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path detection systems. In order to accurately map the coverage of these devices, one
must understand the detection principles of the technology.
Where the target gas cloud method is selected, we often see open paths showing a
simple cylinder of coverage. This is inaccurate of the detection capability of this
technology. This method of coverage is suitable for point gas detectors where the
target gas cloud will intersect with the point device, providing a gas reading for that
point. For an open path device, however, there must be a given concentration of gas
across a given length of that beam in order to result in detection.
Figure 6: Beam Attenuation Principles (Beam detectors B1 and 2 with different
readings provided by concentrations C1-4)
While this is detail you would not typically expect to find in a performance based
guidance document, it shows the potential for inaccurate mapping to be carried out
when following ISA TR 84.00.07 and using some commercially available tools. This
is of particular concern when, at the time of writing, this is rarely accounted for within
mapping software.