Evaluating performance characteristics of machine-mounted methane 

monitors by measuring response time 

C.D. Taylor, J.E. Chilton & T. Mal 

Center for Disease Control and Prevention, NIOSH,Pittsburgh Research Laboratory, Pittsburgh, PA USA 

ABSTRACT: Methane monitoring at the mine face is essential to assure a safe work environment in underground coal mines. Machine-mounted methane monitors must be used to continuously monitor methane levels during mining. Monitors used on mining machines must be approved by MSHA and properly maintained 

to provide protection for workers (30 CFR 75.342). To provide protection the instrument must give accurate 

readings of current methane levels. Accuracy is maintained by calibrating the instrument with a standard gas 

at least once every 31 days. Instrument “response time” determines how quickly measurements will change 

to reflect current concentrations. At present there are no criteria for measuring the response time for a machine-mounted methane monitor while underground. 

 This study examined a procedure for measuring the response time of a machine-mounted methane monitor. Response times were measured using methanometers obtained from three different manufacturers. The 

effects of the measurement technique and methanometer sensor head design on response time measurements 

are investigated. Techniques for reducing variation in response time measurements are discussed. 

1 BACKGROUND 

Federal regulations require that methane monitors be 

mounted on all continuous mining machines used in 

all underground mines. Each type of machinemounted methane monitors must be tested and approved by MSHA before it can be used in a mine. 

The monitors are designed to alert face workers 

when methane concentrations reach 1 pct. The 

monitors must also warn workers and cut off electrical power to the machine when concentrations reach 

2 pct (30CFR 75.342, (b)(1), (c)(1)). 

After installation on the mining machine, the 

mine operator is responsible for periodically checking to assure the instrument is operating properly. 

Methanometer calibration must be checked and the 

results recorded at least once every 31 days (30CFR 

75:342 (a)(4)). Calibration procedures are specified 

by the equipment manufacturer. In general they include exposing the instrument alternatively to an 

atmosphere containing no detectable methane gas, 

and then to an atmosphere containing a known concentration of methane. The instrument readout is set 

to zero while it is exposed to an atmosphere containing no gas or zero gas. The readout is set to the 

concentration of methane calibration gas (usually 

2.5 pct) while exposed to the atmosphere containing 

the gas. 

As a general guideline, a methanometer that reads 

+/- 0.1 pct of the zero gas and +/- 0.2 pct of the 

calibration gas is considered to be in calibration. The 

visual display should stabilize within 2 minutes after 

application of a gas. Application of current 

recommended techniques for single point calibration 

of methanometers is considered adequate for assuring the accuracy of machine-mounted methane 

monitors. 

When a mining machine begins to cut coal, the 

methane concentrations at the face can rise and fall 

rapidly. If the methanometer response time is slow, 

the actual concentrations may be higher than the indicated readings. The monitor must not only read 

the methane concentration accurately but also respond quickly to changes in concentration in order 

to accurately reflect current methane concentrations 

and indicate potentially hazardous conditions. 

The two most important factors that determine 

how quickly a machine-mounted monitor responds 

to gas released at the mining face are: 1) Sample 

transport time - The time for a volume of methane 

released from the coal face to reach the monitor sensor head, and 2). Monitor response time - The time 

for the monitor to respond to the methane after it 

reaches the monitor sensor head. 

Sample transport time is affected mainly by the 

location of the sensor on the mining machine, the 

face ventilation system, and any mechanical barrier 

on the mining machine used to protect the sensor 
 

 

from dust and water. The effect of machine sampling locations on methane readings has been examined during NIOSH studies (Taylor, et al. 2001). 

Monitor response time can be affected by clogging 

of the sensor filter cap with dust, or normal aging of 

the sensor. A primary objective of this research was 

to measure monitor response times for machinemounted methanometers using a technique that

could be employed in an underground mine. 

The response time for a methanometer is defined as 

the time interval for the methanometer output to 

change from a steady state reading in pure air to a 

steady state value for a calibrated gas of known concentration. In this paper response time is expressed 

as the time required for a instrument to display a 

percentage of the final output value (i.e. the concentration of the calibration gas). Since the sensor output approaches the final response limit asymptotically, the response time is usually defined as 90 

percent of the final output value for the known gas 

concentration although lower percentages can also 

be used to define response time. For example, if 2.5 

percent calibration gas is used, the 80-pct-response 

time (T80) is the time required for the concentration 

to reach 2 pct. 

