No modeler need a $185 tachometer.

The Hobbico (which I have), Glowbee and any other photocell optical tachometer, will do the job, as long as the batteries are good and you know what you are doing.

Always use the tachometer in sunlight. Any artificial AC light source flickers and my Hobbico measures even an incandescent source.

Tach the prop from behind, with the sun in front of you, whenever you can.

Check zero, with engine off. If there is a reading, something is wrong.

Never try to tach your engine, when close to high-volage power lines. Readings will most likely be wrong.

If your engine is idling and you read 31.4K RPM, it is wrong.
You must know what range to expect before reading, even by sound frequency alone.

Point the tachometer at mid radius. If the prop is 12" in diameter, point it about 3" in from the blade tips and from 4-12" behind the prop (or in front of it, if you must).

Never point the tachometer at the spinner, or too close to it. The results will not be real.

If your engine is supposed to be around 17K and you see 22K, or 25.5K, the tachometer is lying to you...

If there are props turning in the background, readings may be wrong.

Like the LTI 20/20 police laser speed gun, if you point it at a tree and it reads 22MPH... something is wrong.

Sincerely,