CYA Measurement not valid in dirty water.


Mod Squad
TFP Expert
Apr 4, 2007
SW Indiana
I mentioned this in my "Algae Diary" post, but I thought it deserved some attention on its own. I opened with algae this year, and my first water test showed a dramatic decrease in TA, but CYA was at about the level of last summer, 35ppm. This puzzled me because if the TA had dropped, that indicated major dilution from rainwater over the winter, but CYA should have been affected the same way.

Now that the water is clear, I retested the CYA and found it to be less than 20. I had suspected that the presence of the algae and dirt from the winter had affected the CYA turbidity test. Waterbear felt that this was the case as well. Apparently that is confirmed. Interestingly, the pool water without the CYA reagent seemed very clear in the CYA test tube, with the dot clearly visible and sharp, even when the tube was full.

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
Perhaps the melamine in the CYA test precipitates more than just the CYA but also something in the algae. Also, the test is buffered to low pH so perhaps that has some effect. At any rate, it's good to know to measure the CYA when the pool is free from algae (i.e. has no FC demand overnight and no CC).


LifeTime Supporter
Apr 5, 2007
Joliet, Il.
JohnT said:
CYA Measurement not valid in dirty water.
I had to read your post a few times before I could agree with what you were saying. I was reading your your question to read that the measurement of CYA that you have is of no concern in dirty water.

It took me three reads before I read it correctly. :roll:

boy do I feel silly,


The presence of algae or any other substance that can cloud the water can interfere with the CYA test since it is a turbidity test (you are measuring how cloudy the water gets). The effect of the cloudiness from the test and the initial cloudiness of the water are cumulative, even if the water looks clear before you add the test reagent. We use a colorimeter at work and if the water has algae the colorimeter will register it even though the sample tube looks totally clear! I have had many instances where I could not get a baseline reading when there was algae present and the colorimeter would not calibrate even though the sample looked clear to the eye.


LifeTime Supporter
May 8, 2007
Katy, TX
If CYA is a turbidity test, could a turbidity meter (which gives readings in Nephelometric units, or NTUs) somehow be used to conduct a CYA test? The reason I ask is because I do groundwater sampling for work and we use a turbidity meter to test the turbidity of the water as we sample.

Just curious.



If you could calibrate it to read the CYA concentration based on the turbidity of the sample it would work. This is how many of the electornic testers work but they are calibrated to read ppm CYA.