A STUDY OF THE APPLICATION OF OPEN-PATH INFRARED
METHANE DETECTION METHODS IN
UNDERGROUND COAL MINES
Methane has resulted in more explosions and related loss of life than any other single
cause throughout the recorded history of mining. (McPherson, 1993).
In order to maintain safe coalmine operation it is a necessity that high accuracy, yet
versatile methane detection units are installed at various pertinent locations throughout the
mine, alerting both miners and management to accumulating or dangerous levels of
The most technologically advanced and one of the most accurate methods of methane
detection is Infrared detection. ‘Point’ Infrared gas detectors have been used in
underground coal mines for a number of years now, providing fast, accurate results,
independent of oxygen concentration in the air. ‘Point’ gas detectors are limited though to
providing readings for gas concentration at a single point.
Open-path IR detection systems operate in a similar manner to their point detection
counterparts, in that a beam of IR light is passed through a sample gas, with the amount of
IR energy lost across the span proportional to the concentration of a particular gas in the
Open-path systems however, have the IR transmitter and receiver separated by a span of,
depending on the model chosen, between 2 and 200 metres. This results in a versatile
detector that can equally detect a plume of gas mid way between transmitter and receiver
as it can a steady, homogeneous rise in gas concentration along the entire optical path, as
well as any combination of the two. This would seem to lend the system to an installation
in an underground environment where either case could occur.
However, since the system has (according to commercial suppliers) never been trialed in
the underground coal environment, certain aspects peculiar to that environment may prove
it to be unsuitable.
One of these environmental factors could include dust concentration. Little information
has been gleaned from either commercial suppliers or reference material on dust
concentration and its impact upon effectiveness of the open-path IR system. It is the
impact of dust concentration on the operation of open-path IR gas detection systems
therefore, that forms the bulk of this paper, along with a general discussion of the
technique and its applicability to underground coal mining.
A Dräger GD4003 model open-path IR gas detector was purchased by the University of
New South Wales with the aim of reasonably establishing a dust concentration that such
systems might operate under. The unit was installed into a purpose built apparatus, in
which the IR beam traversed a length of a closed circuit, constantly recirculating a mixture
of air and dust.
The experimental procedure consisted of increasing the concentration of dust within the rig
to the limit of the optics the system, or the point that ‘beam block’ was achieved. The dust
concentration was sampled a number of times, establishing the lowest limit of
concentration above which beam block would continue to be maintained. Conversely, an
estimate for the limit of dust concentration below which the system will continue to
function, was obtained.
The concentrations required for beam block were found to be above those that could be
expected in most situations in underground coal mines, and were certainly above standards
of respirable dust concentrations. This preliminary finding lead to the conclusion that the
system, based on dust concentration alone, would be suitable and indeed beneficial for the
underground coal mine situation.