I bought a refractometer from Marine Depot a few years ago. It's worked perfectly---until this week. Now it seems to register only 1.021 for both of my tanks and fresh seawater. When I put pure water in, it does register 1.000 though.
After looking at the instructions again, it says to clean the prism with a soft, damp cloth or you'll get inaccurate results and damage the prism's coating. I've been using a damp paper towel to clean the instrument.
Given my results, have I damaged the prism's coating with the paper towel? It looks fine to me, unless all the coating is gone. There are no moving parts here, right?
And more importantly, do I need to buy a new one?