Campbell CPEC300, Руководство по эксплуатации продукта

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CPEC300/306/310 Closed-Path Eddy-Covariance Systems
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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 continuous
flow. This allows a dewpoint generator to be connected directly to the
H
2
O Span inlet. The dewpoint generator’s internal pump can push air 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 CPEC310 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 CPEC310 infers the flow rate from this pressure rise.
The EC155 has a pressure sensor in the sample cell to measure this pressure
rise directly, but its accuracy is affected by a small offset drift. The accuracy of
this 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 offset between sample cell and ambient pressures.
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 pressure offset measurement.
The CPEC310 valve module has a proportional control valve to actively
control the flow of zero and span gas to the EC155. The Easyflux DL program
for the CPEC310 adjusts public variable valve_ctrl_press as needed for the
measured flow valve_flow to reach the desired flow, as indicated by
valve_flow_set_pt .
The default value for
valve_flow_set_pt
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.
Even with higher flow rates, the time required to flush and
equilibrate the delivery tubes on an extremely tall tower may make
the automatic zero/span impractical. In this case, a manual
zero/span as described in the EC155 manual should be performed.
The CPEC310 valve module includes a heater and a fan to keep the valves
within their operating range of 0 to 60 °C. The valve heater turns on/off at
2 °C. The valve fan turns on at 50 °C and stays on until the valve temperature
drops to 48 °C. To conserve power, temperature control is active just prior to
and during the time when valves are in use. If the valves cannot be maintained
within the temperature range, the valves are disabled. The valve module
temperature control can be manually activated so that manual zero/span can be
performed by the station operator on site or remotely. If starting from the
minimum ambient temperature (–30 °C), the valves may take as much as
15 min to warm up to the operating range of 0 to 60 °C.
NOTE