How much Chlorine is too much?

[Dave S.]

Hi All,
I've been haunting this site for a few weeks now. Great information!

I installed an in-ground vinyl pool last summer. This summer, I'm having algae issues (it's gone green for the 2nd time since opening it a month ago.

Here's my question;

How much chlorine can I pour in (PPM), and be sure that I'm not bleaching the vinyl?

It's an 18' x 38' pool. 3 - 7' deep, 22,000 gallons. Right now, chlorine is around 6.8 - 6.9ppm.

Thanks!
Dave

[aylad]

Hi, and welcome to the forum!!

How much chlorine you can use depends on your stabilizer level. What's your CYA?

Janet

[Dave S.]

Hi Janet,
Does CYA = Cyanuric Acid?
if so, then 30 ppm

[aylad]

yes, CYA = Cyanuric Acid, aka "stabilizer"

Here's a copy of the "Best Guess Table" that we use as a guide around here...
Stabilizer . . . . . . Min. FC . . . . Max FC . . . 'Shock' FC
=> 0 ppm . . . . . . . 1 ppm . . . . . 3 ppm . . . . 10 ppm
=> 10 - 20 ppm . . . . 2 ppm . . . . . 5 ppm . . . . 12 ppm
=> 30 - 50 ppm . . . . 3 ppm . . . . . 6 ppm . . . . 15 ppm
=> 60 - 90 ppm . . . . 5 ppm . . . . . 10 ppm . . .. 20 ppm
=> 100 - 200 ppm . . . 8 ppm . . . . . 15 ppm . . .. 25 ppm

So with a CYA of 30, you have to get your Cl up to 15 to even achieve a shock level. You probably could to go 20-25 without any problems but I wouldn't go any higher than that just to be on the safe side, unless you increase your CYA level. Realistically, though, anything over 15-20 for a CYA of 30 is overkill anyway.

If you still have green water, then you need to raise your Cl to 15 ppm and keep it there by testing and adding more 2-3 x daily in order to consistently maintain 15 ppm. When your water turns blue/cloudy, then you can let the chlorine drift back down into the 3-5 ppm range and your filter will take out the dead algae.


Janet

[Dave S.]

Great advice, and thank you for the calculations!
My testing kit only goes up to 5ppm for cl.

I can't find any kits locally that will go higher. How can I safely stay within the range?

[aylad]

In a pool your size, know that 1/2 gallon of 6% bleach will raise your Cl by 1 ppm, so if you know your starting point, you can calculate what it will be based on what you add.

In the meantime, to get a ballpark estimate of your current chlorine, you can use CarlD's patented "shotglass method", which consists of mixing one part pool water with 1 part of distilled (non-chlorinated) water. Mix well, run your test, then just multiply your result x 2. If that's still not high enough, you can mix it 1:2 and multiply result x 3, etc. Each time you dilute it you're losing a little accuracy, but it'll give you a good ballpark estimate, and there's enough leeway that even if you overshoot by a little due to the dilutions, you still won't damage your liner.

Janet

[CarlD]

Aha! Do a search within PoolForum for the Shot Glass method! You'll need a bottle of steam distilled water, which your local grocery or discount drug store has.

The idea is you dilute pool water and then measure the chlorine in it. So...if your kit goes to 5 (and I assume it's an OTO kit with yellow reagent), then if you mix a shot glass of pool water with a shot glass of distilled water, when you measure it, when it reads "5" on the tester it's actually 10ppm! Mix 2 shots of distilled to one of pool and it as if the scale goes to 15ppm!

Got it?

[Watermom]

Although it loses accuracy, you can use dilution to get a ballpark figure when you need to test higher chlorine levels than your kit can normally register.

Take one part pool water and mix it with one part distilled water. Then, fill your tester with some of this mixture and test as usual and multiply your results by 2. If that doesn't go high enough to help, mix one part pool water with 2 parts distilled and then multiply results by 3, etc. You do lose accuracy with each dilution so it is not ideal, but better than nothing. What you need to do is to go ahead and get a good test kit like the Taylor K-2006 so you'll have it.