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Monday, September 19, 2016

Chronometric Tachometer


Victory by Design Maserati

I posted a question and received an answer:
Q: What's with the tach needles? They jump from one speed to another. I would expect to see them move smoothly as the engine speeds up or slows down. Is this an artifact of the video, just the way those old instruments worked, or the tachs out of sorts?
A: They are "chronometric" with works like a clock.  They tend to tell you what you were revving two seconds ago. - Rob Mackenzie
A chronometric tachometer? Sounds like somebody got overloaded on steampunk vocabulary, but no, it was a real thing 50 or 60 years ago. They were like a cross between a clock and mechanical calculator. The clock ticks along, just like a regular clock, and between every two ticks, it counts the number of revolutions and at the next clock tick it posts that number on the dial.

Exploded view of the internal mechanism of a chronometric tachometer
THE VELOBANJOGENT has an explanation. Chronometric tachometers were replaced by magnetic ones which work the same way as speedometers: a magnet is spun by a flexible shaft driven by the engine exerts drag on the needle. Magnetic tachs were shortly replaced by electric ones and then by electronic ones. Given that the magnetic speedometer was invented over 100 years ago, it is hard to imagine what prompted the development of the chronometric tach.

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