Maybe someone has an idea about this:
I have maintained the same pool for 5 years. For the first 3, the Calcium Hardness (CH), never exceeded 400.
In March 2015, I purchased new testing kit materials (fresh reagents for the new season). All of a sudden my CH level was 550 with the new kit, and 350 with the old kit. I talked with some folks about this and did some research on it. Pool looked and acted fine. Pool store check was 425. No detectable copper in the pool. Eventually I attributed the CH increase to when we restained the concrete deck, some muriatic acid was used in the prep, then it was power washed. I figured some dissolved calcium got in that way.
We had a rainy period, and there was a lot of rainfall and pumpout, and slowly the CH drifted down. I did not any significant fill water from any source other than rainfall for the rest of the year. In December it was 440. I thought I was close to home on solving the problem.
Until... I bought new testing kit materials again (fresh reagents every year, right?). CH now 580. Copper 0. We did powerwash the deck, but we didn't use any acid or restain it. Certainly some hose water went in with the powerwash, but not even an inch, and the hose water is CH 80. In my 5 years, I have never added calcium or copper containing products to the pool.
Running a few numbers, to go from 440 to 580 in my 13500 gallon pool would take 17+ pounds of calcium product-- so where the heck could this have come from? The only thing I can figure is the pool walls-- where else is 17 pounds of calcium hanging out?
The confession: Due to previous stability and arrogance, I generally only check the CH every 2-3 months. I have learned my lesson here, and will check it more often.
The other theory: perhaps older reagents are more likely to test lower CH levels, and the newer reagents are more accurate, and really my CH is high all along?
Random bits of information:
I use a speedstir for every test.
Pool looks great to me
Pool is in Gainesville,FL
Pool is solar heated, and often kept very warm 86-92ish when we can
Pool is covered from about October to May
It's a salt pool
pH pretty tightly kept 7.4-7.8ish
We suspect copper was previously used in the pool, prior to 5 years ago due to the coloration of the rim around the light, and there are what appear to be some copper stains on the surface too-- stable for years though.
Thanks for any and all ideas.