1 INTRODUCTION 

During coal mining, methane concentrations are 

usually highest near the face where the continuous 

mining machine is extracting coal. To ensure the 

safety of workers, methanometers are mounted on 

the mining machine to continuously monitor methane levels. Each type of machine-mounted methane 

monitor must be tested and approved by MSHA (30 

CFR 27.22 through 27.40) before it can be used in a 

mine. The monitors are designed to alert face workers when methane concentrations reach 1 pct (30 

CFR 75.342, (b) (1)). The monitors must also warn 

workers and cut off electrical power to the machine 

when concentrations reach 2 pct (30 CFR 75.342 (c) 

(1)). 

 Clearly, the methanometer can provide protection 

only if it accurately records the methane readings. 

Accurate readings require the instrument to be calibrated and require that it respond quickly to changes 

in concentration. Calibration procedures are specified by the equipment manufacturer. The mine operator is required to check the methanometer calibration at least once every 31 days (30 CFR 75.342 (a) 

(4)). 

 Currently there are no criteria for the measurement of methanometer response times or response 

time limits. The response time for a methanometer is 

defined as the time interval for the methanometer 

output to change from a steady state or zero reading 

in pure air to a given percent value of the final 

steady state reading upon application of a calibrated 

gas. Since the sensor output approaches the final response limit at a steady state, the response time is 

usually defined as 90 pct of the final output value for 

the known gas concentration, although lower percentages can also be used to define response time. 

 To measure response times 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. The instrument response reading is then measured as a function of 

time from initial exposure to the gas. Techniques 

that have been described for making response time 

measurements with gas sampling instruments are not 

practical for use in an underground mine 

Environment (Hughlett 1982, Foster-Miller 1985)

 During an earlier NIOSH study to measure response times (Taylor 2002), calibration gas was applied directly to the sensor head through the manufacturer-supplied calibration cup at a stated flow 

rate. Use of the calibration cup allowed the delivery 

of a known concentration of methane to the sensor 

head with minimum interference from the surrounding atmosphere and provided a means for measuring 

response times on equipment underground. One 

problem with this technique is that the installation of 

the calibration cup over the dust cap alters the normal airflow around and through the sensor head. 

Therefore, a technique for measuring response time 

Use of a Test Box to Measure Response Times for Machine-Mounted 

Monitors 

C. D. Taylor, and J. E. Chilton, and J. A. Zimmer 

NIOSH/Pittsburgh Research Lab 

ABSTRACT: A technique for use in underground coal mines measures the response times of machinemounted methane monitors. This technique uses a calibration cup to deliver gas to the sensor head to measure response times. For comparison, in the current study, response times were measured by placing methane 

sensor heads in a specially designed test box where they were exposed to an atmosphere containing a constant 

concentration of methane. Response time readings measured in the test box were shorter than the readings 

obtained using the calibration cup. The response times measured in the test box were comparable for sensors 

from three different manufacturers. The effects of dust cap and flame arrestor design on response times were 

also evaluated using the test box. 
that does not require use of the calibration cup was 

desirable for comparison. 

 This report describes a laboratory technique for 

measuring response time in a test box that does not 

require the use of a calibration cup. In the test box, 

response times were measured by exposing methane 

sensor heads supplied by three different manufacturers to a constant methane concentration. Response 

time measurements obtained in the test box and with 

the calibration cups are compared. 

2 TEST PROCEDURE 

2.1 Methanometers 

Methanometers obtained from three different manufacturers, which were also used during an earlier 

study, are identified as “Monitor A,” “Monitor C,” 

and “Monitor G.” Each of the monitors is approved 

by MSHA (CFR 30, part 27) for use on mining 

equipment and included a visual readout unit and 

power supply provided by the manufacturer. Three 

identical sensor heads were obtained from each 

manufacturer. 

 Each manufacturer's sensor head design was different but the basic components included a dust cap, 

a porous flame arrester that covers an internal chamber, and a sensor element. The sensor head for each 

type of monitor used a catalytic heat of combustion 

sensor to measure methane. Figure 1 shows one 

sensor head obtained from each of the three manufacturers. 

2.2 Test Box 

A 14- x 14- x 14- inch plywood box was built to 

contain a methane-air mixture (see figure 2). A 

bracket mounted near the center of the box held a 

single sensor head during testing. The box had a removable top that allowed the placement and 

Figure 1. Sensor heads for monitors A, C and G. 

removal of the particular sensor head being tested. 

A foam seal on the top helped to reduce leakage of 

gas from the box. Methane was injected and air 

samples withdrawn through plastic hoses that were 

inserted through holes on one side of the box. Electrical cables to power the sensor heads and transmit 

data also passed through holes in the opposite side 

of the box. Two 4-inch fans attached to the bottom 

interior of the box were used to mix the injected 

methane gas with box air. 

Figure 2. Test Box. 

