Siemens 7MB1420, Instruction Manual, Page: 7 / 74

4 Mode of Operation
Electrons are released when hydrocarbons are combusted in a hydrogen flame. These electrons
are collected at an electrode by means of an electric field and measured using a highly sensitive
amplifier. The current is proportional to the quantity of organically-bound C atoms in the sample
gas.
A diaphram pump draws the sample gas and generates a specific pressure. The sample gas is
passed into the measuring chamber via an obstruction-resistant fused silica restrictor. The sample
gas is mixed in the measuring chamber with hydrogen or hydrogen/helium (4:6) and with a specific
amount of air and routed via the nozzle into the combustion chamber.
The hydrogen pressure is held constant by a pressure regulator. The balanced system of pump,
restrictors and pressure regulators ensures that the sample gas pressure is kept constant.
The FIDAMAT operates largely automatically. If the parameters (pressures, temperatures) are set,
the device starts up automatically when switched on and ignites when the setpoint temperature has
been reached. The hydrogen and combustion air pressures are measured when switching on, and
the control panel indicates if they are incorrectly set.
1 Flame
2 Quartz nozzle
3 Combustion air
4 Sample gas
5 H
2
6 Measuring electrode
7 Amplifier
Fig. 4 Principle of flame ionization detector
-200 V DC
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