BOREAL LASER GasFinderMC-2, Operation Manual

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9.4
Boreal Laser Inc. GasFinderMC Installation Manual — Appendices July 17th 2017
Appendix C — Explanation of ppmm
Gas concentration is measured in ppm, ‘parts per million’ by volume. If a room measuring
100m x 100m x 100m (1 million cubic metres) has 10 cubic metres of air replaced by a pure
gas, then the gas concentration is expressed as 10ppm. A point sensor measures directly in
ppm. Open path monitors, like the
GasFinderMC , measure the total amount of a specifi c gas,
for example HF, in the path of the laser beam between the transmitter unit and a refl ector.
This is a ‘total path’ measurement. The units are ‘parts per million metres’, or ‘ppmm’.
A uniform background concentration of 10ppm over 50m gives a reading of 500ppmm.
10ppm x 50m = 500ppmm
When the 500ppmm is divided by 50m the result is a value of 10ppm, which is the ‘path
averaged’ concentration.
A concentrated cloud of 50ppm, 10m in diameter, in a background of 0ppm also gives a
reading of 500ppmm. 50ppm x 10m = 500ppmm
50ppm 50ppm
50m
10m
OP3 Remote Head
Retrorefl ector
Retrorefl ector
10ppm
50m
OP3 Remote Head