My TF-100 measured my CH AT 1,100.
I've tested it three times with the same results.
I have had my water tested at 2 pool stores and the CH measured 450 and 550.
I'm starting to wonder which is correct.
Last night I did 3 FC tests within about 15 minutes to see how consistent they were.
The samples came from exactly the same place in the pool.
They measured 3.5...2.0...3.0
The only variable I can think of was the amount of powder added.
The powder I received is full of little white boulders that makes it very difficult to get exactly the same amount each time.