Persistently High CYA... Despite Big Rain and Drain

[LarryG]

Relatively new pool owner / TFP follower here. Not knowing any better, used trichlor tabs for about 1.5 years here.
Stopped tabs on August 8, and switched to liquid chlorine.
Prior to yesterday, got two consistent readings of 104 for CYA after doing conservative water replacements.
Yesterday had big rains (remnants of Erika storm). During the storm, I dumped approximately 2000 gallons.
This morning pH was 7.8 (previously 7.5), FC was 4 (previously 9)... but CYA was still 104... after performing the test twice (dilution method)... what???
Any thoughts / recommendations would be sincerely appreciated. Thank you.

 

[jblizzle]

How are you testing? The recommended CYA test will not give a value of 104.

It is only logical that your CYA must have been much higher than you think.

 

[LarryG]

Hi, JB.
Used equal amounts of pool water and tap water. Multiplied the reading (around 52 on the comparator) by 2, to arrive at 104.

 

[duraleigh]

CYA test results should be rounded to nearest 10 ppm.

 

[Alex_H]

We got a lot of rain on the other coast as well. I bought more CYA because I was low before Erika flooded us. Good luck.

 

[LarryG]

OK... CYA is then approximately 100.
Curious why there was no change in value after a huge storm, and my draining 2K gallons.
Thank you in advance.

 

[Agent99]

The CYA test isn't the easiest to read/gauge, IMO. At at really high levels and really low levels, numbers can be off quite a bit.

You drained off water that was mixing while it was storming. In other words, you didn't drain 2k gallons first...then filled it up. You drained 2k gallons WHILE filling it up (rain).

That will lead to much less CYA coming out of your pool than the approximately 30% you might be thinking from draining 2k gallons. (2/7 = 0.29)

 

[LarryG]

It's correct that I drained while it was raining, Chris. (Really wanted to do the dump earlier, but we were visiting a sick family member.)
However, when I shut off the submersible, the water level was at the bottom of the skimmer. Over night, the rain brought it up almost 2.5 inches.
So I was pretty surprised with today's CYA number.
More rain is expected this week, so I'll likely continue the quest to a CYA below 80.
Thank you for jumping in.

 

[LarryG]

Replaced another 3.5 inches of water last night.
CYA was 120 this morning. It was at 100 yesterday morning.
Is the explanation that the CYA test just isn't accurate above 100?
Any other thoughts?
Thank you.

 

[jblizzle]

Well, it only goes up to 100. Once you start diluting, then the error is multiplied too.

 

[LarryG]

For clarification, both tests were done using equal parts of pool and tap water.
Was surprised with the increase in CYA, since none has been used for almost a month.

 

[jblizzle]

It did not go up. It is in the noise of the test. Lighting plays a large role in the CYA test. Are you pouring the sample back and forth a few times to see if your readings are consistent?

 

[LarryG]

Yup, Jason. Ran the test twice this morning... with identical results.
Just got back from testing the neighbor's pool. His CYA was 60.