Pressure cookers make great vacuum chambers because they're readily available, not too expensive, and are strong enough to hold a full vacuum. Once you've found all of the parts, assembly is straightforward and can be a fun afternoon project.
For this build, I wanted something robust enough to withstand many uses, and reliable enough to work every time. It also needed to be as big as possible, because why not? If your needs are more modest you can use a smaller pressure cooker (they come in many sizes) and maybe use a thinner sheet of acrylic (or switch to a polycarbonate). Although, I'd caution you to do a stress analysis before using anything thinner. Acrylic fails catastrophically, and it's hard to try again while picking plastic out of your eyes. Better safe than sorry. And remember, always wear proper eye protection!
Step 1: Assemble Your Materials
You'll need to find, make, or buy the following items
- 22-qt pressure cooker
- 16"x16"x1" sheet of acrylic
- Vacuum pump
- Several feet of 1/4" copper tubing (the hardware store variety is fine)
- Copper tube cutter
- 3/4" drill bit
- MPT vacuum gauge
- FPT bulkhead fitting
- FPT cross fitting
- NPT T-fitting
- 3x FPT ball valves
- 3x NPT to compression adapters
- 3x MPT couplers
- Compression to Flare adapter
- 3/4" ID rubber gaskets (at least 1/8" thick)
- Teflon tape
I used a Miro 22-quart aluminum pressure cooker, and a Robinair 15300 vacuum pump. Both were available from Amazon and reasonably priced, but there are plenty of other sources for each. That particular pump comes with 3/8" and 1/4" Flare fittings on the inlet port, which is why I needed a compression-to-flare adapter. If your pump has different fittings, you'll need to use the appropriate adapter.
For the 3/4" drill bit, I used a low-end black oxide bit from the local hardware store. This isn't the best choice for this application, but it's all they had, and I only needed it to work once.
All of the fittings are standard 1/4" NPT brass fittings. I found them at a local supply store, but they're generally available online for about the same prices. I used a cross-fitting as the central fitting in this setup so I would have an extra port for future use. You can just as easily replace the cross with another T-fitting, and use one less ball valve, to save a little money. Fewer fittings means less leakage, and better vacuums, so feel free to remove anything you don't need.
Step 2: Assemble the Fittings
Let's get the easy part out of the way first. For this step, you'll need your bag of fittings, the valves, the pressure gauge, a roll of teflon tape, and a crescent wrench.
Assembling brass fittings is pretty easy, but you do have to be careful. Brass is a very soft metal and you can easily strip the threads if you over tighten the fittings. But, at the same time, if you don't tighten them enough, the whole thing will leak. The teflon tape helps seal up the threads with less tightening, but it's not magic. Don't worry about this too much right now. Just get everything assembled in the right order and we can worry about any leaks later.
The fittings go together in the arrangement shown in the picture above. Just assemble the fittings one at a time, in whatever order works for you (but start with the pressure gauge, see below). Ignore the bullhead fitting, and its associated compression fitting, for now. We'll worry about those in a later step.
Make sure you wrap each male thread in at least one layer of teflon before installing. If this is your first time using teflon tape, it can be tricky, and it feels like you need 3 or 4 hands to do it, but you'll get the hang of it. I've found that it works best if I wrap the tape opposite the direction that the fitting will turn while you're tightening it. This helps keep the tape from bunching up inside the female side of the connection. So, if the fittings are "righty-tighty" (aren't they always?), then wrap the tape the other way.
Once you're finished you should have something like the last picture above. I ended up with the pressure gauge on the wrong side because the T-fitting finished tightening upside down, so I just flipped the cross over and swapped the adapter fittings. You should probably start the assembly with the T-fitting and pressure gauge so that you don't get stuck swapping fittings around just to get the gauge right side up.
Step 3: Prepare the Pressure Cooker
Now for the fun part. In this step, you'll be drilling a hole in your nice shiny new pressure cooker. I know, it's tough, but it's for a good cause.
