Use of infrared sensors for monitoring methane in underground 

mines 

C.D. Taylor, J.E. Chilton & A.L. Martikainen 

National Institute for Occupational Safety and Health, Pittsburgh, Pennsylvania, USA 

ABSTRACT: Federal regulations require the use of methane monitors on all mining machines. All machinemounted methane monitors currently used in underground coal mines rely on catalytic heat of combustion 

sensors. As a comparison, although not currently approved for use on underground mining equipment, 

instruments with infrared sensors are evaluated in this study. Potential advantages of using infrared sensors 

include faster response time, long-term stability, and reduced maintenance needs. Tests conducted in the 

laboratory and at a full-scale model mine compare the performance of catalytic heat of combustion and infrared 

sensors operating under test conditions designed to simulate airflow conditions underground. 

1 Background 

Methane monitors mounted on all operating mining 

machines must give warning signals when methane 

concentrations exceed mandatory standards of 1 pct or 2 

pct (30 CFR §75.342). Two types of sensors, infrared and 

catalytic heat of combustion, are commonly used for 

measuring combustible gases such as methane in nonmining applications. In underground applications, only 

heat of combustion sensors are used. 

Infrared methanometers operate on the principle that 

methane gas will absorb infrared light at certain 

wavelengths including 1.33, 1.66, 3.3, and 7.6 microns. In 

the two infrared monitors tested, the light from a tungsten 

filament is transmitted through an enclosure containing the 

sampled gas. A mirror at the end of this enclosure reflects 

the light back to a detector. An optical absorption filter 

placed in front of the detector limits the infrared 

wavelengths to those that are primarily absorbed by gas 

being measured (in this case methane). The detector 

measures the intensity of the filtered light, which varies 

inversely to the concentration of the methane in the 

enclosure. Although other hydrocarbons such as ethane and 

propane will also absorb light in the absorption wavelength 

regions of methane, such as 3.3 microns, the optical filters 

should be carefully selected to minimize the interference 

due to higher hydrocarbons. 

The catalytic heat of combustion sensor uses a very 

fine platinum wire contained within an alumina bead 

coated with a catalyst material, typically platinum or 

palladium. During operation, the bead is heated to a 

temperature sufficient to promote combustion of the 

methane gas with oxygen on the surface of the catalyst. 

The increased heat generated by the combustion increases 

the resistance of the wire inside the bead. The resistance 

change is monitored by a Wheatstone bridge circuit which 

generates an electrical signal proportional to the methane 

concentrations. A second bead in the bridge, not treated 

with the catalyst material, is used to compensate for 

temperature, pressure, and humidity changes. 

All methane monitors currently used on mining 

machines rely on catalytic heat of combustion sensors. 

Although the infrared type sensor is widely available, no 

instrument with this type of sensor is currently approved 

for use on underground mining equipment. However, some 

equipment manufacturers are developing new 

instrumentation and have indicated that the infrared sensor 

has faster response time, longer service life, and requires 

less routine maintenance than the catalytic sensor. 

Therefore, further information comparing the performance 

of the two types of sensors is needed. 

This research was conducted to compare the 

performance, including response time and accuracy, of 

selected instruments using infrared and catalytic sensors. 

Test conditions were designed to simulate gas and airflow 

conditions in an underground mine. Tests were conducted 

in an environmentally controlled box and a full-scale 

model of a mining entry. 

2 Test Procedures 

2.1 Test Instruments 

Two instruments with infrared sensors and one instrument 

with a heat of combustion sensor were selected for these 

tests. The infrared instruments were designated IR-1 and 

IR-2 and the heat of combustion instrument was designated 

HC. Figure 1 shows the three sensor heads. The two 

infrared instruments had similar sensors. However, the 

environmental caps that covered the sensor heads to 

provide protection from dust and water were different. 

