Proper way to test for CYA using TF-100


Well-known member
Apr 6, 2018
I know there is threads on this but to me this measurement with this kit is not revery good. If I face with back to sun and do measurement and then face sun and do it again I get two totally different answers! So which one is correct????

is there another method that’s not as ambiguous to test CYA? Is there a CYA standard solution u can buy with a KNOWN CYA that we could use to practice with then we would know how to properly measure our pool water???


TFP Expert
Platinum Supporter
May 23, 2015
Tucson, AZ
You do the test with bright high noon sun. Back towards the sun and the tube held about waist high in your body’s shadow. Two fingers on the top of the tube to hold it as you don’t want to block light entering the tube. Fill the tube up to each mark, look through the top of the solution, if you see the dot, fill to the next line. Keep doing that until the dot disappears. Do not stare intently at the tube as that can cause you to perceive the dot when it’s not there. Look away then look at the tube. Don’t try to “force” yourself to see the dot.

There is a 50ppm CYA standard available from and other online retailers.

There is no other test for CYA unless you want to drop $500 on a digital turbidity meter with NIST-traceable standard solutions. Test strips are useless as their ranges are far to broad and the color is discrimination is poor.


Mod Squad
TFP Expert
Platinum Supporter
LifeTime Supporter
In The Industry
Apr 1, 2007
Sebring, Florida
Pretty sure the instructions say "back to the sun".

Remember, you are trying to get + or - 10 and should do that easily. Keep practicing.

There is NO other more accurate, easier test