This site has been created to collate general material about
methanometers and only covers a small part of the subject of methanometry. It is
planned for the site to evolve by including selected contributions from readers
and it is hoped that the information presented will be useful to students and
others interested in the coal mining industry.
Presentation of material
is in continuous format to enable printing as one complete document which may be
reproduced for non commercial purposes.
Methane (CH4) is an odourless, tasteless,
colourless, lighter than air gas formed by the decomposition of coal as well as
other carbonaceous materials and is regarded as the most common dangerous gas
found in bituminous coal mines. Because of its low density (0.55 compared to
1.00 for air), methane will rise to the roof and high parts of the mine if
ventilation is insufficient to properly mix it with the mine air.
Whilst
it is a simple asphyxiant its major risk in underground coal mining is that it
is highly flammable within the range 5.3-14.0% (commonly rounded to 5-15%) in
air and also has the potential to explode if a source of ignition is present
when the concentration is within this range. The most explosive mixture is 9.8%
in air and the most easily ignited mixture is 7.5% in air. If coal dust is mixed
with methane and air the potential for a powerful underground explosion is
further enhanced and it is for this reason that methanometers were developed for
detecting and monitoring methane in underground coal mines.
Firedamp is a
traditional name for methane (and sometimes includes other flammable gases) when
it is found in underground coal mines.
In the Illawarra district of
Australia it is common to find mixtures of methane and carbon dioxide which are
capable of forming flammable or explosive atmospheres when mixed with air. Even
though methane is lighter than air, if a mixture with high concentrations of the
much denser carbon dioxide gas issues from the seams under the right conditions,
this "Illawarra Bottom Gas" can flow along the floor and create a significant
gas hazard.
Sir Humphry Davy invented his flame safety lamp in 1816 and,
even though safety lamps were developed independently by Clanney and Stephenson,
it was the Davy lamp that became most widely used in English coal mines. Davy
used a gauze cylinder constructed of copper or iron wire to enclose the flame of
an oil burning lamp. By carefully sizing the spaces in the gauze, heat from the
flame was dissipated and this prevented the flame from propagating to the
external atmosphere. The gauze spacings also allowed light to pass through and
thus illuminate the miner's working area without the risk of explosion that had
been so common with previous lamps used underground for illumination in
hazardous areas. Another important safety attribute of the Davy lamp was that,
if the flame went out on a properly maintained lamp, miners would be alerted to
the fact that dangerously low levels of oxygen may have been
present.
Over the years flame safety lamps were improved and models such
as the Protector Garforth GR6S are still used in many areas of the world for
testing for the presence of methane (by observing differences in the shape,
height and colour of the flame depending on the volume of methane present in the
air sample being tested).
The methanometer is an
instrument to measure the percentage of methane (or firedamp) in the air in
underground coal mines and has been designed to alert miners to the presence of
potentially dangerous concentrations of this gas.
Mine officials carry
methanometers to evaluate gas levels in work areas as well as to inspect other
areas of the mine. If methane is measured at 1.25% then work will cease and
equipment will be shut down and if the methane level is measured at 2.50% then
all personnel will be withdrawn to ensure their safety from risk of fire or
explosion. Methane levels are regularly measured using handheld methanometers in
close proximity to the roof, face, and rib of the working place. Readings are
also taken across intake airways to keep the methane level below 0.25
percent.
Continuously recording methanometers are used at or near upcast
shafts as well as outbye ventilation splits, unsealed goafs and waste workings.
Fixed methanometers are also installed at strategic locations near the face and
on equipment to permanently monitor work areas and initiate alarms or equipment
shutdown.
Diesel vehicles being used in return airways in New South Wales
and Queensland underground coal mines carry methanometers with alarms set at
1.00% and vehicles will be withdrawn to a safe area if this alarm level is
reached.
The first electrical
methanometer for use in coal mines was developed by MSA in 1949. It was known as
the W8 methanometer and was powered by an Edison cap lamp battery. Several types
of hand held electronic methanometers were developed around the world during the
1950's but the first independently powered instrument, the GP (general purpose) methanometer was
not introduced until 1961. The C4 methanometer was introduced by MSA in 1966 and
was later replaced by the D6 which is still the main hand held electronic
methane detector used in British coal mines.
The first recording
methanometer was developed by Maihak in Germany but, as it analysed each sample
over a three minute cycle, continuous recording of methane was not possible
until new instruments were developed at SMRE (Safety in Mines Research
Establishment) in England and the Bureau of Mines in Pittsburgh, USA. As a
result of the work done at SMRE to develop a butane lamp methanometer, in 1961,
the Sigma Recording Flame Methanometer Type 208 was introduced into underground
coal mines in England to continuously chart methane levels.
