Detection Principles - Simtronics GD1 MK3 Operating Manual

10.4.

Detection principles

Unlike a point detector, the GD1 Open Path Detector will not measure the concentration of
the gas in the path between transmitter and receiver. The GD1 measures the total amount
of H S in the path and the value returned by the GD1 is in ppm*m. Below are three
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examples with calculations on what value is expected on the GD1 for different gas clouds.
The drawing above illustrates a GD1 positioned with a gas cloud from a H
laser beam of the GD1. The average concentration of the gas cloud is C ppm H
of the gas cloud is L meters where the beam is crossing. If the path length is smaller than
the gas cloud, the distance L will be equal to the path length. For the gas cloud in this
example, we would expect the following signals from the GD1:
Example 1 - gas cloud characteristics: C = 5 ppm; L = 15 meters
Expected measurement of the GD1 = C * L = 5 ppm * 15 m = 75 ppm*m
Example 2 - gas cloud characteristics: C = 10 ppm; L = 15 meters
Expected measurement of the GD1 = C * L = 10 ppm * 15 m = 150 ppm*m
Example 3 - gas cloud characteristics: C = 150 ppm; L = 1 meters
Expected measurement of the GD1 = C * L = 10 ppm * 15 m = 150 ppm*m
Observation 1! Same cloud size will give same measurement irrespective of path length
(this provided that the gas cloud is smaller than the path length).
Observation 2! If cloud size is known, the average cloud concentration can be calculated by
dividing the ppm*m measurement with path length. This can typically be done only if a
spreading analysis shows that the cloud always will cover the whole path length between
the TX and RX, for example if the GD1 is placed sufficiently far from the leak point.
GD1
Width of cloud: L meters
Path length
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