Troubleshooting
6400-01 CO2 Mixer Problems
20-26 Using the LI-6400
20
doing the mixer calibration (page 18-20), to improve the mixerÕs Òfirst guess-
esÓ.
CanÕt Achieve Low Values
Typically, the minimum value achieved by the mixer is 30 to 50 µmol mol
-1
.
If your minimum value is considerably above that, then here are some things
to check:
1 WhatÕs your maximum flow rate?
In new measurements mode, press 2, then f2, then F, then enter 1000, to re-
quest a fixed flow of 1000 µmol s
-1
. Then monitor flow rate (display line b).
If the value is considerably lower than 700 µmol s
-1
, turn the mixer off. If that
dramatically increases the flow rate, then the ÒproblemÓ is that the mixer cal-
ibration has specified a reduced flow rate to achieve higher CO
2
concentra-
tions, with the side affect of increasing the minimum concentration. Redo the
mixer calibration (Calibrating the CO2 Mixer on page 18-20).
2 Is the soda lime on full scrub and is it good?
Turn off the mixer, and verify that the reference CO
2
goes to zero. If it
doesnÕt, there is a problem with the soda lime, or else the IRGA needs to be
zeroed.
3 Bad valve?
See The Bad Valve Saga on page 20-28, to see if this could be the problem.
4 Valve installed OK?
If the valve discussed in The Bad Valve Saga below has been replaced, but
is installed backwards, the only CO
2
level that can be achieved will be very
high, typically above 2000 µmol mol
-1
.
CanÕt Achieve High Values
The CO
2
mixer should be able to achieve an upper limit in excess of 2000
µmol mol
-1
. 2200 is a typical value. If you canÕt come close to that, or if it
takes a long time to get there, check for the following:
1 Flow from the source
See Step 4, Check the regulator for flow on page 20-23.
2 Calibration problem?
Change to C mode (constant control signal), and set a 5000 mV target. If that
fixes the problem, then re-do the mixer calibration.
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