Evaluation of Person-Wearable Methane MonStors 

J E chiltonl, C D ~a~lor', E E all' and D S yantek2 

ABSTRACT 

Regular monitoring for methane gas is required near working faces in 

gassy underground mines where the potential for methane ignitions is 

greatest. However, high concenuations of methane can also accumulate 

outby the face where methane levels are monitored less frequently. 

Wearing a personal methane monitor equ~pped with an alarm couId 

protect persons working in these outby areas from injury or death due to 

methane ignitions. 

The National Institute for Occupational Safety and Health (NIOSH) 

has conducted studies to evaluate methanometers that couId be used as 

person-wearable monitors. 

For the present study, seven different person-wearable monltors that 

are not currently approved for underground use were evaluated. This 

paper describes test procedures used to evaluate performance of methane 

monitors that could be used underground for continuous personal 

monitoring of methane. The information in this repon provides data that 

allows the reader to make side-by side compansons of these instruments. 

BACKGROUND 

Continuous methane monitoring on a mining machine is required 

near face areas of gassy underground mines (30 CFR 8 

75.323(b)). Except for areas where atmospheric monitoring 

systems (AMS) are located, methane concentrations are not 

monitored on a daily basis in outby areas. In the United States 

methane ignitions continue to occur in work areas that are outby 

the active work faces (McKimey, 2001). A person-wearable 

methane monitor that continuously monitors methane levels 

would give warnings when methane levels were too high at any 

work location. 

Previously NIOSH evaluated a person-wearable methane 

monitor, used in South African mines, that combines the cap 

lamp and methanometer in a single unit (Chilton, Taylor and 

Timko, 2003). The instrument's design made it easy to wear, but 

difficult to calibrate because it did not have a visual readout. 

Personal wearable monitors that could be used to detect methane 

in these outby areas are not used in US underground mines. 

Seven different person-wearable methane monitors, currently not 

used underground, were selected for testing because: 

they are small enough to be worn by an underground worker; 

they all have a digital readout to indicate methane 

concentration; and 

they each have visual, audible, and vibratory alarms to alert 

workers when the detected methane levels exceed preset 

limits. 

GENERAL DESCRIPTION 

The seven methanometers (identified in the text as instruments A 

through G) that were evaluated are shown in Figure 1. The 

weight and size of the instruments varied as shown in Table 1. 

Each instrument has a clip that allows it to be attached to a 

worker's clothing or belt. 

All of the methanometers use catalytic heat of combustion 

sensors. Combustible gas from the surrounding environment 

reaches the sensor element by diffusion. Filters remove any 

1. NIOSWPittsburgh Reseach, Respiratory Hazard Control Branch, 

626 Cochrans Mill Road, Pittsburgh PA 15236, USA. 

2. NlOSWPlnsburgh Reseach, Hearing Loss Prevention Branch, 626 

Cochrans Mill Road, Pittsburgh PA 15236, USA. 

FIG 1 - Methanometers used in NlOSH study. 

TABLE 1 

Methanometer size and weight. 

airborne dust or water mist before it reaches the sensor element. 

Although all methane monitors used in US mines display 

methane concentrations as a per cent by volume, the instruments 

tested display concentration as per cent of the lower explosive 

limit (LEL). All of the instruments measure from zero to 100 per 

cent LEL methane whlch is equivalent to zero to five per cent by 

volume methane. 

INSTRUMENT EVALUATIONS 

Methanometer performance was evaluated based on the 

following parameters: 

run time, 

measurement drift, 

response time, and 

alarm characteristics (visual, audible, vibratory). 

Instrument run time 

New nickel metal hydride (NiMH), nickel cadmium (NiCad), or 

lithium ion (Li-ion) batteries (Table 2) supplied by the 

instrument manufacturer are used to power the instruments. 
J E CHILTON et 01 

/---Plexiglas top TABLE 2 

Methanometer run time comparisons. 

A methanometer's run time is the total amount of time it will 

monitor gas concentrations before its battery voltage drops below 

a minimum level. Each of the instruments has a warning signal to 

indicate low battery voltage. 

To measure run time the batteries were first charged according 

to the manufacturers' instructions. The instruments were turned 

on at the beginning of an eight-hour workday. All testing was 

conducted in fresh air. Any instrument that was still operating 

normally after eight hours was turned off. Without being 

recharged, it was turned on at the beginning of the next workday 

and run times continued to be measured. Run times were 

measured rwice for each instrument and the results averaged. The 

manufacturers' stated run times and average measured run times 

are listed in Table 2. 

