Heath Consultants Optical Methane Detector (OM) - Manual (Page 7)

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Chapter I
OVERVIEW
The Optical Methane Detector (OMD™), is the first of the
“next generation” leak detection systems which address the
high cost presently associated with leak surveying. This new
technology combines sensitivity, selectivity and speed through
the combined use of optics and electronics. The OMD is
mounted on the front of a survey vehicle and detects the pres-
ence of methane in air. It provides for the detection of gas
leaks down to 1 part per million (PPM). It has no moving parts
that will prevent the instrument from operating. It requires no
support gases and is virtually maintenance free.
Based on the absorption of infrared (IR) radiation by methane,
the OMD consists of an IR light source directed at an IR
optical detector. These two are spaced approx. four feet apart
across the width of the vehicle. An optical filter in front of the
detector transmits primarily methane specific IR wavelengths
from the light source. In the absence of methane, these wave-
lengths are unaffected and produce a steady output signal from
the detector. When the vehicle passes through a plume of
natural gas, methane in the plume will absorb some of the IR
light reaching the detector. As a result there is a sudden de-
crease in IR light at the detector causing a decrease in detector
output. This change in detector output is transmitted to the
display panel located next to the vehicle’s driver and becomes
a signal, both audio and visual. Gas concentration is calculated
in the OMD by converting the methane to light ratio to PPM.
The data is displayed in both analog and digital form. The
analog display appears as a peak scrolling across the display
screen. The OMD will also detect other hydrocarbons such as
ethane and propane but with less than half the sensitivity.