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A tachometer is an instrument designed to measure the rotation speed of an object, such as a gauge in an automobile that measures the revolutions per minute (RPMs) of the engine's crankshaft. The word is derived from the Greek words tachos, meaning "speed," and metron, meaning "to measure." This device traditionally is laid out with a dial, a needle that indicates the current reading and markings that indicate safe and dangerous levels. Digital tachometers have become more common, however, and they give numerical readings instead of using dials and needles.
Uses in Automobiles
In their most familiar form, tachometers measure the speed at which mechanical devices rotate, which typically is indicated in RPMs. They are used to monitor the RPMs in automobiles because running the engine at excessively high RPM rates can drastically shorten the life of the engine. In some cases, a small generator is attached to the engine drive shaft, and the RPM measurement is based on the electric current generated by the device. This instrument might also simply measure the rate at which the ignition system sends sparks to the engine.
Use in Airplanes
Airplanes typically have one tachometer for each engine, and in those that use propellers, one is also needed for each. A plane's engines usually operate at higher RPMs than its propellers. By using separate instruments for the different parts, the plane's pilot or crew can know whether there is a problem with any particular part.
Traditional tachometers require physical contact between the instruments and the objects being measured. In applications where this is not feasible for technical or safety reasons, it might be possible for a laser to take measurements from a distance. Laser devices work by pulsing a tight beam of light against the rotating element. The rotating element will have one reflective spot, and the instrument measures the rate at which the light beam is reflected back. They can be permanent parts of the system, or they can be handheld for occasional spot measurements.
A tachometer can even find uses in medicine. By placing a small, turbine-like device called a haematachometer in an artery or vein, a medical professional can use this instrument to estimate 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.