***UPDATED 1 May 2016***

I've decided to shelve this idea, some wise people pointed out a lot of problems that would be difficult (impossible?) to get around. Instead I'll be using a Crooke's radiometer to get a rough idea of how the vacuum is doing. See later project logs. Leaving the original post here for posterity.

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ORIGINAL POST:

First of all, I wanted to say thanks to everyone who followed/liked my project, and especially thanks for the comments and ideas you've shared with me. A lot of this is way out of my normal area of expertise, so I'm learning a lot along the way.

This post is about finding a way to measure the vacuum, so that we have some way of tracking the pump's performance. There are lots of ways to do this, many of which could be done pretty cheaply. All require some sort of electrical pass through though, and to give myself the highest chance of building a working chamber I've been on the lookout for ways to eliminate those wherever I can. I had the idea of using a laser interferometer with one path shining through the vacuum. As the vacuum increased, the path length would change slightly, and might be able to be picked up. So I did some googling, and ran across an even simpler idea here (pdf). All of what follows comes from that paper, so credit is due to [Hasan Fakhruddin] for coming up with the idea. I'll rewrite it here in my own words so I can expound a little.

Here's the setup: shine a laser through the glass vacuum chamber into a diffraction grating placed inside. After the laser passes through the grating and the other side, look at where the 1st order beam lands on the wall. The beam will be deflected varying amounts depending on the index of refraction of the near vacuum inside. The difference in deflection angles is very slight, so we'll have to find a way of amplifying the signal. Not sure yet if this will be possible. The change in deflection angle may be less than the beam width of the laser. But it's worth a shot because it has some distinct advantages:

1. No pass throughs required at all. Just need a transparent chamber wall.

2. Minimal materials required. Laser pointer and an old CD-ROM is about it. And a wall. Might need some mirrors.

Here's some numbers to get an idea of how realistic this is. Here's the deflection equation, from the paper cited above:

I happen to have a 532nm green laser pointer on my bench, so I'll run some numbers with that. I put a small excel file in the project documents section if you want to try these calculations with your own materials at hand. If we use an old CD-ROM as a diffraction grating, that gives us 625 lines/mm, or

d = 1.6x10^-6.

Lambda is 532x10^-9 m for air, and 532.15428x10^-9 m for vacuum.

With M=3 (third order beam), theta (air) = 85.947 degrees, and theta (vacuum) = 86.190 degrees. So dTheta is 0.242 degrees. This is the total width of our measurement "signal" if you will. If we reached perfect vacuum, the beam would move .242 degrees on the screen. So, not much to work with, but we might be able to amplify it. I'm open to ideas. Putting the screen really far away, or bouncing it off a mirror that's really far away and back to a screen close to me is one option. Putting the screen at an extreme angle to the laser is another option, so that a small change in angle moves the dot a large amount across the screen (super simplified sketch below).

Of course this also means the beam dot will kind of "smear" across the screen, it might be pretty wide, and hard to make a specific reading. I guess all I can do is try it!

A quick note on selecting materials. In terms of wavelength or diffraction grating lines, more is not always better. The real goal is to get the deflection as close to 90 degrees as possible, without going over. That seems to maximize dTheta. The wavelength of the laser and the lines/mm of the grating have to be balanced to hit that. Also, if you can get the 1st order to be close to 90, vs. the 2nd or 3rd order, the usable dot will be brighter.