# Thread: Need help understanding the CYA chart

1. ## Need help understanding the CYA chart

My best guess is that let's say I have a CYA level of 30, my minimum FC reading should be 3 and my target FC should ideally be 5? So what does the shock FC mean? Does it mean if I need to shock the pool, should my FC reading be 16? Would shocking temporarily raise it to 16 to kill any algae then recede down to normal levels?

2. ## Re: Need help understanding the CYA chart

Shocking is a process... pool-school/shocking_your_pool/ If your CYA is 30, then the shock value is 12 IF you need to shock it. If there are no apparent water problems and you can pass the OCLT pool-school/overnight_fc_test test then you shouldn't need to shock it. So, the minimum is a level you should not let the chlorine fall below, the target is an ideal. You may need to dose above the target in order to ensure that with the sun you never drop below the minimum. Going below the minimum is what allows bacteria and algae to get a foothold.

I'm new at this too so this is to the best of my understanding! I'm sure those who have been at this longer will be along to confirm (or tell me I got it wrong )

Welcome to TFP

3. ## Re: Need help understanding the CYA chart

What BethyM said^^^^^^^^^^^

4. ## Re: Need help understanding the CYA chart

Originally Posted by pisstol27
My best guess is that let's say I have a CYA level of 30, my minimum FC reading should be 3 and my target FC should ideally be 5? So what does the shock FC mean? Does it mean if I need to shock the pool, should my FC reading be 16? Would shocking temporarily raise it to 16 to kill any algae then recede down to normal levels?
Minimum means just that - FC should never drop below. Target is subjective; you want to be above minimum with some room to spare. You'll lose FC to sunlight and stuff blowing in the pool, and people swimming. So, if you lose 3 ppm per day, you better set your target to at least 6. Or 7, to get a little more safety margin.

Shock level is the point at which it kills algae faster than the algae can reproduce. Think of algae as pool yeast. The shock process means maintaining that shock level by adding chlorine frequently until you've killed all the algae and there's none left to reproduce.

5. ## Re: Need help understanding the CYA chart

Thanks guys, that answers my questions. I will be buying the Leslie's pool kit tomorrow so I am eager to see what the readings say.

6. ## Re: Need help understanding the CYA chart

Be careful buying a kit at leslies....You do not want to buy the K-2005.

You need a kit that contains the FAS/DPD chlorine test.

7. ## Re: Need help understanding the CYA chart

OK, I will make sure. thanks.

8. ## Re: Need help understanding the CYA chart

Yeah, the Leslies kit is confusing, it usually contains the DPD chlorine test, not the FAS-DPD test that you really want.

9. ## Re: Need help understanding the CYA chart

Originally Posted by BethyM
I'm new at this too so this is to the best of my understanding! I'm sure those who have been at this longer will be along to confirm (or tell me I got it wrong )
Wow - that was fast, you sound like any one of the regulars!

The only thing you left off was your test kit recommendation and where to buy it.

Note: I treat the target as my minimum, but that's just me.

10. ## Re: Need help understanding the CYA chart

Originally Posted by Speedo
Originally Posted by BethyM
I'm new at this too so this is to the best of my understanding! I'm sure those who have been at this longer will be along to confirm (or tell me I got it wrong )
Wow - that was fast, you sound like any one of the regulars!

The only thing you left off was your test kit recommendation and where to buy it.

Note: I treat the target as my minimum, but that's just me.

I thought the same thing about BethyM... but I have not been a regular here much longer.

11. ## Re: Need help understanding the CYA chart

Originally Posted by Richard320
Shock level is the point at which it kills algae faster than the algae can reproduce.
Just a minor point of clarification. The shock level is where chlorine kills even larger volumes of green algae faster than they can reproduce, significantly faster so that you get way ahead of the curve and kill off all the algae in a reasonable period of time, usually within some number of hours but certainly within a day. The pool will then be cloudy and need to be cleared of the dead algae through a combination of continued shocking with chlorine and filtration.

The minimum chlorine level for the CYA level kills small amounts of green algae somewhat faster than it can reproduce so essentially prevents it from getting a foothold. If you dump in a bunch of algae or organics into the pool, you can overwhelm the chlorine in the pool having the FC drop (if not overall, then locally in some area of the pool) and algae could grow so the minimum is like a knife-edge and works for normal situations but if you anticipate something unusual to stress your pool, then raise your chlorine level in advance to give yourself some room. Unusual events would include putting a dirty cover on the pool, turning on a solar system for the first time during the season, getting a storm that blows in a lot of debris into the pool, having a very large pool party with people filling the pool (say, 1 person per 1000 gallons), etc.

12. ## Re: Need help understanding the CYA chart

Quick follow up question, how long after adding bleach to the pool should I test it to see my current level? thanks!

13. ## Re: Need help understanding the CYA chart

Give bleach/liquid chlorine at least half an hour with the pump running before retesting.

14. ## Re: Need help understanding the CYA chart

For how long after adding bleach with the BBB method do I need to run the filter? I plan on adding it when I get home from work at 8:00, but by that time the filter has already ran for 8+ hours so I'm hoping not to have to run it a lot more.

15. ## Re: Need help understanding the CYA chart

Same answer, at least half an hour pump run time after adding bleach before you turn the pump off.

16. ## Re: Need help understanding the CYA chart

Thanks, that is good to know!

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•