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Ultrasonic Humidifier Mist

1995 William Beaty

This device generates a pool of fog which resembles dry-ice fog, but uses only water and air. Touch the pool and you touch cloud-stuff. Totally fascinating to play with.
WARNING: ALWAYS EMPTY THE TANK AND THE RESEVOIR BETWEEN USES! This device turns water into microscopic droplets which then evaporate. If you allow bacteria to grow in old stale water, you will soon be breathing invisible clouds of bacteria in the air!

WARNING: YOUR HUMIDIFIER MIGHT OVERHEAT INTERNALLY, AND FAIL! One person has reported that their humidifier stopped working after using it as a cloud-maker. It is possible that the partly- plugged fan hole will let the electrical circuits inside the humidifier get too hot. I've used these humidifiers in museum exhibits running all day with the fan-hole plugged, with no problems. However, some humidifiers might be built differently inside, so that they rely more on their fan to cool their hot transistors. Get an old $5 ultrasonic humidifier from a garage sale. IF YOU CANNOT AFFORD TO DAMAGE YOUR HUMIDIFIER, DON'T USE IT TO MAKE CLOUD-FOG!


  • Ultrasonic Humidifier
  • Aluminum foil
  • Rubber band
Obtain a "Cool Mist" Ultrasonic Humidifier, about $60US, or $5 at garage sales. These are the aquarium-shaped humidifiers which have a rectangular water tank, and which eject cold steam from a spout at the top. Look for them in larger drugstores. Lately they have become hard to find, possibly because of the danger associated with using them with polluted water.

Open up the humidifier's water-well where the float switch and ultrasonic transducer reside. Here there will be an air tube jutting upwards from the base which normally supplies the air flow to blow the mist out the top. Cover this air outlet with aluminum foil and a rubber band. Now poke a 1/2cm hole in the foil, the object being to vastly reduce the flow without stopping it entirely. (Smaller holes will give more dense mist, but with less rate of flow.) If you now resassemble the humidifier, and turn it on to a high setting, you will find that a pool of white fog will fill up and overflow the top. This "fluid" is composed of suspended water droplets. It is not vapor, it is mist. It is the same substance as that which makes up clouds. You can catch this mist in a dark-painted bowl, fill cups with it and pour it on the table, fill cardboard tubes and blow out a blast, make smoke rings, use it in a tornado machine, etc.

WARNING: YOUR HUMIDIFIER MIGHT OVERHEAT INTERNALLY, AND FAIL! One person has reported that their humidifier stopped working after use as a cloud-maker. It is possible that the partly-plugged fan hole will let the electrical circuits inside the humidifier get too hot. I've used these humidifiers in museum exhibits running all day with the fan-hole plugged, with no problems. However, some humidifiers might be built differently inside, so that they rely on their fan much more to cool their hot transistors. IF YOU CANNOT AFFORD TO DAMAGE YOUR HUMIDIFIER, DON'T USE IT TO MAKE CLOUD-FOG!

If you are ambitious, you can arrange a constantly-filled bowl of mist for the kids to play with. Obtain a large plastic bowl. Paint the interior black for contrast. Cut a fairly large hole in the bottom, or drill numerous smaller holes, then cement the bowl over the hole on the top of the humidifier with silicon caulk. You might also put some screening across the hole(s), so small objects do not find their way into the water. You'll find that the humidifier can be adjusted for fast-filling but dilute fog, or for slow-filling dense fog which is as opaque as milk.

If you prefer not to modify your humidifer, you can build a different version. Find a piece of white plastic pipe which you can jam securely into the circular outlet at the top of the humidifier. Jam the pipe in, mark where it passes out of the humidifier, then make another mark 1" farther out. Cut the pipe off at this mark. Use this piece of pipe to draw a circle on the bottom of your plastic bowl. Cut out the circle along the mark. Jam the pipe into the humidifer, jam the bowl onto the pipe, then use RTV silicone caulk (smells like vinegar) to securely seal the pipe into the bowl. After the caulk hardens overnight, paint the inside of the bowl black with oil paint or spray paint, and maybe glue a piece of plastic window screen across the pipe to prevent nasty stuff from finding its way into the water.


WARNING: Ultrasonic Humidifiers break water into small drops. They do not boil and recondense water as teakettles do. Therefore, if the water is not clean, the device will spread the contamination into the air where it will be inhaled. The mist droplets will evaporate and leave invisible clouds of material in the air. If bacteria are allowed to grow in the standing pool of water, they will be inhaled by everyone in the room. Obviously it's a bad idea to use any liquid but water in the tank, since it will be dispersed into the air and inhaled. (No, adding dye to the water does not color the mist, but it might stain anything the mist touches!)

WARNING: The ultrasound beam in the humidifier's internal water pool is many watts of sound concentrated into a tiny spot. If you put your fingers in the water, you can be burned by the intense sound.


If you use soap to wash down the humidifier parts, rinse it very well afterwards. A small amount of soap in the water will change the surface tension and inhibit the mist production. If your device mysteriously stops making mist, try cleaning out the humidifier's water-pool section with non-soapy paper towels. As an experiment, drip some liquid soap into the operating humidifier to see what happens.

Position the device away from drafts, as these will disturb the mist and empty the bowl. If drafts cannot be avoided, use a smaller hole in the aluminum foil plug, so that the mist will be heavier and less prone to drafts (but the device will produce less mist and refill more slowly.)

