
standards would reduce the spacing intervals for CO
monitors to a maximum of 1,000 ft and set alarm levels to
5 and 10 ppm CO above ambient in belt haulage entries
with air velocities less than 300 fpm (3). CO monitors in
mines must be stable and accurate at least to 1 ppm CO
for effective alarm service.

The CO monitors placed on belt haulage entries must
detect fires but could, in addition, be used to verify the
quality of the belt haulage ventilation air with regards to
CO (4). The average concentration of CO in the belt
haulage ventilation air must be below both the exposure
limits for miners and the lower fire alert value.

Commercial in-mine CO monitors that transmit moni-
toring data by electrical signals over copper wires (5-6)
depend on power from underground electrical sta-
tions. Both four-wire systems—two for power and two for
data transfer—and three-wire systems—one for power and
one for data transmission with one wire common to

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

both—are used, Data transmission is cither analog, using
4- to 20-mA current loops, or digital, using frequency shift
keying. Although the approved CO monitors are intrin-
sically safe, the power supplies are not. In the event of a
fire or explosion, all mine power may be disconnected,
Without electricity, the monitors operate on standby
batteries for up to 4h, but after the batteries discharge,
the gas monitors are inoperative. Thus, in mine emer-
gencies when information on mine gas concentrations
could help determine fire status and location, no
information would be available. As part of its goal to im-
prove the health and safety of underground coal mining,
the U.S. Bureau of Mines is working to create improved
CO monitoring systems.

There are several properties of fiber-optic cables that
make them advantageous for use in mines: The fiber is
electrically nonconductive and thus, cannot ignite methane
by sparking. Silica fiber is immune to radio frequency or
electromagnetic pulse interference from electrical sources.
The cable is lightweight, and communication-grade fiber is
inexpensive compared with copper wire.

Several researchers have announced the development of
prototype fiber-optic methane sensors based on the dif-
ferential absorption of infrared energy. Silica fiber has
excellent light transmission characteristics at wavelengths
from 850 to 1,800 nm. For example, the 100-ym-core-
diameter silica fiber used in our tests has a light loss of
only 2.5 dB/km at 1,550 nm wavelength. Methane, ab-
sorbing at 1,668 nm wavelength, is thus a prime candidate
for measurement by differential absorption of infrared
light transmitted by silica optical fibers. CO, however,
has no strong absorption bands in the silica transmission
range. It’s strongest absorption is at 4,700 nm wavelength,
which is above the infrared transmission range for silica.
Therefore, direct estimation of CO by ordinary light ab-
sorption measurements is not possible with silica fiber-
optic cable. Other schemes can be devised for measure-
ment of CO, such as solid-state oxidation or chemical
reaction with color change. However, only the electro-
chemical CO sensor has the required properties of sensi-
tivity, reversibility, and stability necessary for a mineworthy
CO monitor.


HYBRID FIBER-OPTIC ELECTROCHEMICAL CARBON
MONOXIDE MONITOR

By J. E. Chilton’ and C. R. Carpenter?

ABSTRACT

The U.S. Bureau of Mines has developed an intrinsically safe carbon monoxide (CO) monitoring
system for mines by coupling a fiber-optic data telemetry (FODT) system with a prototype electro-
chemical CO monitor. The CO mofitoring system can be used in a coal mine as one component of an
early fire warning system. This system may be used in belt haulage entries when the entries are used
for supplying ventilation air to working places, The FODT accepts a 0.1- to 1.1-V analog signal from
the monitor and converts it to pulses (0 to 5 V) at frequencies from 1.1 to 11 kHz that drive a light-
emitting diode (LED). An electrically nonconducting fiber-optic cable carries the LED signal to a
remote receiver. The receiver analyzes the incoming light signal and displays the measured CO con-
centration in parts per million. This report discusses the performance of the FODT system and char-
acteristics of the CO monitor such as response times, linearity, and temperature and pressure effects,

