Replacing the NPD Bead
Instruction Guide 17
NPD Cleanliness
Below is an overview of typical NPD leakage currents for a
system set to recommended flows and temperature.
When changing insulators and seals, cool the detector to 75 °C
or below. This will minimize the chance of stripping the three
T- 20 Torx screws.
The installation of traps is recommended for the detector,
hydrogen, air, and make- up gas. A moisture trap followed by
chemical traps is highly recommended.
pA Leakage Current System status Comment
0.3-0.9 CLEAN
1.0+ Dirty Decrease in sample response.
10+ Very dirty Almost no sample response.
Cause is usually contaminated ceramic
insulators on the collector - usually the
upper insulator and seal.
NPD Bead Replacement.book Page 17 Monday, October 13, 2008 2:32 PM
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