Calcium Hardness Question

lindend

Silver Supporter
Apr 18, 2020
15
Florida
Just had my pool resurfaced. For the first two weeks, I relied on Leslie's and another local pool store for pool tests until I got my TF-100.

Did my first test today. I've got a SWG (Autopilot)

FC=17
CC=2
PH=7.5
CH=600
TA=80
CYA=30


The Leslie's results were very, very close to the TF-100 results except for CH. Leslie's was consistently in the 200-250 range. I did the CH hardness test three times (first time I didn't use the speed stir because the TF-100 instructions didn't indicate that was necessary but I see on the forums that it should be mixed until the water turns blue). Each time, I was in the 24-26 drop range.

Pool water is crystal clear and not cloudy. I've added CYA via the sock method but I'm puzzled at why there's such a variance (3x)? I'm not convinced I'm doing the test correctly.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
29,619
Laughlin, NV
I would believe your CH test. Many of the 'electronic' test units do a very poor job of testing for CH.
Try this next time you test CH. From Calcium Hardness - Trouble Free Pool
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.
In extreme cases, a fading endpoint may occur even when adding five drops of R-0012 at the start. If that happens, mix pool water with an equal quantity of distilled water, test that, and then multiply the result by two.
 
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lindend

Silver Supporter
Apr 18, 2020
15
Florida
I would believe your CH test. Many of the 'electronic' test units do a very poor job of testing for CH.
The scary thing is the other pool place reported CH values of 75 and 100 respectively. Since both pool places were on the low end of the scale, I figured it was safe to add the remaining calcium hardness increaser I had on hand (5lbs which raised it about 50ppm according to Leslies). I then ordered about 50lb of increaser but decided to wait for the TF-100 results before adding anything. Good thing I did or I would've been in a world of hurt.

I found about TFP after the first round of tests, but even before then, I was skeptical so I got test results from both places to see if they were consistent (and they were wildly different).

Try this next time you test CH. From Calcium Hardness - Trouble Free Pool
Thank you! Those had the ever important mix while adding R012 which were missing from the TF-100 instructions.
 

mgtfp

Bronze Supporter
Mar 5, 2020
158
Melbourne, Australia
There might also be an issue with the dropper of your test kit. I had that problem before, where my test gave a completely different test result to the last pool shop test. My test kit came with a CH standard solution. When testing this reference sample (at CH=200ppm), my test result showed the same deviation I had seen before with my pool water sample. I got in touch with my test kit provider and we worked out that there must have been a problem with the dropper of the titrating agent bottle. Do you have a reference sample with known CH-level that you could use to calibrate your test kit?
 

lindend

Silver Supporter
Apr 18, 2020
15
Florida
Do you have a reference sample with known CH-level that you could use to calibrate your test kit?
Unfortunately, I don't have a reference sample. But my test last Sunday the Calcium hardness increased again so I'm suspicious of the test. We'll see what my next test shows.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
29,619
Laughlin, NV
With a fresh fill, your CH should be pretty straightforward unless your start up procedure allowed your water to get aggressive and dissolve calcium from your new plaster.

What is the CH of your fill water? Test that with TF100. Your water supplier should have that data in a water quality report you can access. Double check your test with their data.
Then how much calcium you have added.
Add them up. What CH should you have?
 

lindend

Silver Supporter
Apr 18, 2020
15
Florida
Haven't been able to obtain my original water quality report yet, but I do have an update. We've had torrential rains in Florida and now that the rain has stopped, my calcium hardness results fell ~300ppm to 425ppm. This makes me more confident the TF100 results are correct and the Leslie's results were just grossly inaccurate.

Lending even more credence to the Leslie's results being inaccurate, I notice that my Autopilot consistently showed a salt level 1000ppm higher than Leslies. Did the Taylor salt test today and it matched the autopilot salt levels. You guys weren't joking when you said store pool tests can't be trusted.
 
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