Research chemist.
*Electronics technician.
US. Bureau of Mines, Pittsburgh Research Center, Pittsburgh, PA.


maritor |, tae
- frequenc’
WMCO100 converter”
. Gate
yimer generator
127 ms

ST
connector

{==
Fiber-
optic
cable

Opto-

Gate
coupler

V+

4C cells

On-off
switch
v-

Figure 2.-CO monitor-optical transmitter diagram.

voltage limiter circuit. The voltage limiter prevents over-
range data signals (as high as 2.5 V) from exceeding the
maximum allowable input signal of 1.5 V to the V/F con-
verter. The output signal from the V/F converter is a 1-
to 11-kHz square wave, swinging from 0 to 5 V. The Tx
board has a 2-min timer that sets the output repetition
rate. The 2-min interval is about twice the response time
of the CO monitors, The timer initiates a 127-ms gating
signal. This signal and the output from the V/F converter
are the inputs to the gate circuit, The output from the
gate is a 127-ms burst of 1- to 11-kHz pulses occurring
every 2 min. This burst of pulses drives the optocoupler
containing an 850-ym-wavelength light-emitting diode
(LED), which converts the electrical energy to light.
ST-type connectors were used on 100/140 pm‘ silica fiber-
optic cables to connect the Tx to a remotely located re-
ceiver (Rx). The enclosure containing the CO monitor
and Tx would be placed at a given location in the mine
(fig. 3). The Rx unit would be located and electrically
powered on the surface.

For safety, the CO monitor and the Tx were designed
to work from low-voltage sources. Such a design helps to
minimize the potential for methane ignition in case of
electrical or mechanical failure. However, MSHA must

“This designation refers to 100-um-core-diameter fiber with 140-
um-diameter cladding,

evaluate this system before it can be used in areas where
permissible equipment is needed.

The Tx and Rx are configured with a CO sensor for
this application. It is apparent, however, that the Tx and
Rx units could be attached to any sensor with a 0- to 1-V
output. Thus mine sensors, such as temperature, pressure,
airflow, oxygen or methane gases, or binary switches, such
as belt haulage on-off switches, could be used with these
fiber-optic-connected Tx and Rx units.

FODT RECEIVER

The Rx (fig. 4) detects the light pulses from the fiber-
optic cable with an optoreceiver. The pulses are counted
and displayed as ppm CO. The Rx unit accurately dis-
plays CO concentrations from 0 to 100 ppm CO. Above
100 ppm, the indication is nonlinear, showing 150 ppm CO
for atmospheres containing 250 ppm CO or greater con-
centrations. Below 0 ppm CO (voltages below 0.1 V), the
Rx shows a numerical value of 999.9 or less. Audible and
visible alarms indicate loss of light signal, underranging,
and overranging conditions. The Rx is powered by 115-V
ac 60-Hz mains that are separate from the mine’s under-
ground power system. Thus, the CO monitor will continue
to operate after a mine emergency or fire for at least
1 month using the built-in batteries.


RI} 9423 REPORT OF INVESTIGATIONS/1992

Hybrid Fiber-Optic Electrochemical
Carbon Monoxide Monitor

By J. E. Chilton and C. R. Carpenter

UNITED STATES DEPARTMENT OF THE INTERIOR

BUREAU OF MINES


cers

MONITOR

ARBON MONOXIDE

=
en
TA aee een eee

CO monitor with transmitter.

Inc.,

—Giner,

Figure 6.

100

807

g @

09 widd ‘asNods3y

Porous paper
disk

20

60 80 100 120

TIME, s

20

Modified calibration fixture

Figure 8,—Etfect of gas flows with 82 ppm CO test gas on CO

monitor response,

Figure 7.—Callbration fixture for CO monitor depicting plastic

holder and metal inlet tube.


