Regarding dielectric oils - It is definitely best to use an oil formulated for HV use, but mineral oil does work well as long as it or any oil is anhydrous - free of water.
Two other oils available in small quantities (~1 - 5 gal containers) are: Silicone Dielectric Fluid - Dow 2xxx or Ge Silicones. Either brand is good stuff - typically above 55- 60 kVAC breakdown in the standard 0.100" gap oil cup.... when clean and dry.
A second type of oil - the dielctric of preference of many in the aerospace - is Castor Oil. You can get it from McMaster-Carr in the Lubrication Oil section.. It comes in 1 Gallon jugs.(price is about $18/gal).. and is usually fairly dry. It's a bit more viscous than transformer oil, so filling things is more tedious, and... it is stickiest gooiest stuff to get off your hands... but, it is NOT a biohazard as Transformer Oil is considered to be.
Dehydrating Oils: To dehydrate oils, you need to heat, - about 80-85C and pull a good vacuum on it, and hold till the pressure stabilizes. You should be able to get down to below 30-50 microns.. when the oil is both dry and degassed. This can take a while - like hours!
With silicone fluid it is particularly important to keep it degassed, since it can dissolve upwards of 18-20% gas by volume. Transformer oil can dissolve about 10 -12% and I think Castor oil is similar. A surprisingly large amount! A painless way to degas is to simply pump down until most of the bubbling stops, and then seal off the vessel with the oil inside. Let stand this way overnight and repump the next day. Pressures will normally go right to minimums.
A vacuum bakeout of the container that will be receiving the oil is also recommended.
Transformer oils typically can absorb to about 25 ppm water. Above this level, water begins to "precipitate out" and collects on things. Surface layers of dielectrics can adsorb water up to nearly 100% concentrations for a few microns depth, so the effects of wet oil are often unnoticed until a flashover occurs. Wet oil also enhances the likelihood of tracking and flashover on surfaces.
A final point here... you can maintain dryness in oil with a small packet of either Activated Carbon (!), Silica Gel or Molecular Sieve material... (just be careful that the packet material is compatible with your oil). Some packets are glued together and the glues may not like your oil.
Also make sure the packet is well dried before you install it Us a good oven bakeout - typically several hours at 150 C or higher. A dessicant pack will keep the oil very, very dry for long periods and provide a good hedge against low level vapor permeation which occurs with all organic materials (i.e.: plastic and rubber seals, bellows, and etc).
Dave Cooper
Created on Thursday, March 29, 2001 10:06 PM EDT by David Cooper