It may work for others but my standard sample only confused me; I could still see the dot but it was fuzzy around the edges....kind of grey looking. After comparing with the Pentair I realized that with the tf-100 I go until I cannot see the dot at all. I have come to realize that the results of this particular test can be interpreted different between users. An average of several tests with the same solution is the best. Unless the solution is right at the line of the tube round up to the next higher number to be safe.
I would think it'd be more helpful to me to figure out exactly what my CYA level should look like in the sight glass. Of course, once I know it for 100% certain, it doesn't matter that much as it should stay the same.
There is a shadow and reflection effect inside the vial once you have an opaque amount of solution in there that looks exactly the same as still being able to see the eyeball. It's difficult for me to determine when I've transitioned from seeing the eyeball to not being able to see it any more, but now see the shadow and glare that somehow still looks like a dark circle in the center with a bright area around it.
So one thing I have noticed is the solution seems to settle. Sometimes when I put the solution in the tube, I will then shake the tube and try to do my glance right after that. Not sure if that is giving me a false high reading or not.
I'm glad someone besides me is also struggling with this particular test. I've drained a decent amount of water out of my pool and refilled to get my CYA lower but I'm still not 100% sure or comfortable that I'm reading it right. To stare or not to stare at the dot? Glance quickly or not? Either option could change the outcome between 80 or 90 on the sight tube...or 100 if in daylight versus inside. I'm so confused what my CYA is other than likely high from last year.
For most, including me, the most difficult test to interpret. The glance method helps because if you stare at the dot the mind can trick the eyes to continue to "see" the dot when it is not actually visible.
I don't really struggle to make a call on the level on a given reading, but day to day, my observation could vary +/-10. I don't care all that much. Long as it's in the 70-90 range I'm good. I chlorinate to meet the higher requirement.
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