I'm sorry but there is too much baloney in this thread to respond to. I wrote a long post for this thread, but it got obliterated for some reason and I'm not going to completely recreate it. All of you who have read previous posts of mine regarding pH and TA understand that relationship. If you want to run a high pH pool, then you can read Ben Powell's
link so you can at least understand the pros and cons of doing so. As for spas, you have to be extra careful to prevent calcium carbonate scaling because it occurs more readily at higher temperatures.
The Hamilton Index uses Total Hardness, not Calcium Hardness, so just consider what it means if water has only magnesium and no calcium. You would have no calcium carbonate saturation at all to protect plaster surfaces. The index does not consider the effect of temperature on calcium carbonate saturation (scale potential). It does not consider the effects of high ionic strength (or roughly TDS) such as in saltwater chlorine generator pools. What a ridiculous index!
The Hamilton index was developed by Jock Hamilton of United Chemical to promote high pH pools, which happen to allow all of United Chemical's bromine based products to work better than at the lower pH normally recommended in pools.
For those who really know chemistry, look at the actual derivation of the Langelier Saturation Index (which I call the Calcite Saturation Index since even Langelier himself didn't want the index named after him) at the bottom of my
Pool Equations spreadsheet. Note that there is absolutely nothing in the derivation specific to boilers, pipes, closed vs. open systems, etc. It is simply a computation of a thermodynamic quantity -- calcium carbonate saturation -- as a function of pH, carbonate alkalinity (adjusted TA), Calcium Hardness (CH) and Total Dissolved Solids (TDS though technically it's really ionic strength and TDS is used as a proxy for that).
For spas in particular, promoting the Hamilton Index with it's 7.8 to 8.2 pH range and 400 TH with 75 ppm TA (which if CH were 300 and the pH 8.2 would be a calcite saturation index of +0.7 at 104ºF and +0.85 at 130ºF at the gas heat exchanger) is a significant risk at developing calcium carbonate scaling.
