[bitwise]

I rinse with tap water, and leave whatever tap water doesn't come out when turned over and shaken in the vial. Then put the cap on loosely. This should keep the water in the vial from evaporating and creating any deposits.

When I want to test again I add a few ml of the water I'm testing to the vial, cap it, shake it, and get as much water out as possible. Then I test. This will ensure the vial is "flushed".