I know 3603 will still "fog" even with pure distilled.

I know there is a post somewhere in here about a piece of 3603 placed into a cup of distilled water. A week later it was fogged. Remove from water and let it air dry, no more fog.

I think it is water absorption by the tubing that makes it look "fogged". I put a lot of heat intop my loop with only a PA 120.2 and maybe that affects this absorption rate in comparison to those of you that do not notice this with pure distilled.

Mine always fog eventually.