"Any thoughts Cees?"
Well ... with respect to CO2 testers I have seen references to having to calibrate them from time to time, and that there may be filters or parts that need replacing after some time. That said, my Gunson Gastester Professional manual does not refer to either (other than the calibrating, but that is done every time it is used fairly simply).
Great idea I think to start a list of tool specifics. The best place on the website for something like that might be (for the time being) the Technical Articles on the homepage. We will have to see about setting this up in a convenient way.
Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic