Richard, (as often happens when I try to keep my 'word count' down), I misrepresented what I was trying to say (go figure ). Though I specificly said I'd look at the SWCG pool areas, it was my own 'shorthand' for 'looking' at them compared to pools of the same age and ~ usage which diddn't have the SWCGs. This also raises the issue of my not being 'blind' (ie 'double-blind experiment') - undoubtably, I'll report some more or less corrosion to the pools in question. However, since I work in a relatively small area, many other factors will be removed from my observations. (in my 'miss-spent' youth, I studied psychology, and as it's a 'soft science', they made sure to 'pound-in' the necessity of using the 'hard science' methodology)