Do you ever try to measure the rpm of your fan? No! Do you know how you can measure the rpm of your fan or what type of instrument can be used to measure the rpm? No!
There is an instrument called stroboscope, which can be used for measuring rpm of a fan, or propeller or frequency of a vibrating object. The stroboscope can flash light at different frequencies. So if we flash light on a rotating object and change the frequency of the flash then if the frequency of the rotating object is a multiple of the flash frequency, the object will then appear stationary. This principle is used to measure the frequency of different rotating object, such as speed of propeller in Aerospace engineering.
There are two kinds of stroboscope :
In its simplest mechanical form, a rotating cylinder (or bowl with a raised edge) with evenly-spaced holes or slots placed in the line of sight between the observer and the moving object. The observer looks through the holes/slots on the near and far side at the same time, with the slots/holes moving in opposite directions. When the holes/slots are aligned on opposite sides, the object is visible to the observer.
Alternately, a single moving hole or slot can be used with a fixed/stationary hole or slot. The stationary hole or slot limits the light to a single viewing path and reduces glare from light passing through other parts of the moving hole/slot.
Viewing through a single line of holes/slots does not work, since the holes/slots appear to just sweep across the object without a strobe effect.
The rotational speed is adjusted so that it becomes synchronised with the movement of the observed system, which seems to slow and stop.
In electronic versions, the perforated disc is replaced by a lamp capable of emitting brief and rapid flashes of light. Typically a gas-discharge or solid-state lamp is used, because they are capable of emitting light nearly instantly when power is applied, and extinguishing just as fast when the power is removed.
By comparison, incandescent lamps have a brief warm-up when energized, followed by a cool-down period when power is removed. These delays result in smearing and blurring of detail of objects partially illuminated during the warm-up and cool-down periods.
The frequency of the flash is adjusted so that it is an equal to, or a unit fraction of the object’s cyclic speed, at which point the object is seen to be either stationary or moving slowly backward or forward, depending on the flash frequency.
Neon lamps or light emitting diodes are commonly used for low-intensity strobe applications, Neon lamps were more common before the development of solid-state electronics, but are being replaced by LEDs in most low-intensity strobe applications.
Xenon flash lamps are used for medium- and high-intensity strobe applications. Sufficiently rapid or bright flashing may require active cooling such as forced-air or water cooling to prevent the xenon flash lamp from melting.