Find RPM of a fan by recording a video

[SentinelAeon]

There is no simple easy way to get a good measurement of the percentage of air back flowing. But you can get a good idea by generating a thin stream of smoke that gets sucked into the fan. Just be aware that the percentage of back flow depends on the back pressure. If the fan is pushing into the wind, or trying to suck air out of a closed room, there will be more back flow. The back flow could range from near zero to 100% depending on the back pressure.

But since this back flow is always occuring since there is such a wide gap, it would probably make sense to make the gap smaller since it takes mere minutes to make a wooden frame. It's hard to judge by hand since air blown into hand is felt a lot more than air being pulled away from hand. If i put a hand behind the fan, i barely notice anything but if i stand besides a window in the other room, its almost like a fan was blowing towards me. Do you have any idea what might be a good way to make such smoke to see ? A single cigarette isnt enough to see at this high speed and mist maker module is not good for the same reason - airflow is so high that it disapears into thin air as soon as you put it behind or infront of a fan.

 

[Tom.G]

For smoke generation, see this for ideas:
https://www.physicsforums.com/posts/5863664/

Or actually the whole thread addresses a similiar situation:
https://www.physicsforums.com/posts/5863327/

And of course:
https://www.google.com/search?hl=en&q=smoke+bombs

Cheers,
Tom

 

[Algr]

Your principle is sound.

I'm not aware that any camera would "Fake" 240 frames per second by shooting at 60 and interpolating in the camera. That would completely moot the point of having a high frame rate! I think photographers would make a fuss if some manufacturer tried that. Interpolation in post makes more sense, since the user could judge when it was appropriate and when it wasn't. I've seen some GoPro demos that had interpolated frames, but I figured they were taking the advertised 120fps and playing it back at 15fps. (1/8 speed)

The timing from your camera should be exactly what it tells you. The only variation might be that 60 is often 59.97, and 30/120 might follow that pattern. 50 fps should be exactly 50. If you are seeing uneven progressions of frames, that might be on the video player.

I disagree about shooting at night. The more light you have, the better, as this will cause a shorter shutter angle and less motion blur. Your photos are backlit. That would make your clock hard to read but in your case that shouldn't be a big deal otherwise. (I'm assuming artistic issues are relevant in this case.)

 

[sophiecentaur]

I'm not aware that any camera would "Fake" 240 frames per second by shooting at 60 and interpolating in the camera.

Dunno if it's available in smart home cameras but a posh TV receiver does Motion Smoothing when it upscales the frame rate. However, the Nyquist limit still applies when there are long lasting bursts of high temporal picture frequencies ( repeating fan blade positions etc.) The aim, in all this processing is to produce an overall best result and sometimes things have to be impaired or blurred in order to make a pretty picture. Jerky motion is annoying so that's dealt with when possible. (The smoothing can be switched off, afaiaa)

Modern image processing has to be very clever when you want to get many HD programmes down a limited bandwidth.