Checking the Mixing Fan

The LI-6400/XT manual recommends checking the leaf fan to ensure it is working properly in step 5 of the Preparation Checklist in Chapter 4: Preparation Check Lists. If the fan is not working, you will not collect valid photosynthesis data. The leaf fan is hidden inside the sensor head itself (we also call it the circulation fan or the mixing fan in other places in the manual). It is different from the fan visible on top of the 2×3 chamber and the two temperature-control fans visible on the sides of the sensor head.

Mixing fan and IRGA components

The manual recommends turning the fan off and on and “listen for sound changes in the sensor head as the fan motor stops and starts.” It is a bit quiet, so it’s hard to hear if it’s running or not, especially if you’re outdoors or in a loud lab, greenhouse, or growth chamber. Here is a more quantitative way to know if the mixing fan is working or not. The mixing fan is powerful enough to push some air back to the console (where the flow meter is located) if the pump is off, so we can look at the measured flow rate and see if there is a difference between when the fan is on or off.

  1. Go into New Measurements and find Flow_uml (line b).
  2. This will be the variable to watch.
  3. Flow rate at 500 uml
  4. Go to menu level 2, and make sure that LeafFan (f1) is set to Fast.
  5. It should be set to Fast by default.
  6. Press f2 to change the flow rate.
  7. Press the letter O to turn off flow completely.
  8. Flow_uml should decrease and settle somewhere between +3 and -3. The variable name might be hidden by an error message saying that the pump is off, but that is fine for testing.
  9. Flow rate measured at -1.0
  10. Press f1 to change the leaf fan speed.
  11. Press the letter O to turn it off completely.
  12. Flow_uml should increase slightly. We are checking to make sure that there is a difference in measured flow rate when the fan is on or off.
    1. If there is no change between when the leaf fan is on or off, then the leaf fan is not working properly and further troubleshooting is required.
    2. The problem could be a blown fuse, a fan jammed with debris, a failed motor, or other problems. Refer to Chapter 20 of the manual: The Mixing Fan.
    3. If you see a difference in flow rate when the leaf fan is on vs. off, the fan is working as expected.
    4. But if Flow_uml is not within 2 µmol/s from zero when the leaf fan is off, then you should calibrate the flow meter. You can escape out of New Measurements, go to the Calib menu (f3), and then go to Flow meter zero.
    5. Zeroing the flow meter
    6. If Flow_uml is close to zero when the leaf fan is off, no further adjustments are needed.
  13. Set the leaf fan (f1) back to fast (by pressing 5 or F) and turn the pump on (f2) to a set flow rate.