Metrohm 875 KF Gas Analyzer - Manual (Page 30)

4.2 Methods
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4.2.3 Explanations regarding the shape of the gas flow and titration
curves
The analysis procedure described in (see Chapter 4.2.1, page 18) results in
a characteristic shape of the gas flow and titration curves. The sample
infeed phase concludes with the gas flow dropping to a value close to
zero. The titration rate (drift) follows this drop with a delay of
approx. 10 seconds. If the gas flow is below a threshold value defined as
common variable for 6 seconds, then the nitrogen valve opens and post-
rinsing commences.
The amount of water detected in the postrinsing phase increases if the
samples contain nonvolatile components that remain in the vaporizer and
the oil filter. The distribution of the liquid and the gas phase balances out
during the infeed phase, so that, at the end of the infeed phase, a part of
the water contained in the sample is still present in the instrument's pip-
ing. Postrinsing serves to remove the retained water. Hydrophilic, nonvola-
tile sample components, such as glycol ester oils used in the refrigerant
industry, for instance, therefore lead to a flattening of the drift curve dur-
ing the infeed phase and as a result to an extension of the analysis time.
As a general rule, the minimum titration time (extraction time) has to
extend beyond the beginning of the postrinsing phase, as the titration
would otherwise be finished in the "trough" between infeed and postrins-
ing. The control program uses the following formula to calculate the
extraction time:
Figure 7 Formula for calculating the extraction time
te Extraction time m Minimum sample amount in mg
v value in mg/min saved under CV.mean
mass flow tn Value in sec entered under CV.time for
postrinsing
The default value of the time for postrinsing common variable is
3 minutes. If a sample requires a longer postrinsing phase, then the value
must be increased accordingly.