What is the proper way to use hanna checkers?

[Salty_Northerner]

My thought process exactly. RO water should suck up anything that might render a test inaccurate.

Yep exactly, when I'm about to test I'll empty out the RO water from the vial and put a small amount of test water in and give it a quick swirl and then empty it out. My thought is by doing that I'm getting rid of any sort of dilution from storing the glass vial with RO water to get a more precise result.

 

[PeterEde]

I fill with water and use a small bottle brush to scrub before rinsing with tank water.
I used to fill with RO but never saw any off readings to keep doing it.
Tap water/scrub/rinse with tank