This information is shared in the spirit of harm reduction and does not endorse dangerous/risky behavior.

Using alcohol (IPA/Ethanol) in an ultrasonic bath is incredibly dangerous. It is possibly more dangerous for makers because most of us are used to being very smart gobos and able to bootstrap together a solution for anything we run into. This makes us uniquely susceptible to the Dunning -Kruger effect. We think we understand the risks, so we think we can figure out how to mitigate them. A perfect example is this video by Teaching Tech: https://www.youtube.com/watch?v=20fbQfsmNKc&t=472s . If you are not terrified of putting a flammable solvent into a device that will readily disperse huge quantities of microdroplets into the air, then you absolutely should not be doing this. A healthy level of distrust of the bubbling fire hazard is a requirement.

Consider this scenario that the video does not address:

• Bath heat + ultrasonic energy put an incredible amount of alcohol vapor in the air.
• Vapors are invisible and cannot be smelled because you are wearing your mask with organic vapor filters like a good goblin.
• Vapors are heavy and fall to hug the ground, so they begin to build up despite a vent fan high up on the table by the bath.
• For IPA the LEL (Lower Explosive Limit) is reached at a mere 2% after only a few minutes.
• You are now standing in a fuel air bomb the size of the room and vapors are beginning to penetrate wall sockets and the enclosures of your computer looking for an ignition source.

The risks of using alcohol in an ultrasonic bath includes sudden death by fireball. In addition to toxic exposure and plain old normal fires.

You should probably not do this, and if you do, you should be using a bath and fume hood system designed by an NFPA engineer. Alternatives exist such as using an initial bath in alcohol followed by a heated ultrasonic bath in water + a cleaner like simple green.

You're going to do it anyway? Despite the fireball warning?

Consulting with two NFPA conversant engineers brought up the below list on how to reduce (but not eliminate) risk.

• Add positive air purge to the electronics enclosure for your ultrasonic bath. This is a control method allowed by the NFPA. Just a small amount of air pumped in can keep vapors from finding the closest source of ignition - the bath itself.
• Purchase and use a VOC Monitor. Test both near the bath and down low where vapors can collect. You cannot tell if you have reached LEL by smell. You should be wearing an organic vapor filter anyway, which blocks the smell. VOC meters commonly give readouts in mg/m^3 which can be converted to ppm using this niosh calculator.
• Actively vent the room. If vent is sucking air out of the room, use only non sparking fans and place low to pick up heavy vapors.
• Set up a separate shed or workspace outside of your home, or better yet do this outside and away from the roof line or anything flammable.
• Use secondary containment for the alcohol. The 3D print goes in a sturdy plastic bag, bag is filled with alcohol and sealed, bag is placed in ultrasonic bath full of water. This is a great way to reduce risk; but the bag can leak from being poked by sharp supports that break off the 3D print or not being closed well, so a VOC monitor is still a must. It is also a good idea to keep the proportion of alcohol in the bag to water in the bath as low as possible. If there is mixing, the amount of alcohol vapor is reduced this way. DOT considers a mixture 25% and below to be non hazardous so this may be a good starting point, but even then a bath that is hot enough will produce sufficient vapor for ignition and possibly explosion. Siraya Tech has illustrations HERE. The NASA brief they discuss is less than a half page of text and provides no meaningful discussion of safety, so be aware that this call to authority is rather misleading.
• Keep and know how to use a fire extinguisher with a type B rating. Most home extinguishers are ABC and cover this, but have you actually used one before? If not, buy one and try using it to put out a fire pit or some other safe and contained fire. It is worth the $25 to feel confident if/when the time comes, and not muck up your one shot with the canister.
• Keep a cover on the bath while in use. Keep the cover nearby when not in use. Putting a cover on the bath is one of the fastest ways to smother a fire.
• Be aware that alcohol fires can be invisible. If you feel radiant heat coming off the bath, place the cover on the bath and turn it off.
• Minimize the use of personal electronics near the bath. One of those little neck fans with a brushed motor on it is a perfect ignition source.
• Print out and keep SDS information for all of the chemicals stored in your workshop. If the fire department has to respond, this information helps to prepare and protect them.

Treat alcohol use in an ultrasonic bath as dangerous and take precautions. The second you start feeling safe the risks will go up. Please dissuade others from sharing stories about how "I've always done it XYZ way and I'VE never been blown up." We should be protecting each other, not letting bravado needlessly expose new goblins to higher risk.