3 of 32
Jun 2001
Detectors
Agilent 6890 Gas Chromatograph Service Manual
Nitrogen Phosphorus Detector (NPD) 320
Theory of operation
Manually controlled detector
The manually controlled NPD has three pneumatic supply lines for air,
hydrogen and makeup gas. All three lines have an on/off solenoid valve and
a non-adjustable restrictor frit. In addition, the makeup gas line has a pressure
regulator to change flow without changing supply pressure. The makeup gas
and hydrogen lines merge before entering the detector.
Figure 320-3 Manual detector flow
On/off
solenoid
valves
Restrictors
Vent
Air in
H
2
in
Makeup in
Pressure
regulator
Electrically
heated bead
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