Connecting the LI-840A to the LI-1400 Datalogger
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The following example shows how you can connect the LI-840A to a datalogging device such as
the LI-COR Model LI-1400 DataLogger to collect analog data and convert to meaningful CO
2
and/or H
2
O values. As mentioned earlier, voltage output from the LI-840A is linear; converting
analog data into ppm CO
2
or mmol/mol H
2
O simply requires multiplying the raw mV output
by a conversion factor, which is dependent upon the selected voltage output range, and the max-
imum CO
2
or H
2
O resolution available (set in software between 0-20,000 ppm).In general, the
conversion takes the form:
3-11
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 5V value, up to 20,000 ppm for CO
2
), X
Z
is the zero value
entered, and V
max
is the full scale DAC output voltage selected (5V or 2.5V).
For example, if the voltage output is set for 0-5V (V
max
), the zero CO
2
value entered is 0 ppm
(X
Z
) and scaled CO
2
range is 2000 ppm (X
F
), and the measured voltage output is 2.9V, the equa-
tion would take the form:
3-12
= 1160 ppm.
Section 3
3-32 Section 3. Operation
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