Andrew Gabriel wrote :

As others have said, this sounds like the oxygen reading.
Combustion analysers often don't measure CO2 directly because it's
hard to do continuously in that type of unit. Instead they measure
O2 and C0 levels, and then calculate the CO2 from that based on the
O2 input assumed to be 21% and type of fuel in use.

Thanks all...

As several have suggested, I think it must be an oxygen measuring
instrument rather than CO2. It was just a description via the phone and
the op insisted it was clearly marked as measuring CO2. Clearly he was
mistaken.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.org