Page 338 - Advanced Mine Ventilation

Measurement and Monitoring of Mine Gases 315

one leg of the bridge is used to burn methane catalytically raising its temperature. The
imbalance in the current flow is a measure of methane concentration. Such detectors
are also sensitive to the presence of higher hydrocarbons, hydrogen, and carbon mon-
oxide, but fortunately, these interfering gases are present in mine air only in parts per
million (ppm) and do not introduce serious error in methane measurements. Catalytic-
oxidation type instruments are liable to get damaged if methane concentration exceeds
5% because of excess heat.
Optical detectors for methane are called “interferometer.” These detectors compare
the speed of light through pure air with that in air contaminated by methane. Light trav-
eling through both media is combined again producing “interference fringes.” The po-
sition of these fringes indicates methane concentrations. These instruments are also
sensitive to other hydrocarbons and hydrogen. Ethane and propane present in air indi-
cate a higher concentration of methane, but CO and H 2 have an effect of lowering the
actual methane concentrations. It is also sensitive to water vapor and carbon dioxide,
but these gases are scrubbed out by passing the inlet gas through a column that absorbs
both of them. One percent of CO 2 in air will indicate 1% methane even if there is no
methane there. Lack of oxygen also impacts an interferometer. Each 1% decrease
below 20.95% (normal O 2 in air) results in 2% methane reading even if there is no
methane in air. As such, it is not a very reliable instrument for 0%e5% methane. It
is much more useful to measure higher concentrations, 5%e100% of methane.

19.1.2 Oxygen Detectors

A flame safety lamp was the most commonly used device to measure oxygen defi-
ciency in past, but it is now replaced by new instruments that work on various princi-
ples, such as liquid absorption, paramagnetic, or electrochemical cells. Many
instruments measure both methane and oxygen concentrations over the range
0%e5% and 0%e21%, respectively. Stain tubes are also available to measure O 2 con-
centration in mine air.

19.1.3 Carbon Monoxide Detectors
As listed in Table 19.1, five different techniques are available to measure CO in mine
air. Ambient CO concentration is normally checked by a handheld instrument that uses
catalytic oxidation, but the readings can be seriously impacted by other gases present
in air, especially higher hydrocarbons. For detecting spontaneous combustion, reliable
measurements are given only by gas chromatographs (GCs). Stain tubes are good in-
dicators, but it must be followed by at least two samples of the mine air for GC anal-
ysis. In many cases, when the handheld CO monitor read 100e200 ppm, the actual CO
concentration as measured by a GC was only 5e10 ppm.

19.1.4 Oxides of Nitrogen

With the introduction of diesel engines in coal mines some 50 years ago, the need to
measure both NO and NO 2 has become urgent. Most commonly stain tubes with a