TESTS OF MONITOR-FODT SYSTEM

Bureau researchers tested the CO monitor-FODT sys-
tem with 400 m of optical fiber cable and four ST-type
connectors joining the Tx and Rx sections. The system
gave equal response as when connected to 1 m of fiber-
optic cable. An optical average power meter measured a
loss of 9 dB light power through the 400 m of fiber-optic
cables with four ST-type connectors. The system uses a
Tx LED yielding -17 dBm light power and an Rx sensitive
down to -32 dBm. These test data are used to calculate
that a maximum distance of 2.5 km between Tx and Rx
units is possible using the 100/140 ym silica optical fiber
with a stated loss of 4.5 dB/km (at 850 nm wavelength).

In addition, underground tests were conducted by plac-
ing the CO monitor and Tx unit in a drift in the Bureau’s
experimental coal mine. The Rx unit was in the mine
supervisor’s office, 200 m from the Tx unit. Two miners
secured the optical fiber cable to the roof and ribs with
wire loops placed every 2 m. The lightweight fiber-optic
cable was easy to deploy and recover. Applying a cali-
bration standard test gas of 60 ppm CO from a portable
gas cylinder to the sensor produced a reading of 57.9 ppm
CO back at the Rx. In laboratory tests, this standard gas
produced an indication of 55 ppm CO.

CARBON MONOXIDE MONITOR RESPONSE TESTS

CO monitors used in mines have many performance
requirements. Accuracy must be within 1 ppm CO at
10 ppm CO. Drift must be minimal so calibration is nec-
essary only every 30 days. Response times must be under
2 min. Low electrical power usage, minimum interference
from other mine gases, and minimum effects of tempera-
ture and pressure changes are all required or recommend-
ed features. Many of these requirements are described in
another publication (21).

The Bureau tested the performance of three Giner,
Inc., model WMCO100 CO monitors (fig. 5). Figure 6
shows a monitor with the fiber-optic Tx. These three
monitors, labeled PT-1, PT-2, and PT-3, were tested ini-
tially using the manufacturer-supplied calibration fixtures
to apply the test gases. The calibration fixtures (figure 7-
upper) were 15/16-in-diameter, Lexan‘ plastic holders
threaded to fit the gas diffusion port at the bottom of the
case. The test gas flowed into the fixture through a 1/16-
in-diameter tube. A 1/16-in hole was drilled into the fix-
ture for a gas exit. The modified calibration fixture
(figure 7-lower) was designed to give minimum test gas
pressure but was not used in the tests. During the first
tests, the case doors were open to accommodate electrical
leads connecting the monitor to a digital multimeter.
Another set of tests was run in a controlled-temperature
chamber with the case closed. Small-diameter wires
running through the door gaskets served as electrical leads
to the multimeter.

RESPONSE TESTS

Tests were conducted to examine the effect of sample
gas flow on the response of the CO monitors. Pressurized

SReference to specific products does not imply endorsement by the
US. Bureau of Mines,

cylinders supplied test gases, and mass flow controllers set
the flow. The mass flow controllers are part of a dynamic
gas-mixing system designed at the Bureau (12). Response
changes of the monitors were measured as a function of
gas flow. The monitors were initially at equilibrium with
air. At the start of the test, CO was fed by tube into the
calibration fixture. Figure 8 shows the effect of test gas

mena mc ot on

kd
vg arte

GINER, INC.

gy, O. 3 64
ae : i ig
a Scale, in a

Figure 5.—Giner, Ine., CO monitor.


ACKNOWLEDGMENT

The authors thank Robert S. Zirkle, physical science
aid, for conducting the chamber tests of the CO monitors,

carefully recording the data, and performing statistical
analyses of the data.

CARBON MONOXIDE MINE MONITOR

The Bureau has developed a CO monitoring system
that couples a prototype electrochemical CO monitor with
a Bureau-developed fiber-optic data telemetry (FODT)
system. Many publications have described the operation
of electrochemical CO sensors (7-9). The CO is oxidized
at the working electrode or anode to form carbon dioxide.
The counter electrode or cathode reduces oxygen from air.
The current from this cell is proportional to the CO con-
centration. In all these sensors, the CO diffuses to the cell
through a membrane, capillary tube, or diffusion chamber.

