Functions of the Tachometer

A tachometer is an instrument designed to measure the speed of an object or substance. The word is formed from Greek roots: tachos, meaning speed, and metron, meaning measure. The traditional tachometer is laid out as a dial, with a needle indicating the current reading and marking safe and dangerous levels. Recently, digital tachometers giving a direct numeric output have become more common.
In its most familiar form, a tachometer measures the speed at which a mechanical device is rotating. A common example is the tachometer found on automobile dashboards. In this application, the tachometer measures the revolutions per minute (RPMs) of the engine drive shaft. It is important to monitor engine RPMs, as running the engine at excessively high rates can drastically shorten engine life.
A tachometer used in this application can be built in multiple ways. It may be a small generator attached to the engine drive shaft, where the RPM measurement is scaled to the electric current generated by the device. Alternately, it may simply measure the rate at which the ignition system sends sparks to the engine.
The traditional tachometer requires physical contact between the instrument and the device being measured. In applications where this is not feasible for technical or safety reasons, it may be possible to use a laser tachometer to take measurements from a distance. A laser tachometer works by pulsing a tight beam of light against the rotating element. The rotating element will have a reflective spot, and the tachometer measures the rate at which the light beam is reflected back. A laser tachometer can be a permanent part of the system, or it can be handheld for occasional spot measurements. 
A tachometer can even find uses in medicine. By placing a small, turbine-like device in an artery or vein, a doctor can use a tachometer to infer the rate of blood flow from the speed at which the turbine spins. This can be used to diagnose circulatory problems such as clogged arteries. This device is known as a haematachometer.