May 9, 2022

Gas extraction from insulating liquids – Part III

Although the headspace extraction method is the most widespread extraction methodology today, it has many commercial advantages.

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Although the headspace extraction method is the most widespread extraction methodology today, it has many commercial advantages for both test providers and users, and it is the most sensitive technique regarding the chemical and physical properties of the mixture. This technique was developed about 30 years ago when the oil type was mainly naphthenic. Nowadays, the number of available oil types has highly increased even in the mineral oil group. Of course, non-mineral oils are composed of completely different atoms and species and their formulae impose a different treatment.

Commercial gas-in-oil standards are indeed indispensable for the performance verification but it must not be used for routine calibration of any type of gas extraction. DGA online and offline devices should be calibrated with gas in oil mixture containing all the range of concentration all measured gases, especially nitrogen and oxygen, which have become crucial gases for modern sealed transformers, especially those using non-minerals liquids. Total gases measured by gas detection devices should not be confused with either total gas concentration or total volatile concentrations measured by vacuum extractions. Examples of such calibration curves for total volatile versus total measured gases are presented in this article, as well as calibration curves for acetylene in different oil types.

The ASTM permits filling the vials by punching the septa, as shown in Fig. 1. Majority of laboratories following the ASTM procedure use this method. It can be seen that this filling practice may create different turbulence and bubbling state. This phenomenon alters the non-equilibrium thermodynamic extraction of gas.

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