HP 5890 Series II Plus Reference Manual page 131

Detector Systems
Nitrogen-phosphorus detector (NPD)
Other gas flow effects of too high flow rates of the hydrogen may allow a
true flame to exist around the active element. This would overheat the
active element severely and destroy the specific response. Too low flow
rates of air tend to quench the background response of the active
element, and this results in a re•equilibrationtime that is too long to
establish proper background response (negative solvent peaks killing the
active element).
Figure 7-4
Final Pressure Regulators
and Gauges
Traps
Compressed
Air
Hydrogen
Gas
Capillary
Makeup
Gas
Flow Diagram, Nitrogen-Phosphorus Detector (NPD)
External Internal
Plumbing Plumbing
Flow Manifold Block
Active Element
Power
Power to
Active Element
AIR
On/Off
HYDROGEN
On/Off
Hydrogen
(Plus Makeup
Gas)
MAKEUP GAS
On/Off
Detector
Air
From
Inlet
Column
131