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

[mwsmith2]

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

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

[aquarium]

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.

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.

[waterbear]

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!

[mwsmith2]

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.

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

[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

[waterbear]

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

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.