I have a GloBee and Tower's Hobbico knockoff. Why two of them?

The GloBee was flakey, plus it wasn't easy to read on a bright day. And one summer day, the sun blackened it's display and it was worthless until it cooled off overnight. And I bought it because it's higher price suggested it would be higher quality.

At Tower, the GloBee and the Hobbico Mini-LCD Digital are both $25. But the Tower brand tach, which appears to be the Hobbico with a Tower label on it, is only $19.95. That's the one I got and it's excellent.

Tips are easy and simple.
Read from behind the prop. It's easier to point up from there. And it's safer and you can also reach the needle valve. And needless to say, have someone else hold the airplane securely while you're doing the reading/adjusting.

They work more dependably if they're pointing up, if they see sky behind the prop. They work by sensing light and dark. If the prop is silhouetted, they count the dark against the light. If the prop is against a similarly lighted background, they have to see the prop as lighter than the background or catch a reflection off the blades. Which doesn't work well for some.