The Butane Lamp Methanometer
developed by SMRE in 1961 used a thermocouple to measure the heat developed by
the combustion of methane in a controlled pure butane flame housed in a modified
flame safety lamp. As the volume of methane in the mine atmosphere varied so did
the heat measured by the thermocouple which was itself connected to an ammeter
or clockwork operated chart recorder calibrated to continuously record the
actual methane level.
It was, however, more common for early
methanometers to use sensors comprising two filaments arranged in a Wheatstone
Bridge circuit and the MSA GP
methanometer used such a sensor. One arm of the Wheatstone Bridge consisted
of an electrical filament that was heated to a high enough temperature to burn
any methane in the air sample that passed over the filament (housed within a
porous flame proof barrier). The process of burning raised the temperature of
the filament further which in turn increased the electrical resistance of this
active filament. This change of resistance could then be calibrated as a current
change proportional to the volume of methane present. The other arm of the
Wheatstone Bridge contained a similar filament that was exposed to the same air
sample but this filament was inactive as it was not heated. By having an active
and inactive filament in the same sample, a balancing control was established to
allow for atmospheric variations such as relative humidity, temperature, and
pressure.
Research to overcome the inherent weaknesses of the filament
detector (filament coils were delicate and could cause inconsistencies in
current output) led to an improved version of the above Wheatstone Bridge
principle known as a "pellistor". The pellistor is now the most commonly used
sensor in modern methanometers.
Despite its disadvantages of
being susceptible to catalyst poisons (including sulphurous gases, silicones and
halogenated hydrocarbons), the pellistor's comparative low cost, simplicity, and
ability to run continuously for over eight hours on one battery charge have
resulted in its widespread use in coal mines as the sensor of choice in
hand-held methanometers.
The concept of the pellistor is also based on
the fact that the most foolproof way to determine whether a flammable gas is
present in air is to test a sample by trying to burn it. A pellistor consists of
a very fine coil of wire suspended between two posts. The coil is embedded in a
pellet of a ceramic material, and on the surface of the pellet (or 'bead') there
is a special catalyst layer.
In operation, a current is passed
through the coil, which heats up the bead to a high temperature. When a
flammable gas molecule comes into contact with the catalyst layer, the gas
"burns" in a controlled environment behind a flameproof barrier known as a
sinter. Just as in a normal burning reaction, heat is released which increases
the temperature of the bead. This rise in temperature causes the electrical
resistance of the coil to rise. There is another bead in the circuit identical
to the detector bead but not containing any catalyst. This bead will react to
changes in humidity, ambient temperature etc, but will not react to flammable
gas. All that is required is to compare the resistance of one bead against
another in a Wheatstone Bridge type circuit in order to obtain a meaningful
signal.
Older methanometers
manufactured from the 1950's to 1980's usually have an analogue display with a
needle indicating a reading within the range 0-2% or 0-5% methane.
Methanometers manufactured since
the 1980's usually have a digital display reading 0.0-4.9% methane or 0.00-4.99%
methane.
The GP
methanometer was powered by two Mallory cells held in nylon mouldings, used SMRE
type filaments, had a probe attachment, and was housed in a stainless steel case
with the complete instrument weighing about three pounds.
MSA GP methanometer
1961
Mine officials were able to carry this
methanometer around the pit and use it to take accurate methane "spot readings"
at specific times throughout each shift.
These days there are many manufacturers of methanometers who
produce their gas detectors in various configurations. Modern hand held
methanometers are usually small, comparatively light weight, sophisticated,
electronic monitors capable of operating for over twelve hours on one battery
charge and able to store large numbers of gas readings. Methanometers can also
be sourced with continuous output to link into mine management systems.
OdaLog CH4
monitor 2002
Apart from being able to provide continuous instantaneous
methane readouts with visual and audible alarms, electronic methanometers now
have the ability to collect useful information for later analysis. Tens of
thousands of methane readings can be stored in onboard datalogging chips and can
be retrieved later on the surface for display on PC's and in printed
format.
Data collected, for example, over a 12 hour period could be very
useful for ventilation officers to establish methane profiles at selected
locations throughout a mine.
MiniGas
graph
With the ability to display the stored data in pictorial format
on a PC (see graph above) mine officials are now able to easily obtain an
overview of methane levels collected by portable methanometers.
Whilst there is a wide
choice in methanometers and most are quite robust instruments, all electronic
gas detectors should be used carefully and regularly maintained.
Most
collieries perform a gas calibration check every week and many of them also test
all hand held methanometers on the surface before every shift by applying a
known level of methane set above the alarm level (usually 2.0% to 2.5%
CH4).
The responsibility for regular inspection and testing of
methanometers to be used underground is usually given to Deputies, Lamp Room
Attendants, or authorised Electrical personnel.
