"Sucking with both of those really didn't help..."

The brilliant Matthias Wandel, seeing how fast he can get his air engine to spin.

His wooden air engine.

(Just about everything Matthias makes, at least since the old stuff, is wooden. Wooden combination lock.
Wooden tank tracks. Wooden marble machines. Wooden Jenga pistol. Wooden pen centrifuge. Wooden pager rotator. Wooden gears. Wooden pipe organ. (Mostly) wooden bandsaw. Wooden binary logic. I don't think the Eyeballing Game and the gear template generator are made of wood, but I haven't examined his server. It could be all ropes and pulleys in there.)

I'm not certain that Matthias' video-based speed-estimation is completely sound. A proper tachometer would probably be a good idea.

There are, of course, a lot of ways to do that. Optical and magnetic sensors, point-and-shoot "non-contact" tachos made to do things like measure the rotational speed of machine tools and model-aeroplane propellers. And of course Lego, shop-bought or home-made (and with or without googly eyes).

Tachometry often involves multipliers or divisors of some sort; the above-linked Lego sensor tops out at only 500RPM, for instance, so you'd need to point it at something geared down from the thing you're actually measuring, to get a reading. And model-plane-prop tachos need to be told how many blades the prop has. For devices like this motor, which spins in the same range of speeds as computer fans, I wonder if you could use a computer fan to measure their speed?

Chop the blades off the rotor of a a standard three-wire computer fan, disable the motor coils (I think the speed sensor is a separate Hall-effect device), connect the de-bladed rotor to the thing whose rotational speed you want to measure, and then either use a frequency counter to measure the pulse rate on the yellow wire, or just plug the cable into a computer motherboard and run a fan-monitor utility.

Anybody tried it?

11 Responses to “"Sucking with both of those really didn't help..."”

  1. TwoHedWlf Says:

    Yeah, pretty questionable way of measuring the RPM he has there. Tachs for RC planes are pretty cheap($20ish US) good for far higher than he's going to be spinning. They also work well for measuring your mains electricity frequency if you have a magnetic ballast fluoro.:)

  2. Erik T Says:

    Oh, I think his method is completely sound in that it won't lie to you (provided you've got a ballpark figure to ensure that you're not missing entire revolutions per frame); it's just going to have a huge associated error bar. Reporting the result as 1000rpm is quite proper (good thing Wincalc rounds well!).

    1.0e3 rpm, of course, would be even more proper.

  3. reyalp Says:

    I agree with Erik, it's a valid way of getting a rough ballpark.

    You should be able to do this quite accurately with a still camera. Put a high contrast mark on the wheel, take an exposure that will be some reasonable fraction of a rotation (e.g. 1/60th should be about 1/4 turn for 1000 rpm), use the streak created by your mark to get an angle.

  4. Shadowex3 Says:

    I'm the least qualified to be asking this so forgive any obvious flaws in my cunning plan but why not use shine a laserpointer or somesuch through that hole onto a detector of some kind and count how many times it's interrupted in a given minute?

  5. TwoHedWlf Says:

    Shadow, rather than quickly and cheaply buying a laser tach, spend ages cludging together something from random components?

    Wiseguy, auto timing light requires a plug lead to trigger the light. If you had a way to fire the light at a known frequency and could adjust it that would work, but you'd spend a lot more time and money setting that up than just buying one.

  6. Shadowex3 Says:

    Didn't know a laser tach existed or I would've suggested that instead.

  7. reyalp Says:

    Shadow, rather than quickly and cheaply buying a laser tach, spend ages cludging together something from random components?

    When you are talking about people who are trying to measure the performance of their wooden air engines, "because I can" is a valid answer :)

    It seems to me you should be able to make a pretty decent optical tach with something similar to the "blinker circuit" used in the CHDK project to record data from a blinking LED with a sound card (serial, parallel and joystick port versions are also possible.) Getting an on-off signal at a few kilohertz should relatively easy.

    Hmm, the shop vac and blower might present a challenge, but an audio tach should also be possible.

  8. Daniel Rutter Says:

    Didn't know a laser tach existed or I would've suggested that instead.

    Not only do they exist, but they can now be startlingly cheap. I thought they all still cost $AU80 or more, and I don't really need one for anything, so I never bothered buying one. But I did a quick eBay troll after writing this post and found one for £7.66 delivered! (About $AU12.66 or $US11.32, at the moment.) It's from this seller; here they are on ebay.com.au.

    A tachometer this cheap may turn out to be rubbish, of course. But if it's only as inaccurate as a cheap multimeter, which is generally still easily within 2%, it could be quite handy. And an optical tach is actually a really easy thing to build out of super-mass-produced electronics modules these days, so it might be quite accurate, even at this price.

  9. TwoHedWlf Says:

    A laser tach really isn't much more than an optical tach with a $.50 laser added on. And should be accurate within a couple hundred RPM at 10K.

  10. Friedrich Says:

    Very interesting I've always wondered how some of these instruments' sounds were made.

    Now, if you'll excuse me, I'm gonna go "play my Cuica". Ahem.

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