Pressure Cooker Vacuum Chamber

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.

Build Instructions

Collapse

1

Step 1

Step 1 - 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 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.
2
Step 2
Step 2: 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.
3
Step 3
Step 3: 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.