Any problem with pool shock that sits on bottom of a plaster pool?

[CarlD]

Tom, I just gave you what I think are reasonable alternatives to explain your problem. If you and the pool store are getting similar results then you are probably running the test correctly. You can always put a handful of the cal hypo in a couple of gallons of water and see if you get really, really high results when you run the test. If you still get "zero" you're doing something wrong or have bad reagents.

[BigDave]

50 lbs in 22,000 gallons should raise the calcium by more than 100 - this should be visible on the test.

Ben describes a method of dosing cal-Hypo designed to prevent buildup for pools with sand filters. Dosing cal-hypo through the skimmer and letting it dissolve on the sand bed encourages the calcium to precipitate out onto the filter. This acts as a filter aid and can help keep some of the calcium out of solution. I can anecdotally confirm this as I used this method for the last half of last season; I didn't measure an increase in calcium (only tested twice), I did find fine white material (finer than sand) on backwash. Warning! cal-hypo must never be put in the skimmer if there's any chance that any stabilized chlorine could be in the circulation system - this can cause an explosion.