I have been running tests myself on Tygon R-3603 to see what was doing it and it has to be the tubing itself because I have tested pretty much everything that could be an outside influence and the damn stuff still fogged up. Thought the filter was helping but in the end it did not. I was using a 5 micron sediment filter to see if it was rad flux, metal corrosion or whatever. Did not work! Still fogged! I know the thread is a few months old but thought I would add to this anyway. Couple pics...