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Appendix B
5/20/2009
21
contaminated with air or oxygen at levels generating an
imbalance of about 450 uv or higher. The safety circuit will
activate should leaks develop or when the helium supply is
depleted. The safety circuit monitors the gross imbalance
between the two sides of the nitrogen bridge. If air or
oxygen is present on both sides of the bridge, the safety
circuit may not activate and damage to the detectors may
occur.
Make certain that helium gas is flowing and that the
instrument is purged before pressing the DETECTOR
RESET button.
The safety circuit is also activated when accidental or
deliberate power interruption occurs. If power has been
interrupted for more than 5 minutes, do not push
DETECTOR RESET until the system has been run as if to
run a blank. Do not hold the DETECTOR RESET button in
or more than one second. If the light stays on when the
button is released, further running is necessary before
pushing the button again. Go through one blank run before
turning on the detector.
17.1.1.10.After the last run go to the Service Menu and monitor the
bridge readings. Adjust the “zero” reading to approximately
2500 µv by turning the respective potentiometers on the
Bridge Balance Card located in the left rear corner of the
“Motherboard”. Typically the bridges should be set well
above negative or zero to approximately + 2500 µv. This is
after the instrument has stabilized. Stability is based on
furnace and oven temperatures being steady for a period of
not less than 1 hour.
17.1.1.11.Check the furnace and oven temperatures. If these have
reached operating levels, let the instrument go through
another three sets of runs in order to purge the system and
condition the reagents. This can be done through the CHN
Run Mode (Run Menu).
17.1.1.12.Turn off the B-valve using the Parameters mode in the
Customize pull-down menu. Continue running helium
blanks until the base line (zero reading) is steady and/or until
the blank for nitrogen and carbon is less than 200 µv, and for
hydrogen less than 1500 µv.
17.1.1.13.Turn ON the B-valve and run oxygen blanks until
consecutive runs agree within 10 µv for nitrogen and carbon,
and 50 µv for hydrogen.
17.1.1.14.Go to the Service pull down Menu and calibrate all of the
CEC-490 again.
17.1.1.15.The instrument is now ready for system calibration with
known standards.