MKS MFC Problem

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Chris Seyfert
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Re: MKS MFC Problem

Post by Chris Seyfert »

Yeah, there's a large leak somewhere. Did you put on a new VCR crush washer when you installed the adapter? They are nominally one-time use, though one can get a few uses if you don't go crazy with the torque. A new crush washer is only supposed to be tightened 1/8 - 1/4 turn past finger tight.
Matt_Gibson
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Re: MKS MFC Problem

Post by Matt_Gibson »

Chris, I did use a crush washer. Not sure where this could be leaking from. All in all, operation doesn’t seem affected.

-Matt
Matt_Gibson
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Re: MKS MFC Problem

Post by Matt_Gibson »

I spoke to MKS about this issue. It sounds as if the MFC is working fine, just can’t use it to pump down the gas lines and regulator due to the tiny orifice. They recommended using a bypass valve to pump down the regulator and gas lines.

-Matt
Chris Seyfert
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Re: MKS MFC Problem

Post by Chris Seyfert »

While a bypass would certainly speed the process, you let your MFC sit for 30 minutes at 5 sccm = 600 cm3 of atmospheric pressure air. It's either the NPT fitting or the hose barbs, and probably the hose barbs.
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Liam David
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Re: MKS MFC Problem

Post by Liam David »

For 1/4" tubing you're at best in transitional flow at a few 10s of mtorr and certainly in molecular flow for anything below. Since the MFC has very small channels and orifices, the transition pressure is some 1-2 orders of magnitude higher and so molecular flow begins in the torr range. The conductance is also reduced by the multiple bends and non-simple geometry in the MFC. You must have a bypass valve if you want low chamber base pressures and high gas purity.
Matt_Gibson
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Re: MKS MFC Problem

Post by Matt_Gibson »

I’ll work on some sort of bypass down the road. For now, I’ll stick with filling my syringe from the regulator and then transferring to the MFC inlet. Seems to be the easiest :-)

-Matt
Chris Seyfert
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Re: MKS MFC Problem

Post by Chris Seyfert »

Liam, what you posted is correct, but doesn't apply to the current problem. Matt let his MFC, with nominally blanked-off inlet, run for 30 minutes at 5 sccm. That's 600 cubic centimeters (at STP) of air that was sucked into his vacuum pump. His system definitely has a leak upstream of the MFC, somewhere, at least at vacuum pressures. Again, it's a simple process of elimination - it's either the hose barbs, the NPT fitting, or the shutoff valve he used during his test.

I completely agree that if one is driving for high purity, a bypass valve would be essential to help pump out the gas system, but it is probably not required for most amateur operation. Pumping down the gas system to 1 Torr has already removed 99.9% of the air . . .

Matt, if you want to stay with plastic tubing, I'd recommend looking for 1/4" Swagelok adapters (with brass or plastic ferrules) and using semi-flexible 1/4" OD polyethylene tubing. It seals much better than hose barbs and is still easy to work with. Ebay has plenty of Swagelok fittings for pennies on the dollar.
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