I did give some more info (suggestions, if you will) and asked to communicate with a technically knowledgeable person in manufacturing, but we'll see what comes of that. I agree with you that it was nice of them to at least respond, though I believe it is a standard response triggered by my use of the word "pool" which I had in my initial inquiry.

In the meantime, I did some quick research at some stores today and found that bottles of Clorox Regular bleach say 6.0% Sodium Hypochlorite and below that it says 5.7% available chlorine. So that strongly implies that the 6.0% is a weight % of sodium hypochlorite while the 5.7% is the weight % of available chlorine as defined in terms of the molecular weight of chlorine gas (which has, by definition, 100% available chlorine). This means that neither number is "Trade %" (grams available chlorine in chlorine gas molecular weight per 100 ml) which in this case would be about 6.2% and would be the number to be used in any simple volume scaling calculations (e.g. BleachCalc).

I also found chlorinating liquid that said 10.0% Sodium Hypochlorite which would imply a Trade % of about 11.0%. On the other hand, my recollection is that the chlorinating liquid I have been using says 12.5% and I believe that to be Trade %, but am not certain. Damn, this is SO confusing -- why did they even come up with this thing called "Trade %" anyway? I know that it makes volume-based calculations of ppm simpler, but it's one more way to accidentally mislabel a product!

I'll keep y'all posted (I can hear the yawns from here).

Richard