LI-COR LI-830, User Manual

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If you are measuring concentrations between 0 and 10,000 ppm, the range is
10,000 ppm. By configuring the DAC range for 0 V = 0 ppm CO
2
and 5 V = 10,000
ppm, the DAC output will have twice the resolution as it would in the previous
configuration.
2-3
20,000
10,000
15,000
5,000
0
0 2 3 4 5 1
Volts
CO
2 Concentration (
μmol mol
-1
)
20,000
10,000
15,000
5,000
0
0 2 3 4 5 1
Volts
CO
2 Concentration (
μmol mol
-1
)
65,536 discreet points over
the 10,000 ppm range gives 2x the resolution 65,536 discreet points over
the 20,000 ppm range
Figure 2-1. The resolution of the analog outputs depends upon the range. If the DACs are
configured to output data over the full 0 to 20,000 ppm range, the data output over the
DACs will have lower resolution (left). If the DACs are configured to output data over a
narrower range (for example, 0 to 10,000 ppm as shown on the right), the data output over
the DACs will have higher resolution.
Also keep in mind that the instrument will not output meaningful information
over the DACs if the measured value is outside of the configured output range.
Computing readings from the DAC output
Here we give some example computations.
Example 1: Computing CO
2
from a voltage output; 0 to 500 ppm range
The CO
2
concentration is calculated from the DAC output. In this example, let's
configure the output range for 0 V = 0 ppm and 5 V = 500 ppm. The concentration
is computed from:
2-4
If the output voltage is 2.9 V, then
2-5
Section 2. Configuring the gas analyzer
2-5 Computing readings from the DAC output