Manual Mode Zeroing; Cell Failure - Teledyne 7120 Operating Instructions Manual

4 Operation
4.4.1.2
Scroll to Zero and enter the Zero function. The screen that appears allows
you to select between automatic or manual zero calibration. Use the UP/DOWN
keys to toggle between AUTO and MAN zero settling. Stop when MANUAL
appears, blinking, on the display.
Enter to begin the zero calibration. After a few seconds the first of three
zeroing screens appears. The number in the upper left hand corner is the first-
stage zero offset. The microprocessor samples the output at a predetermined
rate.
The analyzer goes through C–Zero, F–Zero, and S–Zero. During C–Zero
and F–Zero, use the UP/DOWN keys to adjust displayed Zero adj: value as
close as possible to zero. Then, press Enter.
S–Zero starts. During S–Zero, the Microcontroller takes control as in Auto
Mode Zeroing, above. It calculates the differences between successive sam-
plings and displays the rate of change as Slope= a value in parts per million per
second (ppm/s).
Generally, you have a good zero when Slope is less than 0.05 ppm/s for
about 30 seconds.
Once zero settling completes, the information is stored in the analyzer's
memory, and the instrument automatically returns to the Analyze mode.

4.4.1.3 Cell Failure

Cell failure in the 7100 is usually associated with inability to zero the
instrument with a reasonable voltage differential across the Wheatstone bridge. If
this should ever happen, the 7100 system alarm trips, and the VFD displays a
failure message.
Before replacing the sensor:
a. Check your zero gas to make sure it is within specifications.
4-18

Manual Mode Zeroing

Select zero
mode: MANUAL
####.##
Zero adj:2048 C—Zero
####.## %
Slope=#.###
Cell cannot be balanced
Check your zero gas
Teledyne Analytical Instruments
% CO2
CO2
S—Zero
Model 7120