3-46 | Model 933S UV Analyzer
Hold Time Optimization
Immediately after a column switch, the real-time concentrations of H
2
S,
COS, and MeSH drop temporarily. During this period, the output concen-
trations are held constant until the real-time concentrations reach stable
values. The Hold Time for each species is the duration that the concentra-
tion is held constant after the columns are switched.
At the end of the hold period, the Hold Time for each species should be
adjusted so that the real-time concentration of the gas is between the
T
90
concentration (90 % of the stable plateau value) and the stable value.
Refer to the lower graph in Figure 3-24. Setting the Hold Time to end at,
or slightly after, the T
90
point for the species, will result in the fastest re-
sponse time but will give the noisiest output signal. Setting the Hold Time
to end after the concentration has reached a stable plateau will give the
smoothest output signal, but will result in a longer response time.
For the H
2
S output, the factory default Hold Time should be correct. It
is usually set up so that at the end of the hold time, the H
2
S output is
between the T
90
concentration and the stable value. If a slightly faster
response time is required, the Hold Time can be reduced slightly (see pro-
cedure below). If there is too much noise on the output (the downward
spikes that occur between when the Hold Time ends and the concentra-
tion stabilizes) then the Hold Time can be increased by a few seconds.
If the analyzer is equipped with the optional COS and MeSH Outputs,
their Hold Time have been set at the factory. Before adjusting them, check
the Column Switching Interval (see previous section) and, if necessary,
adjust that parameter. Because there is not usually a requirement for fast
response time on the COS and MeSH outputs, adjusting their parameters
is usually less critical than that for the H
2
S.
The COS Hold Time should be long enough to hold the concentration
constant until the real-time COS concentration has reached a stable pla-
teau. For most applications, the factory default setting should be correct.
The MeSH Hold Time is usually adjusted so that the hold on the con-
centrations comes off several seconds before the columns are switched. If
the Column Switching Interval has been adjusted, the MeSH Hold Time
will also need to be adjusted. Take the new Switching Interval, subtract 2
seconds from it, and use that as the new MeSH Hold Time.
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