A guide to tachometers

Thanks to the school classroom, we’re all familiar with how to measure simple figures. It’s easy enough to reel out the corresponding instrument that should be used to find a length, angle, or even weight. But unless you’ve pursued quantities all the way to a mechanical engineering degree, it’s unlikely that you’re familiar with the special devices used to make more complex measurements.

You might not know what a tachometer is, yet chances are you see one all the time as this particular special device is used in almost every vehicle on the road.

We’ll be diving into exactly what they are, what they’re used for, and what the different types are.

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Table of Contents

What is a Tachometer?

Tachometers are devices that indicate the angular or rotary speed of a rotating object. They usually show instant speed values in revolutions per minute (RPM). They’re also known as revolution counters, rev counters, RPM gauges, or, more colloquially, as a “tach”.

What are tachometers used for?

Tachometers are typically used in motors or other machines. They’re most widely used in the automotive and aviation industries.

In cars, tachometers are typically displayed on dashboards. From here, they tell drivers when to shift gears – taking the rotational speed of the shaft into account.

Similarly, tachometers are used in boats to measure the RPM of the vessel’s engine, informing decisions about performance and fuel consumption.

Aside from this, tachometers can be used to measure the rotational speed of a wide variety of objects, including:

  • Wheels or rollers
  • Disks
  • Motors
  • Shafts
  • Fans
  • Turbines

What are the different types of tachometers?

Tachometers come in various types. Each uses a different mechanism to the same end: measuring the angular speed of a rotating object.

Mechanical tachometers

A mechanical tachometer contains a spring that the shaft on which the device is placed stretches or compresses as it spins, with a centrifugal force proportional to the speed of rotation.

Resonance tachometers

These contain a group of tuned reeds that determine the speed of an engine by indicating its vibration frequency.

Electrical tachometers

There are a couple of variants of electrical tachometers. Eddy-current tachometers use a magnet in the shaft to measure eddy current proportional to the angular speed. Similarly, electric-generator tachometers produce and measure an alternating or direct electrical current.

Digital tachometers

Most modern tachometers are digital tachometers. Whenever a car’s spark plug gives off a pulse of voltage whenever it fires. The digital tachometer measures the average voltage of a series of pulses, which is proportional to the rotational speed of the engine shaft.

Tachometers are relatively simple devices with straightforward uses. Hopefully, you feel more confident about measuring rotational speed after reading this guide.

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