An instrument for measuring the speed of rotation. There are many methods; a typical modern device rotates a magnet near a non-magnetic conductor, exerting a force through the field produced by eddy currents. It is widely used to monitor the driving practices of lorry-drivers and bus-drivers.
A tachometer measures the speed of rotation of a shaft or disk (from Greek: tachos = speed, metron = measure), as in a motor or other machine.
Automotive
Automotive tachometers show the rate of rotation of the engine's crankshaft by measuring the spark rate of the ignition system, typically in revolutions per minute (RPM).
Tachometers fitted to cars, aircraft, and other vehicles typically have markings indicating a safe range of speeds at which the engine may be operated. Prolonged use at high speeds may cause excessive wear and other damage to engines. On an analog tachometer this maximum speed is typically indicated by an area of the gauge marked in red, giving rise to the expression of "redlining" an engine - i.e. The red zone is superfluous on most modern cars, since engine speed is electronically limited to prevent damage (see rev limiter).
In older vehicles, the tachometer is driven by the pulses from the low tension (LT) side of the ignition coil, whilst on others (and all diesel engines, which have no ignition system) engine speed is determined from the alternator output voltage, which is directly proportional to engine speed. With modern engine management systems found in present day vehicles, the tachometer is driven directly from the engine management ECU.
Light Rail Vehicles
Axle mounted electro-mechanical tachometers are used extensively in public transit light rail vehicles. These types of tachometers typically provide 2 to 8 independent channels of output which can be sampled by various systems in the vehicle that need to know the rotational speed of the axle.
Medicine
In medicine, tachometers are used to measure the rate of blood flow at a particular point in the circulatory system.
Analog audio recording
In analog audio recording, a tachometer is a device that measures the speed of audio tape as it passes across the head.
On many recorders the tachometer spindle is connected by an axle to a rotating magnet that induces a changing magnetic field upon a hall effect transistor.
The tape recorder's drive electronics use signals from the tachometer to ensure that the tape is being played back at the proper speed. When the tach signal and the reference signal match, the tape transport is said to be "at speed." This practice is a vestige of the days when recording devices required several seconds to reach a regulated speed.)
Having perfectly regulated tape speed is important because the human ear is very sensitive to changes in pitch, particularly sudden ones, and without a self regulating system to control the speed of tape across the head the pitch could drift several percent.
Tachometers are acceptable for high-fidelity sound playback, but are not acceptable for recording in synchronization with a movie camera.
Tachometer signals can be used to synchronize several tape machines together, but only if in addition to the tach signal, a directional signal is transmitted, to let the slave machines know not only how fast the master is going, but in which direction.
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