A digital tachometer is an electronic instrument that measures rotational speed in revolutions per minute. The spin speed of any spinning object is measurable with a digital tachometer, including propellers, automotive fans and driveshafts, and turbines. Usually small in size and held by hand, a digital tachometer uses laser pulses to count rotations per minute of the spinning object and displays the result on the instrument's LCD screen.
Clean the front end of the digital tachometer to ensure the laser pulses are not confused by debris on the instrument casing. Use water or window cleaner and a soft cloth.
Hold the digital tachometer in one hand and position the instrument in front of the spinning object. Smaller spinning parts, such as those associated with automotive systems, usually require a distance of eight to 10 inches. Larger spinning objects, such as marine engine parts and turbines, require a greater distance for an accurate reading. Refer to the manufacturer's instructions for the particular tachometer to decide how far away from the object to stand. Make sure there is nothing between the instrument and the spinning object.
Point the front end of the digital tachometer directly at the spinning object and press the start button or pull back on the start trigger, depending on the configuration of the particular tachometer in use. Hold the instrument as still and steady as possible.
Release the button or trigger once the beep or alarm sounds, indicating the reading is finished.
Note the reading in revolutions per minute displayed on the LCD screen of the tachometer.
Take a second reading following the steps above to double check the results.