Estimate of Paper Deterioration (Online)
The two methods below should be used together.
9.8.1 CO2 and CO Accumulated Total Gas Values
IEEE Standard C57.104™ Guide for the Interpretation of Gases Generated in Oil-Immersed Transformers  gives status conditions, based on accumulated values of CO2 and CO. Accumulated dissolved gas levels provide four status conditions: normal operation, modest concern (investigate), major concern (more investigation), and imminent risk (nearing failure). The CO2 and CO levels in ppm for each status are given in table 24.
9.8.2 CO2 /CO Ratio
The CO2/CO ratio should also be analyzed in conjunction with the CO2 and CO accumulated values. For analyzing the CO2/CO ratio, see section 6.1.10.
The status from table 24 should be at least in Condition 2 or 3 from one or both gases before a detailed investigation is begun. There is no need to look at the ratios from section 6.1.10 unless a substantial amount of these gases have already been generated. If the transformer is relatively new, CO2 and other atmospheric gases (N2, O2, and even some CO) may be migrating out of the paper into the oil because the paper was stored in air prior to transformer assembly. If the paper was stored in a polluted city atmosphere, a considerable amount of CO may show up in the DGA. This may look like the transformer has a problem and is generating a lot of CO. However, if the transformer has a real problem, H2 and perhaps other heat gases (CH4, C2H6, C2H4) should also be increasing.