How to use a water tester?

[Exanthos]

I've tried everything I can think of, but I can't seem to test the water. What am I missing?

[RalphRoberts]

If I recall correctly, you craft a powered water tester with an unknown water.

[skybornfox]

you need batteries with a charge, and than as the person above me said, craft it,
Although your better off using a pan any time rather than that, the most use you can get from those things is to sell them depending on there condition.