ELECTROCHEMICAL SENSOR

For this effort, a CO monitor that has very low elec-
trical power requirements was specifically designed and
fabricated for the Bureau (10). This monitor uses a cell
with an ion-exchange membrane solid electrolyte with a
three-electrode structure (working, counter, and reference
electrodes). The current measured between the working
and counter electrodes is approximately 0.4 »A/ppm CO.
This solid-membrane electrolyte cell was selected for this
application because of its long life (units have operated for
more than 9 years) and its low working current (64 uA at
0 ppm CO).

The Giner, Inc, model WMCO100 CO monitor has a
range of 0 to 100 ppm CO with an output of 0 to 1 V. As

noted, in the electrical circuit (fig. 1), a constant voltage
source sets a fixed voltage between the reference electrode
and the working electrode. The current between the
working electrode and the counter electrode is converted
to a voltage, and amplified to give an output of 1 V for
100 ppm CO. For our application, the voltage was offset
to 0.1 V for 0 ppm CO and 1.1 V for 100 ppm CO. With
this offset, a 0-V output would signify sensor system fail-
ure. The CO monitor uses a 9-V transistor battery, which
powers the sensor for more than 2 months. The sensor
has a reservoir containing water to keep the membrane hy-
drated. Both the sensor battery and the water supply
would have to be checked, and replaced if necessary, dur-
ing each 30-day calibration. This monitor is housed in a
75- by 7.5- by 5-in fiberglass NEMA 4X enclosure. The
enclosure is coated inside with flame-sprayed nickel metal
to protect from both radio frequency and electromagnetic
interference.

FODT TRANSMITTER

The FODT transmitter (Tx) board (fig. 2) is housed
in the same enclosure as the CO monitor. Four C cells
power the Tx board for up to 2 months. The data signal
(0.1 to 1.1 V) from the CO monitor is fed to the voltage-
to-frequency (V/F) converter on the Tx board through a

Current-
Constant .
to-voltage Amplifier Output
voltage converter P
] O11
Zero Gain
a) c set set
Gas v+ -
sensor \" ,
batter:
Voltage = y
regulator On-off
Tl switch

v-

Figure 1—Giner, Inc., CO monitor system diagram. Gas sensor includes reference (R), counter (C),

and working (W) electrodes.


CONTENTS

Page

Abstract .......
Introduction ....
Acknowledgment
Carbon monoxide mine monitor .

Electrochemical sensor .. . .

FODT transmitter .

FODT receiver .
Tests of monitor-FODT system .....
Carbon monoxide monitor response tests .

Response tests . .

Linearity tests ..

Temperature tests .

Pressure tests ........

Drift tests
Summary ..
References ....

BEBE Scannawuvnunre

ILLUSTRATIONS

1, Giner, Inc, CO monitor system diagram ........... .

2. CO monitor-optical transmitter diagram ...........

3. CO mine monitor telemetry system using fiber-optic cable

4, Fiber-optic receiver diagram . .

5. Giner, Inc., CO monitor . :

6. Giner, Inc, CO monitor with transmitter te eeee .

7. Calibration fixture for CO monitor depicting plastic holder and metal inlet ‘tube os

8. Effect of gas flows with 82 ppm CO test gas on CO monitor response .

9. CO monitor Tecovery measurements with zero air..... 0... 0... ee
10. Chamber test of rise and recovery times with 40 ppm CO test gas .....
11. Effect of temperature on response times with 40 ppm CO test gas oe
12, Linearity of CO monitor response to gas inlet with calibration feature ..............05 .
13. Linearity of PT-1 monitor response to gas in chamber with monitor case doors open and closed
14, Linearity of PT-3 monitor response to gas in chamber with monitor case doors open and closed ..
15. Effect of temperature on CO monitor response to 82 ppm CO test gas and zero air .
16. Effect of temperature on CO monitor-transmitter response to 82 ppm CO test gas
17. Effect of temperature on CO monitor-transmitter response to 40 ppm CO test gas
18. Effect of pressure on CO monitor response ........ sa vaeeseceeene beens
19. Effect of pressure on response of different types of CO monitors . .
20. CO monitor drift test ......
21. Drift test environment .

