The oxygen required to cause oxidation in the gas phase need not to come from oxygen gas. Consider the following reaction:
2CO (g) + O2 (g) = 2CO2 (g)
For this reaction,
or 
or
,
and hence
We see that pO2 is equivalent to a ratio: 
.
Another nomographic scale may be added to the diagram, with a new origin, C where the CO/CO2 line crosses the y-axis.
Similarly for the reaction 2H2 + O2 = 2H2O;
pO2 is equivalent to 
.
Adding a further nomographic scale to the diagram, we see that the equilibrium
pressure ratios of CO and CO2 or H2 and H2O for a given oxidation of metal,
or reduction of an oxide, can be deduced at a given temperature from the diagram.
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