How do you calculate how much Cyanuaric Acid is in your tub

[999]

I am about to do a water change after 3 months using dichlor and MPS shock.

I know how many tea spoons of 55% dichlor that i have used in that time but don't know how many ppm  that is.

I do know that for every 10ppm of dichlor I am adding 9 ppm of CYA..

Is there a standard measure of what 1 teaspoon of dichlor in a 1700 litre tub relates to in dichlor ppm.

[chem geek]

You can use The Pool Calculator by changing the units to "Metric", entering 1700 liters for the volume, going down to the bottom of the page to "Effects of adding chemicals" and selecting "dichlor" from the pull-down menu, and entering the number of grams you have added which is the number of teaspoons times 4.725 so 1 teaspoon (4.725 g) raises the FC by 1.5 ppm and the CYA by 1.4 ppm.

Note that there is a loss of CYA over time as chlorine slowly oxidizes it so figure losing around 5 ppm CYA per month.

[999]

chem geek.. thats just what I was looking for, thanks.

In the last three months I have added 116 tea spoons of 55% Chlorine granules. That equates to 548 g. The calculator shows that my CYA will be at 163. Taking into account a loss of 5 ppm per month, then it will be around 148. I think I am right in saying that anything over 100 is too high, so although the water looks fine I will be changing it today after 101 days.

I have been shocking with MPS, but I have recently done a dichlor shock so that has added to the CYA in recent weeks.

(just by using your estimate of 1 teaspoon of dichor adds 1.4 ppm of CYA also equates to 162.4 ppm (116x1.4) so that is a good guide for me)

[chem geek]

You can always compensate for a high CYA by having a higher FC level, but you are right that at some point that becomes impractical and what usually happens with Dichlor-only users is that the water starts to turn dull which is most noticeable after a water change when the water becomes so noticeably fresh and clear.  With users who manage their CYA level (say, Dichlor-then-bleach), the water stays in good shape longer and after a water change the difference is less noticeable because the water didn't get as dull/cloudy because the active chlorine level is more consistent so keeps up with oxidizing bather waste better.