Tachometer Recommendations

Anybody have any experience with inexpensive tachometers? For those that don’t know, they measure RPM. I am trying to assess the accuracy of a few VEX RPM calculation methods. Under $30 would be ideal because I might be footing the bill instead of my club. If there is a way to do this very accurately using VEX parts that you are willing to share, please let me know.

I bought this one and it works really well