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Jun 2001
Agilent 6890 Gas Chromatograph Service Manual
Thermal Conductivity Detector (TCD)
330
Theory of operation
EPC detector
The EPC TCD has one flow line for the makeup/reference gas. After an initial
filter frit, it splits into two paths. Each line has a proportional valve controlled
by a pressure sensor, and a non-adjustable restrictor frit. In addition, the
switching gas line has a switching valve that switches paths through the TCD
cell several times a second. The switching line oscillates between two
flowpaths, one for each channel of the TCD cell. The makeup gas line sweeps
past the end of the column and carries the column effluent into the TCD cell.
Figure 330-4
EPC TCD flow diagram
Manually controlled detector
The manually controlled TCD has a pneumatic supply line for the switching
gas (also called modulating or reference gas) flow and one for the makeup
gas. Both of the lines have pressure regulators, an on/off solenoid valve and
a non-adjustable restrictor frit. In addition, the switching gas line has a
Filter
frit
Makeup and
reference gas in
Proportional
valves
Pressure
sensors
Restrictors
Reference
switching
valve
Vent
Makeup flow
PS
PS
Pressure
control loops
Summary of Contents for 6890 Series
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