Remote gob gas venthole monitoring and cellular telephone-based

real-time data transmission system

F. Garcia, W. P. Diamond, & J. K. Marshall

National Institute for Occupational Safety and Health (NIOSH), Pittsburgh Research Laboratory, Pittsburgh,

PA, USA

ABSTRACT: Many longwall mines rely on methane drainage to supplement traditional fan-based ventilation

systems. However, the surface production facilities for methane drainage systems, including gob gas ventholes,

are commonly located in remote locations, sometimes without convenient access to electrical power, and they

may be monitored infrequently. The first evidence that a methane drainage system may not be operating

efficiently, or not operating at all, can be an increase in methane concentrations at a return air shaft or

underground where the workforce is exposed to the risk of a methane explosion. A solar powered, remote gob

gas venthole monitoring and cellular telephone-based real-time data transmission system has been developed and

successfully field tested by the National Institute for Occupational Safety and Health (NIOSH) to address this

problem. With this system, individual methane drainage boreholes can be monitored from the mine office to

determine current operational performance parameters, as well as review past data to anticipate maintenance

needs and troubleshoot operational problems.

1 INTRODUCTION

The control of methane emissions in the underground

workplace has long been a safety problem facing the

coal mining industry. Fan-based mine ventilation

systems using dilution as a means to keep methane

concentrations below maximum statutory levels have

long been the primary method for methane control in

underground coal mines. However, with the mining of

deeper and gassier coalbeds, and the increased use of

higher productivity advanced mining technologies, it

becomes more difficult to adequately control

increasingly higher methane emissions with

ventilation alone. As mine ventilation systems

generally reached their maximum practical potential,

methane control technologies were developed to

capture and exhaust methane contained in the mined

coalbed before it could enter the underground

environment, where it would then have to be diluted

with ventilation air (Diamond 1994).

The control of methane associated with longwall

mining is a particular challenge because not only does

the gas present in the mined coalbed have to be

addressed, but gas commonly released from the

overlying and underlying strata as the panel is

extracted must also be controlled (Diamond et al.1992,

Mucho et al. 2000). Generally, little effort is made to

extract gas from underlying strata in the United States,

therefore any gas from this source that migrates to the

mine atmosphere is controlled by the ventilation

system. Vertical boreholes drilled from the surface are

the most common control technology used in the

United States to extract methane from the collapsed

overburden (gob) before it can migrate to the low

pressure sink of the mine ventilation system (Diamond

1994). However, to do this, the ventholes must not

only be adequately designed and located for sitespecific conditions, they must also be monitored and

serviced to ensure that they are in operation to extract

gas from the gob when needed (Mucho et al. 2000). 

Unfortunately, many gob gas ventholes are located

in remote areas, and due to the cost of providing

electrical power to the sites, some of the produced

methane is used to fuel the internal combustion

engines that power the exhausters. The methane-fueled

engines are generally more prone to service problems

than electric motors and may be adversely affected by

varying methane concentrations in the produced gob

gas. If the holes are infrequently visited by

maintenance personnel, the first indication that a

venthole is not operating efficiently or is off-line

completely may be an unexpected increase in methane

concentrations at a return air shaft or in the

underground air courses. This potentially hazardous

scenario can largely be eliminated by adequate

venthole performance monitoring and real-time data

retrieval at the mine office or other designated

location.
Flame

Arrestor

To Micrologger

To Micrologger

To Micrologger

Transducer No.3

Pressure

Pressure

Transducer No. 2

Averaging Pitot Tube

Connected To 

Differental Pressure

Tranducer No. 1

Gob Gas

Venthole

Temperature Gas Inlet Tap

Exhauster Internal

Combustion

Engine

Atmosphere

Gas Vent To

Gas Outlet Tap

Gas Detector

Tubing

Gas flow

Key

Inlet

Temperature

Outlet

Figure 1. Schematic of gob gas venthole monitoring and data transmission system.

National Institute for Occupational Safety and

Health (NIOSH) personnel designed and constructed

a gob gas venthole monitoring system in the mid

1990's. This early system was capable of monitoring

the gas flow volume and methane concentration of the

produced gas stream, and included a hard-wired data

telemetry system (Garcia et al. 1995). However, this

monitoring system required ac power, and the system

components were relatively expensive. To facilitate

NIOSH’s current research efforts in the area of

optimizing methane control technologies for advanced

mining systems and practices, in particular those

associated with longwall mining, a new gob gas

venthole monitoring system was developed that is

capable of operating and transmitting real-time data

from remote locations where ac power is not available.

