T
ROUBLESHOOTING
5-4
October 1999 Rosemount Analytical 748250-L
Model 880A Non-Dispersive Infrared Analyzer
Note:
For applications with very low concentrations (for example Range 3 = 400 ppm
CO), the fullscale value at TP2 and the Diagnostics Display may be considerably
less than 7.5V.
5.5 DIGITAL GAIN ADJUSTMENT
The digitally controlled GAIN amplifier does not normally need adjustment, however, in
the event that the analyzer cannot be spanned, the GAIN must be adjusted as follows:
1. Follow the STEPS for Spanning the Analyzer in Sections 3.9 (standard analyzer)
or 3.13 (analyzer with the Calibration Gas Control Option) and span the analyzer.
If the Run Mode value is not between 51% and 100% of the span gas value,
while keeping the span potentiometer between 5% and 95%, then the digital
GAIN should be adjusted. (The ideal span pot setting is 50%.) Note the final
value of the PS (potentiometer status) for Step 4.
2. Exit to the Run Mode.
3. Follow the steps in Figure 3-6 to obtain the GAIN display in the RANGE
parameters menu.
4. Change the GAIN setting to a value higher or lower than the original value. The
GAIN may be changed to 1, 2, 4, or 8. If the span potentiometer status (PS) was
at the top of its range in Step 2 (95%), then the GAIN should be raised. If the
span potentiometer status (PS) was at the bottom of its range in Step 2 (5%), the
GAIN should be lowered.
5. Press SHIFT/ENTER to return to the Run Mode.
6. Repeat Step 1. If the analyzer still cannot be spanned, repeat steps 2 through 5
for a new GAIN value.
5.6 SOURCE BALANCE SHUTTER ADJUSTMENT
Note:
These adjustments are part of the factory checkout and are not normally
required for routine operation, but must be performed whenever the optical
system is disturbed (i.e., removal of cells for cleaning).
1. Access Diagnostics Mode (See Section 3.7), and access the Detector Signal
display.
2. Flow zero gas (nitrogen) through the SAMPLE CELL until the display reading
stabilizes.
3. Refer to Figure 3-1. Slightly loosen the locking nut on the sample cell shutter
adjust screw. The shutter adjust screw is located on top of the motor source
assembly.
4. Using a screwdriver, rotate the shutter adjust screw until a minimum reading on the
display is obtained. A typical reading is 0.2 to 0.5. Add 0.4 to this value. Use this
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