WCMMIVIANUUAYW

. Rise times for each CO monitor
Chamber test response times
Regression statistics

PRP
© © 0



10

TEMPERATURE TESTS

Temperature tests (fig. 15) were run using the calibra-
tion fixtures to supply zero air or calibration gas to each
monitor. Test gases were fed through 0.3 m of tubing to
each monitor in an environmental chamber at a flow rate
of 100 mL/min. The response increased by 6 ppm CO
when the temperature increased from 25° to 40°C. The
response decreased by 10 ppm CO when the temperature
decreased from 25° to 5° C. In many mine applications,

80 T T T

KEY
—— Case closed
— —Case open

RESPONSE, ppm CO

ce) 20 40 60 80
GAS CONCENTRATION, ppm CO

Figure 13.—Linearlty of PT-1 monitor response to gas In cham-

ber with monitor case doors open and closed.

80 T T T
KEY

8 60; ——Case closed 4
Ee ——Case open 7

& /

w 20r Y 7

v7
a
a 1 1 n
ie} 20 40 60 80

GAS CONCENTRATION, ppm CO

the temperature does not change significantly from the
average of 12°C. These monitors could be used unmodi-
fied in such mines. Nonetheless, the manufacturer has
added a temperature compensation circuit to the amplifier
in some monitors to reduce variations in monitor response.

The combined CO monitor-Tx section was tested in
the chamber for temperature effect. The Rx was placed
outside the chamber. Figure 16 shows the same voltage
response for the monitor but a reduced temperature range
for the Tx. The V/F converter in the Tx has a low-
temperature limit stated to be 0° C; however, the com-
ponent operated only at temperatures above 10°C. A

90 T T T yy
Test gas a
82 ppm CO
—_,
80F 4
Q
12)
& of * Key
uw @PT-1
o + aPpT2 0
z = PT-3
2 10 L 4
n
Ww
« Zero air
ee ee
-10 L 1 1 1
ie} ite} 20 30 40 50

TEMPERATURE, °C

Figure 15.—Effect of temperature on CO monitor response to
82 ppm CO test gas and zero alr.

|
E 100
a Giner output
9 voltage, x10 a
- 90
on
re
a9
zo 80
Receiver display, ppm
« 1 1 n
70, 10 20 30 40

TEMPERATURE, °C

Figure 14.—Linearlty of PT-3 monitor response to gas In cham-

‘ber with monitor case doors open and closed.

Figure 16.—Etfect of temperature on CO monitor-transmitter
response to 82 ppm CO test gas.



2

DRIFT TESTS

Drift tests were conducted with the monitors for
1 month, The test gases flowed into the monitors through
the calibration fixtures for each measurement. Neither the
temperature nor the atmospheric pressure was controlled.
Graphs of the responses of the monitors (fig. 20) and of

84

the atmospheric temperature and pressure during each
measurement (fig. 21) suggest that environmental condi-
tions have a small effect. During the month, the monitors
exposed to 82 ppm CO gas fell an average of 2 ppm CO.
During the same period, the response to zero air fell less
than 1 ppm CO.

A. T T T

8 .
—
E gol KEY oe Sr eS
a e@PT-1 4 r)
ul APT-2 °
3 mw PT-3
5 7. f f :
B 2
an

6 a a 4
fets_sateh tite featt sresae |

-2 1 \ f 1 f

ie) 4 8 12 16 20 24 28 32 36

TIME, days
Figure 20.—CO monitor drift test. A, Span 82 ppm CO in alr; B, zero air.
up 750 + . : 7 : 7 ; :
4 e
32 740+, wee J
8 E ee eo? e.e
ras Or e °° ° ° |
e

a e

720 1 f ' fl f fl
uy 26 T T r T T T T T
5
-E 24 e 4
eo $ oe e eee e ee
ui 22-e ote eoee e eee J
$ e
ef 20 1 1 fl 1 fl 1 1 A

° 16 20 24 28 32 36

_______TIME, days

Figure 21.—Drift test environment.


