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A fog machine is a machine that replicates the visual effect of fog or smoke using one of three methods, each of which has its own benefits and drawbacks. They can use heated glycol and water, atomize fluids into the air, or use dry ice. These machines are usually used in creating cinematic effects, effects for stage shows, or sometimes for Halloween or other small-scale presentations.
The most common way that professional machines work is by heating a mixture of glycol and water to create a fog-like steam. The machine is made up of a pump, a tank, and a heat exchanger, which is an enclosed area with an input hole and a small output nozzle. The exchanger heats up to about 400°F (205°C), and then the pump draws the glycol-water mixture from the tank and sprays it into the heated unit. The mixture turns into steam and is forced out of the nozzle on the front of the fog machine. When the steam makes contact with the relatively cool air outside, it condenses somewhat and turns into fog. This fog is still hotter than the outside air, so it rises up towards the ceiling, creating billowing clouds.
A second type of professional machine works in a similar manner, but instead of relying on heat, it atomizes the fluid and sends it into the air. Usually, the fog particles are much smaller than those made using heat, and they may not even be obviously visible to the naked eye, creating a sort of haze all around for light to reflect off of. This type of fog machine tends to rely more on oil-based liquids, without the high water content of heat-based devices.
The third, and by far cheapest and easiest way to make fog does not even have to use a machine. Solid CO2 — more commonly known as dry ice — can be used to create a fog effect. When the dry ice heats up in the ambient room temperature, it begins to create a hazy smoke. Unlike machines that create a fog that hangs in the air or rises, the fog created by dry ice is cold, and therefore sinks to the ground. While this may be ideal for certain situations, it is generally considered inferior to the more professional devices, and dry ice is usually used only for in the home or by those on a tight budget.