2.2 Environmental Text Box 

Instrument response time was measured using a specially 

designed test box (Taylor, Chilton, et al., 2004). The 

instruments were placed in the 36- x 36- x 36-cm (14- x 

14- x 14-in) wooden box which was then sealed (see 

Figure 2). Approximately 1000 cc of methane calibration 

gas (99 % methane) was injected into the box using a 
 

 

 

plastic syringe. Injection of the gas required less than 4 

seconds. Two four-inch fans attached to the bottom of the 

box quickly mixed the methane and air. The resulting 

concentration in the box was approximately 2 pct methane 

by volume. Although a small amount of gas leaked from 

the box, the methane concentration remained relatively 

constant (approximately 2 pct by volume) for more than 2 

minutes. A computer-based data acquisition system was 

used to record once a second the concentrations measured 

by each instrument. The test was repeated once and the 

results averaged. 

Figure 1. Test sensor heads. 

Figure 2. Environmental test box. 

2.3 Ventilation Test Gallery 

Tests were also conducted in the NIOSH ventilation test 

gallery, a full-scale test facility designed to simulate 

airflow in an underground mine entry. The test gallery has 

been described in detail in previous publications (Taylor, 

Timko et al., 2004). For these tests, regulator doors were 

used to adjust and direct intake flows behind a curtain. The 

2.2-m- (7-ft) high air-intake curtain was supported by a 

wood frame constructed 0.6 m (2 ft) from the left side of 

the entry (see Figure 3). For all tests the distance from the 

end of the curtain to the simulated face was 10.7 m (35 ft). 

Figure 3. Ventilation test gallery. 

The face area was 5 m (16 1⁄2 ft) wide. A model mining 

machine was positioned near the right side of the face. The 

mining machine has a simulated scrubber system that 

includes inlet openings on each side of the machine, a fan 

with ducting to move the air from the inlets to the rear of 

the machine, and a single scrubber exhaust at the right rear 

of the machine (Figure 4). 

Figure 4. Model mining machine with scrubber. 

Ventilation conditions in the test gallery were varied 

between tests by changing the scrubber and intake flow 

rates. Tests were run with scrubber flows of 1.9 m3

/s (4000 

ft3

/min), 2.8 m3 /s (6000 ft3/min), and with the scrubber off. 

Intake flow was either 1.9 m3

/s (4000 ft3/min) or 2.8 m3/s 

(6000 ft3

/min). Tests were conducted for the six different 

operating conditions given in Table 1. 

To simulate liberation of methane from a mining face, 

line gas (see description below) was released from a 

manifold made of four 4.3-m (14-ft) long horizontal copper 

pipes that were located 0.1 m (4 in) away from the face. 
 

 

 

During each test a gas flow of 9.4 l/s (20.2 cfm) was set 

and monitored using a rotameter. 

Table 1. Ventilation gallery test conditions 

Test condition Scrubber flow 

(m3

/s) 

Intake flow 

(m3/s) 

1 No scrubber 1.9 

2 No scrubber 2.8 

3 1.9 1.9 

4 2.8 1.9 

5 1.9 2.8 

6 2.8 2.8 

The three sampling instruments were placed on the 

right side of the mining machine, 2.6 m (8 ft 8 in) from the 

face (see Figure 5). Sensor heads were spaced 

approximately 5 cm (2 in) apart. 

Figure 5. Sampling locations for sampling instruments. 

Methane concentrations measured with each 

instrument were recorded each second using a computerbased data acquisition system. Each test in the gallery was 

10 minutes long. During the first three minutes of each test 

the methane and gas were allowed to mix. Data collected 

during the last seven minutes of each test were used to 

calculate the average concentration for that test. Each test 

was repeated six times and the results averaged. 

3 Results and Discussion 

3.1 Environmental Text Box 

Response time is defined as the time for the monitor to 

respond to the methane after it reaches the monitor sensor 

head. To compare the response times for the three 

instruments, concentrations were measured for 

approximately two minutes after injecting the gas into the 

environmental box. The response time curves (see Figure 

6) were drawn by plotting the percentage response versus 

elapsed time. The percentage response is the concentration 

measured at a given sampling time divided by the average 

high concentration measured for that test. The 90 pct 

response times, shown on Figure 6, were taken directly 

from these curves. 

