OTO and CYA and TC, oh my! What's being measured?

[aquarium]

When using a simple OTO test kit with low levels (10ppm) of CYA in the water, is the chlorine that is being 'buffered' by the CYA detected by the OTO kit?

The reason I ask is that I've just refilled our 15,000 gallon IG concrete pool and now have perfect water as a starting point. I'd like to keep this simple and use the pucks in the auto-chlorinator, setting it to give me about 2ppm per the OTO test kit, then use bleach to adjust after certain events, like a big pool party.

But reading the Best Guess CYA chart leads me to believe that the more CYA I add, the more chlorine I need, well above the 3ppm at the top of the OTO kit. Currently, with 10ppm CYA (calculated by weight of chemical added) the chlorine seems to be holding throughout the day. Why do I want to add more CYA (30-40ppm recommended) when that just means I have to add more chlorine? And if I turn up the auto-chlorinator, the pucks will add more CYA too, which means more chlorine. This seems like a death-spiral.

Why not run the pool with very low CYA if it is able to hold the chlorine?

Thanks,

TW

[mbar]

Read this post:
http://www.poolforum.com/pf2/showthread.php?t=2601

[aquarium]

Heh,

Yeah, that's my post as well. I was trying to ask basically the same question in a different way here and see if I could understand the 'conventional wisdom' better. I'm definitely not grokking on something...

[waterbear]

OTO is just measuring your TC. If the CYA is low then your FC will be burned off by sunlight. If you monitor the FC throughout the day you can see the effect. It is the FC that is the sanitizer. CC is more stable in sunlight and will not burn off as quickly but it is not an effective sanitizer. In fact, early attempts at stabilizing chlorine sometimes used anhydrous ammonia added to the pool to form monochloramine which is more stable than FC in sunlight!

[aquarium]

OTO is just measuring your TC. If the CYA is low then your FC will be burned off by sunlight. If you monitor the FC throughout the day you can see the effect. It is the FC that is the sanitizer.

Thanks,

I guess that's the heart of my confusion. How does CYA work? Does it take the chlorine -completely- out of service, and out of detection by the OTO test kit, or does it just release it more slowly in the presence of sunlight? In brand new fresh water like I now have, it's doubtful there is any CC, so TC=FC?

TW

[waterbear]

The only wat to tell if you have any CC is to use a DPD test or an FAS-DPD titration test.

The way CYA works is that it combines with the chlorine to form cloroisocyanurates in the water. These will test as FC but they do not have the oxidizing power of hypochlorous acid.. This combining is not complete if the CYA and FC levels are kept in the recommended ranges so there is some FC available. When the CYA is too high then most of the FC is combined unless you raise the level with unstabilized chlorine. (This is an oversimplification but it gives you the idea.) CYA has no effect on an OTO, DPD, or FAS-DPD test since the combined chlorine it produces will test as FC on DPD and FAS-DPD and as part of the TC on an OTO test.

[JohnT]

Thanks,

I guess that's the heart of my confusion. How does CYA work? Does it take the chlorine -completely- out of service, and out of detection by the OTO test kit, or does it just release it more slowly in the presence of sunlight? In brand new fresh water like I now have, it's doubtful there is any CC, so TC=FC?

TW

CYA doesn't affect your test readings. Its function is to protect clorine from being consumed by the sunlight. To do this, it makes the chlorine less active. You can't assume anything about the chlorine in your tap water until you test it.

[mbar]

You can think of it like this -The cya is what binds to the chlorine, so if there is too much cya verses not enough chlorine, there will not be any "free chlorine" to fight with problems. If there is not enough "binded chlorine" then your chlorine will be used up by the sun and you won't have any protection. "free chlorine" gets used up by the sun. The "free chlorine" is the "strong" chlorine, the chlorine that is binded with the cya is there to keep the water clean - but if there isn't enough of the "strong" chlorine to attack, then you will have problems. That's why some people can keep thier pool ok for a while with a high cya and low chlorine - a big enough problem didn't start yet, but the opportunity is always there. Keeping enough free chlorine is what you want to keep your pool safe from the "attackers"