' lrstment / Battery I Manufacturer stated 1 Average measured ' I type rmn time (hr) I run time ( hr) 1 

Measurement drift 

.I\ 

B 

C 

D 

E 

F 

G 

A methanometer is calibrated by exposing the instrument to zero 

gas and to an air gas mixture containing a known methane 

concentration. The instrument output is adjusted until the visual 

readout displays the concentration of the applied gases. Over 

time some measurement drift is normal, but the readings should 

remain within specified tolerances. 

After the instruments were calibrated, drift tests were 

conducted to determine how much methane readings varied with 

time when the instrument was exposed to known gas 

concentrations. No further adjustments were made to instrument 

calibration after starting the tests; however, instrument G 

automatically checked the zero reading each time the instrument 

was turned on by way of a fresh air start-up mode. 

NiMH 

Li-ion 

NiCad 

NiMH 

Li-ion 

NiMH 

NiCad 

Drift tests were conducted by placing all seven instruments in 

a 360 x 360 x 360 mm enclosed chamber. Figure 2 shows the 

chamber containing one instrument. The methanometers' visual 

readouts could be seen through the chamber's Plexiglas@ top. 

nice each day, morning and afternoon, a Hamilton 1500 ml 

syringe (reference to specific products does not imply 

endorsement by NIOSH) was used to inject 1300 ml of 99 per 

cent methane gas into the box. Two small fans at the bottom of 

the chamber completely mxed the gas and alr in about four 

seconds. As the gas was injected, leakage of the methane-air 

mixture occurred through small openings in the box. The 

resulting methane concentration in the box was approximately 50 

per cent LEL (2.5 per cent by volume). One minute after the gas 

was injected the instrument readings were recorded. 

An industrial Scientific MDU 420 infrared portable 

methanorneter was dso placed in the chamber to be used as a 

reference instrument. The accuracy of this instrument was 

checked by performing a calibration prior to each test. The 

readings from the reference instrument during calibration and in 

the box were used to normalise concentrations in the chamber to 

50 per cent LEL (2.5 per cent by volume). The drift tests were 

FIG 2 - Enclosed chamber used for measurement drift tests. 

- 6 

12-14 

, 12 

12 

18 

11 

8 

50 

45 

40 

0 4 8 12 16 20 24 

Measurement number 

2.67 

12.17 

10 

11.50 

20 

12 

12.5 

FIG 3 - Methanometer drift test results. 

repeated on 12 consecutive working shifts giving a total of 

24 measurements. Figure 3 shows the normalised first-minute 

LEL per cent concentrations for each measurement taken. 

Instrument response time 

lnstrument 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 

(90 per cent of the full-scale value in this case) of the final steady 

state reading upon application of a known gas concentration. 

Response times were measured for each of the seven 

methanometers using a procedure developed by NOSH (Taylor, 

Chlton and Zimmer, 2004). 

The instruments were tested individually in the enclosed 

chamber described earlier. The Hamilton syringe was used to 

inject 1300 ml of 99 per cent methane. After the gas was 

injected, the concentrations were recorded from the instruments' 

visual readout every five seconds for one minute. After one 

minute, the final or full-scale reading was recorded. 

Measurements were taken twice for each instrument and the 

results averaged. Response time curves (Figure 4) were drawn by 

plotting the percent of full scale readings versus elapsed times 

for that reading. The dashed line on Figure 4 intersects each 

curve at the 90 per cent response time. 
0 5 10 15 20 

Time. s 

FIG 4 - Response time plots. 

Instrument alarms 

All seven methanometers tested have visual, audible, and 

vibratory alarms that produce signals to warn the user when 

preset gas concentrations are exceeded. All of the instruments, 

except instrument A, have two separate alms that can be set 

independently for 'low' and 'high' gas concentrations. The low 

and high alarms were turned on by applying either 20 or 40 per 

cent LEL (one or two per cent by volume) methane gas, 

respectively, to the sensor head. Each type of alm (visual. 

audible, and vibratory) was evaluated separately for each 

instrument. 

Visual alarms 

The methanometers have one or more red light emitting diodes 

(LEDs) that are located on the top, side, or front of the 

instruments (Table 3). Each instrument has a different temporal 

light pulse pattern. 

TABLE 3 

Number and location of LEDs. 

The alarm light intensity and temporal pattern were measured 

by placing the alarm lights approximately 1 cm from a 

photodiode. A constant voltage was supplied to the photodiode. 

The current from the photodiode changes with the amount of 

light. This current is proportional to the voltage drop across a 

50 ohm resistor that was in seoes with the photodiode. The 

voltages were recorded by a computer-based data acquisition 

system (100 Hz sample rate). The magrutudes of the measured 

voltages (signal mv) were proportional to the light intensities as 

shown in Figure 5. The light pulse rates and cycle times (on and 

om are shown in Figures 6 to 8. 

