LI-COR LI-7500, Инструкция по эксплуатации

Страница: 105 / 155

Section 6
6-4 Troubleshooting
Bad CO
2
or H
2
O Readings
Section 2 details what the LI-7500 measures, and how it computes. Most of the relevant
parameters are available for viewing with the LI7500 program, such as calibration
coefficients, absorptances, and zero and span values. The first thing to check when tracking
down bad readings is to make sure all of the coefficients on the "Cal Coeffs" page match those
on the calibration sheet for the head in use. The Band Broadening coefficient should be 1.15.
After that, go to the "Diagnostics" page and check the values of AGC, and also the CO
2
and
H
2
O absorptances. Are any of the diagnostics flags (PLL, etc.) set? Finally, go to the
"Calibration" page and make sure the temperature and pressure make sense, and that the
current values of zero and span are near 1.
Absorptances make sense, densities don't
If the absorptance value displayed on the Diagnostics page seem correct (rough rule of thumb:
absorptance = 0 when density = 0, CO
2
absorptance is about 0.1 with a mole fraction of about
400 ppm, and H
2
O absorptance is about 0.1 with a mole fraction of about 20 mmol/mol), but
if the displayed values of density or mole fraction are obviously bad, then the problem is in
one of the following: calibration coefficients, span parameter, pressure value, and the band
broadening value (CO
2
only).
Readings very noisy
Put the calibration tube in place, just to dampen natural fluctuations. You don't necessarily
need to have anything flowing through the tube. The noise in absorptance values
("Diagnostics" page, below) should be stable, with only the 4th decimal place changing once