Mysterious Calcium Hardness

Naplespool

Well-known member
Mar 26, 2017
78
Naples/FL
#1
For the last month or so, the CH in the pool has been measuring 400 ppm. Today it measured 450 ppm and I don't have a good explanation for where the additional CH came from. I was so surprised I checked a 25 ml sample and counted 10 ppm drops.

No one has added anything with calcium to the pool, expect fill water which measures about 50 ppm. But if I've done the math right I'd have had to evaporate all the water in the pool except a few gallons and refilled with 50 ppm water.

There was a fire in the everglades over the past month which dumped some ash on the deck and I assume in the pool, filtered by the screen cage.

Other than that I can't guess where it came from. Any thoughts?
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
14,509
Tucson, AZ
#2
Let me ask you this - are you titrating to the first moment you see blue or until the blue stops changing color (becoming more intensely blue)? Many people make the mistake and stop the test at the first sight of blue; that's incorrect. You titrate the sample until the blue stops changing; usually the difference is about 2-3 drops of R-0012. If that is not the problem, then try this -

Add 10 drops of R-0010 (assumes you're doing a 10mL water sample test) and then add about 5 drops of R-0012. Finally, add the R-0011L indicator dye and then titrate with the R-0012. Count up all of your R-0012 drops, even the 5 initial ones. That sometimes helps to give a more repeatable result. You can add as much as half of the number of R-0012 drops you expect to use before adding the dye; it helps to lock up some of the calcium and any trace metals that might be in the water sample so that the endpoint is easier to see.
 

Naplespool

Well-known member
Mar 26, 2017
78
Naples/FL
#3
Thanks for this input, I'll give it a try.

In the meantime I ran over to the pool store and had them make the test with their bigger rig and magnetic stirrer. They came up with 405 ppm, nearer my earlier number.
 

JamesW

TFP Expert
Mar 2, 2011
12,833
#4
I never consider the readings to be exact. I always take readings over time and look for an average.

50 ppm is a little bit wide for range but not too concerning.

There are several things that can affect the levels or readings.

If the water sample is not exactly the same, that will throw off precision.

If you had several inches of evaporation without refilling, that can raise your levels.

If your plaster is new, it can add calcium.

If you had scale and it dissolved, that will add calcium.
 

Naplespool

Well-known member
Mar 26, 2017
78
Naples/FL
#6
Thanks for the last two posts. I'm losing about 1/4" per day and the town water has 50 PPM calcium hardness. So to get a 50 PPM increase, nearly all the pool water would have to evaporate and then refill with 50ppm to get a 50 ppm increase.

The pool is new, and recently I've been lowering TA and had the PH down to 7.0-7.2 for a couple days. Still, in my 17250 gallon pool it takes 8 lbs. of calcium chloride to bring the level up 50 ppm.

I'll keep an eye on this over time and also follow the detailed CH instructions more closely.
 

JoyfulNoise

TFP Expert
Platinum Supporter
May 23, 2015
14,509
Tucson, AZ
#7
Thanks for the last two posts. I'm losing about 1/4" per day and the town water has 50 PPM calcium hardness. So to get a 50 PPM increase, nearly all the pool water would have to evaporate and then refill with 50ppm to get a 50 ppm increase.

The pool is new, and recently I've been lowering TA and had the PH down to 7.0-7.2 for a couple days. Still, in my 17250 gallon pool it takes 8 lbs. of calcium chloride to bring the level up 50 ppm.

I'll keep an eye on this over time and also follow the detailed CH instructions more closely.
8 lbs of calcium could easily be removed from the surface of your pool with only a microscopic change in plaster thickness. Calcium carbonate has an average density of 75 lbs/cu. ft. A small pool surface is easily 900 sq. ft of surface area (yours would be significantly larger than that). So if you do the math you'd get a change in plaster thickness of about 0.0002" (0.2mils) per pound of calcium carbonate dissolved. So one can easily dissolve a little bit of their plaster and get a good deal of CH increase.