Additional service may
need to be conducted in accordance with manufacturers' manuals or local
regulations but, as methanometers are usually certified intrinsically safe
devices or housed in flameproof enclosures, it is very important that service is
performed only by authorised technical personnel or approved service
organisations.
In Australia, laboratories that calibrate underground gas
detection equipment are accredited to NATA (National Association of Testing
Authorities) standard and this requires methanometers to meet gas span
tolerances at three points across the range as well as at zero.
Allowable
tolerances are shown below.
With the underground
coal mining environment being extremely hostile to instrumentation, it is
important that inspections, maintenance, and calibrations of methanometers are
carried out on a routine basis.
Methanometers that use
pellistors require oxygen for the sensor to operate correctly. Therefore, in
oxygen levels below 12% (this figure varies with different models) it is
possible for a methanometer to read incorrectly or even display zero when a high
percentage of methane may actually be present.
A methanometer that is not
adjusted correctly in fresh air may carry a "zero error" which could incorrectly
increase or decrease the displayed methane reading whilst a methanometer that
has not recently been accurately calibrated against a certified methane in air
gas mixture may read incorrectly and understate or overstate all
readings.
Because methanometers also respond to flammable gases other
than methane, if gases such as ethane are in the sample, the methanometer will
read higher than the actual methane level present. This should be taken into
consideration when ordering gas calibration cylinders and it should be specified
that nil other flammable gases are to be packaged in addition to the nominated
methane in air level as such an error could result in under-calibration of a
methanometer.
It is theoretically possible to calibrate methanometers to
flammable gases other than methane however the manufacturer should be consulted
for advice before attempting this procedure as there are different types of
pellistors in use and they do not all behave identically.
Methanometers
which are calibrated using cylinders of methane in air packaged at zero relative
humidity could read low when used to measure methane in high relative humidity
conditions due to water vapour in the sample displacing methane. One way some
mines minimise this "apparent discrepancy" is to use small lengths of permapure
tubing between the calibrating cylinder and the instrument sensor cup to
increase the relative humidity of the calibrant gas. It is recommended that
advice be sought from the instrument manufacturer and local authorities before
adopting this method of conditioning the calibration gas. It is also important
to remember the effect of relative humidity when comparing methanometer records
to gas chromatograph records as gas chromatographs measure the sample after
water in the air has first been removed.
It is particularly important to
re-check calibration and fresh air zero after a methanometer has been exposed to
off-scale levels of gas, dunked in water, dropped, heavily bumped, subjected to
extreme vibration, or used in extremely dusty and humid environments. This is
why many collieries insist on physical inspections as well as zero and span
tests for all hand-held methanometers before the start of every shift.
In New South Wales
combustible gas measurement instruments are required to be approved by the
regulator (in this case the Chief Inspector of Coal Mines, Department of Mineral
Resources) and the process in place incorporates intrinsically safe
certification to international standards. The Chief Inspector may accept
"Intrinsic Safety" reports issued by Standards Australia accredited testing
laboratories (SIMTARS in Queensland and TestSafe in New South
Wales) or other test bodies or may apply criteria and processes to address
specific needs.
The example below shows the first page of a recently
issued Standards Australia certificate for a methane gas detector.
Gas detectors
must also be evaluated as to their performance as part of the assessment process
for approval.
The methanometer has been a useful
tool enabling mining personnel to prevent thousands of explosions or fires and
has undoubtedly saved many lives, helped avoid countless injuries, and protected
billions of dollars worth of assets since its introduction.
Methanometers
which monitor continuously and record data have been particularly useful for
ventilation officers and mine officials to help them to optimise atmospheres in
underground coal mines throughout the world.
The methanometer is arguably
the most beneficial safety device ever developed for use in underground coal
mining. It has helped improve the quality of life for hundreds of thousands of
people involved in or dependent upon the world coal mining industry.
This is a new site that is still under
development and it is planned that more material will be added as time
permits.
If you see any errors or wish to supply material relating to
methanometers please e-mail methanometer@apptek.com.au and we
will look at incorporating your suggestions into improving the quality of
information presented.
If your suggestion is used, your name and a link
to your web site will be added to the list of contributors below.
Fresh air reading:
Reading displayed by methanometer when placed in normal clean air (should be
zero).
Zero error: A positive or negative deviation of displayed
reading (away from zero) when an instrument is in fresh air containing nil
methane or other flammable gas.
Because this sponsored site is designed to grow with contributions from
readers its accuracy at any point in time cannot be 100% guaranteed. The
publishers of the site and the contributors to it therefore accept no
responsibility for any negative eventualities from the use of any information
published herein.
Davy Lamp circa 1816 
Protector Lamp circa 1987
MSA GP methanometer
1961
OdaLog CH4
monitor 2002
MiniGas
graph
Worth
calibrating
Last modified on 22nd August 2002
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