A tachometer is an instrument that measures the rotation speed of a shaft or disk. They are commonly used to measure the revolutions per minute of motors or other rotating machines.
Tachometers can be classified into several types based on the acquisition and measurement techniques of rotation speed. Contact type meters are fixed to the rotation shaft and used optical or magnetic sensors to determine speed. Non-contact meters use optical means, such as a beam or laser, and can be handheld.
Further distinctions determine whether the tachometer measures the interval of incoming pulses to determine speed, or makes frequency measurements instead. The method chosen indicates how well the tachometer measures high (frequency) or low (time) speeds.