“Effects of adding “ error/omission?

JimMarshall

Gold Supporter
TFP Guide
Jun 5, 2017
1,003
Oil City PA
Messing around with PoolMath this morning.
2 lbs of Trichlor in my 13,000 gallon pool will raise FC by 17 and CYA by 10. It will drop my pH by .9 and add some salt.

Won’t the acid added in the Trichlor also drop my TA? PoolMath seems to omit this from the effects of adding section. Omitted from dichlor as well.
 
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JamesW

TFP Expert
Mar 2, 2011
20,353
50 lbs of trichlor in a 25,000 gallon pool will:
Raise fc by 219 ppm
Raise CYA by 133 ppm
Raise salt by 179 ppm
Lower TA by 154 ppm
 

mgtfp

Bronze Supporter
Mar 5, 2020
394
Melbourne, Australia
I don't think that adding CYA lowers TA in the same way that adding muriatic acid does.

CYA actually contributes to the TA reading, which is why TA needs to be corrected when used to calculate CSI. CSI needs carbonate alkalinity (CA), not total alkalinity (TA), as an input. Simplified, you have to subtract about 1/3 of the CYA from TA to get CA to calculate CSI.

I suspect (I'm not a chemist...) what's happening is, that the increase in TA by CYA gets more or less compensated by the "acidity effect" of CYA which is only visible in the pH drop. So, CYA does decrease CA, but increases the non-CA part of TA.
 

JamesW

TFP Expert
Mar 2, 2011
20,353
CYA doesn't lower the TA, but trichlor does.

The acidity comes from the chlorine as it connects with water.

The cyanurate that's left after the chlorine leaves the trichlor actually raises the pH by accepting some hydrogen ions.

CYA substitutes cyanurate alkalinity for carbonate alkalinity.
 

mgtfp

Bronze Supporter
Mar 5, 2020
394
Melbourne, Australia
Thanks, James. Now I get it.

When adding 10ppm worth of bleach, it will raise TA by about 7ppm, same for adding 10ppm of FC via SWG. The following use of these 10ppm of chlorine will lower the TA again by about 7ppm, so the whole chlorination cycle is TA neutral.

Trichlor bis different. It doesn't raise TA, but the eventual use of the added chlorine will lower TA, so the whole chlorination cycle is not TA neutral in this case.

Chem Geek solved this in his PoolEquations spreadsheet by adding a "Chlorine Usage" section, so you can calculate the full cycle.

PoolMath focuses on the full cycle, it doesn't tell you that adding bleach actually increases pH and TA, because the following use of chlorine compensates that initial rise again for both, pH and TA.

For Trichlor, PoolMath is not quite consistent. It calculates a pH drop of 0.5 when adding 10ppm FC worth of Trichlor, which seems to be the effect of the whole cycle, about half of that is due to the initial drop when adding Trichlor, and the other half is due to the pH drop when the chlorine has been used.

To be consistent, PoolMath should calculate the TA drop when adding Trichlor as the result of the whole chlorination cycle. Well spotted, Jim!