Thermo Scientific FOCUS GC, Instruction Manual

Page: 175 / 356

Chapter 9
TCD Operating Modes Thermal Conductivity Detector (TCD)
Instruction Manual 175
Columns
The TCD use two separate gas lines. One connects to the analytical column and
the other connects to the reference channel.
The column effluent connects to the analytical cell along with the make-up gas,
from the DGFC pneumatic detector module.
When using capillary columns, the make-up line must be activated. This line has
to feed the analytical channel at the column outlet, thus compensating the low
column flows used with the capillary columns. When using packed columns, the
make-up gas is not required and can be kept closed.
For column installation instructions, refer to Chapter 6, Column .
TCD Operating Modes
The TCD can operate in constant temperature or constant voltage mode. It can
also automatically switch to a constant current mode when the filaments reach the
maximum allowable current value of 125 mA.
Constant Temperature
In constant temperature mode, the filament temperature remains constant at a set
value. A feedback loop circuit changes the voltage as the gas thermal conductivity
changes. If the required voltage reaches the maximum allowable value of 15 V,
the system will automatically switch to the constant voltage mode.
The CT configuration ensures the maximum linearity of the detector up to
concentrations of 1%.
Constant Voltage
In constant voltage mode, the filament voltage remains constant at a set value. The
temperature variation, positive or negative, generates a current variation, negative
or positive, that will give the corresponding signal. The voltage values range from
5 to 15 V. If the current reaches the maximum allowable value of 125 mA, the
system will automatically switch to the constant current mode.
When the constant voltage mode is used, it is necessary to set the filament