The lower-priced tachs being sold use an optical resistor as the sensing element. If the ambient light is not bright enough, they sometimes don't work well. Even the color of the prop can cause erratic readings if it is a dark color. In direct sunlight, they usually work reasonably well. The higher-priced units, like the TNC, use a focused optical diode which is much more sensitive. The TNC will work on cloudy days and you can read the RPM from several feet away. If you ever buy a TNC unit, you will never be sorry, even though they cost quite a bit more than the cheap units.

For what it's worth, Duane.