2.3 Gas Delivery and Mixing 

The three sensor heads were individually exposed to 

gas-air mixtures in the test box. Methane (99 pct) 

was injected into the box using a plastic syringe Super Syringe Hamilton Co Reno NV that had markings to indicate volumes from 0 to 1500 cc. The gas 

was transferred from a cylinder to the syringe (see 

figure 3). The methane flowed through the plastic 

tubing into the water-filled container for approximately 30 seconds prior to the syringe being filled. 

The end of the syringe was capped until the gas was 

injected into the box. Approximately 4 seconds were 

Figure 3. Apparatus for filling syringe. 

required to manually depress the plunger and transfer the gas from the syringe to the box. 

 The quantities of methane gas injected into the 

box were varied (300 to 1200) and the average con-
centrations in the box measured with a Horiba 

Model PIR-2000 infrared gas analyzer. Figure 4 

shows the relationship between the quantity of 

methane gas injected into the box and the final average concentration. For most tests, 1000 cc of gas 

were injected into the box resulting in concentrations between 2.0 and 2.5 pct methane. 

Figure 4. Relationship between gas injected and concentration 

in box. 

 The Horiba Model PIR-2000 infrared gas analyzer, which has a very fast response rate for methane gas, was also used to measure how quickly concentration increased in the box following gas 

injection and how long that concentration was maintained. Air samples from the box were transferred to 

the Horiba analyzer gas cell using a MSA Flowlite 

air pump (see figure 5). The pump was operated at 

1.5 lpm and has inlet and outlet ports that allow the 

direct transfer of gas between the box and the Horiba. The tubing between the box and Horiba was 

as short as practical to minimize transport time. The 

output signal from the Horiba, which is proportional 

to the methane concentration, was recorded by a 

computer-based data acquisition system 

Figure 5. Test setup with Horiba monitor. 

 Figure 6 shows the response time measurement 

obtained with the Horiba monitor for gas injected 

into the test box at time zero. Concentrations 

reached 90 pct of their maximum value in less than 9 

seconds and 99 pct of final value in less than 10 seconds. The 4-second delay between gas injection and 

Figure 6. Response curve measured with Horiba monitor. 

and any signal response was probably due to the 

time required for the gas to travel from the box to 

the Horiba sensor. Assuming this delay in response 

was due to gas travel time, concentrations in the Horiba monitor reached 99 pct of their final level less 

than 6 seconds after the gas was injected. Methane 

levels in the box decreased about 2.5 pct per minute 

after reaching their maximum level due to leakage 

from the test box. 

2.4 Data Acquisition 

Before each series of tests with a sensor head, instrument calibration was checked using 1 pct calibration gas applied by the manufacturer-supplied 

calibration cups. To measure response time the sensor head was placed in the bracket, the lid installed, 

and the fans started. After confirming a zero reading 

on the monitor readout, the gas was injected. A 

large display stopwatch was started when the gas 

was injected. Two observers independently recorded the methane concentrations at 5-second intervals. Data were collected for 2 to3 minutes after 

injection of the gas and each test was repeated one 

time. The response time for each sensor head was 

based on the average of concentrations measured by 

both observers during the two tests. Readings for 

the three sensors from each manufacturer were averaged to determine response times. 

3 RESULTS 

3.1 Response Time Readings 

The response curves (see figure 7) for each type of 

methanometer monitor were drawn by plotting the 

elapsed time versus the response percentages. The 

response percentages were calculated by dividing 

the average methane concentration for each 5-

second sampling interval by the maximum methane 

concentration measured for that test. The response 

curves shown in figure 7 are based on the average 

responses for all tests with the three identical sensor 
heads. The 90, 80, and 40 pct response times, which 

were determined from the response curves, are given 

in table 1. Table 1 also includes the response time 

Figure 7. Response curves for three monitors. 

Measurements made earlier using the calibration 

cup. For these tests 2.5 pct calibration gas was used. 

The 80 and 40 pct response times correspond to 2 

and 1 pct methane readings when using 2.5 pct gas. 

Table 1. Response time, Calibration Cup, and Box. 

Monitor A Monitor C Monitor G 

Cal. 

Cup 

Box Cal. 

Cup 

Box Cal. 

Cup 

Box Resp. 

Time 

 Sec Sec Sec Sec Sec Sec 

90% 29 23 40 25 36 29 

80% 23 17 30 20 28 21 

40% 13 8 14 10 14 10 

4 DISCUSSION 

4.1 Calibration cup 

For each type of monitor tested, response times were 

shorter when measured in the box. Part of the differences in response times can be attributed to how 

fast the gas passes through the sensor head. Flow 

rates recommended by each manufacturer were used 

for the calibration cup tests. Gas must travel through 

the dust cap and flame arrestor to reach the catalytic 

head of combustion sensor whether the measurements are made with the calibration cup or in the 

box. However, the airflow paths are different depending on whether a calibration cup is or is not installed over the dust cap. 

 Figure 8 shows a cross-sectional view of each 

sensor head with the calibration cup installed. The 

arrows show the likely path the calibration gas takes 

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. Installing 

the calibration cup on the dust cap forces the air to 

enter the holes in the top of the cap and exit through 

holes in the side. 

