Troubleshooting the Soil Chamber

Pump Doesn’t Run

When configured for the 6400-09 Soil Flux Chamber, the pump turns on and off automatically during the measurement cycles. If the sample cell CO2 concentration is above the lower window value (Target - Delta), the pump should come on when you start a measurement and stay on until CO2S_μml falls below that value. When CO2S_μml rises above the upper window value (Target + Delta), the pump should turn back on, unless the number of requested cycles has been completed. If this is not happening, it could be one of the following:

  • Fuse
  • The flow board fuse protects the pump.
  • Parameter settings
  • There are four parameters (accessible by f5 level 7, or on old software, AuxParams, f2 level 3) that influence the pump’s behavior.

ddMargin (“Extra Draw Down (ppm)”) can be used to intentionally overshoot the lower limit, providing extra time for things to stabilize before the next measurement cycle.

DeadTime (“Dead Time (secs)”) prevents measurements from starting too soon after the pump turns off.

MnMsrTime (“Min Measure Time (secs)”) is the minimum measurement time, which prevents the pump from turning on again too soon.

ddFlow (“Flow during DrawDown”) lets you set the approximate flow rate when the pump is on.

CO2 Seems Unresponsive

Verify the operation of the IRGA mixing fan. If it isn’t working, the sample cell of the IRGA will never “see” the chamber air.

CO2 Doesn’t Draw Down

Between measurement cycles, the pump should turn on, and draw the chamber CO2 concentration down. Make sure that at least some of the flow is going through the soda lime tube. Note that leaks through the porous soil mean that there is a minimum CO2 concentration that you can achieve, based on the soil CO2 flux rate, the pump flow rate, and how much you are scrubbing the air.

You can adjust the flow rate (f5 level 7, or on old software, Aux Params, f2 level 3) during draw down.

If the CO2 cannot be drawn down, even with full flow going though the soda lime, try capping the end of the chamber with the white plastic cap. If CO2 drops when the chamber is capped, but not when it is on the soil surface, then perhaps you are trying to achieve too low a concentration, or have the flow rate too low, or are not scrubbing enough.

If CO2 doesn’t drop adequately even with the chamber capped, it is likely due to one of the following causes:

  • Chamber fan not functioning
  • Verify the operation of the chamber fan by turning it on and off (f3 level 3) and listening for the noise change.
  • Flow blockage
  • Check for obstructions in the return flow hose barb.
  • Leak
  • It takes two holes in a closed loop to make a leak. With the soil chamber attached to the sensor head in a closed loop, one (big) hole is the chamber itself (porous soil, pressure vent tube, etc.).

If there is a leak somewhere between the pump and the chamber, then air can escape the loop there, and is made up by bringing outside air through the chamber pressure relief port. The primary suspect is the Y connector combining the sample and reference air streams. Make sure all three pieces of Bev-a-line tubing are fully seated into this connector (push in until it stops, then push an additional 1/4 inch).

If the leak is between the soda lime tube on the console, and the pump, then outside air will be sucked in and mixed with the scrubbed air. A good way to test if this is happening is to temporarily route the return flow to the chamber directly to the sample cell of the IRGA, and see if this flow is fully scrubbed when the soda lime tube is on full scrub.

High Humidity Headaches

Chamber air is only potentially dried during drawdown. To maximize the drying that occurs during drawdown, set the desiccant knob to full scrub, and back-off on the soda lime scrubbing, so the CO2 is reduced more slowly.

Note that the system will only give high humidity alerts if the RH in the IRGA (not the chamber) exceeds 95%. If this happens, it will also automatically turn on the sensor head Peltier coolers to warm the IRGAs a bit to try and avoid condensation. You may want to manually increase the target temperature if high humidity in the IRGA remains a problem.