HOT TUBS: Calcium (and Bromine)

[chem geek]

Thanks Evan. The information you provided is great. It all makes sense though it would be interesting to see if chlorine depletes faster than bromine for indoor spas not exposed to sunlight at all. Perhaps the depletion is a "good" thing in that it is oxidizing more organics or killing more pathogens since chlorine is a more powerful oxidant than bromine.

Are you sure about the calcium protecting the copper in heaters? Please refer to this post I wrote on another thread. I know that the conventional wisdom in heat exchangers is to use water balance to slightly scale calcium carbonate to form a protective layer, but that is typically for cooling towers where the water is already quite saturated and where parts of the system are acidic (pH < 7). It is not clear to me that in basic waters (pH > 7) that the protection from calcium is needed or works. If a thin film of scale is required, then the water balance would need to have lots more calcium than 100 ppm (with typical TA of 80-120). If the mere presence of calcium ion somehow inhibits corrosion, then that's new to me.

I think this issue of whether or not non-plaster/grout pools and spas need calcium is critically important to figure out correctly. There has been a lot of advice on this forum that vinyl and fiberglass pools do not need any calcium. If it is needed for spa heaters as well as pool heaters (which typically use copper pipes for heat transfer since copper is very efficient at transferring heat), then we'd better get this sorted out.

Richard