Refraction
1. Refraction is the bending of light at the interface of two substances caused by a difference in
the speed of light. Shooting a laser beam through a pipe having gas and varying temperatures
can be compared to shining a flashlight beam into a pool of water and seeing the light bend.
2. Refraction of the laser is preceded by what appears to be a flaming of the laser spot at the
edges. When this occurs, it is time to mix the air within the pipe by some external means such
as a blower. This occurs on sunny days when a hot pipe is placed in a cool ditch. The pipe
bottom cools first and the warmer air in the upper half of the pipe causes the laser beam to
refract down towards the cooler air.
3. Solvent fumes can also cause refraction from the glue used at joints, or on very cold days when
a cold pipe is placed in a warm trench.
4. Refraction is a small diameter pipe problem. When small diameter pipe is set, it is
recommended to place the hot-sun-baked side of the pipe down in the trench so that it will
cool faster against the earth. This helps create temperature stability in the pipe.
5. Generally, refraction of the laser beam occurs at the last four or five joints of the pipe. This is
because the last few sun-baked 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.
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