Tachometer (Working Principle, Types, Applications)

Tachometer (Working Principle, Types, Applications)

One of the most significant parts of a vehicle is its engine, and a significant part of the engine is a tachometer.

There are several other names for it, such as RPM meter, RPM gauges, revolution counters, rev counters, or just "tachs."

What is this instrument exactly? A RPM meter can help your most important part of a machine avoid complete failure or burning down. Let’s take a closer look…

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What Is a RPM meter And What Does It Measure?

A RPM meter measures and indicates the rotation speed of a shaft or disk, as in a motor or other machines, and it is displayed as RPM.

The device makes the measurement when the machine is in motion. It is also popularly known by different other terms such as a rev-counter and RPM gauge.

Its working principle can be electromagnetic, electronic, or optical-based. RPM meter works to calculate the rotations per minute (RPM) factor on a calibrated analog dial or digital display.

Digital (LCD or LED screen) displays are becoming more and more common as they are more accurate and more precise.

On a vehicle, such as a car or a motorcycle, the rpm meter measures the rate of rotation of the crankshaft of the engine.

By using a rpm meter, drivers can evaluate the speed of the engine speed and change the gearing ratio consequently.

Each rpm meter provides the maximum RPM for that certain engine. Going beyond this limit may result in either engine damage or complete failure.

Tachometer types

There are different tachometer types and categories, but the most common tachometers ( rpm meters) are divided as follows:

  • Digital Tachometers

These rpm meters have electronic circuitry, processor, sensor, and LCD or LED readout. These instruments can perform statistical calculations.

They can measure the rotation speed of the motor more accurately and also are suitable for monitoring any type of time-based quantities.

Digital tachometers are increasingly common these days and instead of using dials and needles, they provide numerical readings on a display.

  • Analog Tachometer

These RPM meters have a needle and dial type of interface. These devices have no provision for saving readings, and they cannot compute information like average deviation.

The speed is converted to voltage with the help of an external frequency to voltage converter. Therefore, this voltage is presented by an analog voltmeter.

Analog Tachometer

  • Contact and non-contact rpm meters

The contact type needs physical contact with the rotating shaft. This type works by using an optical encoder or a magnetic sensor, based on the particular model to measure the RPM.

In order to achieve an accurate reading, direct contact with the shaft or rotating component is necessary.

Non-contact rpm meters that are very efficient, convenient, and accurate have no contact with the rotating shaft.

Also, they do not need to be attached to the device or engine whose speed is measured. In this type, the speed of the engine is measured by using laser light and an optical disk that reflects on a tape and marks a total rotation.

The non-contact type is well suited for applications that are mobile.

  • Time and frequency measuring Tachometers

The time-based digital tachometer calculates the rotational speed of the equipment by measuring the time interval between incoming pulses.

It is more ideal for measuring low speed with high accuracy. The reading time of this tach o meter is dependent on the rotational speed but its resolution is independent on the rotational speed.

The frequency measurement device determines speed by measuring the frequency of the incoming pulses. It is more accurate for high-speed measurements and the revolution of this tach o meter is dependent on the rotating shaft.

Tachometer Working Principle

There are mechanical and electronic tachometers that operate differently. In terms of driving, the operation of a mechanical rpm meter is similar to the operation of a mechanical speedometer.

The rotating shaft connects a moving part in the engine or transmission to the gauge with the help of a flexible cable. Inside the machine, the rotating shaft monitors and controls the position of a needle to show the speed of the engine.

An electronic rpm meter (rpm gauge), on the other hand, uses a magnetic pickup to generate electrical pulses at a frequency corresponding to the velocity of the engine.

The position of this magnetic pickup is near a rotating engine part. Circuitry in the meter helps to convert the pulse frequency for the display of engine RPM with the help of a digital readout.

It is necessary for the user to know the RPM of the engine and its operating range to prevent unnecessary damage. The device operates on either an alternating or direct current.

What Is the Difference Between A Speedometer And A rpm meter?

Generally, the speedometer and rpm meter are utilized to measure and display speed. But they differ in what they represent.

A speedometer is used to display the instantaneous speed of the vehicle whereas a rpm meter is a device that measures the rotating speed of an engine, typically in revolutions per minute.

Tachometer Use

Tachometers can be used in a wide range of applications in industries and environments. Typical RPM meter uses and applications areas include:

  • Automobiles, airplanes, light rail vehicles, trucks, tractors, and trains
  • To display the rate of engine crankshaft rotation in vehicles
  • Laser instruments
  • Medical applications to measure the blood flow rate of the patients
  • Analog audio recording
  • Numerous types of machinery and prime movers
  • To measure the flow of liquid by using an attached wheel with an inclined angle
  • To estimate traffic speed and volume
  • Boats and marine vehicles to identify the best operating range for engine
  • Machinery and mechanical systems
  • Research and development
  • Laboratories, tests, and measurement
  • Maintenance and routine industrial operations
  • They are proper for monitoring turbine and machine health
  • The most common use of the rpm meter is to measure the speed of a rotating shaft being driven by an engine.
  • Large trucks, sports cars, ships, etc., utilize this function of the machine.


What is the purpose of the rpm meter?

The purpose of the rpm meter is to measure the rotation of the engine’s crankshaft, which is displayed in RPM. It is placed next to the speedometer.

Do I mount the meter on the engine?

The meter can be installed anywhere that is at least 8 inches away from the source of the signal. It must be installed on a surface that is not too hot to touch, or over spark plug wires or ignition coils.

Can I extend the wire provided with the meter?

The RPM meter has the ability to extend its wire to eight feet without affecting performance.

Is a rpm meter necessary?

In vehicles with a manual transmission where the driver has to manually change gears, the RPM meter is nearly a "must-have" gauge. It helps the driver know when revolutions are in the optimal range.

Do all cars have rpm meters?

Every car with a manual or automatic transmission contains a RPM meter. Its purpose is not the same in an automatic vehicle and in manual transmission-equipped cars.

What is a normal rpm meter reading?

A good rpm meter has steady reading and doesn't fluctuate too quickly. When the engine is idling, the RPM should be between 1,000 and 1,500. When the speed goes up, the needle will go up.

The following articles may be of interest to you if you would like to read more:

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26th Jul 2022 Saeed Abd

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