2 TEST PROCEDURES 

To measure response time the instrument sensor 

head is first exposed to zero gas and then, instantaneously, exposed to the presence of a constant concentration of a calibration gas. Several techniques 

have been recommended for measuring response 

time in certification agency laboratories, but none of 

them are practical to use underground due to equipment and manpower requirements. For measuring 

response time underground some technique is

needed that will allow application of calibration gas 

without requiring that the sensor head be removed 

from the mining machine. The technique used to 

make response time measurements should: 1) Designed for use by the industry miners or MSHA inspectors while working at the underground face, and 

2) Provide results accurate and precise enough to indicate if methanometer performance is acceptable or 

has deteriorated to the degree that the instrument is 

no longer capable of providing adequate warning for 

the face workers prior to the build up of potentially 

explosive mixtures of methane and air in face areas. 

3 INSTRUMENTATION 

Methanometers were obtained from three different 

manufacturers. In this report, the methanometers are 

identified as “Monitor A,” “Monitor C,” and “Monitor G.” Each methanometer includes a readout unit, 

power supply and sensor head. A sensor head from 

each of the manufacturers is shown in Figure 1. 

Figure 1. Sensor head for three methanometers. 

Three identical sensor heads were obtained from 

each manufacturer for use with the corresponding 

power supply and readout unit. 

The basic components of the sensor head (See 

Fig.2) include: 1) the dust cap that includes screen 

and baffles to help prevent contamination of the sensor elements with dust and water, (“... A suitable filter on the sampling intake to prevent dust and moisture form entering and interfering with normal 

operation.” (30 CFR § 27.22). 2) A porous flame arrester that covers an internal chamber, and 3) Sensor 

elements, where methane gas is oxidized. 

Figure 2. Basic components of sensor head. 

All three manufacturers use catalytic heat of combustion type sensors. When methane reaches the 

sensor elements it oxidizes on the catalytic element. 

The oxidation generates heat, which upsets the electrical balance of a Wheatstone Bridge, sending a 

voltage signal to the monitor unit. The voltage is 

then converted to percent methane. 

4 RESPONSE TIME MEASUREMENTS 

For these tests, response time was measured using a 

large display digital stop watch, a cylinder of 2.5 pct 

calibration gas, a calibration cup for delivering gas 

to the sensor head, and a flow meter for monitoring 

calibration gas flow. Before making a measurement, 

gas flow was directed through the calibration cup at 

the rate recommended by the manufacturer for cali­
bration. The measurement began when the stopwatch was started (time zero) and the calibration cup 

was placed over the sensor head. Two observers independently recorded concentrations at 5-second intervals from the monitor readout and a digital stopwatch. Measurements were made every five

seconds until the values equaled the calibration gas 

concentration (2.5 pct). Each test condition was repeated at least one time. Results from the two observers and repeat tests were averaged to determine 

response times. 

Response time curves were drawn using the percent methane or voltage data from the methanometer 

readouts and the elapsed times obtained with the 

stopwatch. Each measured concentration in percent 

methane was divided by 2.5 (the percent methane 

concentration of the calibration gas) to give the response percentages. The response curve for each 

methanometer was obtained by plotting the elapsed 

time versus the response percentages. The response 

curves shown in Figure 3 represent the average responses for all tests with the three sensor heads.

Figure 3. Response curves for three sensor heads. 

 The 

40, 80 and 90 pct response times determined from 

the response curves are given in Table 1. 

 

 

 

 

 

Table 1. Response time for three monitors. 

Response, pct 

40 

Monitor A 

Time (sec) 

13 

 Monitor C 

 Time (sec)

14 

Monitor G 

 Time (sec)

 14 

80 23 30 28 

90 29 40 36 

The 90-pct 

response times frequently are used as the criteria for 

evaluating performance of air sampling instruments. 

The 80 and 40 pct response times correspond to 2 

and 1 pct concentration readings, when using the 2.5 

pct calibration gas. Alarm signals must be provided 

by the methane monitors whenever methane concentrations reach either 2 or 1 pct methane. 

