Editorial Feature

What is a Tachometer?

Nov 1 2013

Image Credits: Panuwatccn/shutterstock.com

A tachometer is a sensor device used to measure the rotation speed of an object such as the engine shaft in a car, and is usually restricted to mechanical or electrical instruments. This device indicates the revolutions per minute (RPM) performed by the object.

The device comprises of a dial, a needle to indicate the current reading, and markings to indicate safe and dangerous levels. The word comes from the Greek ‘tachos’ meaning speed and ‘metron’ meaning measure so tachometer and speedometer have become interchangeable and essentially both measure speed.

Historically, the first mechanical tachometers were designed based on measuring centrifugal force: an inertial force directing away from an axis of rotation that acts on all objects as viewed from a rotating frame of reference. In 1817, it was adapted to be used for measuring the speed of machines and since 1840, it has been predominantly used to measure the speed of vehicles; specifically locomotives.

Advanced tachometers have novel uses. For example, in the medical field, a haematachometer placed in an artery or vein can estimate the rate of blood flow from the speed at which the turbine spins. The readings can be used to diagnose circulatory problems like clogged arteries.

Types of Tachometers

The types of tachometers commonly found are:

Analog tachometers - Comprised of a needle and dial-type of interface. They do not have provision for storage of readings and cannot compute details such as average and deviation. Here, speed is converted to voltage via use of an external frequency to voltage converter. This voltage is then displayed by an analog voltmeter.
Digital tachometers - Comprised of a LCD or LED readout and a memory for storage. These can perform statistical operations, and are suitable for precision measurement and monitoring of any kind of time-based quantities. Digital tachometers are more common these days and they provide numerical readings instead of using dials and needles.
Contact and non-contact tachometers – The contact type is in contact with the rotating shaft and uses an optical encoder ot magnetic sensor. The non-contact type is ideal for applications that are mobile, and uses a laser or optical disk. Both of these types are data acquisition methods.
Time and frequency measuring tachometers – Both these are based on measurement methods. The time measurement device calculates speed by measuring the time interval between the incoming pulses; whereas, the frequency measurement device calculates speed by measuring the frequency of the incoming pulses. Time measuring tachometers are ideal for low speed measurements and frequency measuring tachometers are ideal for high speed measurements.

Working Principle

The working principle of an electronic tachometer is quite simple. The ignition system triggers a voltage pulse at the output of the tachometer electromechanical part whenever the spark plug fires. The electromechanical part responds to the average voltage of the series of pulses and it shows that the average voltage of the pulse train is proportional to engine speed. The signal from the perception head is transmitted by standard twin screened cable to the indicator.

The tachometers are temperature compensated to be able to handle operations over an ambient temperature range of – 20 to +70°C (-4 to +158°F).

The tachometer in a vehicle enables the driver to select suitable throttle and gear settings for the driving conditions as prolonged use at high speeds can cause insufficient lubrication which will affect the engine. It enables the driver to prevent exceeding speed capability of sub-parts such as spring retracted valves of the engine, and overheating, thereby causing unnecessary wear or permanent damage and even failure of engines.