I have the Tower Hobbies special... uses 2 standard AAA batteries, and gives me rock solid readings in a wide variety of lighting conditions. I even used it once a few minutes after sunset (but still light enough to read the LCD). It doesn't do 4 bladed props, but if you have one, just use the 2 blade reading and divide by 2.

And it's only $21.99.

http://www2.towerhobbies.com/cgi-bin...1&I=LXPT31&P=1


"In the event the readings become erratic, change the direction of the model is facing by 90 degrees and take another reading."

The prop should pass between the light source and the tach. It measures the interuptions, not the reflections.