The new gob gas venthole monitoring system

utilizes lower cost, current state-of-the-art components

with sufficiently low power requirements so that a

rechargeable battery instead of ac power can be used.

The system battery is recharged by a solar panel. The

other significant upgrade to the previous monitoring

system is that the new system utilizes cellular

telephone technology as the communication link to

view and retrieve performance data from the

ventholes. As a test, the system was installed and

operated on a number of gob gas ventholes at a

cooperating mine operating in the Pittsburgh Coalbed

in southwestern Pennsylvania. This paper describes

the system components, operational characteristics,

and ancillary applications for the acquired data.

2 SYSTEM COMPONENTS

2.1 Gas production system

A general schematic of the surface production facility

for a gob gas venthole at the test site is shown in

Figure 1. Gas production from gob gas ventholes is

usually facilitated by the use of exhausters (also

commonly referred to as blowers or vacuum pumps) to

efficiently extract methane from the longwall gob. The

dominant low-pressure sink created in the gob by the

exhauster induces the flow of gob gas to the venthole

instead of the competing pressure sink of the mine

ventilation system (Diamond et al. 1999, Mucho et al.

2000). Electric motor driven exhausters are preferred

for their reliability when electricity is available.

However, when longwall panels are located in remote

areas were providing electric power is very difficult

and/or expensive, methane-fueled internal combustion

engines are often used to power the exhausters. The

exhausters commonly used at the test site where the

new monitoring system was tested are a skid-mounted,

self-contained unit (engine and exhauster) with a

capability of moving up to 45×103

 m3/d (1.6 MMcfd)

of gas while maintaining a vacuum pressure of 15.2

kPa (61 in w.g.)

2.2 Gob gas analyzer system

2.2.1 Data storage and system control

A Campbell CR23X micrologger1with storage module

is the data collection and control component of the gob

gas analyzer system (Figure 2). The micrologger can

read and record dc voltage outputs from the pressure

transducers, barometer, linear temperature devices,

1

Reference to specific products does not imply endorsement by

NIOSH.
Figure 2. Schematic of gob gas analyzer system component.

and the thermal conductivity gas detector (with the

addition of the Campbell Curs100 shunt terminal input

module). The micrologger is also used to control

power distribution to the system components.

The micrologger was programmed to collect data

every 15 seconds from all input devices and to average

these readings over a 15-minute time period. Only the

15-minute averages are sent to the data storage module

(Figure 2) for on-demand downloading to a Windowsbased computer, either on-site to a laptop computer, or

remotely to the mine office. An important feature of

the micrologger is that it uses an external transistortransistor logic (TTL) level trigger voltage to activate

system input devices only at the instant when a

measurement is taken to conserve battery power. The

micrologger was also programmed to calculate and

record the total gas and associated methane flow rates

from the appropriate measured parameters. In addition,

the micrologger’s program corrected these parameters

to the American Gas Association’s STP conditions,

which are 16 deg C at 760 mm mercury atmospheric

pressure (60 deg F at 30 in mercury). The micrologger

is powered directly from a 12-V, 95-amp hr battery.

The optional Campbell Com100 cellular telephone

package and associated support software allows the

micrologger to be contacted remotely from an off-site

location to observe real-time or stored gas production

system performance data. The micrologger can also be

programmed to alert the mine whenever the gob gas

venthole production system goes off-line or when any

measured or calculated parameter falls outside of a

predetermined acceptable range. 

2.2.2 Total gas stream flow rate determination

The total gas stream flow rate is calculated from the

differential pressure measured across the averaging

Pitot tube’s pressure taps and gas stream density

calculated from measured gas temperature and flowing

pressure data. In this application, a 15.24 cm (6 in)

averaging Pitot tube with an accuracy of ± 2% of

actual flow was installed on the inlet side of the

exhauster (Figure 1). The Pitot tube's output pressure

taps were connected across the differential pressure

transducer’s pressure ports to measure the velocity

pressure, which is used to calculate the gas stream

velocity. The transducer used with the Pitot tube has

a pressure range of 0.0 to 1.0 kPa (0 to 4 in w.g.). The

pressure transducer provides an accuracy of ± 1.2%

full scale (FS), comparable to a pressure measurement

of ± 0.01 kPa (.05 in w.g.); therefore, the lower the

flow rate, the less accurate the resultant reading.