Library of Congress Cataloging in Publication Data:

Chilton, J. E.
Hybrid fiber-optic electrochemical carbon monoxide monitor / by J.B. Chilton and
CR Carpenter.
P. cm. — (Report of investigations; 9423)
Includes bibliographical references (p. 13).

Supt. of Docs. no.: I 28.23:9423.

1, Mine gases—Measurement~Equipment and supplies, 2. Carbon monoxide—
Analysis—Equipment and supplies. 3. Gas-monitors—Testing. 4. Optical detectors
—Testing. 5. Electrochemical sensors—Testing. I. Carpenter, C.R. (Clarence R).
IL Title, II. Series: Report of investigations (United States. Bureau of Mines);
9423,

TTN23,U43[TN305] 622 s—de20 [622782] 92-5709 CIP



Report of Investigations 9423

Hybrid Fiber-Optic Electrochemical
Carbon Monoxide Monitor

By J. E. Chilton and C. R. Carpenter

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

BUREAU OF MINES
TS Ary, Director


flow rates from 50 to 200 mL/min on the rise times® and
on the final response values for monitor PT-1. Table 1
lists rise times measured with the three monitors for dif-
ferent test gas flow rates.

Response recovery times’ were measured for each mon-
itor by feeding air at a flow of 100 mL/min through the
calibration fixture after the monitor was at equilibrium in
the test gas. A graph of the responses (fig. 9) shows that
the recovery times were 67, 82, and 80 s. These recovery
times are the same magnitude as the rise times whenever
test gases are fed through calibration fixtures.

Table 1.—Rise times for each
CO monitor, seconds

Flow, mL/min PT-1 PT2 PTS

50... 89 98 96

100 . 7 8 o7t
a 7 6

Response times of these monitors were also measured
in a controlled-temperature chamber. Each monitor was
placed in the air-filled chamber with tape placed over the
monitor’s gas port. The chamber was filled to 40 ppm CO
by mixing 990 ppm CO with air in the chamber. A fan
circulated this mixture, moving the gases from the bottom
of the chamber to the top at about 40 fpm. The rise time
test was started by pulling the tape off the port, using the
chamber gloves to avoid upsetting the atmosphere. After
the rise time tests, recovery time tests were then run by
opening the chamber door, allowing the chamber gases to
be replaced by air. Figure 10 shows both rise and recov-
ery curves for PT-3, Table 2 lists the average of these
values for each monitor.

Table 2.—Chamber test response
times, seconds
Unit Rise Recovery
Prt. 505 769
PT-2. 56.7 72.9
PT-3. 55.6 73.4

The overall average rise time for 3 monitors was 53.8 s
with a 95% confidence interval of 51.6 to 55.9 s for
13 measurements. The overall average recovery time for
three monitors was 75.0 s with a 95% confidence interval
of 66.2 to 83.8 s for eight measurements. For the rise time
tests, the change in gas concentrations was a step function,

SRise time is the time for the response to reach 90% of the final
value.

7TRecovery time is the time for the response to fall to 90% of the
final value.

but for the recovery time tests, it was not. The chamber
required a finite time for room air to dilute the 40 ppm
CO to 0. Furthermore, the chamber door was opened
only partially (4 in) to control the temperature.

