Question on how much dichlor raises chlorine

[watson524]

Hi all,

I'm wondering if anyone knows how much dichlor raises the chlorine ppm per ounce in a fixed amount of water? Say 500 gallons?

The bottle of Spa56 we have doesn't say how much it raises it. It just says to superchlorinate add 1oz / 500 gallons and resume normal treatment when it's back to 2 - 3ppm

In our tub today we were doing the weekly filter cleaning and the chlorine level was 0.4ppm (SLIGHTLY low since we have the Nature2 system but not bad) and I was going to see if I could take it up some with the Spa56 to use some of it up since we had it but I have no idea how much it'll raise it. The store told us to add 2oz / week as part of "weekly maintenance" but I'm opting not to do that after what I learned from posting another thread here.

I guess I'm just wondering if there's a way to know, in the absence of heavy bather load and other "clean up" tasks, what's the expected increase?

thanks!

 

[Richard320]

Poolmath will tell you. Put the tub volume in the box above and then scroll down to the bottom where it says "Effects of Adding Chemicals."

 

[watson524]

Oh heck, that's awesome! Thank you!!! Yeah so I don't want to put 2oz in, that'll take the FC up by 17ppm. YIKES!!!! But this is very helpful since my replenish is about half gone so I'll be able to use the Spa56 in very small doses to keep the FC where I want it.

 

[JasonLion]

The only thing to watch out for is that dichlor raises the CYA level fairly quickly, and you don't want CYA getting too high. Once CYA gets up to 40 or 50 you can switch to using bleach/liquid chlorine (which does not add CYA).

 

[watson524]

The store didn't tell us anything about testing for CYA tho I see it's an option with my Taylor test kit. What should my target CYA level be, I'll test for that tomorrow, and if it's too high, can anything be done to drop it?

 

[JasonLion]

CYA should be around 30, call it 20-40, though 50 isn't really a problem.

The only way to lower CYA is to replace water.

 

[chem geek]

For every 10 ppm Free Chlorine (FC) added by Dichlor, it also increases Cyanuric Acid (CYA) by 9 ppm.

 

[watson524]

For every 10 ppm Free Chlorine (FC) added by Dichlor, it also increases Cyanuric Acid (CYA) by 9 ppm.

Thanks for that info! So as we go through the months, while the water still may look ok and all, am I right to say that we may need to dump and change the water purely because the CYA has gotten too high OR go to using bleach for the remainder of the say 3 or 4 months left in the water OR partially drain and add new water? Sounds like there's a few ways to manage this.

 

[chem geek]

Yes you are correct. You can either change your water more frequently (or dilute as you go) or can switch to using bleach which will roughly let you go twice as long between water changes. This is the Dichlor-then-bleach method described in Using Chlorine in a Spa. Note that if you switch to using bleach then you need to have a lower TA (around 50 ppm) and use 50 ppm Borates. The reason is that the pH will tend to rise due to carbon dioxide outgassing so lowering the TA minimizes this and the borates provide pH buffering without being a source of pH rise. When using Dichlor, it is net acidic when accounting for chlorine usage so tends to keep the pH more in check but it does lower TA over time. For every 10 ppm FC added by Dichlor, it decreases TA by 3.5 ppm (after the chlorine is used/consumed).