Troubleshooting
6400-01 CO2 Mixer Problems
20-28 Using the LI-6400
20
The Bad Valve Saga
If your LI-6400 has serial number 437 or above, then this discussion does not
apply to you.
In 1997, we discovered that a number of mixers were failing in the field. The
typical failure mode was inability to control CO
2
at the specified targets after
prolonged usage. (Example: ask for 300, get 1000). The problem was traced
to an internal valve whose seat was failing when exposed to pure CO
2
. The
valveÕs manufacturer had changed the design from one that worked to one
that didnÕt. LI-COR issued two Service Bulletins (#2 and #3) about the prob-
lem
4
, and notified affected users. A field replacement kit was offered (part
number 6400-909), and by the end of 1997, about half of the potentially af-
fected units had been replaced.
Determining if You Have a Faulty Valve
You do not have a faulty valve, if at least one of the following statements is
true:
¥ Your instrument does not have the 6400-01 CO
2
mixer option.
¥ Your instrumentÕs serial number is 437 or above
A good valve will have already been installed.
¥ A Ò6400-909 InstalledÓ sticker is on the bottom
Normally, this little sticker would be on the bottom plate next to a leg. In the
case of field installations, itÕs up to the user to place this sticker, so thereÕs no
guarantee that it will be there.
¥ White tape on the solenoid
The original design (which does not have the problem) had white tape on the
winding of the valve in question (see Figure 20-5), instead of yellow.
¥ The mixer serial number is CCA-nnn rather than CC-nnn.
See Figure 20-5 for the location.
¥ A red dot is painted on the brass end of the bottom valve (Figure 20-5).
Valves in the 6400-909 kits have a red dot on them.
4.
The information in Service Bulletin #3 is contained in this discussion.
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