Campbell Scientific CPEC200 Closed-Path Eddy-Covariance System - Manual (Page 67)

Appendix A. CPEC200 Diagnostics
If the value of valve_flow = NAN, this indicates a problem with the valve flow
measurement. The valve flow is inferred from the pressure drop in the sample
cell as described in Section 4.2.3, Valve Module.
Check the value of valveControl. This variable determines the size of the
opening of a proportional control valve, from 0 (fully closed) to 1.0 (fully
open). This proportional control valve can be described as an electrically
operated needle valve. In normal operation, when the flow falls below the
setpoint, the CPEC200 will respond by increasing valveControl. This
increases the opening in the proportional control valve and allows valve_flow
to rise to the setpoint. Conversely, if the flow is above the setpoint the
CPEC200 will adjust valveControl downward until the flow matches the
setpoint.
If valveControl = 0, this indicates the CPEC200 has turned the valve flow off.
There are several reasons the CPEC200 may shut the valve flow off:
• No valve is selected
• Measured valve flow is NAN
• Low supply voltage (see notes on bit 9)
• IRGA has been powered down (see notes on bit 8)
If the flow is not at the setpoint, the troubleshooting steps depend on the
situation. There are three cases:
1. Flow is provided from a cylinder of compressed gas. This is the most
common situation. Make sure the cylinder’s regulator is connected to
the proper fitting on the valve module and that it is regulating the
pressure to 10 ± 5 psig. The value of valveControl should be
between 0.2 and 1.0. If valveControl = 0.2 and valve_flow is too
high, this indicates the control valve is unable to provide enough
restriction to limit the flow.
The proportional control valve will be fully closed at any value of
valveControl below approximately 0.3.
If valveControl is 1.0 and the flow is too low and the CPEC200 is
accessible, check the indicator lights on the valve module. One of the
inlet valve lights (Zero Air, CO
2
Span 1, etc.) should be illuminated.
The Flow Control light should also be illuminated. The brightness of
the Flow Control light indicates how hard the proportional control
valve is driven. If valveControl = 1.0 this light should be full
brightness (approximately as bright as the light that indicates which
inlet valve is selected). If the appropriate valve module indicator
lights are not illuminated, make sure the connector on the side of the
valve module is connected.
2. If the H
2
Ospan valve (6) is selected, the flow is not controlled by the
CPEC200 system, but by the dewpoint generator, which pushes flow
through the valve module to the IRGA. The CPEC200 fully opens the
flow control valve by setting valveControl = 1. The acceptable range
for valve_flow is between 0.2 and 2.0 LPM. If the CPEC200 is
accessible, check the indicator lights on the valve module as described
above. Also check the tubing connection between the dewpoint
NOTE
A-15