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322 Advanced Mine Ventilation

1 ppm, the long-term stability of these analyzers, and the frequent sampling, this tech-
nique is best for long-term trending of carbon monoxide to identify a spontaneous
combustion event. With respect to measuring range, it is normally only carbon mon-
oxide that presents problems, with most systems capable of measuring to only
1000 ppm. Because methane and oxygen concentrations can be measured over all
expected concentrations ranges, this technique is the best for automated monitoring
of explosibility of an area so long as a fire or heating does not exist.
To get this improved stability and analytical capability, the immediate availability
of the results is sacrificed. The samples need to be drawn to the surface prior to being
analyzed, meaning the data being generated can be from samples collected from over
an hour before. There is only one bank of analyzers, so only one sample is analyzed at
a time. Depending on the number of tubes in the system and the programmed sampling
sequence, each point may only be sampled once every 30e60 minutes. Add this to the
time taken to draw the sample from underground, which may be as long as an hour,
and it is obvious that this technique is not suitable for the instantaneous detection of
an incident such as a fire.
Because the analyzers in these systems rely on infrared absorbance and paramag-
netic attraction, the gas matrix is not important, making this technique suitable for
the analysis of gases from oxygen-depleted areas such as the gob. The measurement
of oxygen using paramagnetic analyzers is flow rate dependent, and the flow from
each tube must be balanced to be the same, including any calibration gases used.
Otherwise, it is possible that two locations could in fact have the same oxygen concen-
tration, but because of more resistance in one of the tubes, the flow through the
analyzer is at a lower flow rate and as such results in a lower reading than a location
with the same concentration but flowing through the instrument at a faster rate.
In the event of a mine explosion, the tube bundle monitoring system may still
appear to be functional, but the location from which tubes are sampling may not be
the same, due to damage to the tubes. A good tube bundle system will include moni-
toring of the vacuum pressure in each of the tubes, so following an explosion this data
can be used to determine whether a tube has been compromised or not. It is also useful
during routine operation for identifying increased restriction or sudden leakage in a
tube, both of which can compromise the operation of the system.
If the tubes are damaged and not providing any valuable information, it may be
possible to make use of boreholes and connect new tubes to locations of interest as
the surface equipment will still be operational. This is the preferred technique in the
United States, although quite expensive.

19.4.3 Gas Chromatography

Gas chromatography, with regard to gas analysis, involves the separation of all sample
components followed by their measurement on relatively nonspecific detectors.
Specificity is obtained by virtue of the separation process rather than detection.
The use of a GC expands analytical capabilities to include gases crucial in the inter-
pretation of spontaneous combustion events, particularly ethylene and hydrogen. The
GC provides a complete analysis of the gases expected underground and is the only