Campbell Scientific IRGASON IRGASON Integrated CO2/H2O Open-Path Gas Analyzer - Page 49

ConfigCmd 11 Zero-and-span Control, 10.2.2 ConfigCmd 18 Heater Voltage, ConfigCmd 11

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IRGASON® Integrated CO2/H2O Open-Path Gas Analyzer and 3D Sonic Anemometer 10.2.1 ConfigCmd 11 Zero-and-span Control To perform zeroing of CO2 and H2O , 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 CO2 span, the CO2 Span Concentration setting (ConfigCmd 12) must be written to the proper value in ppm CO2 prior to setting the zero-and-span control setting (ConfigCmd 11) to 2. After the CO2 span is completed, the value of the zero-and-span control setting will change to -2. H2O span is similar to CO2. First the H2O 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 (PH) expressed as Watts (W) is given by: PH = V2 5 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 39

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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
39