How to measure TDS correctly
This is a TDS meter made by Hanna. I like having a handheld meter, as I can test water anywhere necessary, even at a club member's home or at the LFS (local fish store).
Let the water run for at least a minute before collecting a sample in a clean glass. Insert the meter and turn it on. The number on the display is the TDS (Total Dissolved Solids) in the water. This sample measures 1, and the DI is several months old. Our goal is typically 0 TDS, but anything under 10 should suffice.
The meter I sell has a Hold button. Once you've immersed the meter to measure the TDS, press Hold and now you can read the display without leaving it in the water. It also measures water temperature, but only in Celcius unfortunately. It comes with a protective case, as you can see below.