Calibrated leak / needle valve wanted.

[Reed Upson]

Hello! I am looking for a calibrated leak or a needle valve for feeding hydrogen gas into a vacuum chamber. I do not need the calibrated leak to be up to calibration, I just need a way to very finely regulate the H2 flow rate. I can handle a leak rate up to 1.5e-3 ATM-CC/s.

I am working on a very constrained budget, so cheaper is better (so long as it works, of course).

Any and all advice is very welcome!

Thanks!
-Reed

[Rich Feldman]

How did you figure that flow rate? I though typical fusors have deuterium flow on the order of 2-10 sccm. Got no personal experience.

Oh, I see you have been talking about making a cyclotron. How much hydrogen do cyclotrons consume, for example the one reported here by Chris Mullins and his son? I wonder how much hydrogen is consumed by Spallation Neutron Source at ORNL, which recently reported beam power of 1.5 megawatts at about 1 GeV. For some engineering reason the accelerator and storage ring use negative hydrogen ions (H-).

For ordinary leak dimensions, choked flow model gives gas flow that varies with square root of molecular weight. Heavier gases have lower volume flow and higher mass flow by that factor.

[Jerry Biehler]

You can get mass flow controllers off ebay used pretty cheap. The low flow ones are a bit more rare but they are out there. They are easy to drive, usually just +/-15v and a pot and switch.

[Chris Mullins]

Yes, I started with a needle valve for our cyclotron, but switched to a used MFC, as described here: viewtopic.php?p=81538#p81538. A needle valve can certainly work, and can be combined with other flow reduction techniques described in other threads (e.g. viewtopic.php?p=20010).

Since my earlier post I've switched to a 1 SCCM flow controller. I operate the cyclotron between ~0.05 and 0.3 SCCM, so even the 10 SCCM MFC was too much for good resolution (but was still much better than the leak valve I had). 10 SCCM MFCs pop up every few weeks on ebay. It took several months for a 1 SCCM controller to appear at a reasonable cost ($70 in 2021). Of course, as described in the earlier thread, getting the fittings and accessories to connect it can be expensive or require more scrounging.

Here's my latest setup:

It's certainly possible to get a cyclotron working with a shoestring flow control system. Just getting it to work at all, demonstrated by showing a peak in the beam current measurement at the cyclotron resonance, doesn't require a precise flow rate. A MFC brings much better repeatability and control, which was important for me as I made other improvements. You want a very low flow though. If you're looking at needle valves check the datasheet to make sure the flow is suitable. The entire usable cyclotron range for my 10 turn valve was the first 1/4 turn.

My complete gas delivery setup includes a lecture bottle of hydrogen and a 2 stage regulation system that feeds into the MFC. A low-budget approach might be creating your own H2 from a PEM cell (described here: viewtopic.php?t=13995) with a home-made flow restriction system.

For reference, the vacuum in my cyclotron with no flow is around 7E-6 to 9E-6 torr, and with hydrogen producing a beam, in the E-5 torr range.

You'll find a lot of good info in the FAQs and older discussions. I spent a long time reading the FAQs and digging through old threads going back many years - well worth the time.

PS - just remembered this is the trading post forum, probably not the best place for such a long response ...

[Richard Hull]

Chris you are right, it will go away eventually. Good stuff like your post might have been a new thread in the vacuum forums. That is a fine setup with the MFC. A great kludge with a great scrounge. MFCs are professional devices and demand professional fittings. VCR fittings are great but terribly expensive, as you note.

Richard Hull

[Jerry Biehler]

MFC also come with swagelok fittings. And most MFCs I have ran across use the same fitting connection, VCR, VCO, or Swagelok all connect to the MFC body the same way, with a 9/16" threaded fitting. If you have one with VCR and another with swagelok you can usually swap the fittings on the MFC.