Campbell Scientific IRGASON Integrated CO2/H2O Open-Path Gas Analyzer and 3D Sonic Anemometer - Manual (Page 49)

IRGASON ® Integrated CO
2
/H
2
O Open-Path Gas Analyzer and 3D Sonic Anemometer
10.2.1 ConfigCmd 11 Zero-and-span Control
To perform zeroing of CO
2
and H
2
O , ConfigCmd 11 is set to 1. After the
IRGASON completes the zero, it will write the value to -1. The datalogger
program can poll this value or simply wait for a period of time to allow the
zeroing to complete. To perform CO
2
span, the CO
2
Span Concentration
setting (ConfigCmd 12) must be written to the proper value in ppm CO
2
prior
to setting the zero-and-span control setting (ConfigCmd 11) to 2. After the
CO
2
span is completed, the value of the zero-and-span control setting will
change to -2. H
2
O span is similar to CO
2
. First the H
2
O Span Dew Point
Temperature value (ConfigCmd 13) must be written to the desired value. Then
the zero-and-span control setting is set to 3. After the IRGASON completes
the span, the span control setting is written as -3. ConfigCmd 14 through 17
automatically store the results of the zero-and-span procedure. Each result is a
coefficient used in the gas analyzer’s algorithms for calculating gas
concentrations.
10.2.2 ConfigCmd 18 Heater Voltage
Normally, the EC100Configure() instruction is run in the processing task.
Skipped scans can occur when the EC100Configure() instruction executes.
When changing operational parameters, these skipped scans are acceptable.
However, they may not be acceptable when changing the heater voltage.
ConfigCmd 218 allows EC100Configure() to operate in the SDM task, thus
avoiding skipped scans. When using ConfigCmd 218, the command must be a
constant and the instruction cannot be placed in a conditional statement.
The IRGASON includes heaters near the optical windows. This setting gives
the voltage applied to the heater. It can be set to -1 to disable the heater, or set
it to any voltage between 0 and 4.5375 V.
The heater resistance is 5 ohms, so the heater power (P
H)
expressed as Watts
(W) is given by:
5
2 V
P
H
=
The maximum power at 4.5375 V is 4.12 W.
Operating the heaters following a precipitation event encourages water droplets
on the windows to evaporate more quickly, thus restoring signal strength.
Heater usage may also be desired to prevent formation of dew and frost on the
windows, which leads to invalid measurements due to differences in absorption
spectra of liquid water and water vapor.
The heaters, however, should be disabled when they are not needed. Heating
near the sample volume may cause air density fluctuations that can affect flux
measurements. Disabling the heaters will also conserve power. Care should be
taken to avoid operating the heaters if the system voltage is too low or if the
ambient temperature is > 45°C. Operation at temperatures > 45°C will
overload the thermoelectric coolers of the detector, as indicated by the TEC
Temp diagnostic flag (see TABLE 10-3, Bits in the Gas Diagnostic Flag).
A datalogger may be used to dynamically control the heaters by using input
from another moisture sensor such as a leaf-wetness sensor. When the sensor
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