Sampling Oil from Hermetically-Sealed Transformers

have a good morning, gentlemen

want to continue topic was partually discussed before. I still have a doubt about getting sample from hermetic Trafo. - for example into manual of Pauwels International NV it is mentioned tt "oil sample may be drawn and send for testing" ones in 5 years- as per maintenance schedule.

first of all lets decide what is "hermetically sealed" - for me it is oil -immersed trafo with no possibility to contact air in normal operation. it is trafo without conservator (or with air bag inside conservator) and air cushion.

there a few questions:

- what is a criteria - do sample/ do not sample -

- if yes - what is procedure for sampling -

is there any clear explanation - standard, norm or something like ?

thank you in advance for any response