This Halloween I set up a new fog system in my front courtyard. That’s right, this is the new fog system, because I already had an old fog system.
A couple of years ago, my wife bought me a fog machine for Halloween, and it was the perfect gift: Something I didn’t know I wanted until she gave it to me. For the last two years, we had that fog machine set up in the small courtyard formed by our home’s U-shaped layout. It was one of those small fog machines they sell in the Home Depot “Seasonal” section, so while you could see a bit of fog in the air, it wasn’t all that impressive.
This Halloween, I wanted to kick it up a notch, so I bought a bigger fog machine. It wasn’t exactly cheap, but it wasn’t all that expensive either. DJs use these kinds of machines to fog dance floors, and there’s a thriving competitive market, which means you can get good equipment at a reasonable price. After doing a bit of research, I ended up with a Chauvet DG Hurricane 1600 fog machine that produces an astonishing amount of fog in just a few seconds.
I also built a fog chiller to cool the fog so it would stay close to the ground. Machine generated fog is normally pretty warm, which caused it to rise up into the air and thin out. But I wanted to produce creepy-looking low-lying graveyard fog, so I routed the output from the fog machine through a box full of ice to cool it off.
Here’s a shot from a test I did in my back yard:
Unfortunately, the Chicago area got hit with rain and snow on Halloween, which kept people away. This was exacerbated by the Village of Bloomingdale’s on-again/off-again attempt to postpone trick-or-treating. Between the confusion and the weather, we only got about 15 visitors.
The other problem was that when I got to Meijer on Halloween, the dry ice freezer was empty, so I had to use regular ice in the fog chiller, and it wasn’t enough to keep the fog low to the ground in such cold weather.
All in all, it was a disappointing Halloween. But it would have been cool if it had worked.
If you think you’d like to try this yourself — or if you’re just curious — the rest of this post goes into some technical detail.
You can find a variety of fog machines, from $15 home systems that produce a short spurt of light fog to $8000 fog machines that can fill a Broadway stage. In the middle are the systems typically sold to DJs to fog a dance floor, like the Chauvet DJ Hurricane series. My Hurricane 1600 produces far more fog than I need, and I’m sure other brands such as American DJ do just as well.
A few things to keep in mind when buying a fog machine:
- Make sure not to get a haze machine, which is designed to produce a thin fog that shows off laser light shows and other beams of light. You want a fog machine that fires a thick burst of fog.
- You probably want one that fires the fog sideways rather than straight up. You can always deflect it later.
- Get a remote control, so you don’t have to stand next to the machine to set it off. I use wireless, but wired will work if you can easily run the wire into your house.
Fog Machine Theory
Fog machines are kind of fussy, so it helps to understand how they work. Basically, a fog machine is a really big steam iron. You know how clothing irons have that button that pumps water into the iron to give you a burst of steam? Fog machines use a small electric pump to do the same thing on a larger scale. There’s a heavy metal block inside that is electrically heated hot enough to boil water, and when you press the “fog” button, the pump feeds water into one or more chambers in the hot metal block, where it flashes to steam which forces itself out the nozzle under pressure.
Boiling the water to steam uses up heat energy, which causes the metal block to cool down. After a 20-30 seconds, the temperature is too low to produce fog, and the pump shuts off to let the metal block to heat up again. This means that fog machines have a duty cycle and cannot run continuously at full output. They cycle back and forth between fog production and reheating.
Consequently, the true fog output is less than the output volume in the machine’s specifications. Even though my fog machine is rated to produce a whopping 25,000 cubic feet of fog per minute, that’s only for the 20-30 seconds it’s operating at full output, before it has to stop and reheat the metal block. The average fog output is a lot lower.
Some fog machines have an adjustable fog volume, and on those machines it’s possible to turn down the volume of fog to extend the length of the duty cycle, possibly even to extend it indefinitely if the heating element can keep up with the reduced cooling effect of the lower fog volume.
Fog Machine Safety
The fog from one of these machines is literally high-temperature steam when it jets out of the nozzle, so don’t trigger the machine if someone is right in front of it. In fact, on some machines, parts of the exterior surface can get hot enough to cause minor burns, so it’s probably best to put the machine out of reach of young children.
Make sure to follow all the usual safety rules for using high-voltage equipment outdoors, including
- Choose heavy-duty outdoor-rated extension cords rated for the the full amperage of the fog machine.
- Plug the extension cord into a GFCI-protected outlet or use a GFCI adapter.
- Use weather-proof enclosures around all cable junctions.
Also, fog machines are not normally weatherproof, so you need to protect them from the rain. I put my fog machine in a plastic storage container with holes cut in the side for the fog nozzle and the extension cord. I also drilled a ton of small ventilation holes under the outer lip of the container so that the fog machine would not overheat in a confined space.
The liquid in fog machines is called fog juice, and it’s a mixture of deionized water and an additive (such as propylene glycol, triethylene glycol, and/or glycerin) that helps form the visible fog cloud.
