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I have a 20,000 gallon pool and water is clear with no signs of algae. I use tablets. My CYA is reading well over 100 using both my Taylor kit and strips. In the spring I changed over 50% of my water. I add one tablet until its disolved and then add another. My chlorine reads 6 to 9. Could I be getting a false reading on my CYA?
One can always been reading the test wrong, but no, I don't think so. Tri-chlor tablets add massive amounts of CYA---6ppm for every 10ppm of chlorine added. If you didn't measure your CYA before the drain and refill, after years of using the tabs, a drain and refill could leave you STILL at around 100ppm for CYA.
You can run the CYA test repeatedly with the same sample. But be sure you are using equal amounts of pool water and R-0013, that you shake the squeeze bottle for at least 30 seconds, face the sun, and hold the black dot tube at waist level. Don't wear sunglasses when you run the test, either.
To re-run the test simply dump what's in the black dot tube back into the squeeze bottle and do it again, as many times as you need to, all with the same sample. I suspect you'll still get a CYA of 100.
Stop using Tri-Chor tablets! And don't use Di-Chlor powder--that's even WORSE! Di-Chlor adds about 9ppm of CYA for every 10ppm of chlorine!
You've gotten into what pool stores mistakenly call "Chlorine Lock". It's not really that, but the danger of algae is there. If your CYA is 100, you need to keep your FC between 8 and 15, not 6 and 9. Use bleach to chlorinate, or, if it's cheaper, Liquid Chlorine, which is really the same stuff, only stronger. Basically, if the bleach is 8.25% and the LC is 12.5%, if the LC is less than 50% more than the bleach it's cheaper. Otherwise, the bleach is.
You probably also find your pH is frequently low and you add Borax or Washing Soda (same as pool store pH Up! products, only far cheaper) to raise it. Switching to bleach will end that as bleach is effectively pH neutral.
Like to see a full range of tests:
FC (free chlorine)
CC (Combined chloramines)
TC (Total Chlorine. TC = FC + CC)
pH
TA (Total Alkalinity)
CYA
and, if your pool isn't vinyl or fiberglass, CH (Calcium Hardness)
FC 9
TOTAL CHLORINE 9
PH 7.6
ALK 90
CYA off the scale
Vinyl pool
The CYA reading prior to refilling was about 120. Pool store said 60.
If you had a reading of CYA 120 before draining off half and refilling then something is wrong.
Did YOU do the CYA test before and after and exactly the same way?
Did you add something?