Hi, Alec!
The CO2 sensor you want is made by Texas Instruments . . operates
at 0-5V . . .costs about 500 bucks, sensor only. You can route it directly
to the PLC - Check out TI's on-line catalog and if you can't find the unit
you want, get back to me and I'll get the part number from one of the
techs.
Phil Nuytten