Which test to trust on CYA

[chem geek]

thanks...
I'm going to get a few more test done as well as perform mine again. Since mine is coming out low, I'd hate to add more and then find out it was already high enough.

Original test showed 0 on my test and 20 at the pool store. I added 2lbs which should have brought it up to more than the 30 i came up with .

How are you figuring that adding 2 pounds of CYA to your 20,000 gallon pool will raise the CYA to 30? I show that you have to add quite a lot more than that. For a 20,000 gallon pool, it would take 10 pounds of CYA to add 60 ppm. Of course, you should add less and retest since as you point out you don't want to go over since that can only be corrected through a partial drain and refill.

If you add 2 pounds, this can raise the CYA by [EDIT] 12 [END-EDIT] so that could be raising it from the 20 measured at the pool store (which you thought was 0) to 30 that you measure now. At any rate, I would just use your measurements which now show 30. Add half the required CYA to get to 70 so that would be about 3.5 pounds, then retest to see that you are at around 50, then add some more. Be sure to wait for circulation in your pool -- at least an hour.

P.S.
How could your original test showed 0? The most common test only has a scale that goes to 30, though some go to 20. If you fill the water to the top of the tube, then you know the CYA is less than the last line near the top of the tube, but you can't really tell how much less.

Richard