Re: Mist Maker Question

[William Beaty <billb@ESKIMO.COM>]

On Tue, 16 May 2000, Tony Wayne wrote:

> *** Background ***
> Here a brain stumper. Florists and nurseries are now selling this little
> device called a mist maker.

Ah, "ultrasonic nebulizers".  I love those things.


  http://www.fernsfountains.com/  High Output Fogger
  http://www.indoorfountain.com/smoker.html  Mist Maker(tm)

Here are articles about "fun" applications:

  Touch The Clouds
  http://www.amasci.com/amateur/clouds.html

  Tornado Chamber
  http://www.amasci.com/amateurtornbox.html



> It have virtually no instructions. It looks like
> a small cylinder about 4 cm high with a diameter of 3 cm. It has, what looks
> like a small round, 1 cm diameter, metal diaphragm on the top.

It's a metal-plated ceramic piezoelectric audio transducer: an underwater
"tweeter" loudspeaker.  These are basically the same as the old "cool mist
humidifiers" that appeared in the early 1980's.  They're gone again,
because the manufacturers fear lawsuits regarding "humidifier fever".

When people use polluted water, or if they let the humidifier sit around
for months so that bacteria grows in the water-well, the humidifier spews
water droplets which evaporate, leaving behind a cloud of infectious
particles.  This is how Legionnaires disease was discovered: a humidifier
in a hotel's heating system caused an outbreak.  Staphlococcus is pretty
common, so its a good idea to always use new tapwater when operating these
devices (and maybe put in a small bit of Clorox.)

> It plugs into
> the wall and runs with no audible sound. The instructions say to place it
> between 5 and 6 cm below the surface of the water. It does not run when the
> water depth is about 1.5 cm above its top or is an inverted glass of air is
> lowered over the unit in under water.

I don't know how the low-level water sensor works.  In the full-sized
humidifiers there was a float switch based on a magnetic reed.  These
"Mist Maker" modules must sense water level somehow.  Perhaps they use
infra-red to detect the air/water interface.  Or maybe they measure
conductivity of the water between metal parts?  Apparently they will be
damaged if they are allowed to run dry. Probably the acoustic equivalent
of a transmitter antenna impedance mismatch?


> When it runs, it creates a water peak at the water's surface directly above
> the diaphragm about 0.6 cm high and 0.4cm wide. It appears that a mist
> emanates from the top of this water peak. It makes a lot of mist very
> quickly.

I've never seen a good explanation of the "nebulizer" effect.  I suspect
that it's like turbulence, or like population fluctuations:  an Emergent
phenomenon covered by Complexity theory.  It's probably a miniature
version of the "dancing water" effect which occurs when a water-filled
metal bowl is vibrated.  A similar thing occurs in the Kundt's tube
demonstration when liquid is used:  the fluid surface humps up and forms
tiny ripples at the peak of the large humps, then the peaks of the ripples
appear to launch droplets upwards.  An old ultrasonics textbook shows that
a similar thing occurs when an oil-wetted glass surface is vibrated: the
oil gathers itself into a regular array of small blobs, and each blob
develops a sharp peak which spews droplets.  To demonstrate a large,
low-freq version of this effect, glue a cardboard plate onto a large
loudspeaker cone, aim the speaker upwards, cover the plate with salt or
baking soda, then drive it at a few hundred hertz.  The powder will form
itself into many fascinating dynamic structures, including conical
"volcanoes" which spray powder from their tips.  I suspect that similar
things will occur if fluid is used instead of powder.  Wear hearing
protection if you try this, and don't expose an Epileptic to continuous
loud sounds (I once witnessed a grand mal seizure triggered by this same
demo.  Same problem as strobe lights.)



> As an aside. It does not shock you when you put your finger in the water.
> However if you place your finger directly over the diaphragm, you will
> receive a searing pain like a long needle being inserted in your finger.

The ultrasound beam has a power level of at least a few watts.  Try
placing the tines of a plastic fork in the underwater acoustic beam.  They
will heat up and soften.  The "hotspot" in the beam can cause the thin
styrene from a jello pudding cup to visibly warp.

When I first saw one of these humidifiers around 1984, I immediately had
nasty visions of acoustic burns occurring in the bone marrow of my finger!
Since then I've never had the guts to stick my hand in the water.  If I
have to receive a burn, I'd rather have it be on my surface!  :)


> *** THE QUESTIONS ***
> How does it work? Why does it work?

I once measured the frequency of an old humidifier.  It ran at about
1.5MHz, if I recall.

Try this: place the Mist Maker in a jar, and fill it with hot water fresh
from the tap (so the water contains a cloud of tiny bubbles.)  Cover the
device with a couple of inches of water, turn it on, and then fill the jar
very slowly.  When the water level hits an integral multiple of half the
wavelength, you'll see a row of parallel disks appear in the underwater
region between the ultrasonic transducer and the water's surface.  These
are nodes (or perhaps antinodes) in the standing wave which is trapped
between the transducer and the water surface.  Bubbles form thin periodic
"disk" layers with 1/2-wavelength spacing, like a Kundt's tube
demonstration tipped on end. A coarse powder suspended in the water would
probably work equally well.  If you know the speed of sound in water, you
can measure the spacing and calculate the frequency.

Another trick: obtain one of those "liquid crystal postcards."  Stick it
in a full aquarium.  Aim your underwater ultrasonic transducer at it, and
you'll see a colored splotch appear at the spot where the sound beam is
heating the plastic.  Try bouncing the beam from a reflector first.
Something I never tried: adjust the water temperature so the liquid
crystal sheet is biased into its "colored" region, then bounce the
ultrasound beam from a curved reflector so the beam forms a 2" spot on the
liquid crystal.  Place your hand in front of the liquid crystal sheet, and
perhaps you will see the shadows of bones!?  Wear a ring and create the
acoustic version of Roentgen's first x-ray photo?  Flesh is supposedly
acoustically transparent when under water.  Or, since the wavelength is
fairly long and the beam is "monochromatic", maybe you'll only see a
complicated diffraction pattern.  Or maybe no image will appear, and
you'll receive an acoustic burn when standing waves build up inside your
fingers.  On second thought, try using hunks of plastic or pieces of
cucumber as test objects.


> Any ideas, scientifically, on why they can claim it to be a "negative ion"
> generator. Wouldn't the water vapor it creates neutralize any charged
> particles it may generate.

I suspect that this is bogus.  Waterfalls are said to create negative
ions.  This is probably a genuine effect:  the polarity of the sky voltage
during clear weather is earth=negative, ionosphere=positive.  The surface
of outdoor bodies of are charged negative, and if anything should cause
droplets to detach from the surface, the droplets will carry away a
negative net charge.  When the droplets evaporate, they leave behind
negatively-charged mineral grains and negative ions.

But the same is not true of the shielded environment inside a house.  The
Mist Maker will only generate negative ions if a positively-charged object
is held above the water, or if the whole device is taken outdoors.  And
during a thunderstorm, when the negatively-charged stormclouds are
hovering above, the Mist Maker will create positive ions!




The "Discovery Channel" store in the local mall had these during last
halloween for $45.  Before that, the prices were around $160-$120.  But
nothing beats the $5 price of old ultrasonic humidifiers found at garage
sales.


((((((((((((((((((((( ( (  (   (    (O)    )   )  ) ) )))))))))))))))))))))
William J. Beaty                                  SCIENCE HOBBYIST website
billb@eskimo.com                                  http://www.amasci.com
EE/programmer/sci-exhibits          science projects, tesla, weird science
Seattle, WA   206-781-3320          freenrg-L taoshum-L vortex-L webhead-L