where I is the measured current, X
F
is the full scale value for CO
2
or H
2
O concentration output
(entered in the Settings Window as the 2.5 or 5V full scale value, up to 20,000 ppm for CO
2
),
and X
Z
is the zero value entered.
Example: 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 current output (I) is 16.25 mA. To convert to ppm CO
2
:
3-10
= 1531.25 ppm.
CO
2
Span Range
The span range entry field determines the maximum CO
2
range over which the LI-840A will
make measurements. The factory calibration range for the LI-840A is 0-20,000 ppm. Note that
there is a benefit to selecting a narrower range if that range is adequate for your experiment. The
narrower range provides better resolution in terms of the digital-to-analog converter (DAC). For
example, if you choose a range of 0-1500 ppm, each step of the DAC is able to resolve twice as
many data points as compared to the 0-3000 ppm range, which results in better resolution.
Alarms
Allows you to enable/disable the alarms, and enter high, low, and dead band values. A complete
discussion can be found in Alarms on page2-1.
3-13Section 3. Operation
Section 3
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