Measurements taken in low-flow gas streams would

require a different equipment configuration or a lower

range differential pressure transducer to maintain

system accuracy. An error of < 3.2% in the calculated

gas flow rates resulted from the use of the Pitot

tube/differential pressure transducer component in the

system installed on the gob gas ventholes monitored at

the test site [total gas flow rate range of 11 to 45×103

m3/d (0.4 - 1.6 MMcfd)]. 

2.2.3 Methane flow rate determination

The methane flow rate is determined by multiplying

the calculated total gas flow rate by the percentage of

methane in the gas stream. The methane concentration

monitoring component of the gob gas analyzer system

(Figure 2) consists of a thermal conductivity gas

detector housed in a container constructed from a

section of 0.3-m (12-in) diameter Polyvinyl Chloride

(PVC) pipe (Figure 3). The gas detector used in the

system has a range of 0 to 100% methane. Typically,

methane concentration values determined by the

thermal conductivity gas detector were within ± 5%

FS compared to periodic gas stream samples analyzed

by gas chromatography methods. The vacuum supplied

by the exhauster is used to circulate a portion of the

produced gas stream through the housing containing

the gas detector (Figures 1 and 3).

2.2.4 Pressure measurement instrumentation

The absolute pressure of the gas stream is required by

the averaging Pitot tube equation (based on the

Bernoulli energy balance equation and the continuity

equation) to calculate the gas flow rate. Two

differential pressure transducers (Nos. 2 and 3, Figure

1) with a range of 0 to 35 kPa (0 to 139 in w.g.) were

placed into the gas stream. The differential pressure

sensors consist of two pressure ports where pipeline

pressure is measured relative to the atmospheric

(gauge) pressure. The absolute pressure of the gas

stream is calculated by adding the gauge pressure

(from transducer No. 2, Figure 1) to the barometric

pressure measurement.
Figure 3. Schematic of gas detector system component.

Figure 4. Schematic of cellular telephone based data

transmission system component.

2.2.5 Temperature measurement instrumentation

A linear temperature sensor is used to measure the

outside temperature and the gas stream temperature on

the inlet and outlet sides of the exhauster, as shown in

Figure 1. The gas temperature measurement taken on

the inlet side of the exhauster near the averaging Pitot

tube/differential pressure transducer location (No. 1,

Figure 1) was an important parameter needed to

calculate the gas flow rate at STP conditions.

Generally, the gas stream temperature on the outlet

side of the exhauster is not used for this purpose since

the gas stream temperature increases substantially as

it passes through the exhauster. However, the gas

stream outlet temperature is a parameter that can assist

in the evaluation of the vacuum pump’s performance.

A significant temperature increase on the outlet side of

the exhauster may indicate that insufficient gas is

available from the gob for the level of applied suction,

thus causing the exhauster to run inefficiently. This

situation can be remedied by adjusting the engine

speed to reduce the suction applied by the exhauster.

2.3 Cellular telephone data transmission system

Cellular telephone technology is a convenient means

of communication for data transmission and system

control purposes from remote locations. The Campbell

Com100 cellular telephone hardware and support

software used in the NIOSH gob gas venthole

monitoring system allows the micrologger to

communicate with a Windows-based computer located

at a remote location, such as the mine office. The mine

operator can monitor and display real-time gas

production system performance data in tabular or

graphical form or download previously collected data

to evaluate longer term production trends for

individual ventholes. The communication system can

also be used to alert the mine operator whenever the

gob gas venthole shuts down or if any measured or

calculated parameter falls outside of a predetermined

operational range.

The major components of the cellular telephone

field station are connected to the micrologger for

system control, as shown in Figure 4. The Motorola

M6000 cellular transceiver requires an external

Motorola RJ11C telephone interface to connect the

Campbell COM200 field modem to the transceiver.

The cellular transceiver transmits a signal at a much

higher capacity and can receive a much weaker signal

than a typical cellular telephone. If necessary, the

transceiver can be used in combination with a

directional (Yagi) antenna placed on a high pole and

pointed toward a cellular telephone tower to increase

signal strength. 

The Crydom relay (Figure 4) allows the micrologger to

apply power to turn on the transceiver (standby mode).

Since power consumption in the standby mode is less

than 0.2 amp hr, the transceiver was left on standby

continuously so that the monitoring system could be

accessed remotely at any time. However, because of

the relatively high power consumption of the

transceiver when transmitting and receiving data (1.8

amp hr), a solar panel was used to maintain a charge

on the system’s 12-V battery.

