Metrohm 875 KF Gas Analyzer - Manual (Page 23)

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4 Operation
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4 Operation
4.1 Arrangement of the gas-carrying system
The valve arrangement mounted on the front plate of the 875 KF Gas
Analyzer permits a safe and complete transfer of the sample and the
water contained in it into the coulometric titration cell. The diagram (see
Figure 4, page 16) shows the schematic arrangement of the gas-carry-
ing system.
The sample is introduced into the apparatus via valve 2 (4-7) and vapor-
ized at the precision control valve (regulator). The heating block (4-11)
compensates the heat that is lost in the system due to the enthalpy of
vaporization and thus prevents the water to be analyzed from condensing
or cooling.
The gas-carrying components are automatically rinsed with nitrogen that
is predried in a drying cartridge (4-1) via valve 1 (4-3) before and after the
sample is introduced. This nitrogen rinsing completely removes sample gas
from the piping, so that no errors resulting from dead volumes can occur.
Furthermore, rinsing with inert gas ensures that the water load on seals
and internal metal surfaces in the apparatus is equal before and after sam-
ple introduction. Memory effects can be ruled out in this way.
The sample amount is metered with a mass flow controller (4-5), which
records the amount of gas flowing in and regulates the volumetric flow.
During the introduction of liquefied gases, no pressure may build up
downstream of the precision control valve, as this would entail the risk of
sample condensing upstream of and within the mass flow controller and
possibly interfere with the flow control and damage the instrument. For
this reason, the precision control valve should be adjusted in such a way
that the setpoint value for the mass flow controller is not achieved. As an
additional safety, the system is equipped with a control that closes the
sample input valve if the gas flow exceeds a threshold value defined as
common variable.
When a new sample is connected, the feed line is first prerinsed with sam-
ple via valve 3 (4-12). This is necessary because, initially, the connection
fittings of gas bottles generally release water into the passing sample and
the results of the first measurement without sample rinsing are generally
higher. At the end of the measurement, the user can release the pressure
from the sample infeed via stopcock 1 (4-2) in a controlled manner. The
infeed line is then no longer under pressure when the gas container is dis-
connected.