Campbell Scientific CPEC200 Closed-Path Eddy-Covariance System - Manual (Page 25)

CPEC200 Closed-Path Eddy-Covariance System
For the three-valve version, the inputs are:
• Zero
• CO
2
Span 1
• H
2
O Span
For the six-valve version, the inputs are:
• Zero
• CO
2
Span 1
• CO
2
Span 2
• CO
2
Span 3
• CO
2
Span 4
• H
2
O Span
The CPEC200’s zero and CO
2
span inlets are not bypass equipped, meaning
that they flow only when selected. This allows the zero and CO
2
span tanks to
be continuously connected for automatic, unattended operation.
The H
2
O Span input is bypassed (vented to the atmosphere through the H
2
O
Span Bypass outlet) when it is not selected, so it permits flow all the time.
This allows a dewpoint generator to be connected directly to the H
2
O Span
inlet. The dewpoint generator’s internal pump can push flow into the valve
module even when the H
2
O Span valve is not selected, minimizing errors
caused by pressurization inside the dewpoint generator. When the H
2
O Span
valve is selected, the dewpoint generator pushes moist air through the valve
module to the EC155.
The CPEC200 pushes the zero/span flow backward through the EC155 sample
cell and exhausts it through the intake tube to the atmosphere. Flow through
the intake tube causes the sample-cell pressure to rise slightly above ambient
pressure. The CPEC200 infers the flow rate from this pressure rise. The
EC155 has a differential pressure sensor to measure this pressure rise directly,
but its accuracy is affected by a small offset drift. The accuracy of this
differential pressure measurement can be improved by stopping all flow
through the EC155, allowing the pressure in the sample cell to equilibrate with
ambient pressure, and measuring the differential pressure offset. This offset is
then subtracted from subsequent measurements used to control the flow.
Because the pressure sensor offset can change over time, this offset is
measured at the beginning of every zero/span cycle. This step requires at least
10 seconds to complete; 5 seconds for the pressure to equilibrate, and 5
seconds to average and store the differential pressure measurement.
Either of the CPEC200 valve modules have a proportional control valve to
actively control the flow of zero and span gas to the EC155. The CPEC200
program adjusts public variable valveControl between 0 (closed) and 1 (fully
open) as needed for the measured flow valve_flow to reach the desired flow, as
indicated by CAL_FLOW_SETPT. The default value for
CAL_FLOW_SETPT is 1.0 LPM. This flow is adequate for lower
measurement heights (allowing a shorter tube between the valve module and
the EC155), but setting a higher flow rate may be required with long zero/span
delivery tubes used on tall towers. The proportional valve is opened fully
during an H
2
O span operation to prevent pressurizing the dewpoint generator.
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