Furals in Oil

Furaldehydes (Furals) are chemicals formed when cellulose paper degrades due to overheating. The status of solid insulation can be measured using liquid oil insulation.

Degrees of Polymerization (DP) is a test done on the paper to reveal its mechanical strength. An inverse relationship exists between the D (paper test) and the Furals (oil test); the higher the Furals in the oil, the lower the DP of the cellulose paper. This relationship holds, however, only if the paper is degrading evenly. The DP of the paper usually varies within the solid insulation.

Since it is difficult to sample paper from an operating transformer, Fural Analysis in the insulating oil can indirectly reveal the status of the solid insulation.


We require 20 mL of insulating oil, which should be transported in a small, dark bottle, as Furals are light sensitive. Sample oil before reclaiming it, since Furals will be absorbed.

There are two ways to do Fural Analysis

  • Screening Test  (low cost, colour test)
    • This test will measure only 2-furaldehyde which is also the most prominent of the furaldehydes.
  • HPLC Test  (complete analysis of furaldehyde and phenols)
    • High Pressure Liquid Chromatography will measure all the furaldehydes (2-Furaldehyde, 5-Hydroxy-Methyl Furaldehyde, 2-Furyl Alcohol, 2-Acetyl Furan, 5 Methyl-2-Furaldehyde and Phenols, m- Cresol, O-Cresol and Dimethyl Phenols).