Heath Consultants Optical Methane Detector (OM) - Manual (Page 8)

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The OMD operates reliably under a variety of environmental
conditions including dry weather, snow, ice and temperatures
from -22° F to +122° F. Its sensitivity, calibration or response |is
not affected by small fluctuations in the light beam or by reasonable
amounts of dust, or snow on the optics. An internal calibration test
cell is included so the operator can verify proper operation from the
vehicle cab at any time before, during or after a leak survey. An
Installation Mounting Kit is available for mounting the OMD to the
front of a vehicle. All interconnecting cabling is provided with the
OMD including the cable to obtain power to operate the instrument
from the survey vehicle 12 volt battery. An RS232 port is included
whereby a personal computer may be connected to acquire and
save the survey data with the appropriate software.
Depending on local meteorological conditions, a given amount
of gas escaping from the ground will produce a plume that
varies in size and uniformity of concentration levels. The
plume, of its very nature, defies quantification by fixed or
moving detectors. Regardless of these uncertainties, it is of
paramount importance to be able to detect gas when gas is
present. Concentration levels of the measured gas are of
interest, but are of secondary importance. Consequently, the
ability of a detector to detect the plume with accuracy and
precision is not as important as its ability to do so with good
sensitivity (or a low detection limit) and with a minimum of
false alarms.
There are two kinds of false alarms: false negatives and false
positives. False negatives occur when a leak is missed. The
detector is saying there is no gas present when, in fact, a leak
was present but undetected. False positives occur when the
detector says that a gas leak is present when it really is not.
Examples of false positives experienced include the detection of
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VERVIEW