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7 Refraction
68 DG711 and DG511 Pipe Laser User Guide
joints of the pipe have not had a
chance to cool in the shade of, and
against the earth of the trench.
6 If possible, always back-fill the
pipe as you go. The earth is a great
temperature stabilizer as it
surrounds the pipe. The earth tends
to stabilize the temperature of the
pipe and the air inside. Use of a
blower in conjunction with back-
filling accelerates temperature
stabilization. Always back-fill
immediately unless the jobsite
condition makes it impossible.
7 To fully minimize the effects of
refraction, it is advisable to use a
blower after the pipe run exceeds
15 m (50 ft). Operate the blower at
a speed that minimizes the effects
of refraction. As the pipe length
increases, increase the speed of the
blower.
8 When using a blower, make
sure it is taking in fresh air through
the intake. On the discharge end,
the nozzle should be angled to
bounce the air off the side of the
pipe thereby creating a spiral or
corkscrew effect.
9 If possible, mount the nozzle on
the side of the pipe or in the invert.
Avoid attaching the nozzle at the
top of the pipe unless there is no
other choice, as this will push the
warmer gas through the pipe. If a
strong wind is blowing down the
pipe and fighting the blower, it may
not be needed.
10 In large diameter pipe, 45 cm
(18 inches ) and larger, it is better
to position the blower nozzle off
the invert below the laminar
surface of the gas. If this is
insufficient, three alternatives are
possible. Use two electric blowers,
a Homelite Model III-B (or
equivalent) gas driven blower or a
short blast of air from an air
compressor, to clean out the line.
Nozzle