Tachometer showing engine revolutions per minute, and a redline near 5800 RPM
A tachometer is a device used for measuring the speed of a moving body or substance (from Greek: tachos = speed, metron = measure). The most common form of the tachometer is one that measures the speed of a rotating shaft, as in an engine or other machine. The readout is traditionally in the form of an analog dial, but digital displays are increasingly common.
The first, mechanical, tachometers were based on measuring the centrifugal force. The German engineer Dietrich Uhlhorn is assumed to be the inventor, he used it for measuring the speed of machines in 1817. Since 1840 it was used to measure the speed of locomotives.
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 (RPMs). This can assist the driver in selecting the most appropriate throttle and gear settings (more applicable to manual transmissions than automatics) that the driving conditions call for; or, perhaps more frequently, allow automotive enthusiasts to admire how fast they are "revving" the engine.
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. running it at (dangerously) high speed. 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 are completely encased to prevent ingress from the outside environment. The only exposed parts outside of the case is a sealing cannon plug connector and drive fork which is attached to a slotted disk internally through a bearing and seal. The slotted disk is typically sandwiched between 2 circuit boards containing a photo-diode, photo-transistor, amplifier, and filtering circuits which produce a square wave pulse train output customized to the customers voltage and pulses per revolution requirements. 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. Examples of such systems include automatic train control systems and propulsion/braking controllers.
In medicine, tachometers are used to measure the rate of blood flow at a particular point in the circulatory system. The specific name for these devices is haematachometer.
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 most audio tape recorders the tachometer (or simply "tach") is a relatively large spindle near the ERP head stack, isolated from the feed and take-up spindles by tension idlers.
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. Other systems connect the tach spindle to a stroboscope which alternates light and dark upon a photodiode.
The tape recorder's drive electronics use signals from the tachometer to ensure that the tape is being played back at the proper speed. The signal from the tachometer is compared against a reference signal (either a quartz crystal or alternating current from the mains). The comparison of the two frequencies drives the speed of the tape transport. When the tach signal and the reference signal match, the tape transport is said to be "at speed." (To this day on film sets, the director calls "Roll Sound!" A moment later the sound man calls back "Sound 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. A modern, tachometer-regulated cassette deck has a wow-and-flutter (as the measurement is called) of 0.07%.
Tachometers are acceptable for high-fidelity sound playback, but are not acceptable for recording in synchronization with a movie camera. For such purposes, special recorders that record pilottone must be used.
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.