I might know this answer ...

[wpturner05]

I opened this year to a green pool - SLAMd longer than I needed to JUST TO BE SURE ... brought up my CYA and allowed my FC to drop to about 5. End of school got me and I got busy. I have a solar cover on my pool and have not removed it or brushed or vacuumed in a little over a week. - I was peeking in to check the floor every couple days and I was checking pH and FC regularly which was always stable. I had more time today and decided to roll off the solar cover and drop in the robotic cleaner. I see bright green along the vinyl seams here and there and some patches here and there in the divets of the liner ... water is crystal clear, glass-like otherwise.
My numbers are as follows:

pH 7.2
FC 5
CC 0
TA 70
CYA 70
salt 3000

Do I need to SLAM?

 

[AimeeH]

Yup.

But the you knew that

 

[wpturner05]

I wanted a DIFFERENT answer

UGH!

 

[AimeeH]

I know. I've been there too. But if you aren't all green with algae maybe get right on it and this will go fast.

 

[wpturner05]

Admittedly - I should know this answer - but I dont. Why did I get the algae in the first place? because of no brushing or swimming? My FC levels were in range.
Was it because I wasnt brushing? Could it have been the leaves and pollen that got in the pool?

 

[BattleOfYakima]

According to our handy CYA chart, you'll see that the bare minimum FC for a CYA of 70 is 5ppm FC. So anywhere in the pool that is under 5ppm you can think about as not having any defense against algae formation and you're on the highway to green swamp. This is why the target range is higher than the minimum, so we have a little buffer and never fall under the minimum. As for why you "get algae" I share the immortal words of chem geek from this thread:

There is no spontaneous growth, however. If algae never gets into your pool, it will never grow there. The problem is that there is algae almost everywhere outdoors as a small amount is readily blown by the wind and it's also growing near the pool where it is moist (from dew, for example), has at least some indirect light, and some nutrients. So for practical purposes, you can consider it to already be in your pool just waiting for the right conditions to grow even though technically it's the other way around -- the conditions become conducive, some new algae blows in or drops in, then it takes off in growth.

 

[RadEngr70]

You're most likely a victim of what is known as an error budget deficiency or precision stack up error. The measurement we rely on most to keep an algae free pool is the CYA test. This test determines what level of free chlorine is required to keep our pools free of algae and other biological agents.

The best CYA test we have available to us now, as pool owners, is by most accepted to be accurate to +\- 10 to 15 ppm (if you do it right). So it's possible what you measured as a CYA of 70 was actually 80, which means your minimum FC was actually 6, and not 5 - and that doesn't even count the precision errors that might be inherent in the CYA/FC tables themselves. This could have allowed the algae to take hold.

Not the end of the world though... TFP is the resource to fix these minor hiccups... imagine if you had to rely on the pool stores!?!?

 

[BattleOfYakima]

Awesome post RadEngr70! Great points! Perfect wordings! Thank you!

 

[pooldv]

Good answers folks! I find that with my solar cover on that I need to run my FC a bit higher. We live in the woods and have lots of trees and critters helping me keep my pool dirty too though. With CYA at 80 I run my FC at 8-10 just to be on the safe side. More is always better. It is safe to swim with FC up to shock level for your CYA, [FC/CYA][/FC/CYA], so no reason to risk being too close to minimum.