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1. The problem with the system is that the inlet
pressure setpoint is 1.5 psi (see pressure/
flow calculator on the previous page). The
actual inlet pressure is 5.9 psi. This results
in a "Not Ready" status on the GC and signif-
icantly increased column flow and linear
velocity:
A column flow of 13.17 mL/min instead of
the desired 2.93 mL/min
An average linear velocity of 130 cm/s
instead of the desired 33.3 cm/s
2. Proportional valve 2, which functions as the
inlet back-pressure regulator is set fully open
by the EPC control in order to lower the inlet
pressure to the setpoint of 1.5 psi. The inlet
pressure cannot go down because of restric-
tion in the split vent flow path. There are
five possible sources of this restriction:
Released: April 2004
6850 Series Gas Chromatograph Service Procedures
• Insufficient clearance for flow at the
interface of the glass injection port liner
and the gold seal at the base of the inlet
• The clearance between the outside diam-
eter of the glass liner and the inner sur-
face of the injection port body
• The 1/8-inch copper tube between the
injection port body and the split vent
trap may have condensed sample mate-
rial that has accumulated after repeated
injections of viscous samples
• The split vent trap may be of the older,
more restrictive design or may be getting
plugged by condensed sample
272
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