The DACs in the LI-840A are bipolar, and will go slightly negative (~-0.100V). This can happen,
for example, if the cell becomes contaminated or from small random perturbations when the
CO
2
concentration is near zero. See Cleaning the Optical Bench on page5-1 for instructions on
cleaning the cell should it become contaminated.
Voltage output is measured by attaching the positive lead from the logging device to terminal 9
(V Out 1), or terminal 7 (V Out 2) on the LI-840A terminal strip. Connect the negative lead to
position 10 (GND), or position 8 (GND).
The CO
2
or H
2
O concentration can be calculated from the DAC output voltage as follows:
3-1
where V is the measured voltage, X
F
is the full scale value for CO
2
or H
2
O output (entered in
the Settings Window as the 2.5 or 5 V value, up to 20,000 ppm for CO
2
), X
Z
is the zero value
entered, and Vmax is the full scale DAC output voltage selected (5V or 2.5V).
Example: The DAC output range selected is 0-5V (V
max
), the zero CO
2
value entered is 0 ppm
(X
Z
), the full scale CO
2
value entered is 2000 ppm (X
F
), and the measured output voltage (V) is
2.9V.To calculate the CO
2
concentration from 3-1 above,
3-2
= 1160 ppm.
3-9Section 3. Operation
Section 3
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