Sciencemadness Discussion Board

Vacuum pump for Cathode Ray Tube (CRT)

blogfast25 - 10-3-2022 at 07:57

Hi All,

Long time, no see!

I'm working on an electron diffraction experiment which involves hacking into a CRT and slightly modifying it. It's a Soviet era CRT, probably once part of a piece of scientific equipment. (but I see SM now no longer allows uploading images? Shugs!)

I've got almost everything together, so the hack can soon commence!

But the one thing I still need to procure is a source of high vacuum because after the hack, high vacuum needs to be restored.

Acc. Wiki the vacuum for a TV CRT is between 0.01 Pa to about 75000 times lower.

I suspect that the required vacuum depends somewhat on the length of the electron flight path, which for my device will only be about half of that of a TV CRT.

This dinky toy from eBay would probably be OK for vacuum distillation but its maximum vacuum of 5 Pa is high compared to the Wiki prescribed one.

Frankly, I'm having trouble sourcing anything that could deliver about, say 1 - 0.1 Pa...

[Edited on 10-3-2022 by blogfast25]

Antigua - 10-3-2022 at 08:20

These HVAC vacuum pumps from AliExpress/eBay/whatever else are only bragging about these final vacuums and these values are nowhere near realistic. If you're looking for a high vacuum pump, you're in for a quite expensive party. You'd need to buy a two stage pump (like an Edwards or Welsh), use an oil with very low partial pressure (Inland 19/Ultragrade 19/anything else meant for high vacuums) and that's pretty much it. Oh, and also run it for quite a long time depending on 1. the diameter of your vacuum connection and 2. the size of the chamber to evacuate.
You can find high quality Edwards' on eBay from time to time. Varian would be okay too, Leybold sometimes. They at least report realistic final vacuum values like 1x10^-3 mbar.
Good luck.

violet sin - 10-3-2022 at 08:21

Hey! You are back! Nice! I don't have any advice on the pump, but I will respond to say hello once again :)

Sulaiman - 10-3-2022 at 09:34

The simplest would be to find a small business somewhere that re-conditions CRTs and get them to do it for a fee.
There used to be many such businesses, but I've not come across one for a long time.
Worth a check ?

Rainwater - 10-3-2022 at 09:36

Old tried and true. My grandfather had one that could do 4 tubes at once. It will take a long time to reach the level of vacuum you're wanting but except for the mercury, it's cheap to build and operate.

https://en.m.wikipedia.org/wiki/Sprengel_pump

Images still work. Click preview post and add the file there

blogfast25 - 10-3-2022 at 12:20

Quote: Originally posted by Antigua  
These HVAC vacuum pumps from AliExpress/eBay/whatever else are only bragging about these final vacuums and these values are nowhere near realistic. If you're looking for a high vacuum pump, you're in for a quite expensive party. You'd need to buy a two stage pump (like an Edwards or Welsh), use an oil with very low partial pressure (Inland 19/Ultragrade 19/anything else meant for high vacuums) and that's pretty much it. Oh, and also run it for quite a long time depending on 1. the diameter of your vacuum connection and 2. the size of the chamber to evacuate.
You can find high quality Edwards' on eBay from time to time. Varian would be okay too, Leybold sometimes. They at least report realistic final vacuum values like 1x10^-3 mbar.
Good luck.


Yes, I'm sure you're right.

The level of the descriptions of technical stuff in eBay is often appaling.
Quote: Originally posted by violet sin  
Hey! You are back! Nice! I don't have any advice on the pump, but I will respond to say hello once again :)


Thanks for the welcome!
Quote: Originally posted by Sulaiman  
The simplest would be to find a small business somewhere that re-conditions CRTs and get them to do it for a fee.
There used to be many such businesses, but I've not come across one for a long time.
Worth a check ?



GREAT idea! But with classic(TM) CRTs now out of fashion it may be hard to find the Guy with the Pump. But will definitely try!
Quote: Originally posted by Rainwater  
Old tried and true. My grandfather had one that could do 4 tubes at once. It will take a long time to reach the level of vacuum you're wanting but except for the mercury, it's cheap to build and operate.

https://en.m.wikipedia.org/wiki/Sprengel_pump

Images still work. Click preview post and add the file there


Great type of pump. Combined with a first stage from a basic vac pump this may be my best option yet.

Did you mean images through links? I used to upload loads of images from my laptop.




