A tach is short for the word tachometer, which is a combination of the Greek and Latin words for measure and speed. A tach is used to measure angular speed of something that is rotating. The most common kind measures revolutions per minute (RPM) of a crank shaft on the engine of an automobile, aircraft, tractor or truck.
Many tachometers also have an hours meter that measures how many hours an engine has been run at a certain RPM. The hours measurement is primarily used for service intervals and is mostly seen on aircraft and tractors. For example, a tractor will need its oil changed every 30 hours or six months instead of a certain number of miles or kilometers.
A tach usually displays RPM on a calibrated analog or digital display. The display on most tachometers indicates safe operation zones, with speeds past the maximum operating speed marked in red. Operating a vehicle or aircraft past the maximum operating speed (also called “redlining”) of these safe zones may cause excessive wear and/or severe damage to the engine that includes the crankshaft, valves and pistons to possibly fail or break. The engine may explode in the most serious of situations.
All the different jobs that a tach can perform cause it to come in a variety of types, including mechanical, electric, infrared, stroboscopic, and reed-type. Tachs are used by operators and mechanics for numerous reasons. First, they alert the driver whenever there is a severe loss of engine speed, so he may make an adjustment to avoid redlining the vehicle. A tach allows a mechanic to make precise engine adjustments in the form of a tune-up and it is also used to measure the output of exhaust gases during an emissions test, which is required in many states in the United States.
German inventor Diedrich Uhlhorn is credited for inventing the tach which was first use to measure centrifugal force. Uhlhorn started using the tach to measure machines in 1817 and began to measure the engines of locomotives in 1840. In modern times, the tach is used to measure multiple things besides the RPM of an airplane, automobile, tractor or train. In medicine, tachometers are used to measure the rate of blood flow at a specific spot in the circulatory system. A tach may also be used to measure the speed of audiotape as it passes across the head in analog audio recording.