dave@onfcanim.UUCP (02/14/87)
I recently bought an ultrasonic humidifier (Bionaire BT-201) because I
have a cold. It works by agitating the water so it breaks up into tiny
droplets which are then carried away by a moving airstream.
There is a small ceramic transducer at the bottom of a water reservior,
with a depth of perhaps an inch of water over top of it. When the
humidifier is in operation, a stream of water is ejected upwards from
the surface of the water, just as if there was a nozzle located down
where the transducer is. At full output, this stream reaches several
inches above the surface of the water. This was quite an amazing sight
when I knew there was nothing other than sound propelling the water.
Also, there are warnings in the manual and on the unit itself that you
should never touch the water when power is applied to the transducer in
order to avoid burns.
Being the curious sort, I took it apart. The piezoelectric transducer
is driven at 1.7 MHz! No wonder the drive circuitry is enclosed in an
RF shielding can. The drive voltage reaches 140V P-P (50VAC) at one
point in the AC power cycle when the intensity is set to FULL. (The
drive circuitry runs on unfiltered full-wave-rectified AC, so the drive
voltage is 100% modulated at 120 Hz).
Does anyone know more about these things? How does the ultrasound
"push" the water into a stream? I can see why the stream is narrow -
the transducer is about 20 wavelengths in diameter. Can you really get
a burn from RF-frequency sound waves conducted by water?
Dave Martindale
{watmath,musocs,micomvax}!onfcanim!davebp@pixar.UUCP (02/17/87)
Yes, it's interesting how ultrasonic humidifiers work, but are they good for you? Unlike the old steam vaporizers, they don't boil the water - so whatever is growing in that water just gets thrown into the air, and eventually winds up in your lungs. I don't know how effective high-frequency sound is at killing bacteria, and how much of the atomized water has been subjected to the high-amplitude vibration. It would be interesting to let some tap water sit in one of these for a few days and then place a microscope slide in the air stream and look for bugs. I haven't done this, because I wouldn't buy the vaporizer. I'm concerned that some of the people who buy such a vaporizer for their colds could get pnumonia from the vaporizer. I'd like to hear more about this from someone who has studied it. Bruce Perens bp@pixar
gardner@uxc.cso.uiuc.edu.UUCP (02/18/87)
Ultrasound form focused transducers operating at similiar frequencies have been used as an experimental cancer treatment. By focusing the power, you can go through the skin and "cook" the tissue below. I've seen these transducers operate just in a tank of water, and they can literally boil the water at the focal point with enough power applied. I don't believe that is quite how the himidifier types work, but your sure transfer a lot of energy to the water. mgg
ken@rochester.UUCP (02/18/87)
You don't have to boil water to vaporize it. Face it, we don't live in a sterile environment. Although it might be pushing luck to put dirty water in an ultrasonic himidifier, I doubt if the bacteria wouldn't escape with the vapor anyway. Ken
david@sun.UUCP (02/18/87)
In article <555@pixar.UUCP> bp@pixar.UUCP (Bruce Perens) writes: >Yes, it's interesting how ultrasonic humidifiers work, but are they >good for you? You've got it backwards-- ultrasonic humidifiers are much healthier. The details were in the Consumer Reports article. -- David DiGiacomo {decvax, ihnp4, ucbvax}!sun!david david@sun.arpa Sun Microsystems, Mt. View, CA (415) 691-7495
henry@utzoo.UUCP (Henry Spencer) (02/20/87)
> Unlike the old steam vaporizers, they don't boil the water - so whatever > is growing in that water just gets thrown into the air, and eventually > winds up in your lungs. Most household humidifiers are not steam vaporizers, if for no other reason than because boiling water is expensive in energy. They generally rely on room-temperature evaporation. > I don't know how effective high-frequency sound is at killing bacteria, > and how much of the atomized water has been subjected to the high-amplitude > vibration. I seem to recall that Consumer Reports looked at this and concluded that the bacteria count coming out of an ultrasonic humidifier was significantly less than that from ordinary humidifiers. The ultrasound seems to kill the beasties moderately well, although not perfectly. -- Legalize Henry Spencer @ U of Toronto Zoology freedom! {allegra,ihnp4,decvax,pyramid}!utzoo!henry
davidw@lpi.UUCP (02/21/87)
On not touching the water: I was curious about this, so I hooked up a diode and capacitor and voltmeter and tried to measure a RF field in the water. Nothing (I also checked the circuitry with a RF generator and it worked). Feeling a bit safer, I tried touching the water and all I could feel was some warmth. The humidifier says to never use water > 104 F; I imagine this is to insure the water can absorb enough energy to effectively cool the transducer. Ideas??? DW
agn@unh.cs.cmu.edu.UUCP (02/23/87)
The warning concering ultrasonic humidifiers (and similar devices like cleaners) is due to the extreme pressures of the sound wave in the water. These pressures can be 20 bar and up (that's about 600 psi), which is more than enough to damage cells mechanically. The net result is similar to a burn or exposure to extrem cold (say liquid nitrogen). That warning is real. -- Andreas agn@vlsi.cs.cmu.edu