2.4 Battery/solar-powered charging system

The major components of the battery and solarpowered charging system used to power the remote

gob gas venthole monitoring and communication

system are shown in Figure 5. A Solarex SX-50

photovoltaic solar panel, which generates a peak

power of 50 watts, keeps the system’s 12-V, 95-amp hr

battery charged. The output from the solar panel is

connected to the input of a SunSaver-6 solar

controller. Whenever sunlight is available, the

controller provides a precise charging voltage to

protect the battery from overcharging. The controller

also prevents current leakage at night that would

discharge the battery. The dc output voltage to power

the cellular transceiver is regulated by the controller.
Figure 5. Schematic of battery/charging system component.

The controller will automatically disconnect the power

to operate the transceiver whenever the battery

capacity falls below 11.5 volts to prevent damage to

the battery. If the transceiver is disconnected due to

low battery voltage, the communication system cannot

transmit data or send an alarm to the mine office until

the battery is recharged. However, all other system

components and data storage and device control

functions can continue to operate normally with less

than 11.5 volts of battery capacity. The transceiver is

automatically reconnected once the battery is

recharged. In extended tests of the system through all

seasonal weather conditions and usage demands, the

50 watt solar panel and 12-volt, 95-amp hr battery

were sufficient to keep the cellular telephone

communication system on-line.

3 ANCILLARY SYSTEM APPLICATIONS

3.1 Troubleshooting gas production system

 performance problems

In addition to providing real-time gas production data,

a major advantage of the gob gas venthole monitoring

system is that historical data can be reviewed to

diagnose production problems, in some cases before

they become acute. It was common practice at the test

site that when a venthole was found off-line by

maintenance personnel, they try to restart the methanefueled engine using propane. In some cases the engine

would restart and run normally. Typically, the

maintenance personnel would only remain on-site long

enough to complete their routine servicing of the

system and then leave, assuming that the gas

production system was running properly. However, it

was not uncommon for some trouble-prone production

systems to go off-line again shortly after the

maintenance personnel had left the site.With the gob

gas venthole monitoring and communication system in

place, the operational status of the ventholes can be

checked more frequently from the mine office than the

sometimes weekly on-site visits by the maintenance

crew to ensure that all holes are running properly.

Because of the possibility of variable methane

concentrations in the produced gas stream powering

the methane-fueled engines, the mine operator

sometimes incorrectly assumed that trouble-prone gas

production systems were going off-line frequently due

to methane concentrations being outside of the

optimum range for the engines. Thus, the gob gas

production system would sometimes remain off-line

for long periods of time due to an incorrect assumption

as to the reason for the problem. Once the gob gas

venthole monitoring system was in place on holes that

developed operational problems, it was quite obvious

that the methane concentration in the produced gas

stream was not varying significantly and was within

the operational range for the engine.The problem with

these holes was then determined to be mechanical, not

fuel-quality-related. By having access to historical gas

quality data from the monitoring system, the time

required to diagnose the cause of gas production

system failures can be significantly reduced. Thus, the

venthole can be put back on production in a timely

manner to avoid adversely affecting the methane

concentration levels in the underground workplace.

An operational problem that sometimes occurs

with the gob gas venthole production system is that the

flame arrestor becomes clogged with particulate matter

(probably drill cuttings) entrained in the gas stream

soon after a venthole is completed and put on

production. With pressure transducers installed on

both sides of the flame arrestor (Nos. 2 and 3, Figure

1), a comparison of the pressure readings from the two

sensors enables the mine operator to evaluate the

potential of a clogged flame arrestor being responsible

for an unexpected decline in gas production. Under

normal operating conditions (gas flowing freely

through the flame arrestor), the pressure differential

between the two transducers will be minimal.However,

as the flame arrestor becomes clogged, the pressure

will increase above the flame arrestor (transducer No.

2, Figure 1) and decrease below (transducer No. 3,

Figure 1), resulting in an increasingly larger pressure

differential between the two monitoring points.

