A tachometer is a measuring device with the purpose of measuring the speed of a circular plate or disk, or sometimes of a shaft. The word comes from the Greek words meaning speed (tachos) and measure (metron).

A tachometer is typically used in measurement of engine speed in an automotive engine, but was originally used to describe centrifugal force. The German engineer Dietrich Uhlhorn is believed to be the inventor of the tachometer. His interest was in measuring the speed of machines. The use of the tachometer in measuring the speed of locomotives has been common since about 1940, and it is also used in measurements of light rail axle speeds.

The early tachometers were entirely mechanical, but it is interesting to note that the modern day version relies entirely on electronic measurements to register as part of a computerized system. Find more info at http://www.tachometerworld.info

For instance, the tachometer, although described in revolutions per minute (RPM’s) is actually derived in older vehicles from impulses recorded from the low tension side of the ignition coil. Engines which have no ignition system, such as diesel engines, measure the alternator output voltage. In neither case is the engine actually “revolving”, although the pistons perform a somewhat circular dance, when in operation.

Originally, tachometers were used to register and display the range where the engine could be safely operated. To “red line” an engine meant to run the engine at a dangerously high speed. The theory was that over racing the engine would lead to excessive wear and damage to the equipment. Even knowing the possibility of engine damage was never enough to prevent the sound of “revving up” the engine as an invitation to drag race, or perhaps just to let others know that your engine was “hot”

In modern engines, a rev limiter prevents damage due to engine speed over safe levels.

In the most basic and familiar form, a tachometer measures rotating speed of any mechanical device. One of the best examples is the tachometer fixed in automobile dashboards.

How fast the engine of an automobile is turning is measured in revolutions per minute (RPM) by a tachometer. Most cars have got a standard shift transmission and when it is necessary to shift at the best possible RPM to get a better acceleration and to economize on gas then the indicator in the tachometer is very helpful. If the car is fitted with automatic transmission then the tachometer is not used frequently. Tachometer indicators move to the red zone when the engine is running very fast. The indicator in the red zone is an alert or an alarm to the driver, to safeguard the engine from damage. Most of the tachometers show single digit indicators with a small mark, which reads RPM X 1000. It explains that the reading in the tachometer should be multiplied by 1000 to get the correct RPM. A tachometer can also measure the rate at which the sparks to the engine is transmitted by the ignition system.

Even medical science uses tachometers. If a turbine like small device is placed in the vein or on an artery, the doctor can make use of the tachometer to measure the blood flow rate from the spinning speed of the turbine in the device. Circulatory problems such as clogged arteries can be diagnosed with this device. This device is known as a Haematachometer.

A physical contact between the instrument and the device being measured is needed in the traditional tachometer. If it is not possible to provide physical contact in applications where safety is more important then a laser tachometer can be used to take measurement over a distance. A tight beam of light from the laser tachometer is pulsed against a rotating element. Since the rotating element will have a reflective spot, the tachometer will measure the rate at which the light beam is reflected back, which in turn will help in calculating the speed of the device.

There are wide ranges of hand-held portable tachometers, which can be used to measure speeds up to 250,000 RPM.

As the name implies the infrared tachometer uses an infra red-beam which is not affected by ambient light, to measure the rotating speeds. The built in quartz crystal display ensures accuracy. It can be used to measure the rotating speed of drills, saws, hones, gears, motors, engines, fans, compressors and generators.