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Agilent 6890 Gas Chromatograph Service Manual
Volatiles Interface 250
Theory of operation
Before Pre Run, the interface is forward pressure controlled; pressure is
sensed downstream from the flow proportional valve. See Figure 250-4.
During sampling, pressure upsets caused by switching valves in the external
sampler can cause fluctuations in column flow rates. To compensate for this,
the interface is flow controlled during sampling time. The sampling flow rate
is calculated from the pressure setpoint that is active when sample
introduction begins. This flow control starts when the GC goes into the Pre
Run state (when your system is automated and the Pre Run light is on or
during manual operation when you press [Prep Run]) and ends after the
interface’s Sampling end setpoint expires.
Flow to the interface is measured by a flow sensor and controlled by a
proportional valve. See Figure 250-3.
After sampling end, the interface is forward pressure controlled; pressure
is sensed downstream from the proportional valve. See Figure 250-4.
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