Water_man said:
smuggs: After you sprayed in the stuff, was the flow meter clear of the WS for a longer period of time as compared to the effect of acid treatment? If this is indeed consistent, then desacling may be better, and I wonder if someone (chemgeek?) can explain what's going on.
If the DeScale It product is what is shown
here, then I suspect it still has low pH and they avoid the use of "harsh mineral acids" by simply using an organic acid. So the pH still gets quite low when sprayed on locally, but not so much as to be irritating nor affect overall pool pH. Basically, it takes low pH to initially dissolve the scale and bubble away the carbonate as carbon dioxide and then sequestrants can bind to the calcium to keep it soluble.
I can't find an MSDS or more detailed descriptions than the link above on DeScale It, but
this link to a product which sounds similar shows how one can claim to not be toxic nor corrosive yet have a pH of 1 which is clearly very acidic. It's not one of the "common" organic acids which they list, but is still a moderately strong acid. The pKa of 0.176 that they list is similar to the pKa of urea.
I can't tell you which would be more effective either in the short-run for immediate removal nor in the long-run. If the calcium level in the water were an issue, then the sequestrant could help, but realistically one would add that separately to the bulk pool water if the CH were truly too high.
To prevent scale, one can use scale inhibitors that work by slowing down the buildup of scale, rather than by sequestering calcium. For example, having lots of magnesium in the water (i.e. high Total Hardness relative to Calcium Hardness) does this because magnesium gets bound into the calcium carbonate matrix replacing calcium, at least temporarily (magnesium carbonate is more soluble than calcium carbonate so this process only affects the rate, not the thermodyanmics). On the other hand, magnesium sulfate has an extremely high salt crystallization pressure so splash out of the two with subsequent evaporation could be very harsh to cement and stone -- there's no free lunch here.
Ironically, this means that those with natural water that is "hard" and that did not need additional CH but where the CH wasn't too high might find less scaling than those whose fill water was "soft" or low in total hardness and where CH was increased by addition of calcium chloride. This may be why the Hamilton Index that uses Total Hardness is reasonably successful (but not as good as the real calcite saturation index). Their higher total hardness levels say to have lower TA but not low enough to have the saturation index be near zero, but at such higher hardness levels the rate of scale formation may be slower even though the tendency to scale is still there.
Richard