The problem is that the additives in fog juice can gum up the works inside the fog machine, causing it to clog and stop working. To prevent this, you should periodically run cleaning fluid through the fog machine. Also, you should not let fog juice sit in the machine when you’re not using it. So before you put it away for the year, you should empty the fog juice tank and run cleaning fluid through the machine to ensure that it works correctly when you need it next year.
Fog juice is is surprisingly expensive — $20 to $30 per gallon — but I don’t recommend trying to cut corners. Don’t dilute the fog juice to make it last longer, and don’t add anything else to it. That will just make it jam up faster. You don’t necessarily have to use the fog juice sold by the fog machine manufacturer, but don’t buy the really cheap stuff either. Use one of the well known brands that other people are using and recommending. I’ve been using Froggy’s Freezin’ Fog, which is formulated to work with a fog chiller.
Fog Juice Safety
In case you’re wondering, fog juice is mostly safe. It’s known to be a minor throat irritant if you inhale a little too much, but there are no lasting health effects from short-term exposure. Long term exposure is a little less certain: There is some evidence of reduced lung function in people with long-term occupational exposure — e.g. professional fog machine operators for theaters and concerts.
Remember, if you’re working with a fog machine, you will be breathing fog juice. Think of a fog machine as a giant refillable vaping system — whatever you put in the tank ends up in your lungs. So, as with vaping products, you want to use a reputable supplier to make sure it’s safe. And don’t add anything to the fog juice tank that you wouldn’t want in your lungs.
(Because I know my readers: Yes, you probably could use this as a drug delivery device by adding cannabis oil to the tank. But please don’t. The vapor comes out really hot and you could scald your throat taking a hit off the nozzle. I know you potheads are ingenious when it comes to ways to get high, so you might be able to construct some contraption to cool the vapor on its way to your mouth, but… I don’t know if you can die from a cannabis oil overdose, but trying to huff cannabis vapor from a high-volume fog machine would be the way to find out.)
The fog coming out of the nozzle begins to cool immediately, but it still stays warmer than the air long enough for its lighter density to make it rise up. That’s not the kind of spooky ground-hugging fog you want on Halloween. If you want the fog to stay low, you have to cool it off. Thus, the fog chiller.
The basic idea is to route the fog through a long winding tube surrounded by ice, or to force the fog to flow directly through a pile of ice. You can buy ice chillers from the same places that sell fog machines, and you can even buy fog machines with built-in ice chillers, but it’s easy enough to build your own.
You can find instructions for building a variety of fog chillers all over the web. I based my chiller on this design:
I made a few changes:
- I used a cheap Coleman cooler instead of an uninsulated storage bin.
- I used dry ice instead of regular ice.
- I routed the tubing so I could place the fog machine enclosure on top of the cooler.
Here’s what it looked like before I painted it black:
You can use 3-inch PVC pipe like this to route the fog to wherever you need it. Since you’re not using these as water pipes, you can save time by using hot glue rather than PVC cement to stick everything together. You can also route fog with flexible vacuum hoses or the kind of hose you’d use to vent a clothes dryer.
To get the most fog flow through the pipe, it helps if you leave a gap of an inch or two between the fog machine nozzle and the pipe opening to allow air to be entrained in the system. The fog travels pretty well, and I’ve seen people use a couple dozen feet of pipe and/or hose to put fog wherever they need it. I’m thinking I might use a long hose next year, so I can keep the fog machine in the garage.
You can use regular ice (water ice) or dry ice (carbon dioxide, aka CO2), depending on what you want to achieve and how much work you are willing to do.
Regular ice is cheap — I got 22 pounds for $6 — and available everywhere. But since water freezes at 32°F, there are limits to how much a bag of ice can cool the fog. This is why my fog chiller didn’t work: The outdoor air was already as cold as the ice, so even after passing through the chiller, the fog was still warm enough to rise. In addition, regular ice also melts into water, which can make a mess or damage the fog machine if you aren’t careful — water and electricity don’t mix.
Dry ice solves both of those problems. First of all, it’s very cold, -109°F (-78°C), so it can quickly cool the fog way down. The ground-hugging fog pictured above was made using dry ice for cooling. Second, dry ice doesn’t melt, it sublimates. That is, the freezing point of dry ice and the boiling point of dry ice are the same, so there is no range of temperatures at which dry ice is a liquid (under normal atmospheric pressure). Consequently, as dry ice warms up, it just evaporates directly to a gas, leaving no mess behind.
On the other hand, dry ice brings a few problems of its own. For one thing, it’s relatively hard to find. Google for “dry ice suppliers,” and you will mostly find industrial suppliers that sell to businesses. However, Airgas does sell dry ice for residential users under the brand name Penguin, which I found in my local Meijer store. You can look for retailers near you with their online store locator.