Figure 11 shows the results of single response time
tests, run at different temperatures. The rise times were
a weak function of temperature, while recovery times ap-
peared to depend more on temperature. In reality, much
of the time dependence for the recovery measurements
resulted from the gas temperature affecting how fast air

replaced the gas in the chamber.
100
ow! KEY
80 ‘’ == PTH, 675 1
8 WV "—'— PT-2, 82s
€ \\ —— PT-3, 80s
& 60 \\ 1
i \\
g \
2 \\
8 40 \\ 1
ray \
* \
20 \ 4
eS
S—>>
0 20 40 +60 80 (00 120
TIME, s

Figure 9.-CO monitor recovery measurements with zero alr.

40
ao
° “
8 30 4 4
€
& / KEY
yg 20 / \ Re ver
2 ” ¥
a /
wlop 4
Yu
7
7 1 n 1 n
) 20.40 60 80 100
TIME, s

Figure 10.—Chamber test of rise and recovery times with
40 ppm CO test gas.


different converter is available with a low-temperature
limit of -21° C.

Monitor PT-3 was tested in the environmental chamber
with a closed case and without calibration fixtures at dif-
ferent temperatures with gases ranging from 35 to 40 ppm
CO, Figure 17 shows that the effect of temperature on
response has the same general characteristic shape as the
other tests run with calibration fixtures and open cases.

PRESSURE TESTS

The dependence of the CO response on atmospheric
pressure was measured by placing a monitor in a vacuum
jar and reducing the pressure by a vacuum pump. Test
gas flowed to the monitor through the calibration fixture.
A mass flow controller held the gas flow to 100 mL/min.
Figure 18 shows the monitor response from 737 mm Hg
(the pressure at the laboratory elevation of 1,000 ft) to
239 mm Hg for both 82- and 40-ppm CO gas mixtures.
The ratios of the average percent change in response to
the percent change in pressure are 0.44 for both concen-
trations, The monitor’s response depends on atmospheric
pressure and thus altitude. Monitors must be calibrated at
the elevation of use to minimize the bias due to change of
ambient pressure. Figure 19 shows an interesting compari-
son of the pressure dependence of different sensors. The
Giner cell, with a solid-membrane electrolyte and a tubu-
lar diffusion chamber, had much less pressure dependence
then did a cell with liquid electrolyte and a capillary gas
entrance. The membrane cell with enclosed liquid electro-
lyte had the greatest response to pressure changes.

For this study of the low pressure response of the CO
monitors, the test gas was fed in succession into each
monitor’s calibration fixtures with the gas flow set to a
100-mL/min rate by a mass flow controller. Each unit was
placed in a vacuum jar, with calibration fixture in place,
and the pressure within the jar was maintained by ad-
justment to the airflow from the jar to a vacuum pump.
The pressure within the jar was measured with a mercury
manometer. In this dynamic pressure test the test gases
were continually flowing, and thus the responses involved

11

both the effects of pressure and of gas flow on the CO
monitors. These dynamic pressure tests were performed
as a laboratory evaluation of different types of CO sensors,
and the test has no relation to the monitors’ operation in
a mining environment. It should also be emphasized that
these tests were performed on laboratory monitors and not
on fixed point monitors approved for use in mines.

90 r , T ,

80 J

70 4
o
8
— 60 KEY 4
a 82 ppm CO
3 ——— 40 ppm co
S 50 4
a
«

40 wee 4

eo
_—
30 — 4
a
20 -
200 300 400 500 600 700 800

PRESSURE, mmHg

Figure 18.—Effect of pressure on CO monitor response,

90 T T T

70

KEY
TM Tube
© Capillary
4 Membrane

RESPONSE, ppm CO
8

10

L

1

TEMPERATURE, °C

Q 50 T

E

&

B 1
2

2

8 30 f fl .

« “So 10 20. 30. 40

1
300 400 600

500
PRESSURE, mm Hg

700

800

Figure 17.—Effect of temperature on CO monitor-transmitter
response to 40 ppm CO test gas.

Figure 19.—Effect of pressure on response of different typ
of CO monitors, (Tube = solid-membrane electrolyte and tubul
diffusion chamber; capillary = liquid electrolyte and capillary g:
entrance; membrane = enclosed liquid electrolyte.)