High CYA, Chlorine & Phenol Red

[Roguewave]

Good afternoon,

My question is...

With a high CYA level, does the chlorine level effect phenol red in the usual way? In other words, should I be concerned about accuracy when testing for pH with a CYA level of 100 and FC of above 10?

No need to read further unless you're wondering why the high CYA level...

So I have discovered my pool is likely not 20,000 US gallons as previously thought. I wasn't careful and ran the CYA up to near 100 when opening a few weeks back when I was shooting for 80. With testing via 2006C, I'm calling it 95. I'm pretty confident in this range since I also ran a test with 50% tap water, which gave me a reading of around 50. I have no plans to drain any water. Instead I will perform at least weekly backwashes, which, won't do a ton, but I'm not concerned about this CYA level. The winter rain should bring it down a fair amount and I won't have to spend much on CYA next season. Also... everyone is loving the water.

Per Pool Math, I'm maintaining a FC level of 4-13 with a target of 7. It was 6.5 this morning. (Edited to change 4-11 to 4-13 after looking at the app again)

Given the amount of salt added, and the beginning and ending salt level as tested with a Taylor kit, I now estimate the pool to be around 18,000 gallons. Signature updated.

 

[HermanTX]

With a high CYA level, does the chlorine level effect phenol red in the usual way? In other words, should I be concerned about accuracy when testing for pH with a CYA level of 100 and FC of above 10?

When FC is at 10 or higher, the drop test for pH is not accurate. If you have an electronic pH meter, then most are fine with elevated FC levels. The CYA/FC states a min. of 5ppm and target of 7-13 for CYA of 100 and SWCG. FC/CYA Levels
So keeping it at 7-9ppm should allow you to get reasonable result on your pH drop test.

 

[JoyfulNoise]

Even at 10ppm FC the Taylor phenol red will work. You just need to read the test right away, no letting it sit around while you do other things. The de-chlorinating agents in the R-0004 can handle the higher FC. It’s only when you get above 20ppm FC that the test is too hard to read. If you want to be really careful, you can dilute the water sample 1:1 with distilled water and test the pH, the dilution will not affect the pH much at all.

 

[Roguewave]

Even at 10ppm FC the Taylor phenol red will work. You just need to read the test right away, no letting it sit around while you do other things. The de-chlorinating agents in the R-0004 can handle the higher FC. It’s only when you get above 20ppm FC that the test is too hard to read. If you want to be really careful, you can dilute the water sample 1:1 with distilled water and test the pH, the dilution will not affect the pH much at all.

Well you had me right up into the distilled water thing. Distilled water has a pH of 7.0. You may be right. It would just be surprising, is all. I'll have to test that theory at some point.

That's interesting news about Taylor's phenol red. I'll have to test that too. Thanks!

I'll test it at around 9.0 and then bump it up the next day and retest.... and maybe bump it up a second time and retest.

That said... I wonder how much bounce I'll get by bumping the chlorine via the SWCG. For proper science I should use some bleach, I suppose.