High daily chlorine loss and CYA disappeared

[Ritaelyn]

Current test kit: TF-Pro Salt
Pool history:

• 11,000 gallon plaster pool, first time pool owner.
• Using a solar cover.
• Originally had a pool guy, told him in writing that we were going to run it hot at 84 degrees through winter. He required us to provide chemicals - fair for heavy use during winter/etc. Promised in writing weekly testing/etc.
• Pool drained in November by pool guy.
• Things were fine for a month, then mustard algae outbreak January.
• Tested water, turned out NO chemicals, no FC, no CYA, pure tap water. Pool guy just threw in one puck a week, didn't even skim or do anything else promised.
• Pool guy fired.
• Went to pool store, got pool stored, cal-hypo shock sold to us immediately clogged cartridge filter with additives.
• Found TFP, switched to liquid chlorine. Received TF-Pro kit thanks to TFTestkits.net!
• Added CYA to 50 according to TF-Pro kit. Pool store tests also showed 50~ cya (yes, I know they aren't reliable, but they seem to be accurate for CYA, FC, and PH. They are wildly off for CH.)
• Adjusted PH down to 7.0, SLAM didn't remove mustard algae. Removed lights (tons of mustard behind the lights.) Mustard algae shock did. Passed OCLT back at regular SLAM levels after mustard shock.
• Tons of red-yellow stains remained, discovered PH was 8.2+, TA 180. Basically PH went to 8.2 overnight, it was that bad.
• Got TA to 70 with muriatic acid treatments and running waterfall feature, PH holding for over a week at 7.2. Using Pool Math, enabled CSI, kept under 0.30.
• Did full tests - CH 425, CYA 50 still.
• Red-yellow stains remain, vitamin C removed them. Iron Rust traced to a pool light that was completely rusted out in the back.
• Let FC drop to 1.0, added polyquat-60 at FC 1.0 at the max active algae dose. Turned off pool heater, pool was 60-65, CSI index ~0.60
• Did an Ascorbic Acid bath treatment per wiki. Stains lifted a ton but not completely with 1lb of ascorbic acid. Ended up doing 2 lbs which made the pool look INCREDIBLE.
• Added tons of liquid chlorine to get rid of AA. Finally passed OCLT at min target FC 4 per wiki.
• Started swimming again march, heater turned back to 84 degrees. Put target FC at 6 for 50 CYA. Pool very clear.
• Started getting a Crud ton of daily chlorine use. We're talking FC down to 2 at 10 AM, put chlorine enough to hit FC 6-8, and by 5pm, FC of 0.
• OCLT @ FC 6-8 was 0.5 - 1ppm test which considered passing.
• CYA measured - still 50 per TF-Pro and pool store tests (ignoring everything else on pool store test.)
• Read a lot on TFP about nascent algae blooms.
• Decided to SLAM to 20 FC, PH 7.2. Kept swimming daily at SLAM levels at 84 degrees.
• Failed OCLT @ SLAM levels, 3ppm FC loss overnight.
• SLAM lasted 4 days until OCLT passed. Pool looks incredibly crystal clear now, that TFP look. The water is almost invisible now. Huge night and day difference. No iron stains, no signs of mustard algae.
• Haven't swam since stopping SLAM.

Yesterday's test results: FC 15.5, 100 TA, CH 525, CYA 30. Added 16 oz of stabilizer to raise CYA 10ppm per pool math. Pool @ 84 degrees.

Today's test results: FC 11.5, 100 TA, CH 525, CYA 40. Can't test PH yet. Pool @ 84 degrees.

Concerns:

I'm still having a 4ppm chlorine loss per day despite all the above. Somehow 20ppm of my CYA disappeared. I don't suspect any pool leaks as CH increased from 425 to 525 (10ml sample size), so we had evaporation and probably some plaster loss due to the AA bath. I had my partner independently do the CH and CYA tests for each reading and he got the same results.

We took the same sample to the pool store and pool store results say we have 59.5 ppm CYA. Yes, yes, I know pool tests are unreliable, they told us we have a 295 CH, and so on, but they seemed accurate for FC, TA, and I was expecting to have a 60 CYA with zero losses. They previously were accurate for CYA. We didn't tell them we had any problems with the pool water, they just ran the test and said our pool water looks great. Due to the high FC usage, I can't test PH yet. Pool store says 7.4.

Does the CYA test fail under high chlorine usage? I found this thread that indicates the results might be reduced for each 1ppm of free chlorine - Superchlorination & CYA

In order to test CYA, today at FC 11.5, I prepared three samples:

• 7.5ml pool water, 0 drops, CYA test. CYA test reads as 40ppm.
• 25ml pool water, 1 drop R-0007 sodium thiosulfate. 7.5ml withdrawn for CYA test. CYA test reads as 50ppm.
• 10ml pool water, 1 drop R-0007 sodium thiosulfate. 7.5ml withdrawn for CYA test. CYA test reads as 60ppm. It was so cloudy I almost read it as 70ppm but could still see the dot glancing away. Near 60 it's fully disappeared.

Should I worry about my still huge FC losses? Should I worry that my CYA is 40 while the pool store is 60? Should I try eliminating FC with hydrogen peroxide and see how the CYA test goes?


Which CYA test value should I use? 40 or 60?

Thanks!

 

[mknauss]

Your FC loss of 4 ppm per day, with a CYA of 40, right now is pretty realistic to me. My pool, which I have a solar cover on right now, with a CYA of 60, has a 3 ppm FC loss per day. We get alot of UV here. And you are heating the pool which increases the loss some.

As far as CYA test, I trust yours. Not sure why you got different readings using the R-0007. Never have seen that mentioned before. @JoyfulNoise can help us if there is any chemistry explanation.

Also, our water has iron in it. Look north - red cliffs, etc. from the Virgin River and some on the Colorado. I get iron stains in my pool. Not bad, but some. When I drained the last time (before adding my water softener for the pool water) I did a AA treatment. Since I am seeing some discoloration of the white goods. My fiberglass pool does not show the minor iron. I think the water softener is removing some of the iron versus before.

 

[JoyfulNoise]

I would use the value for CYA as determined by the actual CYA test protocol - no thiosulfate reduction. Thiosulfate reactions with chlorine are complex but generally increase the pH of the resulting mixture one the chlorine is neutralized. The CYA melamine test must be done under acidic conditions to get accurate results and so the R-0013 drops contain pH adjusters to ensure that the mixture of R-0013 and pool water is acidic. By adding thiosulfate you could be inadvertently changing the pH of the solution so that it is not in the correct range.

Yes, there is some literature to indicate that high chlorine levels can interfere with the formation of the cyanurate-melamine complex but the errors in measurement from that are smaller than the tolerance of the test which is roughly +/-15ppm.

 

[Ritaelyn]

Thanks everyone! I'll use the CYA result of 40. I wasn't aware that the reaction needs to be acidic. I wasn't aware that the tolerance of the test is +- 15ppm.

I'm relieved to hear that the FC loss is considered normal for our UV conditions! I'm still amazed that I went from 50 to 30 CYA before adding 10 ppm but it makes sense with the tolerance of the test and so on.

 

[mknauss]

With your pool covered the water is pretty warm. CYA degrades quicker in warmer water. At 90F water, you will lose 10-15% CYA level each month.

 

[Ritaelyn]

With your pool covered the water is pretty warm. CYA degrades quicker in warmer water. At 90F water, you will lose 10-15% CYA level each month.

Makes sense!