Campbell Scientific AP200 CO2/H2O Atmospheric Profile System - Manual (Page 76)

Appendix B. AP200 Diagnostics
Zero/Span Inlets
The sample flow for zero/span inlets is set by restriction in the small, stainless
steel tubes that connect the bulkhead fittings to the valve manifold. See Section
4.2.2, Valve Manifold, for details.
If the flow goes too high at a valve switch, but comes into the normal range in
a few seconds, this is normal. It is caused by the pressure regulator on the
cylinder. See notes on pressure regulators in Section 7.5, Zero/Span Flow. As
long as the sample flow is within the normal range by the time the data are
included in the average, this is acceptable.
If the flow for a zero or CO
2
span inlet stabilizes at a flow that is too high or
too low, this is most likely caused by the pressure setting on the regulator.
Adjust the pressure regulator to give the desired sample flow with the
corresponding inlet selected.
Bit 6: Cell temperature is outside its operating range
If bit 6 of diag_AP200 is set, this indicates the IRGA sample cell temperature
is outside the normal range. To confirm this error, check the value of
cell_tmpr, which is measured by the IRGA. The normal range for the sample
cell temperature is 48°C to 52°C.
The IRGA heats the sample cell to maintain its temperature at a nominal 50°C.
It may take several minutes to stabilize at this temperature when the system is
powered up (as much as 20 or 30 min if starting from –20°C). If the system
has been powered for at least 30 min and the cell temperature is still below
48°C, contact LI-COR ® .
The IRGA does not actively cool the sample cell when the ambient temperature
is too high. If the sample cell temperature is above 52°C, make sure the
ambient temperature is no higher than 45°C and contact LI-COR ® if the
problem persists.
Bit 7: Cell pressure is not at the setpoint
If bit 7 of diag_AP200 is set, this indicates the IRGA sample cell pressure is
not at the setpoint. Compare the value of cell_press to PUMP_P_SETPT.
Diagnostic bit 7 indicates they differ by more than 2.0 kPa.
The AP200 will control the speed of the sample pump to maintain pump_press
(measured at the inlet of the pump) at the setpoint pressure PUMP_P_SETPT.
If there is a problem with the pump, this will normally result in both bits 4 and
7 being set (see the notes above on bit 4). Having two independent diagnostics
allows some insurance that both of the pressure sensors are working properly.
Compare cell_press (pressure measured by the IRGA) to pump_press
(pressure measured at the pump inlet). These two points are physically
connected by a tube with relatively low flow, such that they should be at
similar pressures. The pressure values should agree within the combined
uncertainty of the respective pressure sensors. If they disagree by more than 4
kPa, turn the pump off (set pump_ON = False) and allow the system to
stabilize at ambient pressure. Compare each pressure sensor to the pressure
expected for the given elevation. This test may help to diagnose a problem
with the pump’s pressure sensor or the IRGA’s pressure sensor.
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