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Tachometer is a device to measure angular speed of a rotating machine in the unit of revolution/minute (RPM). There are two types of tachometer: contact type and non contact type. Measurements of angular speed using contact type tachometer is done by making a firm contact of the knob of the tachometer with the centre of the rotating machine. A non contact tachometer generally uses infrared light to measure the speed of rotation of a shaft or wheel or any other rotating object. What is necessary for a infrared tachometer to work is that there be some contrast in colour or lustre on the surface of object. If there is no such contrast or if the surface is highly polished then a spot is marked on the surface using a marker or sticking a piece of paper onto the surface. The infrared light from tachometer falls on the surface and gets reflected back to the detector on the tachometer. The detector detects the changes in frequency of the reflected light as reflected alternately by the object surface and the contrast spot. The number of frequency changes per unit time gives the speed of rotation of the object.

Calibration of contact type tachometer is done using a variable speed (but stable at that particular speed) motor having an arrangement to hold firmly the knob of the tachometer at the centre and a calibrated tachometer (standard) to monitor the speed of the motor. Calibration is done by comparison method. Calibration of non contact tachometer is done as per the above explained method by comparison.

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