CYA Discrepancy

[AfterTen]

Split by Moderator from HERE. Please start your own thread for your own questions. Thanks, jblizzle

I'm going to hook into this thread since I was about to post a similar question.
First my story.
Just before the 4th of July I let the pool go for a couple of days. You know, busy.
Look under solar cover to get a sample and it is a complete cloudy mess.
FC at 0, CC at 3.5
Just as a check, I take the sample to pool store and get the same story.
They run some test called "Chlorine Demand" and it prints out I need to put in 48 pounds of "BurnOut 3"
I start dumping in bleach. I test a couple hours later and FC is 0. Dump bleach, test and FC is 0.
Do this for a a day and a half. Swore I dumped in 20 gallons of bleach in 36 hours and the FC was 0 each time.
I read somewhere that the FC could be so high it would bleach out the test and never show red. I hoped this is what I did.
But on a whim I did my CYA test. I had no reason to think it was low. Pool store had it at a good number (30 I think) and I was holding chlorine prior to this cloudiness.
CYA was 0. Seriously, I couldn't believe it. My tube was crystal clear. Took a sample back to the store. CYA was 30.
I dumped a pound of CYA in that I had on hand and a pound more a day later. 72 hours later my test shows 30 and the store shows 60.
I'm still SLAMing right now and trusting my numbers for the work.

How can the pool store numbers be so bad?

 

[Richard320]

There's no mystery. What you're describing is perfectly normal.
inconsistency-in-readings-t61728.html

 

[jblizzle]

Welcome to TFP!!!

Typcially they do not do the CYA test with the correct lighting. So with not enough light, they are getting a higher result than you ... assuming you are doing it correctly outside with your back to the sun.

 

[JasonLion]

This is quite rare, but it does happen. Once in a very long while, when FC levels fall to zero, a kind of bacteria that can break down CYA grows in the pool and some, or all, of your CYA disappears. The is often, but not always, accompanied by ammonia showing up in the water (ammonia is one of the possible breakdown products from CYA). Ammonia creates a huge chlorine demand, so that FC always reads zero until you finally add enough chlorine to burn off the last of the ammonia.

As for pool stores, what is amazing is that some of them are correct once in a while