[Edited on 10-3-2022 by blogfast25]

Rainwater - 10-3-2022 at 17:32



Screenshot_20220310-203144_Samsung Internet.jpg - 416kB

blogfast25 - 11-3-2022 at 04:58

Thanks Rainwater! Old age and poverty have affected my memory...:(

Here's the CRT (oscilloscope type) in question:

CRT03.png - 437kB

[Edited on 11-3-2022 by blogfast25]

[Edited on 11-3-2022 by blogfast25]

Rainwater - 11-3-2022 at 05:04

There is usually an inlet port on the bottom... a sealed glass tube... You have to drill it out, then attach another glass tube before it can be sucked and sealed. Yours may he different. They're usually a borosilicate type glass. But a jewelers torch will make quick work of it.
When drilling it out it is helpful to sand it flat first

unionised - 11-3-2022 at 09:26

Also, as soon as you open it there's a good chance that the getter will oxidise leaving a hygroscopic surface.
For best results, you will need to work out how to bake the tube, and re-fire the getter.

The good news is that, if you can work out how to do that, you can massively reduce your pumping problem.
Fill the tube with something like CO2 or O2 by repeatedly evacuating and filling it to flush out any other gases like nitrogen and argon.
Then pump it out as well as you can with a "not really good enough" pump and seal it, then fire the getter to remove the residual gas.
That should drop the pressure by a few orders of magnitude.

If you are planning on opening up the tube to put a diffraction "grating" in it the you might be able to find a suitable getter to put in while you are at it.
It needn't be anything complicated- magnesium will do the job.


neptunium - 11-3-2022 at 10:27

I`ve used these for a very good roughing pump
https://www.ebay.com/itm/313908597485?hash=item49166912ed:g:...
https://www.ebay.com/itm/115287900192?epid=659945437&has...
but to get below 10e-3 Torr (or 0.1 Pa) you`ll need a diffusion pump or a turbo pump
Diffusion are cheaper but such a pain to use, trust me a good turbo pump is the way to go. Yes, you`ll have to pony up the money and they need a special controller, but from my experience ,you`ll save big time in the long run..
https://www.ebay.com/itm/224458191887?hash=item3442c0340f:g:...
good luck!

blogfast25 - 11-3-2022 at 12:41

@unionised:

I'm pretty sure mine is barium. Not sure how Al would work: it passivates very quickly and is hardly volatile.

blogfast25 - 11-3-2022 at 12:44

@neptunium:

The option of first stage pumping, combined with a second stage Sprengler pump (see comment by Rainwater) actually appeals the most to me right now.

Rainwater - 11-3-2022 at 16:17

Great.
The general working principle is that each drop of mercury captures and forced a drop of gas down the pipe and out of the apparatus.

Dont know where you can source mercury but you will also need a positive displacement pump,
This is the basic design
Screenshot_20220311-184957_Samsung Notes.jpg - 76kB
Here is a good video
https://youtu.be/viJ3T-1KZqY

Take special note of your construction materials as even glass outgasses at these pressures, it will eventually stop, but if it sucks the gas out of glass any tipe of plastic will prevent you from reaching the vacuum you need

If it was me and i had a few days, i would use a 24/40 vacuum adapter. Like this
amazon link
Screenshot_20220311-190220_Samsung Internet.jpg - 90kB
I would draw that center tube out thin and long.
Then amazon link
Screenshot_20220311-190721_Samsung Internet.jpg - 249kB
Draw out the middle section thin and long.
Place the two together with grease and your a professional glass blower.

But the video shows an easier and cheaper way to do it.
Edit: removed personal information

[Edited on 12-3-2022 by Rainwater]

blogfast25 - 12-3-2022 at 07:03

Quote: Originally posted by Rainwater  
Great.
The general working principle is that each drop of mercury captures and forced a drop of gas down the pipe and out of the apparatus.

Dont know where you can source mercury but you will also need a positive displacement pump,
This is the basic design

Here is a good video
https://youtu.be/viJ3T-1KZqY

Take special note of your construction materials as even glass outgasses at these pressures, it will eventually stop, but if it sucks the gas out of glass any tipe of plastic will prevent you from reaching the vacuum you need

If it was me and i had a few days, i would use a 24/40 vacuum adapter. Like this
amazon link

I would draw that center tube out thin and long.
Then amazon link

Draw out the middle section thin and long.
Place the two together with grease and your a professional glass blower.

But the video shows an easier and cheaper way to do it.
Edit: removed personal information

[Edited on 12-3-2022 by Rainwater]


Great video, Rainwater, thanks for that!

[Edited on 12-3-2022 by blogfast25]