94 Advanced User Guide
5 Detectors
Turning off the detector
Setting the initial bead voltage for new beads
Before you turn on the bead for the first time, manually set its
voltage to a safe value so that the new bead is not destroyed.
1 Make sure Adjust Offset is turned Off.
2 After the temperature stabilizes at setpoint, set the initial
Bead Voltage, depending on bead type:
• Ceramic bead (white or black): 0.0 V to 2.0 V
Setting NPD bead voltage manually (optional)
Bead voltage shows the voltage used to heat the bead. It can be a
value derived from the Adjust offset value, or can be entered as a
setpoint. Entering a setpoint causes the voltage to change at
13 mV/second until it reaches the setpoint provided that
• 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
CAUTION
If you turn Adjust offset [Off] at any time, the bead voltage,
hydrogen, and air flows all turn off.
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