The History of the Tachometer

The first Tachometer is widely considered to have been developed by the German engineer, Dietrich Uhlhorn in 1817.  Uhlhorn needed a gauge to measure the speed of machines.  Little did he know that within 200 years  this simple device was going to be a standard feature on vehicles driving in every continent on the Earth.

The Tachometer is an instrument that measures the rotation speed of a shaft or disk.  It is also known by several other terms including a rev-counter and RPM gauge.  The gauge usually displays the revolutions per minute (PRM) on a calibrated analogue dial.  On a vehicle, such as a Car or Motorcycle, the Tachometer measures the rate of rotation of the engine's crankshaft.  With a Tachometer, driver's are able to assess the 'engine speed' and change the gearing ratio accordingly.  Each Tachometer indicates the maximum RPM for that particular engine.  Exceeding this limit may result in either engine damage or total failure.

The Tachometer was first used to measure speed on a vehicle (a locomotive) in 1840.  Even though the first petrol or gasoline powered automobile was developed in 1886 (by Karl Benz), it is unclear when the first car featured a Tachometer.

Types of Tachometers

The types of tachometers commonly found are mentioned below:

  • Analog tachometers - Comprise 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 - Comprise LCD or LED readout and a memory for storage. These can perform statistical operations, and are very 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. The non-contact type is ideal for applications that are mobile, and uses a laser or optical disk. In the contact type, an optical encoder or magnetic sensor is used. Both 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.

How the Tachometer Works

Early models of tachometers depended on a geared rotating mechanical drive taken from some moving part of the engine, such as the flywheel, camshaft, fan pulley, etc. The drive rotated a magnet, thus inducing eddy currents in an aluminium disc, much like a speedometer, but the scale was marked in revolutions per minute. The modern type tachometer is electronic, operated by impulse from the low-tension 'make and break' of the distributor points.

The Smith's Instruments version of the Impulse Tachometer is featured on the E-Type Jaguar. Two pairs of connections were required, one pair for the supply leads and the other for the pulse pick-up leads. The pulse leads were, in fact, one continuous wire with a loop (forming the primary winding of a transformer) taken around a soft iron core projecting from the rear of the tachometer casing. The advantages of this type of signal pick-up were that there was no break in the ignition wiring, so a break in the tachometer circuit would not affect the ignition; and that there is no direct electrical connection to the ignition from the tachometer circuit.

CAI and Smiths have designed and supplied Tachometers for specialist vehicles producers including:

  • Caterham
  • Mini
  • Morgan Motor Company
  • Land Rover
  • Ginetta Cars Ltd
  • Noble Automotive
  • Old Empire Motorcycles
  • Metisse Motorcycles

Each Tachometer is designed to reflect the style and unique look and feel of each vehicle. For us, a Tachometer is far more than just a gauge to measure the RPM.

For further information on Tachometers for Cars or Motorcycles, please view the dedicated sections on our website, or gauges in general please contact us by:

Phone: =44 1639 732200

Email: info@caigauge.com

Via our website

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