Figure 6. Response time curves for sampling instruments. 

The differences between the 90 pct response times are 

large. At least part of the difference was believed to be due 

to the design of the environmental caps. Earlier tests with 

heat of combustion sensors showed that the flow path 

through the environmental cap can have a significant effect 

on instrument response time (Taylor, Chilton et al., 2004). 

Therefore, the environmental caps used with the two 

infrared sensors were examined further. It was noted that 

each of the infrared caps has narrow openings that allow 

passage of air through the cap (Figure 7). Only the IR-2 

instrument also has a filter material (180 micron pore size) 

that lines inner surface of the IR-2 cap. The composition of 

filter is not known, but the movement of the air is clearly 

slowed as it moves through the material. After these 

observations were made, the environmental caps were 

removed from both infrared instruments and response time 

measurements were made again to determine how much 

the caps affected response times. With the caps removed, 

the 90 pct response times for both instruments were 

approximately 3 seconds (Figure 8). 

Figure 7. Environmental caps and flow paths. 

One additional test was conducted in the 

environmental box to determine if ethane gas would affect 

instrument readings. Line gas containing ethane and 

methane is obtained from a commercial supplier and is 

released from the pipe manifold during tests in the 

ventilation test gallery. A sample of this line gas, analyzed 

using gas chromatography, confirmed that it contained 
 

 

 

 

approximately 95 pct methane, 4 pct ethane, and less than 

one pct higher hydrocarbons and inert gases. 

Figure 8. IR-1 and IR-2 response times with environmental 

caps off. 

Approximately 1000 cc of line gas was injected into 

the box using a plastic syringe. Figure 9 shows the 

concentrations measured by the three instruments. The 

highest concentration measured by IR-1 was about 1 5 

times higher than the readings obtained with IR-2 and HC. 

To correct for this overestimate of methane concentrations, 

all measurements made in the gallery with IR-1 were 

reduced by 1/3. 

Figure 9. Average peak concentrations measured in 

environmental box using line gas. 

3.2 Ventilation Test Gallery 

The three machine-mounted test instruments were used to 

measure the average methane concentrations for the 6 test 

conditions (Table 1). The average readings were 

determined using the final 420 one-second readings 

(representing the last seven minutes) obtained for each 10­

minute test. The readings varied for the different test 

conditions while for each individual test condition the 

readings for the three instruments were approximately the 

same (see Figure 10). 

Figure 10. Average concentrations measured on the mining 

machine. 

The 420 one-second readings obtained from each 10­

minute test were plotted to show how the concentrations 

varied during each test (Figure 11). The data shown in 

Figure 11 were obtained for test condition 2 when the 

scrubber was off and the intake flow was 2.8 m3

/s. For this 

test the average concentrations for IR-1, IR-2, and HC 

were .37, .38, and .41 pct methane, respectively. The graph 

show how the measured concentrations varied for each 

instrument. Each of the instruments showed similar 

patterns of change in concentration during the test. Figure 

12, expands two minutes of data taken from Figure 11. 

The expanded data shows how quickly the instrument 

readings changed in response to changes in gas 

concentrations. 

Figure 11. Variation in gas concentrations measured on the 

mining machine. 

In general, the instrument with the fastest response 

time (IR-1) showed changes more quickly. Also, peak 

values were usually highest for the IR-1 instrument due to 

the faster response time. 
 

 

 

 

 

 

Figure 12. Variation in gas concentrations (120 to 240 

seconds). 

4 Conclusions 

Response times for the infrared instruments were largely 

dependent on the construction of the environmental cap. 

