Alright, when we left off 4 months ago, I had been thinking we'd have to use multiple stages to reach our goal vacuum. However, the seals between stages were going to introduce a LOT of complications. I think. I didn't build anything, so far this is all just intuition. The other development is that I learned, thanks to [DeepSOIC], that discs tend to warp badly at high speed. He graciously proved experimentally that this warping is not an issue in rough vacuum, and the discs will spin smoothly and easily. See discussion on project page if you're interested. This does mean that a rough backing pump will be required just to get our disc pump up to speed safely. It's not the end of the world, decent backing pumps are like $100 at Harbor Freight, and can be found for free in old air conditioners.

4 months ago I also mentioned I thought I had a way to get back to a simple single stage pump design. Here it is.

The multiple stages was originally predicated on assumptions about the compression ratio of each stage, given a specific outer edge speed of the rotating discs. The max speed we're realistically (safely?) going to get is about 300 m/s with aluminum. At best. That gives a 1.6 compression ratio, 14 stages to a rough vacuum. However, as [Comedicles] astutely noted, the notion of pressure in the typical sense is basically meaningless when describing the flow of gas molecules at high-mid vacuum. There is no boundary layer, and the pump's operation can be better understood kinetically. As long as the mean free path of the molecules is larger than the gap width between the discs, they will not interact with each other in any "pressure gradient" producing way. They will simply (mostly) bounce between the two discs, picking up energy and gradually moving outwards.

Here's the key. If our roughing pump can bring the pressure low enough, and our discs are spaced closely enough, the pump will work. How low of a vacuum it will produce is more a function of how quickly it can move molecules to the outer edges, and what kind of chamber outgassing we're dealing with. If anyone knows how to do that kinda math I'm all ears. It'd be pretty cool to run a simulation of the particle kinetics.

Here's the new design, with a few numbers thrown in to make it sound more real.

1. Get a nice donut shaped cookie cutter from the kitchen, a cutting board, and a roll of aluminum foil. And some kind of thin star shaped cookie cutter too. Not sure if those exist yet.

2. Stack a bajillion alternating donuts and stars until we get about an inch thick total. The donuts are the discs of course. The stars act as hubs, spokes, and spacers between the discs.

3. Mount this mess to a small aluminum rod I got from Lowe's, stuff it inside an empty pickle jar, add a motor, and we have a pump! Hey, this is HACKaDay, right? haha. In all seriousness...

Roughing pump pulls chamber down to 100 Pa. The Harbor Freight pumps are rated down to 10 Pa, but let's assume that with the disc pump chamber and tubing and everything attached we only get 100 Pa. That put's the mean free path of air at .07 mm. Width of standard household aluminum foil is .016 mm. Perfect. We can use aluminum foil as both the discs and spacers.

That covers the axle and discs. Next:

Bearings: The bottom we can simply file the aluminum axle to a point and set it on a concave piece of glass as a makeshift needle bearing. I did actually try this, it works. Didn't try it at 60,000 RPM yet though... At the top, we can use a hollow cylindrical magnet. It might work like an electrodynamic bearing, I hope.

Motor: This will be a rudimentary, horribly inefficient, brushless DC motor. I think that's the easiest way of spinning this thing without creating the need for extra seals or electrical pass throughs and craziness (i.e., sources for leaks). The bottom disc will not be foil, but actually 3D printed PLA or maybe machined aluminum disc with 3 spots to place strong permanent magnets. Directly below, but OUTSIDE the chamber walls are our 3 coils. This way, the simplest part of the motor (the magnets) are the only thing inside the vacuum chamber, and we don't need any electrical or rotating mechanical pass throughs. It will not be very efficient at all, because of the distance between the magnets and coils, but who cares as long as it can still get enough power through the chamber wall to spin up ok.

Motor controller: I had toyed with the idea of using my FPGA, it seemed my mBed microcontroller would be too slow to handle the timing for extremely high rotation speeds. However regular BLDC quadcopter motors are so darn cheap that it's worth trying one of those just to see if it can be made to work. Thanks to [Ben Krasnow] for pointing this out in one of his recent videos.

Chamber: Oh boy. This part needs some work. You guys that know vacuum stuff are going to get a kick out of this. I'm glad most people have stopped reading by now, this post is so darn long. Here's what I'm thinking so far. Regular food jars stay vacuum sealed for months or years. Yes, they're not at high vacuum, but if the seal holds for that long with even a rough vacuum they're good enough that I really want to try it out. It might be a waste of time, but I'm curious. So, start with one big pickle jar, right side up, sitting on top of the 3 motor coils. In the lid, we drill two holes. One large one for the high vacuum chamber tube, and a small one for the roughing pump to plug into. Then solder this lid to a nice thick sheet of aluminum stock, lined up on a hole through the sheet for the high vacuum again. Above the aluminum is another jar lid, soldered on upside down with a hole lined up on the aluminum again. Screw a jar onto that second lid upside down for the high vacuum chamber. Ridiculous I know. If I can figure out a better way to seal the glass jars to aluminum I could skip the lids altogether. The coating inside those lids is probably no good for high vacuum stuff.

Anyways, those are the latest thoughts. Sorry this was a long post, but it'd been like 4 months so had a lot to catch up on. Next time I'll talk about how to measure the vacuum. ttfn!