Massive CYA discrepancy

[Khlorine]

Hi everyone! Been loving all the guidance on this forum, inherited a very dirty above ground pool after purchasing a new home that we're slowly making good progress on.

Long story short, I am going to SLAM the pool and need to calculate the FC needed, which is why I need an accurate CYA. I actually received my TF-100 today (thanks for the super fast shipping before the holiday weekend btw!) and the first thing I tested was to verify the CYA against what the pool store test gave.

To my surprise, I'm testing less than 20 where the pool store tested at 50. I can still see the dot at waist level even once the test tube is full (pretty hazy though, so must be close).

Is this typical being this off from the stores test? And I'll add I'm actually inclined to believe my testing as we initially drained the pool down to 18 inches and refilled and have NOT added any stabilizer myself at this point.

Any advice on this? I'm just worried I'll add too much stabilizer.
Thanks!

 

[Jimrahbe]

K,

If you take the same water sample to three different pool stores, none of them will agree.. At least one and most likely many of the tests will be off by 25% or more.

What you have found is the main reason that we require one of our approved test kits before we can give valid chemical advice..

Thanks,

Jim R.

 

[Khlorine]

Thanks for the reassurance Jim, it was my understanding that (good) home testing would definitely be more accurate - was just so surprised how much it differed by!

I calculated and added just a bit of stabilizer in and will check again tomorrow to give myself some extra assurance I am in fact doing it right (not that it's that hard ).

 

[Dtkokay]

It can take a day or two for stabilizer to show up in testing, so don’t be surprised if your reading tomorrow is the same as today.

To test CYA, I use the following method that I learned on this forum. Fill the test tube to 100, then place it at your waist level and see if you can see the dot. If yes, then fill the tube to 90, etc. Basically, rather than filling up the tube until you can’t see the dot, fill the tube up to each increment of 10.

 

[Khlorine]

Good to know! Didn't realize it could take that long to show up. Thanks for that tip on testing, I'll definitely try that out also.