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

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

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

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...

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!

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Originally Posted by 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.

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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

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.

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Originally Posted by aquarium

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

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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.

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"

Okay, got it.

I'm actually trying to run the pool with very low CYA (about 10ppm) so I can also run relatively low chlorine - 2-3ppm per the OTO kit. I have the pump (with auto-chlorinator) set on a timer so it's adding chlorine during the day. So far it looks like it holds the chlorine level constant both day and night - I'm testing AM and PM to set the chlorinator. I'd like to find NONstabilized trichlor pucks but so far no luck.

Thanks Everyone!

TW

There are no non-stabilized trichlor pucks.......

There are non-stabilized cal-hypo pucks, but you can't use them in a trichlor feeder, and since hth changed the packaging and removed the plastic capsule they're junk anyway because they go to mush within 12 hours.

Janet

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Originally Posted by aquarium

Okay, got it.

I'm actually trying to run the pool with very low CYA (about 10ppm) so I can also run relatively low chlorine - 2-3ppm

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With 10 ppm cya, i wouldn't consider 2-3 ppm "low chlorine". I'd consider that "normal chlorine", and for you cya level, it might even fall into the category of "high chlorine". I think you need to do A LOT more research on the chemistry, along with the hazards of a pressurized chlorine feed system before you go any further. There are MUCH safer ways to do what you are doing.

That's just my $0.02. :)

Michael

Thanks Janet,

Yeah, and we have -very- high calcium hardness here, so I don't want to add more.

Just took a couple of tests at high noon here. I can see the dip in the chlorine when taken across the pool from the returns, so the low CYA is having the expected effect. Near the returns it's almost up where I want it. I'm letting the pump/chlorinator run a couple of hours after the sun is off to punch it back up for the night.

The way it's set now a single 8 ounce puck will last most of a week. (15,000 gallon IG concrete) Before this water change I was going through nearly one a day. :eek:

Tom

PS: It's not a 'pressurized' chlorine system, just a vertical PVC pipe with water in/out and a ball valve, with the pucks inside. I think it's also called an erosion type feeder.

the problem you are going to run into as you continue to use trichlor is that your CYA will continue to rise and as it does you will have to run your chlorine at a higher level to compensate which will cause your CYA to rise even faster. That is the downside to using stabilzed chlorine. That is also the reason most of us here in the forum use bleach (sodium hypochlorite) for our chlorine source.

Trichlor is actually a compound made from cyanuric acid and chlorine and when it dissolves it forms some hypochlorious acid (unstabilized chlorine), some cyanuric acid and some chloroisocyanurates (stabilized chlorine). These componants will be in equalibrim depending on the amount of cyanuric acid in the water. the higher the CYA less hypochlorous acid and more cloroisocyanurate are formed. It is a losing battle that will eventually force you to either switch exclusively to unstabilized chlorine or to drain and refill the pool and start over!

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Originally Posted by aquarium

PS: It's not a 'pressurized' chlorine system, just a vertical PVC pipe with water in/out and a ball valve, with the pucks inside. I think it's also called an erosion type feeder.

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Ahhhh ok. I see what you are saying now. Well, I won't repeat what waterbear said, because he hit it right on the head.

Michael

Yep,

I understand the problem now. With aquariums the similar problem is with a slow build-up of nitrate, the end product of the ammonia cycle, which is diluted by water changes. So my thought was to apply that technique, albeit on a much larger scale.

I'm hoping that by starting with low CYA to begin with, that the rate of increase CYA>increase Cl>increase CYA can be held down to a flatter curve. Then smaller water changes to control the CYA level. We're about to go into dry season, so watering the yard with pool water gets double use of the water. The downside then becomes the higher use of muratic acid for the new water makeup. (EDIT: We already have high ALK, calcium and pH in the tapwater.)

TW

Quote:

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Originally Posted by aquarium

Yep,

I understand the problem now. With aquariums the similar problem is with a slow build-up of nitrate, the end product of the ammonia cycle, which is diluted by water changes. So my thought was to apply that technique, albeit on a much larger scale.

I'm hoping that by starting with low CYA to begin with, that the rate of increase CYA>increase Cl>increase CYA can be held down to a flatter curve. Then smaller water changes to control the CYA level. We're about to go into dry season, so watering the yard with pool water gets double use of the water. The downside then becomes the higher use of muratic acid for the new water makeup. (EDIT: We already have high ALK, calcium and pH in the tapwater.)

TW

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If you have a sand filter the backwashing will help the dilution. If you are using a cartridge or non backwashing DE filter you will be doing partial water changes frequently and usually have to rebalance the pool a bit each time.
On a related note, If you close the pool for the winter sometimes anerobic bacteria will degrade the CYA in much the same way that anerobic denitrification can take place in an aquairium with a denitrating coil or filter or a plenum. The problem is that the denitrification does not seem to proceed all the way to N2 which gasses off but seems to stop and urea and ammonia compounds which create a HUGE chlorine demand when you open the pool.