Calculating Adjusted TA due to high levels of CYA

[nicole12]

I read a post that talked a bit about TA numbers that could be misleading due to high CYA making up the chunk of the TA. Is there a calculation that I can determine Adjusted TA?

Last nights numbers just in case you need them:

FC 12
CC 0
CYA 100
PH 7.2
TA 90

[waterbear]

The correction for TA based on CYA is pH dependant ...to simplyfy the math for you at pH of 7.2 subtract a quarter of you CYA reading from you TA to get the adjusted TA.....not exact but close enough for government work!
so you would subtract 25ppm from your TA (1/4 of 100 pppm) and you would get an ajusted TA of 65. (the actual adjusted TA would be 64, pretty darn close) As you can see this indicates that your TA is actually a bit low. IMHO, you should bump it up a bit.
The quick and dirty way to do this calc is to subtract 1/4 of the CYA reading for a pH of 7.0-7.2 and 1/3 of the CYA for a pH of 7.4-8.0 . This will be accurate enough for you to get your TA in the correct range! If you CYA is 50 ppm or less I wouldn't even bother with the correction since it won't change your TA that much unless your TA is at the very low end of the acceptable range.

There are some that think this adjustment is not even necessary at all, the jury seems to be out on it's usefulness but there is some sound chemisty behind it.. Just remember that there is nothing 'Magic" about the accepted range of 80-120 ppm for plaster or 100-150 ppm for vinyl and fiberglass. Being a bit outside of these ranges is ususally not going to cause any major problems!