Drained pool 1/2 way twice still too high CYA

[DavidDury]

Pool is 28,000 gal with a large sand filter.
It turned green and no matter how much chlorine I added I couldn’t fix it. After much research I found my CYA levels were far out of wack 120+ from the pool store test. I bought a tf100 kit drained my pool 1/2 way refilled and checked it. Still was over 100. Drained it over 1/2 way filled again and checked still over 90. Is this possible? Should I drain it again? Help!

 

[mknauss]

Welcome to the forum!
It is very possible your CYA was well above 200 ppm if you had used trichlor and dichlor in the past.
100 is the limit of the CYA test, so you have to do a diluted test. Mix 50% pool water with 50% tap water. Use this mixed sample as your test water. Multiply the result by 2 for your CYA level.
I suggest you read ABC's of Pool Water Chemistry.

 

[tim5055]

When I took over my pool the CYA was somewhere between 250 and 300. As Marty said, a diluted test will get you in the ballpark, but with each dilution the errror factor in the test goes up. Above 200 it's almost a guess.....

If you are now registering on the test (CYA=90) you are getting it where you need to be. A little over 50% drain should get you to 40...

 

[DavidDury]

Thanks! I did add 1/2 tap water and was still over 100 but thought I was crazy for trying that. Looks like it was a good way to get close. Now 1/2 tap water showed about 40-50. Looks like I’m on the right track. I will drain some more. I did read the ABC’s and they helped a lot. Just was unsure that CYA levels could be that high when the pool store said mine were only 110-120. Like all the post say don’t believe them! Thanks again!!

 

[mknauss]

pool store said mine were only 110-120

That was the limit of their equipment. And it is not accurate to begin with.

Stay at it.