Currently there are no MSHA written response time 

criteria for methanometers, which are mounted, on 

mining machines. The response curves and data in 

Table 1 show that the response times varied for the 

three different methanometers. Response times vary 

for different reasons. After use underground, response times may vary because of aging of the sensor element, and exposure to dust and water in the 

mine environment. The instruments used in these 

studies were practically new and clean. Additional 

testing was conducted to determine what factors 

might cause differences in response times for new 

instruments. Factors related to the measurement 

technique and instrument design were also examined. 

5	 EVALUATING RESPONSE TIME 

MEASUREMENT TECHNIQUE 

To make response time measurements underground, 

the technique needed to be simple and require minimal equipment. The measurement technique was 

designed to reduce variation in the test results. Each 

methanometer was calibrated prior to making response time measurements. Calibration procedures 

were similar to those recommended by the manufacturer. The same cylinder of 2.5 percent methane 

calibration gas was used for all tests. Calibration 

does not affect instrument response time, but it is 

important to calibrate the instrument in order to determine the final output value for the response time. 

(In this case 2.5 pct methane). 

5.1 Data Acquisition 

The methane sensor head provides a continuous 

voltage signal that is proportional to the methane 

concentration. Underground the only way to observe the sensor head output is with the visual display. To simulate response time measurements underground, methane concentrations were obtained by 

two observers who recorded the methane concentrations from the visual display every 5 seconds. The 

two observers recorded the data independently, but 

concurrently. There was a high correlation between 

the data collected by the two individuals. 

Above ground it is possible to disassemble the 

sensor head for monitor A and G and record the output signal directly. The output signals from monitors A and G were recorded every 1⁄2 second using a 

computer-based data acquisition system while, concurrently, the two observers recorded the voltage 

signal from the sensor head every 5 seconds using a 

digital voltmeter. All voltage readings were converted to the corresponding concentrations in pct 

methane and the response percentages plotted versus 

elapsed time. The response curves (Fig. 4), based on 

the 1⁄2 and 5-second readings, were very similar. 
 

Therefore, the collection of data by the observers at 

5-second intervals did not cause differences in the 

response time measurements. 

Figure 4. Comparison of response time data obtained with 

computer and visual observations. 

5.2 Control of Gas Flow Rate 

Increasing gas flow rate through the sensor head will 

reduce the time it takes for gas to reach the sensor 

elements. The effect of flow rate on response time 

was demonstrated by applying the calibration gas at 

flow rates 40-pct greater, equal to, and 40 pct less 

than the recommended calibration gas flow. Only 

one of the sensor heads for each of the methanometers was used for these tests. The response times for 

each of the monitors decreased as the flow rates increased (Table 2). 

Table 2. Effect of calibration gas flow rate on response time 

(seconds). 

Monitor A 

 Pct Response 

Flow rate (m1/min) 40 80 90 

60 19 35 47 

100 14 28 37 

140 11 20 26 

Monitor C 

 Pct Response 

Flow rate (ml/min) 40 80 90 

300 20 45 60 

500 13 26 34 

700 10 19 24 

Monitor G 

 Pct Response 

Flow rate (ml/min) 40 80 90 

300 20 35 47 

500 14 26 33 

700 13 25 31 

During testing the effects of gas flow rate on response time was minimized by maintaining the flow 

rate as close as practical to the recommended calibration flow (0.1 lpm for monitor A and 0.5 lpm for 

monitors C and G). Flow rates were monitored and 

adjusted using Dwyer Visi-Float flowmeters 1

1

Identification of manufacturer does not imply endorsement 

by NIOSH 

(0.06 

to 0.5 and .15 to 1 lpm ranges) and an inline flow 

regulator to dampen flow fluctuations. A calibration 

gas flow of 0.500 +/- .05 lpm was maintained during 

tests with monitors C and G, and 0.100 lpm +/- .02 

lpm for monitor A. These small variations in flow 

did not have any measurable effects on response 

times. Gas flow rates were checked before each test 

and adjusted when necessary. 

6 EVALUATING SENSOR HEAD DESIGN 

Monitor response time has been defined as the time 

for the monitor to respond to the methane once it 

reaches the methanometer sensor head. Specifically 

the sensor head design could affect how long it takes 

for the gas to travel through the sensor head to the 

sensor elements. Design features of the sensor 

heads that could affect response time were exam

ined. 