 Figure 9 shows the same sensor heads with the 

calibration cups removed from the dust caps. Without the cup, air can enter or exit through holes in the 

sides and top of the dust cap. The improved exchange of air provided by calibration cup removal 

was probably a factor in the reduced response times. 

Figure 8. Sensor heads (monitors A, C, and G) with calibration 

cup attached. 

Figure 9. Sensor heads (monitors A, C, and G) with calibration 

cup removed. 

4.2 Dust cap 

Response time measurements were made in the box, 

as described above, except that the dust caps were 

removed. Dust caps must be installed on the sensor 

head when operating the machine-mounted 

methanometers underground. Comparative measurements were made without the dust cap to estimate 

how much of the response time is due to gas flow 

through the cap. The 90, 80, and 40 pct response 

times for each monitor, with and without the dust 
caps in place, are given in Table 2. For all response 

times a large part of the total response time can be 

Table 2. Response Time Cap On and Cap Off. 

Monitor A Monitor C Monitor G 

Cap 

On 

Cap 

Off 

Cap 

On 

Cap 

Off 

Cap 

On 

Cap 

Off 

Resp. 

Time 

 Sec Sec Sec Sec Sec Sec 

90% 23 12 25 4 29 14 

80% 17 9 20 3 21 10 

40% 8 4 10 2 10 4 

attributed to the time required for the gas to flow 

through the dust cap. 

4.3 Ambient air velocity 

The amount of air moving in and out of the sensor 

head is affected by the ambient air velocity and orientation of the sensor head relative to the flow. 

Flow conditions in the box were measured at three 

locations (see Figure 10) using a hot wire anemometer. Flows measured in the vertical direction varied 

from 35 to 45 fpm and from 65 to 150 fpm measured 

in the horizontal direction. These were within the 

range of conditions that would be encountered near 

the head of an operating mining machine. 

Figure 10. Flow measurement in test box. 

4.4 Flame arrestor 

The 90 pct response times for the three monitors 

varied from 4 to 14 seconds when the dust cap was 

removed. The differences in time are likely due to 

how quickly the gas traveled through the flame arrestor and reached the sensor elements. The time for 

the instrument to electronically record voltage 

changes due to the heating of the sensing element 

would be similar for each instrument. 

 The flame arrestors used with monitors A, C, and 

G are made either of porous metal or a fine mesh 

screen. The sensor elements are located close to the 

flame arrestors. Monitor A has a cylindrically 

shaped porous metal filter that surrounds the sensor 

element. In Monitor C, two small round porous 

metal plugs are located in a plate above the sensor 

elements. In Monitor G, a screen is located above 

the sensor element. 

5 CONCLUSIONS 

Response time readings were made by placing 

methane monitors inside a 14-inch square fabricated 

test box and exposing them to an atmosphere containing methane. These readings were compared to 

response times measured earlier by applying methane directly through the calibration cup to the sensor 

head. Use of the calibration cup was designed for 

underground use where the technique must be simple and require minimal equipment. The use of the 

calibration cup is a satisfactory method for making 

response time readings underground. Obtaining this 

data periodically will also help to indicate if instrument performance has deteriorated and whether 

there is need for repairing or replacing the instrumentation. 

 Use of the test box provided a way to expose the 

sensor heads to gas without using the calibration 

cup, and also provided conditions that closely simulate underground airflow conditions on a mining 

machine where a methane monitor is mounted. 

The response time readings made in the box were 

approximately equivalent for all three monitors and 

were faster than the response measurements made 

with the calibration cups. 

 The effects of dust cap and flame arrester design 

on response time were also examined. It was found 

that shortening response time is one way to improve 

instrument performance, and response time is affected by the design of the dust cap and the flame arrestor. Shortening the flow path through the dust 

cap and decreasing the porosity of the flame arrestor 

might shorten response time. However, it must be 

kept in mind that the dust cap helps prevent water 

spray and dust from contacting the sensor element 

and the flame arrestor prevents the propagation of 

any flame outside the sensor head. Therefore, no 

changes should be made to the dust cap or flame arrestor that would lessen the protection of the sensor 

element. 
6 REFERENCES 

1. Statistical Approach to the analysis of Bias and Precision in 

Reusable Gas-Detecting Devices, R. Emmett Hughlett and 

George H. Schnakenberg, Jr., BuMines Rpt of Investigation 

8715, 1982, 27 pp 

2. Improved Diffuser and Sprayfan Systems for Ventilation of 

Coal Mine Working Faces, Foster-Miller, Inc., BuMines OFR 

18-86, Jul. 1985, pp. 190-210 

3. Taylor, C. D., Chilton, J.E. and Mal, T 2002, Evaluating 

Performance Characteristics of Machine-Mounted Methane 

Monitors by Measuring Response Time, Proc. 9th US Mine 

Vent Symp, pp 315-321.