Wet Oil Sucks

Not many first time veggie oil users are aware that cooking oil absorbs water when food is cooked in it.

Potatoes are mostly water, so is fish, shrimp, chicken, and just about everything else you cook in a deep fryer. What's how it cooks in fact - the oil instantly vaporises the water at the surface of the food. That vapour appears as bubbles in the oil. Some fraction of the vapour doesn't make it to the surface and you in to the air; it is absorbed by the oil.

Filtering, and settling of the oil will help separate the water from the oil (fuel). But how do we know when it's done? In short - it'll never get done. See: Chemical Equilibrium.

How to test for water in oil

1. Pour your suspect oil in to a kitchen fry pan (get a cheap one from the dollar store)
2. Heat the oil on high on your kitchen range
3. If you see bubbles popping, then there's water in your oil

Methods to remove water from oil

1. Centrifuge
2. Evaporation
3. Evaporation under vacuum (uses less energy than #2)
4. Chemically (may leave impurities which will then need to be removed)

I use option #2 above because it's simple, and inexpensive for the volume of oil I dewater.
220 litres of filtered oil, $0.05 per kWh, 1kW element, 10 hours, equals less than 1/5 of a penny to dewater 1L of oil.