Ultra-sonic Fog Generation / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Ultra-sonic Fog Generation

1 2 next

Move Thread

LAN_403

Frosty90

Mon Oct 26 2009, 06:49AM

Registered Member #1617 Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139

Hi all. Recently I got it into my head that I want to build something to generate 'fog' from water. I've seen those ultra-sonic mist/humidifier things that you can put into little ornamental ponds to generate fog and thought I'd try something similar. Just as an initial experiment, I purchased a few of these:

I built a circuit to drive it at about 1MHz, with a 15v pk-pk squareish wave, mounted it in a small container with water over it, and all I've managed to get so far is the water surface directly above the transducer rises up into a sort of 'hill'/standing wave, but no 'fog' is produced. I next tried it with an aproximately 50v signal, i got a bit of a bigger wave pattern in the water, but still no fog. Does anyone know how much power would be needed to actually produce fog at all? The original plan was to gang heaps of these up, to make large quantities of 'fog', but its looking like these things wont do it.

On the data sheet it mentions 'center-frequency 40khz' what does this mean? It certainly works at way higher frequencys, because I get the standing wave patterns in the water. Anyone else ever built something like this or pulled appart a commercial unit?

Cheers,
Jesse

Proud Mary

Mon Oct 26 2009, 07:20AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Why not drive the transducer at its centre frequency of 40kHz (as recommended) instead of 1MHz, where I'm surprised that it functions at all?

Steve Conner

Mon Oct 26 2009, 08:10AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes, they presumably mean the transducer will be mechanically resonant at 40kHz, so that's where you need to drive it for maximum output.

Frosty90

Mon Oct 26 2009, 09:01AM

Registered Member #1617 Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139

...mechanically resonant at 40kHz...

Thats what Iwas thinking, (other transducer data-sheets usually specify a resonant frequency, so thats probably what this is), but, when I drive it at 40khz, I get nothing happen at all! It definitely produces sound output, it can be detected with another one, but there is absoluteley no effect on the water untill above about 800khz, and the effects get better as you go higher. The highest I've gone is a bit over 1.5mhz at about 50v peak to peak, and I only get a funny little standing wave in the water, but no fog. I'll have to build something to drive it at its full rated voltage...but thats too hard for now...

Proud Mary

Mon Oct 26 2009, 09:25AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

How is the the transducer coupled into the water, Jesse?

mikeselectricstuff

Mon Oct 26 2009, 09:33AM

Registered Member #311 Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253

Those transducers are designed to couple to air - putting them in water may well shift the resonance significantly, but they are not designed to handle the power needed to make fog so I think you are wasting your time. You can buy replacement discs for foggers - here look at bottom 'replacement membranes'
You may be able to do something with the discs designed for audio sounders, although I suspect the capacitance may be a problem.

3l3ctrici7y

Mon Oct 26 2009, 09:35AM

Registered Member #1806 Joined: Sun Nov 09 2008, 04:58AM
Location: USA
Posts: 136

The water level probably needs to be within a certain range.

I took a commercial unit apart many years ago, and I found that it did not
produce much fog unless the water was just the right depth.. too little depth, or too much and it did not produce much or any fog.

Proud Mary

Mon Oct 26 2009, 11:37AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

I feel Mike is right here: the acoustic impedance of air and water are radically different.

Bjørn

Mon Oct 26 2009, 01:30PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

We have 3-4 of these threads in the archives. You need a special ultrasonic element, you need the right depth of water and you need a proper driver that drives the element at the edge of destruction at the resonant frequency. The power level means that even the elements designed for this have a limited lifetime. Any other element is unlikely to last more than a minute at the required power.

If you spend some time on google reading the datasheets and application notes for the elements and driver you will find a lot of information.

klugesmith

Mon Oct 26 2009, 06:45PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

I once noticed that a regular ultrasonic cleaning bath will instantly kill mosquito larvae.

[edit] That comment was not intended to be off-topic, but to suggest a way to compare ultrasonic outputs in water, and control pests at the same time.

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob