It has been observed, that the absorption of oil takes place in three stages, duration of which are respectively τ1≈0,66 h, τ2≈109 h and τ3≈524 h. It was established that the time required to complete the oil absorption process takes more than 46 days. This means that at least as much time should take oil impregnation process of pressboard samples made for electrical measurements.
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Przemysław Rogalski, "Optical registration of transformer oil absorption processes in electrical pressboard nano-capillaries," Proc. SPIE 10010, Advanced Topics in Optoelectronics, Microelectronics, and Nanotechnologies VIII, 100101R (14 December 2016);