Using an electronic tachometer to measure fan speed

Fan speed can be quickly determined using an eletronic tachometer.  The tachometer is pointed at the rotating fan blades and within a few seconds a reading of "blades per minute" is given.  This number can then be divided by the number of blades to obtain fan rpm which can then be compared to recommended fan speed of the manufacturer.  If the fan speed is more than 5% slower than recommended, the fan belt should be replaced.