8
TheOMDoperatesreliablyunderavarietyof environmental
conditions including dry weather, snow, ice and temperatures
from-22° F to+122° F. Itssensitivity,calibration or response|is
notaffectedbysmall fluctuations in thelightbeamorbyreasonable
amountsofdust,or snow ontheoptics. Aninternalcalibrationtest
cellisincludedso the operatorcanverifyproperoperationfrom the
vehiclecabat any timebefore,duringor after aleaksurvey. An
InstallationMountingKitisavailableformountingtheOMDtothe
frontofavehicle. Allinterconnectingcablingisprovidedwith the
OMDincludingthecableto obtain powerto operatetheinstrument
fromthesurveyvehicle 12 voltbattery. AnRS232 port isincluded
wherebya personal computermaybe connected toacquire 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.
Examplesoffalsepositives experienced includethedetectionof
OVERVIEW