If your device does not work well, let it run for five minutes and see if it improves. Tiny amounts of oil or soap on the water surface will slow down mist production, but will be eliminated in a few minutes.


Fill a cup with mist. Pour it over someone's head. Pour it onto a dark-colored table. Fill a pitcher. Fill a garbage can. What will you do with a garbage-can full of "nothing?"

Fill a bowl with extremely dense mist (use a smaller hole in the aluminum foil.) Let everyone slowly place their hands in it. It looks like milk, but feels like nothing.

Fill an empty aquarium with mist. Place a square of cardboard vertically in the aquarium, scrape it sideways, then lift it vertically. The pool of mist will show surface waves, it will slosh back and forth. Stick your hand in the pool and make it slosh.

Make a smoke-ring launcher from a coffee can and a balloon: Use a can opener to remove both ends from the can. Cut the balloon open, stretch it across the open end of the can, and secure it with tape. Place the plastic coffee-can lid on the other end of the can. Cut a hole (2cm to start) in the lid. If you gently tap or pluck the rubber membrane, the can will launch a transparent ring-vortex through the air. Shoot these invisible smoke rings into a pool of fog. Or fill the can with mist and launch visible smoke rings!

Build a crude model of a small town, then let the fog roll in!

Fill an empty aquarium with mist. Place a clear lightbulb a few feet away, then play with lenses above and within the mist. The focussing effect of convex lenses is totally obvious. Take your fog-quarium out into the sun and see how a magnifying glass can focus light. Obtain a pair of colored light bulbs, red and green, and place them near each other a couple of feet away from the mist. A convex lens will then clearly display all the geometry of real-image formation and camera operation! Try this with a string of christmas lights instead of the two lightbulbs. Try it with a laser...

Place a large, lightweight cup or bag upon a beam-balance and weigh it. Fill it with mist. What is the weight per unit volume? How big would a pound of humidifier mist be?



This air/droplet "fluid" is quite dense, yet it is the same stuff as clouds and fog. Why do clouds stay up? Why does fog hug the ground? The answer is below(1), but it might present a topic of considerable discussion for your kids.

A hint for visualizing our world: air is not invisible, it is transparent, and air to us is like water to a fish: we can't see it, so we think it's not there. If a plume of dirty water invades a pool of clear water, a fish might see the dirt the same way that we humans think of smoke. To a fish, dirty water looks like "smoke". We who live outside the water know that the fish is really just seeing dirty water mixing in with the clean. Without the water, the dirt wouldn't act like underwater smoke, it would fall to the bottom. So, what is humidifier mist? It's not just water droplets, since water droplets alone would instantly fall like rain. Actually, humidifier mist is "dirty" air. It is air that's been stained white with some other substance. Humidifer mist makes air visible. Adding mist to the air is like adding dirt to the water. If the fish's world was without water, the dirt would not float and couldn't look like smoke. If we lived in an airless vacuum, the mist droplets would fall like so much sand. Smoke and fog cannot exist in vacuum, because smoke and fog really are air which is stained. Fires in a vacuum would not smoke, instead they would shoot out fine grey powder at high speed. Clouds and fog in a vacuum would instantly fall and turn into puddles on the ground.


Ultrasonic Humidifiers are becoming scarce. Some drug stores still carry them, but most appliance stores have "warm mist" or "boiling water" humidifiers, which don't produce the dense, ground-hugging mist.

Try for used ultrasonic humidifiers.

The Mist-Maker(tm) can also be found at Mainland Mart website

High-volume water fog for special effects: MEE FOG INC.

I haven't tried any of these, let me know if you get good results.

(1)Clouds are water droplets, and water is heavy. Why do clouds stay up, yet mist from the humidifier falls downwards? There are several reasons.

Evaporated water is a gas like any other. Does it rise like Helium, or fall like Carbon Dioxide? Here are some molecular weights:

   H2      2
   He      4
   H2O    18
   N2     24
   O2     32
   CO2    44

So, water-gas or "evaporated water" is lots heavier than hydrogen or helium gas. Yet it's lighter than air: it is lighter than Nitrogen or Oxygen. Therefore, if evaporated water is mixed with air, that air will rise. Cumulus clouds, the white puffy clouds seen in fair weather, are created by rising plumes of humid air, and the bouyant water-vapor helps keep the small water droplets suspended. The humid air always goes upwards like helium. When humid air becomes filled with water droplets, it turns white, yet it still goes upwards.

A second and more important reason why clouds stay up there: warm air rises. When any air rises in the open atmosphere, it moves into a region of lower pressure, expands, and the temperature of the air falls (in other words, rising air normally gets colder.) However, if the rising air is full of evaporated water, the falling temperature can cause the water-vapor to condense and become a white mist made of tiny droplets. When condensation takes place, energy is released, so THE WATER-FILLED AIR BECOMES WARMER THAN SURROUNDING AIR as it rises.

Remember, it *takes* energy to boil water, it also *takes* energy to cause evaporation. The opposite occurs during condensation, and energy is *returned.* As a result of this energy release, rising air which contains condensing droplets will stay warmer than the surrounding air, so it will expand more than it should. As a result, it ends up becoming less dense than the surrounding air. The lighter, low-density, foggy air is driven upwards like a balloon. In other words, "hot air rises." Clouds are warm, and they remain aloft for the same reason that hot air balloons do. If you could heat up the humidifier mist enough, it would rise too. Or, if you could cool a real cloud down, so that it had the same temperature as the rest of the air up there, it would come pouring downwards. It might end up as ground-hugging fog.


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