BOREAL LASER GasFinder FC, Operation Manual

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7.4
Boreal Laser Inc. GasFinderFCr Operation Manual — Troubleshooting July 17th 2017
Stability of Support Structures
The diameter of the laser beam when it exits the remote head is about 1.5mm and the
divergence is approximately 1.5mrad (milliradians). This means that at a distance of 100m
the beam diameter is 1.5 x 100 = 150mm (6”). Similarly, at a distance of 1,000m, the beam
diameter would be 1,500mm (59”). The beam intensity falls off towards the edge.
The remote head can be mounted on a tripod, post, or platform, and directs the laser beam
to a refl ector, which can be from a few metres to several hundred metres distant. This article
deals with the stability requirements for the mounting structure.
Calculating the Effects of Movement
Consider the example of a 150mm-diameter retrorefl ector array located 100m due north of
the remote head.
The post, and therefore the remote head, can move vertically or horizontally 150mm (6”)
either side of the centreline before the beam will move completely off the retro array. This
movement is referred to as
translational . In practice, signifi cant translational movement is
unlikely to occur, as the post is usually fastened to a solid base. However, because it is fi xed
at the bottom,
torsional or bending movement is much more likely, due to wind loading or
thermal distortion. Any East - West bending (i.e. perpendicular to the centreline) is manifested
as a lateral motion and will have a minimal effect. Conversely, any North - South bending will
have a serious negative effect: the top of a 3m (~10’) tall post can only move 4.5mm (0.177”)
before the beam will have moved 150mm all the way off the retro. For a 500m path length,
with the retrorefl ector array doubled in diameter to 300mm (12”), the top of the 3m post can
only move 1.8mm (0.071”) before the beam will completely miss the retrorefl ector. A 10m post
can move 6mm (0.236”) and cause the same result.
Note: Movement of the retrorefl ectors has little effect compared to movement of
the remote head. The retro support does not require the same structural stability
as that required by the remote head support.