Vacuum measurement

A project log for Everyman's turbomolecular pump

(Maybe) access to the high vacuum environment for the rest of us

keegan-reillyKeegan Reilly 03/22/2016 at 22:047 Comments

***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.



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.


DeepSOIC wrote 04/11/2016 at 14:35 point

I'm sorry to come up with potential problems again...

I think measuring tiny angle changes is waay harder than watching for reflection off a chamber wall. As the vacuum is created, the wall is stressed, will bend, and that can be detected with light. Or with a tensoresistor.

Furthermore, that very effect may dwarf all your changes due to changing index of refraction. But... it may not, I'm not sure.

Next, this setup may happen to be sensitive to atmospheric pressure variations, which are many orders of magnitude larger than the vacuum turbopumps are supposed to achieve. There can be two sources, again: refraction when going through the wall, and walls bending again.

I highly recommend you don't make too many innovations in a single project. I've never seen optical vacuum gauges yet, which makes me think they are not easy to make, compared to membrane, pirani/thermocouple, and ionization gauges.

Last but not least. How do you expect calibrating against total vacuum? Most types (excluding ionization gauges) require finding their readout for zero pressure, otherwise you can't tell what ultimate pressure does your pump achieve. You can only observe changes...

And thanks to Ben Krasnow, we all know a cheap way of making electrical pass-throughs. Spark plugs!

  Are you sure? yes | no

Keegan Reilly wrote 05/01/2016 at 10:56 point

"I highly recommend you don't make too many innovations in a single project"

Very wise words, thank you!  I don't mind you coming up with problems, that's actually why I bother posting anything at all on here, so other people can keep me from wasting time on dead ends!  

I've decided to shelve this idea for now, I agree I think the effect I was going for would be too hard to isolate to measure accurately.  I've decided to go with building a Crooke's radiometer instead.  Those are simple to make, and well understood.  Not inventing anything new.  They just happen to work best right around the vacuum level I'm shooting for for the rough vacuum chamber.  It shares the same advantage that no pass through's are required (though as you pointed out spark plugs are a great option for that! I'm just trying to make the chamber as simple as possible right now.  I'm not good at building these things, so I want as few possible failure points as I can get.  

The radiometer should work for a simple prototype.  Obviously anyone actually using this setup in the future would want a real gauge.  This is just a short term hack, to find out whether discs can actually pump down a vacuum at all or not.  The nice thing is, the thermal effects for the Crooke's disappear at high vacuum, so I can use it to tell if the high vacuum setup is working too! 

  Are you sure? yes | no

M. Bindhammer wrote 04/10/2016 at 16:41 point

The use of the LaTeX equation above is mathematically not correct. Correct would be:

\arcsin\left ( \frac{M\cdot \lambda}{d}  \right )=\Theta

  Are you sure? yes | no

Keegan Reilly wrote 05/01/2016 at 11:01 point

Thank you! Fixed.

  Are you sure? yes | no

Eric Hertz wrote 03/23/2016 at 01:20 point

I feel like I read something recently about some sort of optical system for measuring a tiny change in angle... dagnabbit what was that...? "optical lever" or something?

  Are you sure? yes | no

Keegan Reilly wrote 03/26/2016 at 05:02 point

Hmm, I google'd that, looked like a method of measuring physical displacement.  At least that was the first result, I'll do some more digging though.  Thanks!

  Are you sure? yes | no

Eric Hertz wrote 03/26/2016 at 06:21 point

haha, I hope you got the "feel like" "sort of" and whatnots as a sign that I know absolutely nothing about whether this thing actually exists. Now that you mention it, I think you might be right, it might've been about measuring physical displacement in much the same way you're measuring already, by shining a light across a room.

  Are you sure? yes | no