CIRAS-3 Operation Manual V. 1.09
104
and Offset recorded at each step of the calibration. The first row of data represents the lowest limit of
reference readings that are allowed for these Stored Diff Bal Coefficients. The lower limit is fixed in
software to 100 ppm less than the lowest CO
2
Ref reading and 4 mbar less than the H
2
O Ref reading.
The last row of data represents the upper limit of reference readings that are allowed – 100 ppm more
than the highest CO
2
Ref reading and 4 mbar higher than the highest H
2
O reading. The column labeled
“Stored Offset” represents the calculated offset computed for that row’s Reference reading using the new
Stored Diff Bal Coefficients. These first and last rows also show the calculated offsets for these
upper/lower limit reference readings which are extrapolated past the actual recorded offsets.
Users should review this lower screen prior to accepting data for the following criteria:
1. Measured offsets at each point should be less than 20 ppm and 0.5 mbar.
2. Measured offsets should show a trend - gradually increasing or decreasing over the six
points.
3. The computed Stored Offsets in the 3
rd
and 6
th
columns should match the measured
offsets in the 2
nd
and 5
th
columns within about 1 ppm or 0.5 mbar.
4. The first and last row Stored Offsets should not be significantly higher or lower than the
other values, and should follow the trend observed in the actual data. Since these
calculated offsets are extrapolated beyond the actual recorded offsets, they might be
unrealistic if the least square curve fit has a high quadratic term. If this does occur, the
new coefficients could still be used, but the user should be sure to limit the operational
settings CO2Ref and H2ORef to be within the range where the Stored Offsets seem
valid.
Tip
If you receive the “Cuvette RH > 70%, decrease reference H2O” message during the Stored Diff Bal
calibration you can simply ignore it. This message is more relevant when performing measurements on
live plants when chamber humidity can get quite high depending on the environmental conditions and
state of the plant.
