The CO / CO2 ratio, is the ratio of measured CO divivded by CO2.

It gives an indication of the following:

* How good a gas sample the instrument is reading.

* How clean the boiler is running.

For example: A new or clean domestic boiler will display a ratio of less than 0.004, a unit in need of cleaning 0.004 - 0.008 and a unit in need of major overhaul will show greater than 0.008.

This only shows a reading when a combustion test is being carried out. 0.0000 is displayed while in fresh air.