50 

0 

ABCDEFG 

Instrument 

FIG 5 - L~ght signals 1 crn from photodiode 

a High alarm 

Q 3 

2 

0 

ABCDEFG 

Instrument 

FIG 6 - Methane alarm light pulses. 

ABCDEFG 

Instrument 

FIG 7 - Low methane alarm pulse duration. 

01 

ABCDEFG 

Instrument 

FIG 8 - High methane alarm pulse duration. 
J E CHILTON et a1 

Audible and vibratory alarms 

An audible alarm is produced by a speaker located in either the 

front or top of the methanometer (Table 4). A vibratory alarm is 

produced by a motor that rotates an eccentrically weighted shaft. 

The vibratory alarm was located inside methanometers A, B, E, 

F, and G. Instruments C and D have external vibrators that are 

controlled by signals from the methanometers. 

TABLE 4 

Speaker locations 

- - 

%lethanometer 

A 

B 

Sound and vibration measurements were made inside the 

NIOSH anechoic chamber. During each test the methanometer 

was placed on two small pieces of soft foam with the visual 

readout facing up. Supporting the methanometer in this manner 

in the anechoic chamber simulates free boundary conditions and 

has little impact on the measured vibration. 

Sound level and temporal pattems for each alarm's audible 

signal were measured using two ripe-1 ICP-powered 

microphones. The microphones were positioned 0.3 metres from 

the top and 0.3 metres from the front centre of the methanometer 

(Figure 9). The recorded data was used to determine the sound 

levels and temporal pattems for each alarm's audible signal. 

Speaker location 

Front 

Front 

E 

FIG 9 - Microphone orlentation. 

Front 

A lightweight triaxial accelerometer was used to measure 

vibration and mounted on the case of the methanometer or the 

external vibrator. The accelerometer axes were aligned with the 

axes of the instrument cases (Figure 10) or the external vibrators 

(Figure 11). 

A 16-bit resolution LMS Pimento portable data acquisition 

system was used to simultaneously record sound and vibratory 

acceleration data Sample rates for the sound and vibration data 

were 50000 and5000 samples per second, respectively. Each 

recording was approximately ten seconds in length. The recorded 

FIG 10 - Accelerometer placement on methanometer case 

(instrument D). 

F 

FIG 11 - Accelerometer mounted on external vibrators C and D 

Front and top 

sound pressures were post-processed to determine the maximum 

A-weighted sound levels, the frequency content, and temporal 

characteristics of each alarm signal. 

Sound level 

The maximum A-weighted sound levels with fast response 

measured at the top and front microphone locations for the entire 

frequency distribution are shown in Table 5. 

TABLE 5 

Maximum A-weighted sound levels. 

Alarms Maximum A-weighted sound level with 1 

fast response (dB re 20 @a) 
hVALUA1 ION Ob PERSON-WEARABLE Mk1 HANh MONlTORS 

Sound frequency 

The 113-octave band sound pressure level (SPL) spectra were 

calculated for each recording using a fast time constant. The 

spectra at the times corresponding to the maximum A-weighted 

sound level (Table 5) were examined for top and front 

microphone locations. The 113-octave bands yielding the highest 

and second highest sound pressure levels were determined. The 

sound pressure levels and the associared 113-octave band centre 

frequencies are given in Table 6 for the top measurement 

location. Similar results were obtained for the front sampling 

location. 

TABLE 6 

Highest and second highest 1B-octave band sound pressure 

levels at top microphone. 

Sound temporal patterns 

E 

F 

F 

G 

G 

Temporal patterns of the audible alarm refer to both the rate at 

which the audible signal repeats itself and the changes in sound 

frequencylpitch. Dominant and fundamental frequency tones for 

each alarm signal were determined using narrowband frequency 

analysis (Table 7). For instruments A, C, D, E, F, and G, the 

repeating signal consisted of alternating pulses with fixed 

frequency content, whereas for instrument B the signal consisted 

of alternating pulses that began at one frequency and swept to a 

higher frequency. Table 7 also shows the cycle time (seconds 

needed for the signal pattern to repeat one time). 

High 

Low 

High 

Low 

High 

Vibration 

The measured accelerations were filtered and double-integrated 

to yield vibratory displacement in micrometers. The RMS 

displacement was calculated for each axis. The 113-octave band 

vibratory displacements were calculated for the steady state 

vibration signal in the direction of the axis having the highest 

displacement. The steady state displacement and corresponding 

113-octave band centre frequencies are shown in Table 8. 

