I have noticed that over the month or two that I've been using my TF100 test kit that I'm developing a ring around the Cl Only plastic vial that is used for the FAS / DPD test. It is a cloudiness or frosted look from about the 10ml line, up to around the 20 - 30ml line, where it slowly fades out, which I would say roughly matches the area that would be intermittently covered while swirling the test to mix between drops. The area below the 10ml line, where the test water itself sits is still clear.
I've tried gently scrubbing at it, and it doesn't seem to come off. It seems like the plastic is getting etched a bit. I rinse the vial before and after each test w/ pool water, and let it air dry in between tests.
The only real problem it causes is the frosting seems to make the water sit unevenly in the tube, making it a bit hard to read the meniscus to see what the water level is. As a result I'm guessing that my water volume for the test is only good to + / - about 0.5ml - not sure if it makes a difference.
1. What is the cause of this?
2. Is it normal?
3. Does it pose any problem for future testing, if so what needs to be done about it?