Setting Npd Bead Voltage Manually (Optional) - Agilent Technologies 7890 Series Advanced Operation Manual

5
Detectors

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 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.
Advanced Operation Manual 199