Min Recommended Calcium level is WHAT?

[PoolDoc]

While it probably doesn't hurt anything to have a calcium level of 200, or even 400, I suspect a preference for those levels originates with an excess of confidence in the Langlier index, and other saturation indexes.

Please remember that those indexes were designed, not to protect plaster, but to avoid excessive deposition of scale on the interior of hot water and steam boilers!

In my own experience, I've never seen plaster damage that was associated primarily with low calcium (less than 120 ppm). The plaster damage I've seen has been associated, in order of frequency and severity, with

1. acid washing (#1 cause by a HUGE margin);
2. low pH from use of chlorine gas or trichlor;
3. low alkalinity, from use of excess aeration due to oxonators, water falls, slides, or other features;
4. erosive damage from high velocity water flow from a jet directed at a plaster surface.

It's probably worth noting that I've NEVER seen corrosive plaster damage on a pool that had principally been treated with either calcium hypochlorite or bleach. Even though cal hyp adds both calcium and alkalinity, in my experience, bleach treated pools have had no higher rate of corrosive plaster problems.

There's some lab work and theoretical analysis, done a few years ago by John Wojitowizc, that suggests that calcium levels by themselves have little effect on plaster loss unless those levels are extremely low, in the absence of low pH or low alkalinity. This correlates well with my own field experience.

So, if you want to put 200 ppm of calcium in your pool, and don't have a heater, that's OK. And levels of 400 ppm can be compensated for, though it's not a level I'd ever waste money creating.

But, overall, given the tendency of people to use heaters, whether solar or otherwise, or fountains and falls, which create thin films which concentrate the calcium salts present, I think lower levels even on concrete pools will reduce maintenance and problems.

There's no perfect answer here. There are too many possible variations, and too little real research. But please keep in mind, when recommending specific calcium levels, that there's a substantial 'cost' to calcium levels that are too high. This cost applies to ALL pools, but especially those with some sort of heating, or with thin water layer features (like falls or slides) or which are in low humidity climates.

By contrast, any 'cost' to low calcium affects only a small portion of pools.

Consequently, you can't err "on the safe side" by going high: that 'side' is not safe, either!

Ben