🚀 go-pugleaf

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!dave