The IR-1 response was faster than the response for HC, but 

the response for the IR-2 was slower than the response for 

the HC. The explanation for the latter was that the filter 

material in the environmental cap of IR-2 increased the 

time required for the gas to reach the sensor. However, the 

filter material provides better protection for the sensor 

from coal dust and water sprays. Underground testing is 

needed to determine the level of protection required to 

maintain the performance of the infrared sensor. Based on 

the current study, if protection provided to the sensor head 

is reduced, instrument response would be faster. 

The results showed that the ethane in the line gas had a 

great effect on measurements made with instrument IR-1. 

Instrument IR-1 overestimated methane content in the line 

gas by 1/3. A correction was made for all readings made in 

the ventilation gallery with IR-1. Most gas in underground 

mines contains methane and ethane. If the percentage of 

ethane in the mine gas is not known, it is important to use 

either heat of combustion or infrared instruments such as 

IR-2 which are not affected by the presence of ethane gas. 

To clarify, the filter used in the infrared instrument 

should, as much as possible, remove wavelengths of the 

transmitted light that are absorbed by ethane and higher 

hydrocarbons. Specifications for the filters used in IR-1 

and IR-2 are not known. However, it is likely that certain 

wavelengths of light passing through the IR-1 sensor were 

absorbed by the ethane, resulting in overestimates of the 

methane concentration. On the other hand, the filter in IR-2 

effectively removed light having wavelengths absorbed by 

ethane. Additional work is needed to specify requirements 

for filters used in infrared instruments. 

As shown in Figure 10, average gas concentrations 

measured by all three instruments were affected by 

changes in intake and scrubber flow. For each test 

condition, the average readings of all three instruments 

were similar. However, the differences in response times 

were evident when measured concentration was plotted 

versus time for individual tests (Figures 11 and 12). In 

general, the faster the instrument response, the quicker the 

change in concentration, and the higher the peak values. 

Peak concentrations measured at the mining face are 

indications of poorer mixing which usually is the result of 

inadequate face ventilation (Kissell et al., 1974). 

Therefore, the ability to monitor peaks provides 

information that can be used to predict the buildup of 

methane at the face. To demonstrate, while collecting data 

on the mining machine, a fast-response infrared monitor, 

similar to instrument IR-1, was used to measure methane 

levels one foot from the face. This instrument was 

calibrated to measure methane concentrations up to 100 pct 

by volume. Figure 13 shows how methane levels at this 

face location varied during the same time frame when 

Figure 12 data were collected. 

Figure 13. Variations in methane concentration one foot 

from the mining face. 

As was expected, the average and peak concentrations 

were much higher at the face. However, it was not possible 

to correlate concentrations measured on the machine and at 

the face. Further studies are needed to establish whether 

concentrations measured on the machine (average and 

peak) can be used to predict face concentrations. 

When looking specifically looking at the response 

time data, the results of this study do not clearly show 

advantages for using infrared sensors instead of catalytic 

heat of combustion sensors. Long-term testing in 

underground coal mines is needed to determine whether 

monitors with infrared sensors have better long-term 

stability and require less maintenance as suggested by 

equipment manufacturers. A critical factor for the use of 

infrared instruments is the type of filter required to protect 

the sensor head from exposure to dust and water. The 

potential benefit of faster response will depend on the type 

of filter material required for the environmental cap. 
 

References 

Kissell, F N, Banfield, J L, Dalzell, R W, and Zabetakis, 

M G, 1974. Peak methane concentrations during coal 

mining, the U.S. Bureau of Mines RI 7885. 

Taylor, C D, Chilton, J E and Zimmer, J A, 2004. Use of a 

Test box to measure response times for machinemounted monitors, in Proceedings of the 10th US/NA 

Mine Ventilation Symposium 2004, pp 177-182 

(Taylor and Francis Group plc.) 

Taylor, C D, Timko, R J, Senk, M J and Lusin, A, 2004. 

Measurement of airflow in a simulated underground 

mine environment using an ultrasonic anemometer, 

SME Transactions, Vol. 316.