New pool owner, very high Calcium Hardness

flynwill

Well-known member
Sep 18, 2021
46
Los Angeles, CA
Greetings everyone:

I've been reading this site for several months now as construction has on our new pool was under way, and now it's finally finished (just in time for cooler weather :().

The pool is small -- 6100 gallons, Pebbletek finish, Jandy SWG not yet online as we need to wait several more weeks before adding salt per Pebbletek. The pool has a motorized cover and will be kept covered except when it is in used (probably 30-60 minutes per day).

I have the TF-100 test kit, purchased last summer. Most recent test results (yesterday morning):
PH 7.6
Free Chlorine 2ppm, Combined Chlorine < 0.5ppm (FAS/DPT test)
CYA 0 (Adding CYA is on this weekend's to do list)
Total Alkalinity 150ppm
Calcium Hardness 1000ppm.

I've been adding acid daily to keep the PH in check, which several sources indicate is normal for a new Pebbletek pool.

That last number is where it think "That can't be right". In order to get the CH test to work at all I'm diluting the sample with an equal part of distilled water, and then doubling the result. Without that step the solution never turns blue, it just goes clear after a 30-40 drops of the test solution. (I did run the test on my distilled water supply and the solution stays blue -- no calcium).

When I test the city water supply I get the same result. Checking water company's water quality report they report Total Hardness of ranging from 84-274ppm with and average of 181ppm. Is there any "magical additive" our water company could be adding to allow them to report total hardness much lower than the calcium hardness? I'm in the Los Angeles basin (Palos Verdes Peninsula for any locals reading), and our water is notorious for being very hard as it is source from the Colorado River, Eastern Sierra, and local wells.

The pool service company my builder brought in to do the startup also tested shortly after we filled the pool and said he got "1500". They also added Beautec which is a product that claims to be able to sequester the calcium and prevent scaling. They recommend a reapplication every 6 months. I tend to be a bit skeptical of products that make claims like this, particularly when there is no real explanation of their ingredients or the chemistry.

I took a sample to the pool store to see what they might come up with. They tested with a test strip and declared it was "probably about 200". I looked at the strip and the last color patch on the bottle was for 200 and the test patch looked to me to be several shades off the end of the scale, so I don't give this much credence.

I have ordered a bottle of CH reference solution from TF Test Kits which should sort out if there is something wrong with my reagents or technique.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
44,923
Laughlin, NV
Pool Size
6000
Surface
Fiberglass
Chlorine
Salt Water Generator
SWG Type
Pentair Intellichlor IC-40
Your CH level cannot be that high.

The test can be interfered with any metals in the water. Also, if you added a scale 'inhibitor', that will mess with the test.

For metals - read Calcium Hardness
 

flynwill

Well-known member
Sep 18, 2021
46
Los Angeles, CA
I agree there must be something interfering with the test, but whatever it is must be in the city water supply as that also tests way high.

I purchased some Clorox brand test strips from the local home center, and those seem to indicate a much more expected ~250 ppm Total Hardness for both the current pool water and the city water supply so I guess I will take those as a reference for now.

I had read the Calcium Hardness page already -- that's what led me to the dilute the sample with distilled water version of the test. But that page seems to imply that if you get a clear indicator color change then you have managed to avoid the interference from metals.

I also tried another variation of the test, by diluting my sample 1 part sample 4 parts distilled water, and then used the more sensitive version of the CH test (25 ml sample) which gets back to the same 50 ppm/drop resolution. For the current pool water this came up with ~950ppm -- ie pretty much the same result, but for the tap water sample I got 450 ppm, still a lot higher than expected, but closer.

We'll see how the standard solution tests when it gets here, maybe I just got a bum batch of reagent.
 

TampaKathy

Gold Supporter
Oct 2, 2020
580
Tampa, FL
Did you try the "fading endpoint" method? Not sure if that applies in your situation but I know that's what I need to do...read post #7 from link below to see if it applies.

From post #7.
The sample may turn purple during the test, or go to blue for a moment and then turn back to red/pink. This is called a "fading endpoint" and is caused by interference from metal ions. If this happens, do the test again, but this time add five drops of R-0012 before adding any R-0010 or R-0011L. Remember to count the initial five drops in the total.
 
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