A tachometer is an instrument designed to measure the rotation speed of a shaft or disk. Tachometers generally measure rotations per minute (RPM) though some models also serve as rate meters and/or totalizers. Measuring the rotational speed of a spinning object is important for a number of reasons. Parts are often rated to a maximum rotational speed beyond which they may be subject to failure or system breakdown. Other parts may operate more efficiently at a particular speed.

Tachometers are available in a number of styles. While many are handheld models meant for spot checking conditions, others are meant to be permanently mounted, constantly monitoring rotational speed and able to trigger an alarm if speeds are too high or too low.

Contact tachometers work by placing a freely spinning wheel in direct contact with a rotating shaft or disc. The shaft or disc propels the wheel creating pulses that are read by the tachometer and converted into RPM. Linear speed and distance can also be calculated.

Like many instruments, tachometers are becoming increasingly digital, which requires the application of membrane swicth or graphic overlay as the control panel. Though it’s still possible to find analog tachometers, especially in cars, digital models are much more prevalent. Digital tachometers offer a number of advantages over their analog cousins, primarily in the advanced functions available with each type. Analog tachometers have no provision for features such as memory, or statistical functions such as averaging or changing measurement units. Besides the membrane switch is widely used for the need of Human-Machine interaction.These functions are common on most digital instruments.