70 dB at 5000 Hz 

84 dB at 2000 Hz 

86 dB at 2000 Hz 

79 dB at 4000 Hz 

78 dB at 4000 Hz 

DISCUSSION 

65 at 2500 Hz 

83 at 4000 Hz 

80 at 4000 Hz 

76 at 2000 Hz 

74 at 2000 Hz 

General description 

Toe seven methanometers tested are all small enough to be worn 

by. a miner, either attached to the belt or the jacket. Sampling 

location on the miner is an important factor that can affect the 

accuracy of the methane readings and the ability of the user to 

react to instrument warnings of high methane concentrations. 

The instrument should be located on the miner so that air can 

pass freely over the sensor head and the user can recognise one 

or more of the alarm signals. 

Instrument run time 

Measured run times varied from less than three hours to 20 

hours. Four of the instruments had run times that exceeded the 

manufacturers' stated run times. The battery charge for 

instrument E was not depleted until the third day. Although 

instrument A had an initial run time of 2.67 hours, after 

recharging several times, the instrument ran for five hours. To 

provide full-shift methane monitoring, instrument run times 

should be ten hours or longer. Aging of the instruments will 

reduce run time as will operation of the alarms during a shift. 

The user must monitor battery condition to ensure that power to 

the methanometer is adequate. 

TABLE 7 

Temporal patterns determined by narrowband frequency analysis. 
J E CHLTON et a1 

TABLE 8 

RMS displacement. 

Measurement drift 

G / Low 1 X / 18.6 1 100 

Average readings for six of the seven methanometers were equal 

to or greater than the 50 per cent LEL (2.5 per cent by volume) 

concentration in the test chamber. The instrument drift test data 

were analysed to determine average values and standard 

deviations (Table 9) for the 22 measurements made during the 

drift tests (Figure 3). A ten per cent deviation is acceptable for 

the accuracy of methanometers approved for use in underground 

mines when measuring 50 per cent LEL (2.5 per cent by volume) 

methane gas. (30 CFR 9 22.7 (d)(2)). Five of the seven 

instruments fell within the ten per cent criteria. The dnft as 

indicated by the standard deviation about the individual means is 

less than three per cent for six of the seven instruments. 

TABLE 9 

Instrument drift calculations. 

G 

When using methanometers, calibration to correct for 

instrument drift should be performed as needed, but at intervals 

not to exceed 31 days (30 CFR 8 75.342 (aI(4)). The amount of 

instrument drift would be expected to increase with instrument 

age and exposure to dust and water. 

High 

Response time measurements 

X 80 

Currently there are no response time criteria for methanometers 

used in mines. However, the faster the response time the more 

quickly the instrument will provide a warning alarm signal. The 

measured response time for 90 per cent of the full-scale readings, 

T,, (Table 10). were determined from the response curves. 

Response times for the person-wearable methanometers varied 

from eight to 20 seconds. These times were faster than response 

10.5 

TABLE lo 

Ninety per cent response times. 

times measured for mining-machine-mounted methane monitors 

equipped with filters for removing airborne water and dust 

(Taylor, Chilton and Zimmer, 2004). 

Measured response times 

lostrument Tw (set) ! 

Visual alarms 

A 

The effectiveness of a visual alarm depends on the location, 

intensity, and the temporal flashing pattern of the light(s). For a 

visual alarm to be recognised by the user, the instrument must be 

worn so that the light or lights are in line of sight. 

15 

In general the more intense the light the more likely it will be 

recognised by the user. Ln a dark underground environment the 

temporal pattern may be however, as important as the intensity. 

Light pulse rate and pulse duration are two key factors that affect 

the recognition of visual alarm signals. As shown in Figures 7 

and 8, instruments B, E, F, and G ail have pulse rates that differ 

at low and high alarms; instrument E has the largest difference in 

pulse rates. 

Audible alarms 

I 

Sound level 

B 

In general a signal sound will be clearly audible if the 

A-weighted sound level of the signal exceeds the level of the 

ambient noise by 15 dB or more (IS0 Standard 773 I). 

The maximum A-weighted sound levels (Table 6) ranged from 

72 to 88 dB for the top microphone location. Since ambient 

A-weighted sound levels near operating machinery in 

underground mines are generally in excess of 85 dB, the audible 

alarms would probably not be heard by miners working in face 

areas. In areas outby the face, where machinery is not however 

operating, the audible alarms would probably be heard by the 

person wearing the instrument. 

