Campbell Scientific EC155 CO2 and H2O Closed-Path Gas Analyzer and EC100 Electronics with Optional CSAT3A 3D Sonic Anemometer - Manual (Page 52)

EC155 CO
2
and H
2
O Closed-Path Gas Analyzer
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
EC155 completes the zero, it will write the value to -1. The datalogger 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 Span/Zero Control setting (ConfigCmd 11) to 2. After the CO
2
span
is completed, the value of the Span/Zero Control setting will change to -2.
H
2
O span is similar to CO
2
. First the H
2
O dew point value (ConfigCmd 13)
must be written to the desired value. Then the Span/Zero Control setting is set
to 3. After the EC155 completes the span, the span control setting is written as
-3. ConfigCmd’s 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 datalogger’s
processing task. Skipped scans can occur when the EC100Configure()
instruction executes. When changing operational parameters, these skipped
scans are acceptable. However, it may not be acceptable when changing the
heater voltage. ConfigCmd 218 allows the EC100Configure() instruction 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.
If the EC155 includes the optional heated intake assembly, 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 prevents condensation in
the intake tube.
The resistance of the heater in the intake assembly is 30 ohms, so the heater
power will be given by:
30
2 V
P
H
= (W)
The maximum power (at 4.5 V) is 0.7 (W). The heater may be operated
continuously at full power, over the full range of operating temperatures. If
ambient conditions are dry enough to prevent condensation without heating the
intake, the power may be turned down to conserve power. Note that the
CPEC200 system automatically controls the intake heater power as needed to
prevent condensation.
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