The three cross sectional drawings in Figure 5 

show the basic internal configurations of each of the 

three sensor heads tested. The sensor heads are 

shown with the calibration cups attached. Arrows 

show the most likely path the calibration gas would 

take as it enters the sensor head from the calibration 

cup, travels through the sensor head to the sensor 

elements, and finally exits the sensor head. The internal design determines how far the gas must travel 

through the sensor head before reaching the sensor 

elements. In general the greater the distance, the 

longer the response time. 

The response time is also affected by the direction of the gas flow as it passes through the sensor 

head. 

­
Figure 5. Sensor head for methanometer A. 

Flow direction is affected by the baffles and screens 

as well as the locations where the gas exits the sensor. Gas enters each of the sensor head through 

holes in the top and or sides of the filter caps. Flow 

will generally be toward the exits holes, which is the 

path of least resistance. In sensor head A (Fig. 5) 

the exit holes are located in the side of the dust cap, 

below the level of the sensor element. In sensor 

head C (Fig. 6) the exit holes are in the top of the 

calibration cup, above the level of the sensor element. 

Figure 6. Sensor head for methanometer C. 

 Exit holes for sensor head G (Fig. 7) are in 

the sides of the calibration cup, above the level of 

the sensor element Changing the location of the 

holes where the gas exits the sensor head may affect 

response time by varying the amount of gas that 

passes over the sensor elements. 

Figure 7. Sensor head for methanometer G. 

Sensor head C was used to study the influence of 

exit hole position and flow direction on response 

times. With this sensor head, most of the gas exits 

the sensor head through three holes located in the 

top of the calibration cup (See Fig. 6). There are 

three holes near the bottom of the dust cap. When 

the dust cap is only screwed part way onto the body 

of the sensor head (location A), the holes are above 

the threads and open to the inside of the sensor body 

(Fig. 8A). At this position the holes are near the location of the sensor elements. With the cap turned 

all the way onto the body of the sensor cap (location 

B), the three holes are over top of the threads (Fig. 

8B). 

Figure 8. Dust cap positions tested for sensor head C. 

Response times were measured with the dust cap 

positioned so that the holes were either above the 

threads (location A1), or over top of the threads, (location B). The response curves in Figure 9 show 

that the response times were much shorter when the 

dust cap was at location A and the holes provided 

exit locations for the gas flow. 

With the dust cap at location B, some of the gas 

would have leaked around the cap threads even 

though the holes were covered. To determine what 

effect this leakage would have had response times a 

third series of the tests was conducted with the cap 

in location B and the threads sealed with Teflon 

tape. Response times were slower when the threads 

were sealed (Fig. 9). Sealing the threads forced all 

airflow to move toward the holes in the top of the 

calibration cup and away from the sensor elements. 

Leakage around the threads allowed some of the gas 

to pass by the sensor elements. 

Tests were conducted with the holes at the top of 

the calibration cup covered with tape, and uncovered 

as during the prior tests. The dust cap was positioned so that the bottom holes in the dust cap were 

over the threads, but the threads were not sealed. 

The response curves (Fig.10) show that response 
times were shorter when the exit holes in the top of 

the calibration cup were covered. Covering the 

holes forced the gas to exit around the threads and 

past the sensor elements. Flow direction in the sensor head was again the factor affecting variation in 

the response times. 

Figure 9. Effect of dust cap position on response time (sensor 

C). 

Figure 10. Effect of sealing holes in calibration cup (sensor C). 

7 DISCUSSION AND CONCLUSION 

This research demonstrated a technique for measuring response times underground. Response times 

were measured for three types of methanometers 

that are currently approved for use underground. 

Response curves were drawn to compare instrument 

performance. The curves were similar to the “S 

shaped curves”2

2

Introduction to Nonlinear Differential and Integral Equations, H. T. Davis, Dover Publications, NY, 1962 

 obtained with other air monitoring 

devices. 

