Talk:Tachometer

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Anyone know how a diesel tacho works? --219.88.29.108 06:18, 24 April 2006 (UTC)

Diesel engines do not have a spark ignition system that could provide a signal for a tachometer. Diesel-engined automobiles have alternators which generate ac power at frequency proportional to engine rpm, and this can drive a tachometer. (cuddlyable3)84.210.139.189 00:07, 15 December 2006 (UTC)

Has the section "History" been vandalised? Now it says only "The first were mechanical" without a stop. I think stuff has been removed here! (cuddlyable3)84.210.139.189 00:07, 15 December 2006 (UTC)

Is it worth mentioning that the word "tachometer" is used in some other languages (e.g. German) to refer to a car's "speedometer"? Might be a "false friend".--Millard73 22:29, 21 July 2006 (UTC)

This seems a bit opinionated for my tastes: that the driving conditions call for; or, perhaps more frequently, allow automotive enthusiasts to admire how fast they are 'revving' the engine.

24.69.25.201 09:41, 21 December 2006 (UTC) colin

[edit] Measure rate not speed

All the tachometers described measure repetition rates, usually rotation speed (except the obscure haematachometer?). The rotation speed may or may not be linked to a vehicle speed or flow of tape or a fluid depending on the application, but I suggest it is misleading to define the tachometer as essentially an instrument to measure speed.

While the Greek root "tachos" can mean speed, the earliest use of the word tachometer seems to be in 1810. This indicates to me that tachometers are an offspring of the Industrial Revolution that brought us reciprocating machines. The first of these machines were steam engines that stayed in the place they were built.

The automobile tachometer familiar today is not intended to display vehicle speed, for which another instrument the speedometer is required by road laws.

The measure of revolutions per minute of an engine, electric motor etc. is usually called the speed of rotation or just engine speed, motor speed etc. It is not usually called a rate. The repetition rates are usually expressed as pulses per minute, beats per minute etc. However, the actual unit of measurement is 1/time in either case. --C J Cowie 00:42, 1 September 2006 (UTC)

The concept "speed" signifies general movement without implying repetitive movement. Introducing a distance unit (and a time unit) allows speed to be expressed as a (repetition) rate, such as miles per hour or feet per second, but that is an arbitrary introduction not inherent in "speed". A tachometer is essentially restricted to sensing a rate, and a rate must have the dimension of 1/time. That fact is being neglected if we call the tachometer a device to measure speed.

C. J. Cowie finds this to be unusual use of the word "rate" and seems to prefer to use "speed + a modifier" such as "speed of rotation". I have no objection to that.

I suggest deleting the vagueness of "used for measuring the speed of a moving body or substance" and instead defining like this:

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. The device often displays the rate of revolutions per minute on a calibrated analog dial, but digital displays are increasingly common.

I think it is good to lead with the most common definition, but also good to make some reference to the medical and audio recording usages. --C J Cowie 00:36, 10 September 2006 (UTC)