LI-COR LI-6400, Manual

Page: 544 / 846

Troubleshooting
IRGA Problems
20-18 Using the LI-6400
20
for reference concentrations, while the average of block and air temperatures
are used for the sample cell. (The equations are on pages 14-5 and 14-6.)
If the culprit is pressure or temperature fluctuations due to sensor or circuitry
problems, thereÕs not much to do except the final step in this sequence, which
is to contact LI-COR. There is a stop-gap work-around, however, that could
keep you going, and thatÕs to use a constant value instead of a measured one
for the offending sensor. You could, for example, change the pressure sen-
sorÕs calibration coefficients to 98 and 0 (offset and slope), which would
cause pressure to remain fixed at 98 kPa.
9 Contact LI-COR
If you have gotten to this point by carefully following the logic, you have de-
termined that the instability is not due to fluctuations of concentration in the
incoming air, leaks, debris in the cell, or unstable temperature or pressure sig-
nals. An IRGA hardware problem is suggested, so contact LI-COR.
Readings Obviously Wrong
If you just donÕt believe the IRGA readings, then try these steps:
1 Is it responsive?
Watch reference readings, and go from full bypass on both soda lime and des-
iccant, to full scrub. If the IRGAs donÕt respond, turn to
IRGA(s) Unrespon-
sive on page 20-16.
2 Zero and Span
Check the present values found in ÒView,Store Zeros & SpansÓ in the Calib
Menu. (For a discussion of these numbers, see
View, Store Zeros & Spans
on page 18-18.) Try resetting them to the factory defaults, or follow the zero
and span setting procedures starting on page 18-4. A common cause of prob-
lems here is zeroing the IRGAs without having a truly CO
2
-free or H
2
O-free
air stream.
3 Verify that the chamber fan is operating
Without mixing the sample cell, a leaf will have little effect on the sample
IRGA readings, which can make for strange behavior. The verify the fanÕs
operation, use your ears: turn the fan off (
3 F3 O (thatÕs a letter, not a number))
and on again (
F3 F
), and listen for the sound changes, if any. (No sound
change - no fan - no good.)