DO NOT DO THIS
These are not instructions on how to do the thing I am describing. They are an explanation of the dangerous thing I did, so that future accident investigators can figure out exactly where I went wrong. The first thing I did that was wrong was even thinking about doing this, which I implore you not to do. Please, unless you are some kind of certified professional, don't attempt any of this.
What this was
Okay, so I would like my parade float to have a low-lying cloud of fog coming out from underneath. The hope is that at night, with suitable lighting, that it'll be spooky. "So buy a fog machine," I hear you saying. The issue is that my float is human-powered and small, and I'd like it be quiet, which means I don't want to bring a gas-powered generator. And electric fog machines require more than 1000 watts of power, which means running them off a battery for any length of time is not ideal. So, what to do?
Well, I saw this video online:
The gist is that you can repurpose a propane-burning bug fogger into a fog machine if you fill the reservoir with baby oil or mineral oil instead of pesticide. These foggers are about $50-$70 at your local home improvement warehouse store. They're designed to be handheld, and once they've warmed up, you cause fog to come out of them by pulling a trigger, which pumps the oil into the hot coil and vaporizes it.
Can you see where this is going?
What I need is for this to sit on the float and work automatically. While I think it would be extremely spooky to have someone walking around at night spraying mysterious fog out of a hand-held device, I also think it's likely to get the cops called on you. So, I need to convert the device into something that can be attached to the float (hopefully in a way that won't catch anything on fire) and that will automaticaly pump the fog juice through the coil at appropriate intervals.
This is where I tell you once again not to do any of this. As I hope is obvoious, modifying a commercially available propane-burning device is STUPID and nobody should do it. You've got open flames and flammable gas involved and a lot can go wrong. Don't.
To trigger an automatic burst of fog juice at regular intervals, here's what I hit upon:
- A DC powered programmable timer DROK Time Delay Relay Controller Board 0.01s-9999mins Switching Relay Module with LCD Display – I paid $13.99.
- A low-voltage DC pump (I bought a 4 pack) Sipytoph 4Pcs DC 3-5V Micro Submersible Mini Water Pump – $11.39
- Silicone tubing 3/16" ID Silicone Tubing 3 Feet High Temp – $6.59
- A check valve (I bought a 3 pack) "Metaland One Way Check Valve, 3/16" Hose Barb (Pack of 3)" – $7.99
- A rechargable USB power supply that was laying around the house.
The silicone tubing and the check valve I discovered are needed because once the fog juice hits the hot coils, it has a tendency to want to go backwards and back into your reservoir rather than out the buisiness end. So, to avoid heat and backflow issues, you need heat-resistant silicone tubing and a check valve.
I've also found it was important to get a timer that can be programmed and reprogrammed easily. This type of timer allows you to set it up to run the pump for some very short interval (probably in the 0.7s to 1.5s range) and then not run the pump for a longer interval (probably in the 7-10s range). You can't continuously run the pump because the coils will cool off and you'll start spraying non-vaporized but still very hot oil out. The exact durations will depend in part on ambient conditions, so it's very helpful to find a timer that can be adjusted on the fly. I started with a cheaper one that wasn't as smart or easy to adjust and it didn't work out.
I wired all of this together and put it into a case. I added a toggle switch so it could be turned off easily without needing to unplug it. It looked like this. The black wire is a usb cable which will go to the battery and the grey wire runs to the pump. The battery supplies 5V and just barely enough power to run the timer and the pump.
So, the water pump sits in a container of mineral oil and is hooked up to the silicone tubing. When this timer thing is powered up and turned on, it sends a short (and kinda weak, tbh) jet of oil out past the check valve. Every 7 to 10 seconds. Great.
Now the dangerous bit
Having disassembled the bug fogger, we find that it's actually quite a simple device. Here's what my test rig looked like. Note the FIRE EXTINGUISHER. Also DO NOT DO THIS.
The propane flow is controlled by a valve (A). The fogger is designed to attach directly to a smallish propane tank (B), but this is impacticable for my application, so I have interposed a hose. Note that propane is DANGEROUS in both gas and liquid form. Don't dispense it out of tanks that are upside-down, that's how you get very cold liquid propane in your system. Note also that if your valve (A) is at any distance at all from your tank you're going to have a release of some propane when you unhook everything, so don't do this at all. Disconnect these things only long after everything is cooled down (and ALWAYS disconnect the tank from the hose first, since these small tanks are self-sealing.)
There's a tiny orifice at the end (C) which can actually be unscrewed. The buisiness end is made up of four seperable parts (D).
- the cage which does a tiny bit to prevent things from coming into contact with the flame. Don't rely on this! Don't let anything come near this thing when it's operating!
- the propane burner
- the ignition sparker
- the coil, which runs outside the burner and inside the cage.
The entry point for the coil is (E), and that's where you hook up the silicone tubing coming from the pump. Be sure to use a check valve between here and the pump.
Note: you should NOT disassemble these things and re-bend the tubing to make it fit your setup, that's STUPID and only an IDIOT would attempt this. Note also that if someone was so foolish as to attempt this, they might find that the coil and the burner could get misaligned, which will result in bad performance. Spend some time understanding how all of the parts fit together before leaving them alone and wisely walking away from this project entirely. The alignment is finicky and the sparker can easily get jostled. Probably best not to even think about doing anything like this.
Making it work
I've found that you can run this really hot and the coil will start glowing. That's too hot for mineral oil. Turn down the flow of gas. I've also found that one of the most dangerous things is configuring it in such a way that hot mineral oil comes out. Mineral oil boils at over 500º, which means even oil that's not hot enough to vaporize can make very serious burns and set other stuff on fire. Also, the oil will catch fire itself at just a little over its boiling point. Keeping the temperature within limits requires both manually adjusting the flow of gas and getting the timer dialed in just right. This is why an adjustable timer is so useful. If you see ANY liquid oil coming out, whether spraying out or dripping, you have a serious problem and you need to figure it out because you have a dangerous condition. Even better, DON'T EVER BUILD THIS KIND OF THING.