Metrohm 875 KF Gas Analyzer - Manual (Page 32)

4.2 Methods
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NOTE
Please note:
This method does not use the calibration factor that is assigned to this
gas type. The mass flow displayed during the subsequent analysis may
therefore considerably deviate from the value that was set when the
precision control valve was adjusted.
4.2.6 Calibrating a new gas type
At the factory, the mass flow controller is calibrated to nitrogen. If the
instrument is to be operated with a different gas, then the flow value has
to be corrected by an appropriate factor. These correction factors are
determined gravimetrically by letting larger amounts of gas flow through
the MFC and monitoring the weight reduction of the gas container. The
quotient of the gas volume indicated and the weight difference is the cor-
rection factor. This factor is in the range between 0.5 and 1.5 mL for most
liquefied gases. The correction factors have to be individually determined
for each flow controller using the Gas calibration method. This method
saves the correction factor in the tiamo™ configuration as common vari-
able. In order to achieve a sufficient level of accuracy, the sample weight
difference should have at least three significant places. The balance used
therefore has to offer a corresponding resolution and maximum weight in
accordance with the gas bottle size. For the determination of the calibra-
tion factor, the gas container has to be connected to the 875 KF Gas Ana-
lyzer with the flexible plastic capillary (OD 1/16'') enclosed in the scope of
delivery, as steel capillaries transmit vibrations to the balance.
Samples should be taken from the gas phase Gas calibration_gas rather
than the liquid phase of the gas container Gas calibration_liquefied
gas for calibrations, because the flow pattern is much more uniform if
vaporization does not take place in the 875 KF Gas Analyzer. The Gas cal-
ibration_liquefied gas method is only to be used if a water content
determination is to be done for the same gas container after calibration.
The procedure to determine the calibration factor is described below step
by step using butadiene as an example:
1 You can find the correction factors for the gases you have used so
far in the Common Variable subwindow in the tiamo™ configura-
tion. Templates with the designation "additional gas type x" (x = 1 to
9) are stored for adding further gases. The common variables can be
rendered editable via Edit
▶ Properties. Replace the blank variable
additional gas type x with the lowest number x by the term buta-
diene.