Direct Evaluation
The mechanical properties of insulating paper can be established by direct measurement of its tensile strength or degree of polymerization(DP). These properties are used to evaluate the end of reliable life or paper insulation. It is generally suggested that DP values of 150 - 250 represent the lower limits for end-of-life criteria for paper insulation; for values below 150, the paper is without mechanical strength. Direct measurement of these properties is not practical for in-service transformer.
Analysis of paper insulaton for its DP value requires removal of a few strips of paper from suspect sites. This procedure can conveniently be carried out during tranformer repairs. The results of these tests will be a deciding factor in rebuilding or scrapping a transformer.
Note: Since it is usually not practical(and often dangerous to the transformer) to obtain a paper sample from a de-energised, in-service transformer an alternative method has been found.
When a cellulose molecule de-polymerises(breaks into smaller lengths or ring structures), a chemical compound known as a furan is formed.
Furan Analysis
By measuring the quantity and types of furans present in a transformer oil sample, the paper insulation overall DP can be inferred with a high degree of confidence. The types and concentration of furans is in an oil sample can also indicate abnormal stress in a transformer, whether intense, short duration overheating or prolonged, general overheating. Furan analysis can be used to confirm Dissovled Gas Analysis(DGA) where carbon monoxide present indicates problems with solid insulation.

It has been shown that the amount of 2-furaldehyde in oil(usually the most prominent component of paper decomposition) is directly related to the DP of the paper inside the transformer.
Paper in a transformer does not age uniformly and variations are expected with temperature, moisture distribution, oxygen levels and other operating conditions. The levels of 2-furaldehyde in oil relate to the average deterioration of the insulating paper.
Consequently, the extent of paper deterioration resulting from a "hot spot" will be greater than indicated by levels of 2-furaldehyde in the oil.
For typical power transformer, with an oil to paper ratio of 20:1, the 2-furaldehyde levels have teh following significance:

(Written by I.A.R. Gray)