CC defying pool physics?

[EricH]

Hi Forum,

I'm happy with my clear water, but consistently am reading non-zero CC levels (and a funky smell.) If I believe the Taylor manual, raising FC to 10x the CC is supposed to make it go away. So, that's what I've been trying using 6% walmart bleach as shock, and the BleachCalc program to guide my efforts.

So, it keeps going like this: I test CC and get something like .4 or .6. I dump enough bleach, at night, to raise FC to about 10 (2+ gallons), and see what happens. Well, the FC gets nice and high (and even holds it for a few days - 2 days later I'm still at 6FC) but that pesky CC is still reading high!

Here's the weird phyics - one time I measured the FC at 10 after adding the bleach (so I know it got up there) but the CC was still there at .5! Thought that was impossible.

My pH stays pretty low due to the TriChlor pucks (I can't help using the feeder) and the CYA stays low too, I guess due to lots of kids splashing out water? The low pH, high calcium, and high temp keep me stable according to the Taylor saturation index.

Anyone know how to lick that CC out of there? Is there a way to add it when shocking other than just pouring it in? I even "stirred" the pool last time I tried.

The #s:
TA=90
CYA= undetectable with Taylor, but must be there
FC=6
CC=.8
CH=475
pH=7.0
temp=85 degrees

12,700 Gallons, Plaster, Cartridge Filter, Inline Chlorine feeder (3 1in tabs/week), solid safety cover always on when we're not swimming.

Thanks!
E

[mbar]

If you have been using the chlorine pucks, your cya is probably very high. We can't help you until you find out yur cya levels, because you have to know them in order to see how high you have to shock the pool. Here is the "best guess chart"

Use the info in this chart to help you figure out what levels of chlorine you need to maintain in your pool based on the amount of CYA (cyanuric acid, also called stabilizer) that you have in your pool. (FC = free chlorine)

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

The chlorine pucks have cya in them, so you are constantly putting in cya, and the only way to get rid of it is by partial drains. Knowing your cya level is one of the most important things to know about your water - it will help you to know how much chlorine you need to keep a sanitized pool. On the other hand a cc of .5 is not something to worry about. You have to worry when it goes to 1. or above. Feel free to ask any other questions you may have

[EricH]

Yeah, chem_geek tried to help me with the CYA over on this thread:
http://www.poolforum.com/pf2/showthread.php?t=7705

Unfortunately, my Taylor reagent never shows enough reaction with my water to block the dot from view - so my assumption is no CYA.

To the point of your answer, though: is it that CYA will inhibit the ability of free chlorine to "neutralize" combined chlorine? So at the very least, I could try driving my shock level up to 25 or more to see if that helps (and maybe tell me what my true CYA is despite the Taylor test!)

Thanks,
E

[chem geek]

You are correct that the rate of Free Chlorine reaction to break Combined Chlorine will be slowed down by more CYA since it is the disinfecting chlorine (hypochlorous acid) that does the actual breakpoint. Ironically, the formation of new Combined Chlorine also comes from the disinfecting chlorine so exactly what happens depends on the types of substances that are in the water since disinfecting chlorine combines with ammonia very, very quickly, but breaks monochloramine (chlorine combined with ammonia) more slowly. Other organics may have the opposite behavior.

In any event, it is sunlight that helps break down combined chlorine at reasonable rates, along with the chlorine of course. So if you keep your pool covered or if it is not exposed to sun, then you may find more CCs that are hard to get rid of. It is also possible to have "persistent combined chlorine" that doesn't go away, but again this is more typical in indoor pools with no sunlight.

I'm not sure why your Taylor CYA test doesn't have the black dot disappear. If it gets cloudy at all, then there is CYA in the water. This link to the Taylor website shows how the black dot goes from being visible, to clouded, to obscured as you add more sample to the tube.

Richard