Heath Consultants Optical Methane Detector (OM) - Manual (Page 48)

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PPENDIX
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low gas to vent rather well. These cracks are subject to
filling with water if there is thawing or rain which tem-
porarily reduces venting.
The most ideal condition for venting during a winter
survey is sustained weather below freezing with clear,
dry streets. The presence of frozen ground will cause
leak locations to spread further in the soil, which often
results in gas seeping into more available openings. As
a result, your sampling pattern should include as many
such openings as possible.
G. Wind Velocity
Wind velocity has, generally, more influence on the abil-
ity to perform a successful survey than any other envi-
ronment factor. Ideal conditions are, of course, at such
times as the wind velocity is zero M.P.H. (miles per hour).
Very heavy winds, on the other hand, dilute the sample
and disperse the sample in the air. In several parts of the
country, the problem of prevailing winds can be nulli-
fied by starting the survey earlier in the morning and
ending earlier in the afternoon. If on a survey where the
environmental conditions are hindering his ability to do
a good job, the technician should contact his immediate
supervisor.
H. Location of Underground Structures
The consultant must know the general location of the
gas main and other subsurface structures. Gas will tend
to follow water lines and other substructures, vent near
valves or water meter boxes, etc. In general, the best
sample points are those cracks in the pavement nearest
the main and curb line and any vertical structures such
as risers, building walls, poles, sign posts, etc.