Using a crookes radiometer as a LPM

[sopark4000]

First of all for those of you that don't know what a crookes radiometer is heres some short reading.

Crookes radiometer - Wikipedia, the free encyclopedia

So I was messing around and shining my DL Hulk100 at my radiometer trying to see how fast it would spin from about 12 inches away and it got me thinking. If you could figure out a reliable method of detecting the number of rotations you would have a crude LPM (which is why it's called a radiometer). So just out of curiosity can anybody think of a reliable way to detect the number of rotations.

 

[Sigurthr]

Interesting idea, since it is heat based like a thermopile. However, I don't think they have a linear spectral responce. I seem to remember seeing a science demonstration that showed it spins faster for certain wavelengths than other.

It could probably be used for comparative non quantitative analysis, but not quantitatively.

Also, remember the glass will reduce the power that reaches the vane.

 

[Flaminpyro]

Sure you will have to put a small white spot on one of the vanes and use an optical pickup to detect it every time it comes around you will need to connect the pickup to a counter and then you can count the rev's with out any contact.

Let me how you are going to open the radiometer case :crackup:

 

[lasersbee]

The detection of the rotational speed is an easy solution..
The problem lies in the following....

It has been noted that shining a higher powered Laser
on the black side of the vanes seems to burn the coating
and making the vanes spin with less speed.
The black coating on the vanes is not meant to handle
the power of a Laser but merely the power of the direct
sun's radiated energy.

I think I mentioned this idea as a possible LPM as a joke
about 2-3 years ago....:evil:

Jerry

 

[sopark4000]

Yea this is all just hypothetical. I have no intention of trying to actually use my radiometer as a LPM lol. Interesting input from everyone so far.

 

[Cyparagon]

They also vary from piece to piece. Some have better vacuum conditions and will spin better than others.

 

[charliebruce]

RC helicopter people use a simple meter which has a photodiode and just counts the "pulses" of light it observes (off when blade overhead, on otherwise, pulses per minute/number of blades = RPM). Obviously only works in sunlight/incandescent light and has to respond pretty fast, but it works accurately for them measuring rotational speed. A similar thing but at lower frequency would be perfect to measure the rotational speed - maybe even a lower-powered laser shining vertically would be useful, instead of using direct light. It'd have major issues (frequency response, linearity, burning plates, reflection, change over time/temperature etc) but as a gimmick it'd be quite fun, and I like the idea of measuring a laser, using a laser.

 

[DrSid]

I would try using sound. The radiometer makes very little sound .. but it makes some. Microphone touching it could record some squeeking, which could be then analyzed in computer.

 

[lasersbee]

I would try using sound. The radiometer makes very little sound .. but it makes some. Microphone touching it could record some squeeking, which could be then analyzed in computer.

That seems like a very expensive and complicated way of detecting
the speed of a visible mechanical motion....


Jerry

 

[sopark4000]

RC helicopter people use a simple meter which has a photodiode and just counts the "pulses" of light it observes (off when blade overhead, on otherwise, pulses per minute/number of blades = RPM). Obviously only works in sunlight/incandescent light and has to respond pretty fast, but it works accurately for them measuring rotational speed. A similar thing but at lower frequency would be perfect to measure the rotational speed - maybe even a lower-powered laser shining vertically would be useful, instead of using direct light. It'd have major issues (frequency response, linearity, burning plates, reflection, change over time/temperature etc) but as a gimmick it'd be quite fun, and I like the idea of measuring a laser, using a laser.

This would probably work especially because one side of the radiometer is covered in silver foil (white paint on some) so it would reflect the light really well. Like you said it would be fun as a gimmick to try to do it.

 

[DrSid]

That seems like a very expensive and complicated way of detecting
the speed of a visible mechanical motion....

Jerry

I think analyzing visual data is usually more complicated. I could set up this in 30 seconds and it cost me nothing.
Problem is the readiometer indeed is very quiet. It rattles as bit, but only at high speeds. At lower speed there are just no sounds at all.

 

[Bluefan]

If you shine a low power laser on the reflecting blades and let it reflect to a photodiode you get a pulsed signal out with at the rotation frequency*number of blades. Still it's more a gimmick than a power meter.