Of course, once you’ve found a dry ice retailer, you’ll face the second problem with dry ice: It’s expensive. The ten pounds of dry ice I used in my chiller ended up costing me about $22.
And not only is dry ice expensive, it’s also hard to store. From the moment dry ice is manufactured, it begins to evaporate away, and there’s not much you can do about it. Your household freezer isn’t nearly cold enough, and putting dry ice in your freezer could damage both your food and the freezer itself. (If you have a refrigerator/freezer, putting dry ice in your freezer could keep it so cold that the compressor doesn’t run, which would allow the food in the refrigerator compartment to warm to unsafe temperatures.)
As far as I can tell, nobody uses dry ice freezers. Instead, everybody keeps dry ice cold by placing it in an insulated container and letting it keep itself cool. Some amount of the dry ice evaporates every day, and evaporative cooling keeps the remaining dry ice cold enough to stay frozen.
The upshot of all this is that you either have to buy your dry ice right before you need it, or you have to buy enough extra dry ice so that there’s plenty left by the time you need it. I’ve read that well-stored dry ice evaporates at the rate of about 10 pounds per day, but you’ll probably have to experiment to figure out if that solves your storage problem.
Dry Ice Safety
The very first time I handled dry ice was when I ordered some food online and it was shipped with a pack of dry ice for cooling. Not knowing what it was, I grabbed the ice pack with my hand to pull it out. Even though it was wrapped in plastic, I felt an intense sting within a second or two and I dropped it. No harm done, but it was a lesson: Dry ice is cold enough to hurt you.
Be careful handling dry ice. Use insulated gloves, and keep it away from children or pets. To use it in a fog chiller, you’ll probably want to break it into smaller pieces. This is easy enough to do with a hammer — dry ice is not as strong as regular ice — but be sure to do it inside a plastic bag, so pieces of it don’t fly around and end up in your eyes or sliding down the front of your shirt. Remember, dry ice may look like hard-packed snow, but it’s much colder and therefore more dangerous.
The second danger from dry ice comes from the fact that it evaporates into carbon dioxide. Or more accurately, dry ice is carbon dioxide. It’s been cooled down to -109°F (-78°C), where it freezes solid, and then it turns back into a gas as it warms up.
This is the basis of dry ice bombs — sealed plastic bottles with dry ice and water inside. The warm water causes the dry ice to quickly evaporate into a gas, which raises the gas pressure inside the bottle. Eventually, the bottle explodes from the pressure. It’s loud enough to damage your hearing and powerful enough to drive fragments of the bottle into your skin.
You don’t want to create something like that by accident, so you need to be careful not to put dry ice in a sealed container. Putting 10 or 20 pounds of ice in a cooler with a lid that latches tightly could cause pressure to build up to dangerous levels. Maybe that pressure will just seep out past the seal, but maybe it will rupture the cooler walls with explosive force.
(Carbon dioxide’s boiling point increases as pressure increases, so eventually the gas pressure will raise the boiling point to the ambient air temperature, which will shut down gas production. However, if I’m reading the CO2 phase diagram correctly, that doesn’t happen until the gas reaches a pressure of at least 500 pounds per square inch, even on a day that’s freezing cold. That works out to 36 tons of force on a 1-foot-square cooler wall — more than enough to blow it apart.)
The other problem is that you can’t breath carbon dioxide. It’s not exactly poison gas, but breathing too much can cause a variety symptoms. Your body produces carbon dioxide as a byproduct of how it produces energy from food, so roughly 3% of your exhaled breath is carbon dioxide. By comparison, the earth’s atmosphere is only 0.04% carbon dioxide, so you breath in much less carbon dioxide than you exhale.
If you let a block of dry ice evaporate in an enclosed space, it can dramatically raise the concentration of carbon dioxide, and if the concentration gets above 3%, you will be breathing in more carbon dioxide than you are exhaling, which can lead to a dangerous build-up of carbon dioxide in your bloodstream. It’s not a firm dividing line: You might notice sleepiness, headaches, and difficulty concentrating as low as 0.2%, but even amounts as high as 5% won’t kill you right away.
Let me emphasize that this is not a problem you are likely to run into. Your house’s normal circulation of fresh air is more than enough to handle a few dozen pounds of dry ice slowly evaporating. On the other hand, if you’re driving home from the store with 100 pounds of dry ice in the back seat of your car, you might want to crack open the window.
Does Dry Ice Contribute to Global Warming?
As near as I can tell, the answer is “Yes, but no.”
When a block of dry ice evaporates, it certainly ends up as atmospheric carbon dioxide, one of the principle causes of global warming. However, carbon dioxide is a waste product from a number of industrial processes. Dry ice makers (and beverage carbonation suppliers) capture this waste carbon dioxide from the processes before it is released into the atmosphere. So nobody is creating new carbon dioxide to create dry ice, and using it in your fog chiller does not increase the net amount of atmospheric carbon dioxide.
So go ahead an enjoy your cool Halloween fog.