Match Valve Problems

“CO2 has Changed” Message

This message appears in match mode when the sample CO2 concentration changes by more than 3 μmol-1 since entry. This can be caused by two things:

  • Match mode entered at the wrong time
  • If the light just changed, or you just closed the chamber, or the CO2 mixer target just changed, or you just made an adjustment on a chemical tube flow knob, then the change in CO2 is not indicative of a problem with the system, just with your timing. Wait for CO2S_μml to stabilize, then match.
  • Chamber leak
  • If CO2S_μml never does stabilize, then you very likely have a chamber leak. See Sensor Head Leaks.

“Excessive Deltas” Message

This message appears when you try to match, but the differences between sample and reference are too large. This could be due to very poorly set IRGA span or zero, but may also indicate a problem with the match valve or its plumbing. The default limits are 10 μmol mol-1 for CO2, and 1 mmol mol-1 for H2O.

  • Is the Match Valve functioning?
  • Figure 4‑2 shows how the match valve should be positioned in and out of match mode.
  • Is the return flow tubing in place?
  • Check to be sure there is a piece of tubing connecting the chamber bottom with the match valve (Figure 20‑6).

“CO2R Didn’t Change” Message

After the initial delay when entering match mode, during which the H2O reference reading is supposed to stabilize, if the message

appears, it is because the CO2 reference reading changed less than 1.5 μmol mol-1 after the match valve closed, and the expected change (the pre-match difference between sample and reference CO2) was larger than 10 μmol mol-1. Reasons for this would be a match valve that is sticking, or the air flow tube connecting the chamber to the match valve not being in place, or some other flow related problem. Note: This message could be spurious the first time you match. Suppose the IRGAs badly need matching (ΔCO2>10), and the chamber is empty. When you enter match mode, the reference concentration won’t change (because the chamber is empty), and the system gets fooled into displaying this warning.

Match Valve Doesn’t Move

Stuck match valve? See Match Valve Maintenance.

Figure 20‑6. The exhaust from the leaf chamber is sent to the match valve.