3.2 Control of gas production rate

Methane concentrations in the produced gas stream

typically vary over the life of a gob gas venthole

(Diamond 1994). Once the methane in the produced

gas stream reaches ~ 45 to 55%, the methane-fueled

internal combustion engines operate less reliably. At

25% methane, the gob gas production system is

usually de-energized for safety reasons, and the holes

are either shut-in or allowed to free flow. The

extraction rate of gas from the longwall gob depends

on the amount of suction applied to the individual gob

gas ventholes.When the recharge rate of methane to

the gob is less than the production capacity of the

venthole, mine air is pulled into the gob gas production

system, thereby reducing the methane concentration in

the produced gas stream (Mucho et al. 2000). 
It is desirable to keep the methane concentration in

the produced gas stream within the optimal range of

the engine powering the gas extraction system.

Keeping the gob gas ventholes operating is a key

safety issue for many longwall mines. Additionally,

for gob gas production systems that capture and sell

the produced methane, it is desirable to maintain the

methane concentration at contractual quality levels.

The amount of suction applied to the gob gas venthole

by the exhauster can be controlled by adjusting the

engine speed. This is currently done at the test site

when the gob gas production system is periodically

serviced by manually adjusting the engine throttle

setting for optimum performance for the current

methane concentration in the produced gas stream.

However, the gob gas analyzer system could be

configured and the micrologger programmed to

monitor engine RPM and to activate a control circuit

to adjust the engine speed to accommodate varying

gas production conditions. A similar system (but using

electric motors and a hard-wired communication link)

has been used by a mine operator in Virginia to

monitor and control their gob gas production system

performance (Smallwood et al. 1993).

4 SUMMARY

A remote gob gas venthole monitoring and cellular

telephone based real-time data transmission system

has been designed and successfully field tested under

varying seasonal conditions by NIOSH. This system

can provide an additional margin of safety for the

underground workforce by continuously monitoring

the operational status and critical production

parameters associated with longwall gob gas

ventholes. By including an alarm capability in the

system, the mine operator can automatically be

notified that the gas production system is off-line or a

critical production parameter is outside its optimal

range. Thus, the first evidence of a malfunction in the

gob gas venthole production system is not an increase

in methane concentrations at a return air shaft or in the

underground environment. 

By taking advantage of the real-time data

transmission capability of the cellular telephone-based

communication system and historical gas production

data for a particular venthole, the mine operator can

evaluate and spot potential operational problems or

maintenance needs before they result in a shut-down

of the gas production system. The historical data can

also be of use in diagnosing the cause of production

problems after they have occurred, which can

significantly impact the amount of time required to

correct the problem. When this system is fully

deployed at a mine site, the mine operator will have

the necessary data to make timely decisions to more

efficiently control methane gas in the underground

environment.

Modifications to this system, such as controlling

the gas extraction rate by automatically adjusting the

engine speed powering the exhauster to keep the

methane concentration in an optimal range, may

provide additional benefits to the mining industry. The

gob gas venthole monitoring system can also be used

for other mine-site methane drainage applications

where accurately determining the total gas and

methane flow rates is desirable. This includes

monitoring other surface gas production facilities such

as those associated with cross-measure, horizontal, or

vertical borehole gas drainage systems.

REFERENCES

Diamond, W.P. 1994. Methane control for

underground coal mines. US Bureau of Mines

Information Circular 9395.

Diamond, W.P., Schatzel, S.J., Garcia, F., LaScola,

J.C., McCall, F.E. & Jeran, P.W. 1999.

Characterization of gas flow in longwall gobs:

Pittsburgh Coalbed, PA. Proc. intern. Coalbed

Methane Symposium, Tuscaloosa, May 1999: 233-

244.

Diamond, W.P., Ulery, J.P. & Kravits, S.J. 1992.

Determining the source of longwall gob gas:

Lower Kittanning Coalbed, Cambria County, PA.

US Bureau of Mines Report of Investigation 9430.

Garcia, F., McCall, F.E. & Marshall J.K. 1995. Gob

gas analyzer system. In A.M. Walla (ed), Proc. of

the 7th US Mine Ventilation Symposium, Lexington,

June 1995: 371-376.

Mucho, T.P., Diamond, W.P., Garcia, F., Byars, J.D.

& Cario, S.L. 2000. Implications of recent NIOSH

tracer gas studies on bleeder and gob gas

ventilation design.Preprint 00-8, SME annual

meeting, Salt Lake City, Feb. 2000.

Smallwood, M.L., Horn, M., & Meade, K. 1993.

Programmable logic controller based mine wide

monitoring system. In R. Bhaskar (ed),Proceeding

of 6th US Mine Ventilation Symposium, Salt Lake

City, June 1993: 495-500.