Mettler Toledo M400 Guide Rapide page 25

Therefore the following procedure should be used. Start with a very high flow rate
and gradually decrease it. The measurement value will then start at a low value and
increase with decreasing purge flow. At some point it will level out and stay constant
for a while and then again start increasing. Choose a purge flow in the middle of the
constant region.
2
1
Decreasing purge flow
Optimizing the purge flow
On the x-axis there is purge flow and on the y-axis there is the instrument concentra-
tion reading.
1 Concentration reading with high purge flow. The path length is now shorter than
the effective path length since the purge tubes is completely filled with purging gas
and some of the purging gas is flowing into the measurement path.
2 Concentration reading with optimized purge flow. The path length is now equal to
the effective path length since the purge tubes are completely filled with purge gas.
See the illustration below.
3 Concentration reading with no purge flow. The path length is now equal to the
nominal path length since the probe is completely filled with process gas.
4 The optimized purge flow.
h
WARNING: Always start purging at maximum flow before starting the process.
h
WARNING: Purging must always be switched on in order to avoid dust deposition
onto the optical surfaces.
4
3
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