You're going to need:
- A metalworking drill press
- The pressure cooker
- A 3/4" drill bit
- Several smaller drill bits
- Various clamps and bits of wood
- A deburring tool
Because all of my other fittings are 1/4" NPT, I chose to use a 1/4" NPT bulkhead fitting for the vacuum port into the pressure cooker. Of course, the fitting itself is much larger than 1/4", so we'll need to drill a bigger hole, namely a 3/4" hole. To do this you'll need a drill press big enough to handle your pressure cooker. You might be able to use a hand drill, but with a 3/4" bit, I wouldn't recommend it.
Start by removing the lid from your pressure cooker (mine came with the lid attached), and notice that there's a rubber gasket inside the lid. Don't damage the gasket; we'll be using it later. We won't be using the lid, but it's protecting the gasket, so for now just set them aside so we can come back to them later.
Next, set up your drill press in whatever manner is appropriate for your equipment, shop rules, etc. If you're at a shared workspace, make sure you don't use the drill press that's set aside for woodworking. You'll get in trouble, and you'll probably ruin it.
We're going to be making several cuts because you can't just go straight for a 3/4" inch hole. Not easily anyway. Pick out several smaller bits that you can use to work up to the final size. I started with a 1/4" bit, and a few others that I found in a drawer of old bits. But don't use overly dull bits, or you'll have a bad day.
With that in mind, lower the table until the pressure cooker fits beneath your first drill bit, and then find some wood and clamps to clamp everything to the table. Get everything lined up so that your final hole will be an inch or two above the bottom of the pressure cooker. If yours has a rounded bottom, like mine, then move the hole an inch or two above the top of the rounded part. Once you get everything lined up, tighten the clamps and lock the table in place.
At this point you should have everything lined up, clamped down, and locked in place. Load up your smallest drill bit and set the drill press to a speed that's appropriate for whatever bit size you're starting with. Now, drill your first hole. If you're using a huge pressure cooker, it will probably vibrate and chatter a lot. Just go slow, and back off if things get too crazy.
Once your first hole is finished, admire your handiwork for a minute, then move up to a larger bit. Keep going until you have a 3/4" hole. Don't forget to lower the drill speed as the bits get bigger.
When you're finished, find a deburring tool and clean up the edges. That last bit probably made a mess.
Congrats! That was the hardest part. It's all Easy St. after this.
Step 4: Install the Bulkhead Fitting
This step is very easy, but it's also your biggest potential leak. Don't worry about that too much now; leak hunting is great sport in the world of vacuum chambers, and there's plenty of time for it later.
- Find your bulkhead fitting and two of the rubber gaskets
- Remove the nut from the fitting
- Put one of the gaskets on the fitting (it should be a tight fit)
- Install the fitting+gasket in the hole in the pressure cooker that you drilled in the previous step
- Reach inside the pressure cooker and put the other gasket on the end of the fitting
- Install the nut on the fitting and hand tighten
Proper tightening of the nut is somewhat critical when it comes to leaks. Too loose and it will leak. Too tight, and it will leak. You want it to be tight enough that the brass makes full contact with the rubber, and the rubber makes full contact with the aluminum wall, but not so tight that the rubber appears to be buckling or deforming.
Once the vacuum pump begins removing air, the outside air pressure will push the fitting against the gasket and make a seal. But, if the gasket is warped, it won't seal properly and you'll have a small leak. If you have a super strong pump, or aren't trying for a very hard vacuum, you may never notice smaller leaks. But if you have problems achieving the desired vacuum, this is the first place to look.
Step 5: Copper Tubing and Compression Fittings
It's finally time to put the pieces together.
Cut two lengths of copper tubing. One length connects the fittings assembly to the pressure cooker. The other, longer, length connects the fittings to the vacuum pump. I made the shorter length about 12" long, and the longer one can be a couple of feet. The exact lengths aren't too important. Just make sure you have enough room to get a wrench between the various parts; you'll need the space when it's time to hunt for leaks.
- If you didn't already, attach the NPT-to-compression adapter to the bulkhead fitting that you just added to the pressure cooker. Be sure to use the teflon tape.
- Connect another NPT-to-compression adapter to one of the ball valves that's attached to the fittings assembly. You probably did that earlier, but if you didn't, then you should do it now. Don't forget the teflon tape.
