Page 344 - Advanced Mine Ventilation

Measurement and Monitoring of Mine Gases 321

19.4.1 Real-Time Monitoring

Real-time sensor systems are ideal for telling us what is happening now. The sensors
must be located where the gas needs to be measured, and the measurement signal is
sent to the surface. This means having multiple sensors underground. These sensors
are exposed to the harsh underground environment which is not ideal for precise
analytical measurements. This is not really a major problem as these systems are
used to detect step changes, such as the onset of a fire, a sudden increase in a mine
gas in the general body, or reduction in oxygen. They offer real-time warning and
are the best system for identifying a sudden event such as a belt fire. The situation
is reported when it happens. Generally, sensors included are for methane, carbon mon-
oxide, carbon dioxide, and oxygen.
These types of sensors employed underground tend to have limited measuring
ranges: carbon monoxide is often only capable of being measured up to 50 ppm,
methane to 5%, and carbon dioxide to several percent. This range is fine while no
problems exist and indeed to alert the onset of a problem. But if a fire or other major
incident involving generated gases occurs, these sensors may quickly reach full scale
and become unable to return a true indication of the concentrations.
Most of these sensors require the presence of oxygen to work and are therefore un-
suitable for monitoring areas of low oxygen concentration such as sealed or nonventi-
lated gobs.
As each individual sensor needs to be calibrated regularly (at least monthly), they
are not suited to being located for long-term monitoring in inaccessible areas such as
the gob.
Some of these sensors also suffer from cross sensitivities, as the reactions they rely
on to give a response that can be common to other gases found underground, such as
carbon monoxide sensors being cross sensitive to hydrogen sulphide and hydrogen.
In the case of an explosion, it is likely that the real-time monitoring system will be
rendered inoperable, requiring other techniques for the determination of the status of
the underground environment.

19.4.2 Tube Bundle

Tube bundle systems draw gas samples from designated sampling locations under-
ground to the surface through plastic tubes using vacuum pumps and analyzed sequen-
tially using infrared and paramagnetic techniques. Gases measured are carbon
monoxide, carbon dioxide, methane, and oxygen.
Because the analyzers are on the surface, tubes can be located in the gob as once
positioned there is no requirement to access the end sampling point.
Tube bundle systems are suited to long-term trend analysis. Very good analytical
equipment is available and can be housed in dedicated air-conditioned rooms on the
surface with the samples dried and passed through particulate filters prior to entering
the analyzer.
Generally tube bundle systems are set up to measure oxygen, carbon monoxide, car-
bon dioxide, and methane. Given their ability to measure carbon monoxide down to