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Ultrasonic "fog" makers like in humidifiers

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LAN_403

Bored Chemist

Sun Sept 09 2007, 08:12AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

"water will disassociate best at 2.48mhz"
Why?

Experimentonomen

Sun Sept 09 2007, 10:59AM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

2.48millihertz?! LOL Thats a low frequency!!

Sorry, couldent resist ;)

Terry Fritz

Mon Sept 10 2007, 12:09AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

You might try adding propylene glycol or non-toxic RV antifreeze for a real fog machine (Warning! pie in the sky untested idea). Might not be good for the ultrasonic transducers or something...). Fog machines use glycol since it does not evaporate into the air. The mist lasts a long time.

Those transducers use "nasty" sound energy. You probably don't want to touch them since they might vaporize your blood or do totally unknown things to you

Cheers,

Terry

Carbon_Rod

Mon Sept 10 2007, 01:55AM

Registered Member #65 Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155

Deep tissue burns suck... Don't put your fingers in there....

The "cold" plastic welder built out of one of these 1" discs worked well with vinyl (also boils holes in CDRs.)

The foil plating of the disc damages easily... Don't try to bond a small metal parabolic dish to the center X-[

Marko

Mon Sept 10 2007, 02:10PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Col stuff.. I had no idea what can these small postcard speakers do.

And they are resonant in Mhz range? (Sounds like a lot for such a large piezo).
I guess they wouldn't mind to be hard driven..?

EDIT:

I just tried to drive small postcard piezo with 1Mhz sinewave. With just few watts, most power seems to be converted into heat by dielectric losses. Placing some water atop of it did nothing; all that happened is that it boiled when piezo got too hot.

The "cold" plastic welder built out of one of these 1" discs worked well with vinyl (also boils holes in CDRs.)

Can you tell more about what you did? Those small piezos look like they are going to overheat much sooner than doing anything. Unless those used in humidifiers are different construction.

EDIT again:

OK, weird things are definitely going on. I've put as much water on the surface of piezo as surface tension allows. It starts bubbling like it's boiling, but piezo is still touchable at that point! Not for long, but touchable, not close to 100 degrees C. More like 50-60.

The problem is that steam is given of instead of mist; it's not very visible, and obivously carries enthalpy making it hotter to feel than piezo itself.

As far as I remember humidifiers gave off foggy mist that felt cold.

Is there some kind of power threshold before mist starts to form?

How do commercial humidifiers isolate the piezo form water?

Anyone had luck to disassemble one? ;)

CT2

Tue Sept 11 2007, 03:17AM

Registered Member #180 Joined: Thu Feb 16 2006, 02:12AM
Location: Ontario, Canada
Posts: 187

Ok I finally got around to getting some pics of the "fog maker". The unit is old (I remember using it when I was little and asthma was bad) and it has a fan to blow the fog out, a transformer to supply the power, and then some other dials to control power. The actual piezo element is attached to the board that controls it and it runs off 48V AC (there is a rectifier on it though). The element needs to be in water that is atleast an inch or so deep, if not the water just kinda hops up but doesn't turn into fog. If there is no water on the element or if the water is really low (like just a small layer) then the element gets pretty hot, maybe even hot enough to boil the water thats left.

Sorry about the low quality images I took them with my phone.

Marko

Tue Sept 11 2007, 09:42PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hey, thanks a lot CT2. I've managed to solder a mini coax to the piezo and insulate the hot side with Dow Corning RTV silicone. I hope to be able to dip it into water as such without creating shorts.

I'l try it tomorrow, there are pics if anyone is interested...

Marko

Thu Sept 20 2007, 11:06PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I have managed to replicate it to some point, but I lost the piezo before I was able to take any good pics.

Dow corning silicone I used did great job insulating the piezo and I was able to dip it under water.

I carefully turned the power up. At 30V there was noticeable cavitation, in form of bubbles forming on piezo ang rising up.

I upped the power to 70V, and piezo was instantly shivering in bubbles, like water is boiling around it -but it was cold! Frequency was 1Mhz sine, power unknown, but somewhere under 50W.

Still there wasn't any myst forming, vapor produced from bubbles seemed to be invisible.

I tried to increase coupling with ferrite rod, but at that point the crystal cracked in half and started arcing massively at the point of crack. Silicone held out, but that portion of piezo got electrically disconnected. I ran it on low power trying to photograph it but at that point only like 1/4 of it worked, and poorly.

Soon after it went completely open.
I wonder if I may carefully solder or electroplate something to foil layer of piezo to make it stronger.. although I still fear tha tit won't save the crystal from breaking.

I have a feeling I'm doing something very wrong there.

Conundrum

Wed Sept 26 2007, 07:47AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Interesting. I've done something similar using crystals from Ebay (sold as replacements for foggers) and a modified Colpitts oscillator.

I also got mine to nearly work and "bulge" the water, the limiting factor was the transistor used (2N2222) but apparently the circuit used in commercial foggers is a single transistor oscillator with a very simple feedback system.

Future experiments include using a diode-based tuning system similar to that used in an FM radio.

If you want to try this, I advise using a spring and metal disc to connect to the transducer, as a point contact seems to be required to reduce mechanical stress (without this it will rapidly disassemble itself)

-A

Marko

Fri Sept 28 2007, 03:19PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Here are some schematics and piezo's, not much information but can be started from. I can't figure the transistor out.

Piezos are told to work at 30W so I was at about right power level, although I don't understand what means '48V 30mA max' in driver circuit spec?

Water level is said to be 40mm, so I was probably too low. I don't understand although why is water level important?

Should the piezo be driven self-resonant? I don't see any particular virtue in that.

Since it behaves more like parallel LC it's not too easy to drive.

How should I fix the piezo to assure it stays alive? They seem to use some kind of rubber packing, spring contact and watertight seal. Piezo is also mounted at an angle for some reason.

I also don't know if postcard piezo is capable for that at all. It looks very similar but I don't know.

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