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Bucket of Fog

Most home growers have difficulty properly controlling the humidity around their plants. You will see recommendations such as using a humidity tray or closely grouping the plants together. These steps may help slightly, but anyone who has actually measured their humidity with an accurate meter can tell you that it's a very slight effect.

As a side note, the cheap electronic temperature/humidity sensors are quite accurate for temperature, but grossly inaccurate for humidity. Just buy two and put them next to each other to see that they are probably reading more than 20% different. One of mine, which claims ±5%, has a remote sensor, which generally reads 10% off when placed side by side.

To give you an idea of what it takes to accurately read humidity electronically, search the web for prices of the Sensirion SHT-75 component. Knowing that manufacturers are not paying orders of magnitude less than the ~$30 you'll see here lets you know that full instruments with good accuracy are not going to be cheap.

Assuming you have an accurate humidistat, here's a < $100 way of providing enough fine fog to humidify inside the house without wetting your plants, walls, floor, or furniture.

Materials List

  1. 5 gallon bucket with lid: < $10
  2. Small AC computer case cooling fan: < $10
  3. Ultrasonic mist generator with float buoy: price variable
  4. Bolts, nuts, wire: Probably in your toolkit

The most expensive item, will be the ultrasonic mist/fog generator. I bought mine on eBay for $66 plus $15 shipping. Considering the weight of transformer alone, I didn't feel bad about the shipping cost. The model I bought looks identical to the MCL05 5 disk > 1500ml/hour model sold at MainLandMart. This one has no problem humidifying a 10 x 12 room, even on the dryest days. You may be able to go smaller, but when I used a set of one 110ml/hour and one 300ml/hour generator, it was only able to keep my room above 60% humidity on the relatively moist days.

The fan I use is rated for 73 cubic feet per minute. It's a little louder than I'd like if it was in a room I'm habitually in, and you could probably go smaller, or reduce the voltage to slow it down and still get good fog dispersal.


This picture should give you an idea of where I am going:

Bucket of Fog
Large Huge

Most of the work is on the bucket lid. I cut a disk out of the middle of the lid to allow the fan air to be pushed down into the bucket. It should be the same size as the fan disk. I just placed the fan on the lid and traced the inner contour, then used a knife to cut out the disk. Save the disk to use as a splash guard.

Then drill holes for bolts to hold the fan to the top of the lid. Make sure your bolts are long enough to hold the fan to the lid, with a little extra for the splash guard mount.

Make a splash guard mount. Mine is crude but functional. It's just a pair of wires bent to hold the disk two inches below the hole in the lid. They have little loops in the ends for the nuts to hold onto the bolts.

If you have an AC fan, wire it to the AC outlet of the humidistat, in parallel with the mist generator transformer. Usually, an electrical octopus works fine for this.

If you have a DC fan, wire it to a DC power supply of the appropriate voltage. See the previous paragraph, treat the fan power supply as though it were the AC fan.

Now when the humidistat switches its outlet on when it's too dry, your fan and mist generator will turn on simultaneously.


You are mixing water and electricity. Make sure the humidistat is plugged into a GFCI outlet.

If the splash guard is not properly positioned, the fan will get wet.

Don't overfill the bucket. The ultrasonic waves in the jet of water that comes up from the disk are powerful, and can melt holes in the plastic of the splash guard. Keep at least three inches between the surface of the water and the splash guard. I am considering tethering the buoy to the bottom so that overfilling won't allow the jets to destroy the splash guard.

Use the purest water you can get. Reverse Osmosis, distilled, or rain water will keep the ultrasonic disks clean for as long as possible. Deposits in the water will reduce the amount of fog generated.

Evaporation and Temperature

Graph of humidity and temperature This is a cold mist device, so when it is running, it will cool the air in your room. The latent heat of evaporation is 620 calories/gram from 20°C. My 1500ml/hour fogger removes 915 kcalories of heat per hour of operation. Of course, since it switches on and off, I'm not removing that much heat from the room per elapsed hour. If you look at the graph to the right, you'll see that the light blue line (relative humidity) moves up and down rapidly when the humidifier goes on and off. The two top lines are the temperature in the room, measured in two locations. You'll see that they move down as the humidity rises. This is because the heat in the air is being used to evaporate the mist.

Because rising air temperature makes relative humidity drop, a humidistat and effective humidifier acts as a crude swamp cooler. You are not controlling the temperature directly, but you are indirectly. Of course, just as with a swamp cooler, if you never allow fresh air in, your air will continue to get hotter and moister, as long as there is some source of heat, such as sunlight or artificial lights.


For less than the cost of a few nice plants, you can now control your humidity, and perhaps grow plants you never could before.

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Last Modified: Mar 31 2005