Fluid Dynamics—Building a better MIDI Breath Controller

[Freixas]

As it was already pointed out, wire will be hot enough to not allow any condensation on its surface. Condensing water will not cool it, quite the opposite - but even if, it will just shift the calibration which you have to do anyway.

But yes, water can be a problem in general, as it will condense on other parts of the duct. I wonder if keeping it heated to about 40°C would not keep whole thing dry, exhaled air has a dew point well below the temperature of the human body. Add few degrees to allow drying of saliva. It will again shift the calibration and it again doesn't matter.

Hey, thanks for continuing to think about this.

I've observed what happens in a clear vinyl tube as I blow into it. Water droplets form and are blown this way and that. If one has a hot wire in the path, no condensation will form on it, but I imagine that big, fat water droplets will fall on it...and get blown off, all in a random pattern that would defy calibration.

If you heat the whole tube, that's another story--and another complication. I have a CPAP with a heated humidifier hose, so it's clear it's doable, but I don't want to try to build one (nor would I want to try to adapt a CPAP hose).

I do know of one melodica player who has installed a water trap on his hose and is thinking about heating it. This gets way too DIY for my tastes. My dream device would be a small black box that connects with standard, off-the-shelf melodica hoses and mouthpieces.

By the way, contrary to what some people think, this condensation is water, not spit. One should not spit into one's instrument--and, yes, the term "spit valve" is a misnomer.

 

[Borek]

I know it is mostly condensation, but now and then a droplet of saliva is unavoidable. We do exhale them all the time, the stronger you blow, the more of them. Even if the tube is heated and there is no condensation such droplets would appear - and they have to dry out.

 

[Tom.G]

Hmm... I wonder if some baffling in the air stream would work... it would probably take some creative experiments though.

My thinking is the more massive water droplets would tend to flow in more of a straight line, hopefully impinging on baffles where gravity can do its work.

Another possibility is have the main airway a straight-thru path with the hot-wire in a (smaller?) parallel path; inlet side plumbed with a "T" and outlet with a "Y" to blend the two airstreams (and supply some pneumatic gain?). There could be a user-operated valve for calibration, etc. in the straigh-thru segment if desired.

Ah well, enough brainstorming.

Cheers,
Tom

 

[Freixas]

Hmm... I wonder if some baffling in the air stream would work... it would probably take some creative experiments though.

My thinking is the more massive water droplets would tend to flow in more of a straight line, hopefully impinging on baffles where gravity can do its work.

Another possibility is have the main airway a straight-thru path with the hot-wire in a (smaller?) parallel path; inlet side plumbed with a "T" and outlet with a "Y" to blend the two airstreams (and supply some pneumatic gain?). There could be a user-operated valve for calibration, etc. in the straigh-thru segment if desired.

Ah well, enough brainstorming.

Cheers,
Tom

I don't mind the brainstorming. My OP was kind of a theoretical question, although if someone were to give me an answer within my skill levels, I would indeed want to build this.

No solution can require a fixed orientation. Nor is it acceptable that it works "most of the time". If I am giving a live performance (even for friends), I wouldn't it to go screwy because a water drop found its way to the hot wire.

It's certainly an interesting problem. How do you run a hot wire anemometer in a super-humid environment? Heating the tube is probably the only thing that would really work.

You know, I thought I'd get more ideas along the lines of the pinwheel or flap ideas that I posted in the OP. The flap idea seemed particularly good, although I don't know how I measure the angle of the flap. I'm sure there's something. The flap would need to be on a spring and it would be great if there were a way to easily adjust the tension. It seems as though it would work at any angle and in a humid environment. It would certainly require a ton of prototyping.

 

[Borek]

If the flap were to react to the slow flow/low pressure spring should be rather weak, so the flap could easily dangle when you move the instrument.

 

[Freixas]

If the flap were to react to the slow flow/low pressure spring should be rather weak, so the flap could easily dangle when you move the instrument.

Yep--that would be a problem. The pinwheel idea probably has similar problems.

Well, I can see why everyone uses a pressure sensor. :-)