Metrohm 875 KF Gas Analyzer - Manual (Page 31)

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4 Operation
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0
200
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600
800
1,000
1,200
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0
5
10
15
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Gas flow [mL/min]
Time (min)
Drift [mL/min]
Start of post-rinsing
with nitrogen
Drift [mL/min]
Gas f
low [mg/min - mL/min]
Figure 8 Typical shape of the gas flow curve and drift curve
Gas type
Propene Sample amount
1.25 g
Minimum sample amount
0.5 g Vaporization temperature
70 °C
4.2.4 Method "Reference measurement"
The trueness of the analysis can be checked by measuring water-spiked
reference gases using the Reference measurement method.
Control gases with certified water contents are commercially available.
The Reference measurement method relies on the nitrogen calibration
of the mass flow controller integrated in the instrument; i.e., it only deliv-
ers correct values if nitrogen is used as reference gas. The procedure for
reference measurement is the same as the one applied for sample mea-
surement. The result is indicated as a recovery rate in percent.
4.2.5 Changing the gas type
If the measurement of a new sample coincides with a change of the gas
type, then the flow rate of the precision control valve has to be adjusted
to the current sample using the Precision control valve setting
method. This method sets the setpoint value at the MFC to the maximum
value of 5 L/min and graphically displays the current flow by utilizing the
internal nitrogen calibration. In order to prevent a pressure rise in the area
after the regulator, the precision control valve has to be set in such a way
that its vaporization rate is lower than 5 L/min and the setpoint value is
not reached at the MFC. After the start of the method, follow the instruc-
tions of the text messages and adjust the precision control valve so that
the gas flow is within the required limits (definition by common variable).