- Similarly, connect the compression-to-flare adapter to the inlet port of the vacuum pump. Don't use any teflon with Flare fittings.
In general, it's a lot easier to install the NPT and Flare fittings first, and then do the compression fittings last. Both NPT and Flare fittings need to be rotated in order to tighten them, which you can't do while they're attached to the copper tube. The compression fittings have a nut for tightening, so you can save those for last.
If you're not used to compression fittings, they can be a bit daunting at first, but they're actually very easy. If you unscrew the nut from one of the fittings you'll see a ferule inside. Don't lose it. Take the nut and slide it onto the copper tube and push it out of the way. The "open" end of the nut should point towards the end of the tube. Next, take the ferule and slide it onto the end of the copper tube, but don't push it any further. It should be right at the end and it should fit snugly, but not tight. Then, insert the tube into the fitting and gently push the tube in as far as it will go. The ferule should seat against the body of the fitting and the tube should slide through the ferule a little ways. Finally, slide the nut over the ferule and screw it into position. You can hand tighten it for now, and then tighten it further when the joint leaks.
Now, using the assembled fittings, and the two lengths of copper tube, connect the pressure cooker and the vacuum pump to the fittings assembly. Use the short tube to connect the pressure cooker to the port on the cross that has the compression adapter. Connect the longer tube from the vacuum pump to the ball valve that you attached the compression adapter to.
The final result should look similar to the first picture above.
Hang in there, we're almost finished!
Step 6: Final Assembly
Now is the time for that pressure cooker lid that you set aside earlier.
The gasket that's in the lid of the Miro pressure cooker is very easy to remove. Just give it a gentle tug and work your way around the lid. It should come right out. Check that the gasket is clean, dry, and undamaged.
Next, place the gasket on top of the pressure cooker. Try to get it evenly spaced around the edge. If it's off center it could pop out of place as the differential pressure builds up.
Once you have the gasket where you'd like it, take your sheet of acrylic and place it gently on top of the gasket. Try to keep it centered, otherwise it will get stressed funny. In the best case you'll just get a bad seal, in the worst case it could wear out early, and fail.
Congratulations! You've built a Pressure Cooker Vacuum Chamber!
In the next step we get to turn it on and see what happens.
Step 7: Vacuum Time
Well, we're finally here. It's time to turn on the vacuum pump, and run for cover. Maybe.
Before we actually flip the switch, a few words of caution from our sponsor.
First, be sure to read the instructions that came with your pump. Most, if not all, high vacuum pumps are very finicky and have a special "first use" startup procedure. Follow the directions and get that out of the way before going any further. If you need to, disconnect the pump from the copper tube before running the startup procedure. Once it's ready to go, reconnect it to the copper tube.
Second, high vacuum is serious business. If anything goes wrong there's the potential for a very bad day. With that said, vacuum failures are usually just a leaky joint, and not the least bit exciting. However, we are using a sizable chunk of acrylic here, and acrylic tends to turn into high speed shrapnel when it fails. But, a 1"-thick sheet is nearly bullet proof, so you'd have to do something amazing to get this this thing to break. In all likelihood there's nothing to worry about, but I have to mention it anyway, just to be safe.
Did you put on your safety glasses? Good. Just checking.
If you're ready to go, give everything one last check (did you check the valve positions?), and then flip the switch. The pressure gauge needle should begin moving immediately and the acrylic sheet should visibly compress the gasket. That part is a little surprising the first time, but you'll get used to it.
Within a few seconds the gauge should be showing a serious vacuum. Within a minute or two the action should start slowing down. Eventually the needle will settle on the final vacuum and you should hear the sound of the pump change. If everything went well, you now have a functioning vacuum chamber!
At this point, go ahead and turn off the pump. The pressure should immediately begin rising. The faster it rises, the bigger your leaks. Now you get to begin the never ending task of leak hunting. While the pump is running you can apply drops of the pump oil to the various joints to try to find the leak. If you have an air hose, you can also try pressurizing the chamber and spraying soapy water on the joints (look for bubbles). Tighten any joint that's leaking and try again. Good luck!