Works, stable readings compared to RPM readings from my lathe. I haven't checked if it draws power when it's off (how much cheap electronics just eats up batteries even when it's off). Edit: still using the first battery, in a few years I think? It doesn't consume much juice when it's off.
π Neiko 20713A Contact Digital Tachometer
3 Review
FITNATE Tachometer Batteries Reflective Instruction
3 Review
π Enhanced Precision and Functionality: Protmex Reflective Tachometer
3 Review
Handheld Digital Laser Tachometer Non-Contact RPM Meter Motor Speed Gauge Gun Measuring Range: 2
3 Review