31
Using a Blower to Eliminate Refraction
Certain conditions can cause refraction and
scintillation to occur inside the pipe, with the
laser dot appearing to be dancing all around
the target.
The laser light passes through a medium – air.
Anything that disturbs the air will cause the
light to scintillate (dance). The two main
causes of disturbed air are temperature
difference and glue used to connect the pipes.
Before being put in the ground, the pipe is
stored at the job site in the sun and heat. The
pipe is placed into the ground, which is much
cooler than the road or dirt surface. The pipe
has heated air and will continue to heat the air
until it cools off. The turbulent flowing air in
the pipe will cause the laser dot to dance.
A solution is to use a blower to move cool air
through the pipe. Set the blower at about 5 mph. A dangling dollar bill placed at the outflow
end of the pipe will be at a 45° angle in 5 mph of flow – sufficient to cool the pipe in about 20
minutes.
Glue, as it dries, gives off vapors. These vapors are a difference density, and possibly
different temperature, than the air. This also causes the dancing dot. Wait until the glue is
dried enough so that the laser dot is fixed for a good reading.
OPTIONAL EQUIPMENT
Blower for Refraction Control
Minimizes laser drift for long pipe runs where there is more that 20° F temperature difference
between the trench top and bottom. P/N 1-08915
Target Panel
For 12 inch and 15 inch (300-400 mm) pipe. Can be used with standard frame. P/N 1-11254
Target Panel Assembly
For 15 inch ( 375 mm) and larger. P/N 1-11217
Tall Leg Set
For 15 inch to 24 inch pipe: Order two (2) side legs (9-09622) and one (1) center leg (9-
09623).
For 375 mm to 600 mm pipe (metric): Order two (2) side legs (9-09606) and one (1) center
leg (9-09607).
Optical Instruments for Checking Elevation
Digital Level and Bar Code Rod
One push of a button provides digital elevation and distance. Eliminates rod reading errors.
P/N 1-16612
AL26-C Automatic Level
High accuracy, wire hung magnetic compensator with 26tX magnification. P/N 1-16613
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