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how do you build a rpm meter and what is it called? Answered

iv been tring to design a time mechine for a few months now and the best solution to this problem that i could find was gravitational time dialation this is the thery inwich gravity slows time down inside the gravitational field. to do this i decide to build a big centrofuge with a power full motor but to do this i will need a rpm meter to mesure the angular velocty of the centrofuge but the bad part about this is i dont know where to buy 1 so i ducided to build 1 but i need 1 that feed data into a laptop through a usb port 



Best Answer 8 years ago

depends where it is, but if its somewhere relatively dark then you can do it like your strobing an engine to set the timing.
You need some sort of signal generator, could be a simple oscillator or something more complex, and a bulb/xenon tube/led.
You mark one point of the rotating object and adjust the frequency of the the sig gen until the mark appears stationary, coinciding to the same point each time the light flashes.
This is easier than it sounds, at first it'll be whipping round but you just tune the frequency and it'll quickly slow down, then go back, then stop as you fine tune it.
The frequency in Hz is the revs per second. Multiply that by 60 to get rpm.
To check the frequency you either need a scope or computer with a scope program and interface.

Love to know if there's a simpler way

You said you'd love to know if there's an easier way.  Does this method/device:

Harbor Freight Item 66632:  Digital Photo Sensor Tachometer

qualify as easier?

The strobe light method is fun though.  Probably the most entertaining periodic signal I ever observed via strobe light was a washing machine in the middle of a spin cycle.  I mean the clothes were zipping by at I don't know how many RPM, but with the strobe tuned right I could see clearly enough to read the labels on the underwear.

BTW, just to be pedantic, I thought I'd mention that the strobe light method is susceptible to aliasing.  Frequencies of 2*f, 3*f, ..., aka harmonics, look just like f.  That is: if the wheel is moving really fast, it can make 2, 3, ... n,  complete revolutions during the dark period.


ahhh, I thought I recognised that image  :) 

Yes that would be easier. A lot easier. But no fun to make  :)

In this situation a reed switch may not be able to operate quickly enough.  I don't know what the max. duty interval of a reed switch is. 

But he should include at least one just on principle.

I agree on all counts. It ain't a project if there's not a reed switch.

Yep it's a tachometer, or rev counter, but if you're building a time machine you can't call it that - Non technical people would understand what it was.  You'd be better off calling it a 'sampling rotational enumerator'.  That'll baffle them.

Here's what you need.  It's used to check the rpms of a model air plane motor.   It works by aiming it at the prop. and counts as as the prop. blanks out the photocell.  You can work up something similar.

I've heard such things called "tachometer".  The usual trick to constructing such a device is to rig up some kind of sensor that gives you a pulse for each revolution, and then feed those pulses to a frequency counter.  A frequency counter is just a counter that resets itself periodically. For example, suppose the counter resets itself with a period of one second.  The result it displays is the number of counts it recorded during the previous sample 1s sample period.  Then it multiplies  the 1s count,  by 60, and calls that result RPM. 

The trick for producing the pulses can be as simple as putting a piece of reflective tape on a rotating shaft, and then watching the shaft with an optical sensor.  Every time the piece of reflective tape comes into view, it produces a pulse in the output of the light sensor.  This optical tachometer trick is kinda neat in that you can measure angular speed of a shaft without actually touching it.

Example product manual:


I don't know how to make one but I do know what it is called. The word you are looking for is Tachometer. or RevCounter.

Your theory depends upon the equivalence principle that Gravitational mass is indistinguishable from inertial mass. And that Gravity is equal to acceleration.

Let's hope you are right and that your time machine works. Unfortunately it will only permit time travel into the future - but, hey, its a start.