User guide

330 Advanced User Guide
10 Detectors
• the detector is at the temperature setpoint
• temperature is at least 150 °C
• gas flows are on
• Dry Bead time, if On, has elapsed
Bead voltage is also useful for small adjustments between runs. If
you observe a baseline drift, you can enter a small, one- time
change to compensate for the drift.
If you are not using the recommended Adjust offset process,
note that large voltage jumps reduce bead life. Use increments
no greater than 0.05 V, spaced 10 seconds apart, until you reach
the desired offset.
New beads
After a new bead reaches the initial voltage, begin to increase
the voltage value in 0.05 V increments until the bead ignites.
Wait about 10 seconds between each voltage adjustment.
Monitor the detector output. When the bead ignites, the output
will rise suddenly, then decrease towards a more stable value. It
is best to allow the NPD to remain in this state without further
adjustment for about 24 hours. Then you may adjust the bead
voltage in small increments (0.05 to 0.1 V) until reaching the
desired offset. With a clean environment, clean gas supplies,
and low bleed column, a typical offset may decrease 6- 12 pA
during a 24 hour period.
Typical voltages for new ceramic beads range from 2.5 to
3.7 volts. Higher values reduce bead life.
Typical voltages for new Blos beads range from 0.5 to 1.0 V.