Shortly: It is indeed the reference cell concentration, and it should probably be kept somewhat higher than what you want in your sample chamber. It is faster and stabler to control for CO2R than for CO2S, which is why it is often preferred. Do you use a CO2 mixer with external gas source? You might want to check for flow restrictions such as the CO2 regulator's filter assembly, o-rings etc. Also, consulting the manual might be a good place to start.

Antti Tenkanen Hi, thanks for your reply. I didn't use a CO2 mixer. I have read the manual and it didn't tell me what value I should expect. So the CO2R is the same as the atmosphere concentration? Then why is it so much lower? Thanks.

Well, CO2R is not the same as atm. [CO2], but if you do not have a mixer, then outside air will be where your CO2 comes from, and this should definitely be higher than what you get, (but it will depend on the season and whether people have stayed in the room etc.)

I'm not very familiar with using the instrument without a CO2 supply, and indeed I've only used the older LI-6400 version, so the following might not apply to you (and I can't guarantee anything will work anyways after doing these, even if they do apply). But here's a couple of things you could check:

- Bypass soda lime or indeed remove the chemical tube completely. Even if you have the scrub in the middle position, it will still remove quite a bit of CO2.

- Check that the air inlet on the side of the instrument is not blocked (there may be a rubber plug in place, especially if the machine has been in storage).

Also playing around with the desiccant-scrub, flow and similar things might have a minor influence, but if the above do not work, I'd recommend asking techsupport.

Thanks for your reply. I think it is something wrong with the Licor machine. Some suggested it's to do with the optical path. I have tried it with CO2 mixer and it can reach a higher CO2 concentration but since it can't even sense the ambient atmosphere CO2, I don't trust the sensor.

I have check that the air inlet is not block, we test it outdoor so it shouldn't be a problem.

I do agree with Antti Tenkanen . First you should check whether you are scrubbing the inlet air with sodalime (such low values are possible in that instance). You should "By pass" it when taking measurements without the CO2 mixer. Just by pass it and check the CO2R and CO2S before clamping a leaf. There cannot be much variation between the two readings if the machine is working properly. Further just exhale into the leaf chamber and observe the drastic increase of the CO2S.