10 

Signal frequency 

An alarm signal should be based on frequencies in the 300 HZ to 

3 kHz frequency range, and the signal should have sufficient 

sound energy below 1500 Hz to be heard by workers with 

hearing loss and workers wearing hearing protection (IS0 

Standard 7731). The more the frequency at which the alarm 

signal generates its highest sound level differs from the 

frequency where the ambient sound level is highest, the easier it 

will be for workers to hear the warning signal. 

All of the instruments except B produced audible alarms 

having frequencies in either the highest or second highest 

113-octave bands that were in the 300 Hz to 3 kHz range, but 

none generated sound energy below 1500 Hz. Instrument B 

generated a pulse that swept from a low to high frequency, while 

the other six instruments generated warning signals with fixed 

frequency content. 

C 20 
EVALUATION OF PERSON-WEARABLE METHANE MONlTORS 

Temporal characteristics The monitors all have digital readouts that display current 

Recognition of an audible alarm also depends on the pulse rate of 

the signal (Table 7). Six of the instruments (A, C, D. E, F, and G) 

had pulse frequencies in the range of the 0.2 Hz to 5 Hz (period 

of 0.2 to five seconds), which is recommended range for audible 

alarms (IS0 Standard 7731). 

Vibratory alarm 

The slun is most sensitive to vibration frequencies from 200 Hz 

to 300 Hz (Salvendy, 1987). Recognition of the vibratory alarms 

also depends on displacement. Table 8 shows that whereas the 

vibratory frequencies ranged to 80 Hz to 160 Hz, for these tests, 

displacements ranged from approximately 5.4 to 22.2 p. 

External vibrators can be placed closer to the user's skin while 

keeping the methanometer on outer clothing. 

CONCLUSIONS 

This study investigated seven different types of person-wearable 

methane monitors. Factors of instrument performance and alarm 

function that would be important in protecting a worker from 

exposure to high methane concentrations in an underground mine 

were examined. 

Six of the methanometers had run times that were ten hours or 

greater. The drift for instrument G was more than three times the 

accepted range for accuracy (ten per cent), and the standard 

deviation was three times greater than the other instruments. 

The 90 per cent response times for these methanometers were 

eight to 20 seconds. 

Six of the methanometers tested have separate signals for high 

and low alarms. The first or low alarm was set at 20 per cent LEL 

(one per cent by volume). The second or high alarm was set at 

40 per cent LEL (two per cent by volume). These levels 

correspond to warning and danger levels prescribed for machine 

monitoring systems (30 CFR 3 27.22 (b)(2)). The methanometers 

are designed to alert the user of potenual problems due to the 

accumulation of methane. The low alarm is a warning signal and 

the high alarm is a danger signal. Both require action on the part 

of the miner. 

methane concentrations. All of the monitors have three alarm 

modes: visual (pulsating LEDs), audible (72 to 88 dBa with 

dominant or fundamental tones in the 2500 to 4500 Hz bands) 

and vibratory (80 to 120 Hz). Current requirements for methane 

alarms only include audible or coloured lights (CFR 5 30 27.23 

(a)). The addition of the vibratow alarm for personal monitors is 

a feature that will help the recobition of mdthane in high noise 

and flashing light work areas. Multiple alarm modes increase the 

chances that a worker will recognise exposure to high methane 

levels. 

The monitors are small and light enough to be worn by a miner 

during normal work activities. Any electronic instrument used in 

US underground mines must be approved however, by Mine 

Safety and Health Administration for intrinsic safety and 

performance. All of these instruments do have Underwriters 

Laboratories andlor Canadian Safety Association approval for 

use in atmospheres containing flammable methane gas mixtures. 

Future field study work will include application for experimental 

permits from MSHA for the methanometers. Upon instrument 

approval, additional testing wiIl be conducted on-site or 

underground. 

REFERENCES 

Chilton. J E, Taylor, C D and 'Iimko, R J, 2003. Evaluation of IYONl I1 

methanometen, in Proceedings 30th International Conference of 

Safety in Mines Research Institutes. Johannesburg, South Africa 

International Organization for Standardization, 1986. International 

Standard IS0 7731, Fist edition - 1986-12-15, Danger signals for 

work places - Auditory danger signals, 10 p. 

McKinney, R, Crocco, W and Stricklin, KG, 2002. Department of Labor, 

Report of Investigation, CAI-2001-20. 

Salvendy, G, 1987. Hundbook of Human Factors (Wiey-Interscience: 

New York). 

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

measure response times for machine-mounted monitors, in 

Proceedings Tenth US/North American Mine Ventilation Symposium, 

Anchorage, Alaska, pp 177- 182.