A difference in response times between instruments was attributed primarily to differences in the 

designs of the sensor heads. The different configurations caused changes in flow patterns in the dust 

caps. Tests were conducted to show how changing 

flow patterns affected response times. In general 

shorter response times are more desirable for methane sampling instruments. However, the purpose of 

these tests was not to modify methanometer design, 

but response times for the methanomters. When 

used and properly maintained in the mine environment, the methanometers can help to ensure a safe 

work environment for underground workers. 

 

The primary reason for measuring response times 

is to document changes in methanometer performance that occur during normal usage. Normal usage 

includes aging of the methanometer and exposure of 

the sensor head to dust and water. When normal usage results in longer response times, the level of 

safety provided to the worker can be reduced. The 

data obtained from this study can be used as basis 

for criteria to evaluate instrument performance using 

response time measurements. 

The ranges of response times measured for each 

of the three methanometers are typical for new instruments. Any increases in response time beyond 

this range could indicate a deterioration in instrument performance and the need to either, provide 

routine maintenance, repair, or replace the instrument. 

For these tests, methanometer performance was 

determined by either comparing the response time 

curves (which were plots of response percentages 

versus elapsed time from application of the calibration gas), or the 90, 80, or 40 response percentages 

for the final output values of the calibration gas. 

The 40 and 80 percent responses correspond to 

methane concentrations of 1 and 2 pct respectively 

because 2.5 pct methane calibration gas was used. 

As long as a 2.5 pct calibration gas is used, the entire response time curve would not have to be drawn 

to determine the 40 or 80 pct response times. Instead, the time from application of the gas, until a 

reading of 1 or 2 pct could be measured directly. 

Moreover, the instrument alarms for 1 or 2 pct could 

be used to indicate when the concentration had 

reached the 40 or 80 pct response times. 

8 FUTURE WORK 

8.1 Flame Arrester Permeability 

The research investigated the effects of the travel 

path on the response times, but the movement of the 

gas through the flame arrester was not evaluated. 

Porous metal flame arresters surrounded each of the 

sensor elements. The time required for gas to travel 

though the flame arresters is not known but would 

vary with the porosity and thickness of the metal. 

During these tests the porous metal was clean and 

the time for the gas to pass through the flame arresters considered small. Future testing will be conducted to examine how exposure of the flame ar

rester to dust and water affects permeability and 

travel time for the gas. 

­

8.2 Effects of Exposure to Dust and Water 

Exposure to dust and water can also affect the permeability of screens in the sensor head. Water can 
 

damage sensor elements and dust can block holes 

that allow movement of gas in and out of the sensor 

head. It is not possible to design a laboratory test to 

evaluate the effects of dust and water on 

methanometer performance because exposure to 

these airborne contaminants is different for each 

mining situation. Underground testing that includes 

periodic measurements of response time for the machine-mounted methanometer should be conducted. 

Using either total dust exposure or feet of mining 

advance as an index of exposure, the effects dust and 

water response times would be determined. 

8.3	 Gas Application without Calibration Cup 

Measurements made during these tests required the 

use of calibration gas, which was applied to the sensor head through a calibration cup. Applying gas 

through the calibration cup affected the flow patterns in the sensor head. In the mine environment 

gas would enter the sensor head via convection and 

diffusion, and flow patterns would not be the same 

as during response time measurements. Additional 

testing will be conducted to determine if response 

times measured by applying calibration gas with and 

without the calibration cup are significantly different. The three sensor heads will be placed side by 

side in a sealed box and exposed to a step input of a 

methane gas. The response times will be compared 

to the times obtained using the calibration cup. 

8.4	 Effect of Travel Time on Methanometer 

Response 

The location of the instrument on the mining machine and the amount of instrument shielding are 

important factors. Increasing the distance of the 

methanometer from the face and increasing shielding can reduce methanometer exposure to dust and 

water but it can also increase response time due the 

additional time required for methane to travel from 

the face to the sensor head. Measurements will be 

taken in a model mine to compare times for gas to 

travel from a gas release point at the face to different 

sampling locations on a model-mining machine. 

REFERENCES 

Taylor, C.D., Thimons, E.D. & Zimmer, J.A. 2001. Factors affecting the location of methanometers on mining 

equipment. Proceedings: 7th International Mine Ventilation Congress, June: 683-687. 

U.S. Code	 of Federal Regulations. Title 30 - Mineral Resources, Part 75