MKS MFC Problem

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Anze A Ursic
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MKS MFC Problem

Post by Anze A Ursic »

Hello,

I have an MKS gm50a (gm50a006500ram020, 5sccm) that is connected to my chamber. My vacuum chamber pumps down to 5E-5 within 20 minutes now, but when I open the MFC's valve even slightly, the pressure rises dramatically, even if it's blanked off on the other side, so it's not like it's evacuating the remaining air on the other side or I'm accidentally feeding it deuterium. I'm trying to figure out what to do since my pressure goes from that 5E-5 range to 7E-4 and refuses to pump down lower. Not sure what to do, I also have some other models, indlucing an MKS 1480A ALTA MFC. Is this a normal occurence or do I have to get another MFC?

Anze
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Liam David
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Re: MKS MFC Problem

Post by Liam David »

It could be contamination. The MFC orifice is very small and the flow path is somewhat complex, so any problematic substances on the inlet side take forever to pump out. Give it a good cleaning and you might consider adding a bypass valve to improve the pump-down of the gas line.
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Anze A Ursic
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Re: MKS MFC Problem

Post by Anze A Ursic »

I spoke to another person who uses the exact same device. Apparently he gets the same thing. May be device issue. It's not like we're using these things as intended. But it contaminates the gas line with air so now I have to find another way.
Matt_Gibson
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Re: MKS MFC Problem

Post by Matt_Gibson »

I wonder if it has to do with not having enough pressure on the inlet? Maybe these things don’t like to have a vacuum pulled through them?

Mine does this, but seems okay when I have deuterium on the other end. If I try to pull a vacuum through it with its valve open, pressure will rise (I usually stop trying once I hit 10 microns, up from 0ish). If I connect my deuterium filled syringe, I can open the valve and maintain down to around 1 micron.

Closing the valve, I can pull a strong vacuum.

Regardless of what’s the case, it doesn’t seem to affect operation.

-Matt
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Anze A Ursic
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Re: MKS MFC Problem

Post by Anze A Ursic »

But that would imply that, even without Deuterium on the other side, it leaks air into the system right? Wouldn't that mean that the Deuterium would be contaminated with air anyway once it enters? Also what's the pressure on the inlet side (the deuterium inlet), that works as you mentioned?

Anze
Chris Seyfert
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Re: MKS MFC Problem

Post by Chris Seyfert »

Hello Anze,

Your problem is related to your other MFC question that I had previously responded to. An MFC cannot regulate the gas without sufficient pressure drop across it. Without a pressure difference, the MFC control loop has nothing to regulate on and the valve position will go to an unknown position, but likely fully open.

This is the procedure for setting up an MFC on a system. This assumes a basic setup of cylinder - regulator - MFC - chamber. Start with the cylinder valve closed.
1) Pump down chamber to at least rough vacuum
2) Command MFC to max flow rate
3) Set regulator to a non-zero pressure (note - D2 cylinder valve is still CLOSED)
4) Monitor MFC feedback signal until flow drops to zero (this could take some time)

The above four steps have drained the regulator and tubing to the MFC of residual air. Now we can continue:

5) Set MFC to zero flow rate
6) Set regulator to zero pressure
7) Open D2 cylinder valve
8) Set regulator to approximately 20 psig
9) Set MFC to desired flow rate
10) Monitor MFC feedback and verify that flow rate agrees with setpoint

Try that out and see how it works for you.
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Anze A Ursic
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Re: MKS MFC Problem

Post by Anze A Ursic »

Hey Chris, awesome info! Will give it a shot tomorrow, thanks!
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Re: MKS MFC Problem

Post by Anze A Ursic »

Chris, I am currently doing what you said although I noticed that when measuring the voltage output signal from the MFC, the output voltage value is just the same as the one being fed to it by my power supply. There are 9 pins in total:

1. Valve Override
2. Flow Signal Output
3. +15 to +24VDC
4. Power Common
5. No Connection
6. Setpoint input
7. Signal Common
8. Signal Common
9. Valve Test Point

So I apply the 0-5V signal to the MFC between Setpoint input (6) and signal common (7). Do I then read the flow rate between Flow Signal Output (2) and Signal Common, (7) or (8)? Because currently that voltage signal between (2) and (8) is pretty much the same as the voltage applied between (6) and (7) and it is not dropping.
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Re: MKS MFC Problem

Post by Matt_Gibson »

Chris,

Even if the valve goes to fully open, shouldn’t a sealed system maintain a “0” pressure?

I am unable to get beyond step 4, pressure never drops. Air is getting in from somewhere, teflon tubing and hose barbs maybe?

-Matt
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Anze A Ursic
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Re: MKS MFC Problem

Post by Anze A Ursic »

I have the exact same problem as Matt.
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Re: MKS MFC Problem

Post by Chris Seyfert »

Anze, yes, you measure the flow output between pins 2 and 7/8. Also there should be a connection between signal common and power common for best accuracy, though that is just good practice, not likely the source of the problem.

I am not sure at the moment why the pressure is not dropping. Obviously a leak is a possibility but 5 sccm is not a lot of flow. How long have you let it sit at full flow rate without seeing any decrease in flow?


Matt, can you post some more details of your gas handling system? Apologies if you've previously posted that information, just let me know where to look. Teflon tubing and hose barbs may be an issue, Teflon is a fairly rigid plastic and may not be sealing well on hose barbs. Compression or Swagelok-type fittings are preferred if you can get them.
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Re: MKS MFC Problem

Post by Matt_Gibson »

Hi Chris,

I’m using a GM50A from MKS. The inlet has a kf25 converter which goes to a kf25 to hose barb adapter. I’ve been connecting a gas syringe via some pvc tubing directly to this hose barb, but recently tried out some Teflon tubing when I tried switching directly over to a deuterium cylinder and regulator.

Both the regulator (with Teflon tubing) and the syringe (with pvc tubing) behave the same as far as pulling a vacuum through the MFC goes.

I have even tried just blocking off the inlet to the MFC and still can’t pull a vacuum through it. Fusion seems to work alright with a syringe, just can’t start with a vacuum. I have been flushing (wasting) the line+MFC with deuterium before I fire it up.

Air seems to be getting in from somewhere in the MFC body?

-Matt
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Re: MKS MFC Problem

Post by John Futter »

Is this a new MFC??
if it is secondhand it is possible someone has over pressured it in its past or even worse disassembled it and left a seal out??
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Re: MKS MFC Problem

Post by Matt_Gibson »

John,

Mine was second hand, but not sure if it was ever used. When I downloaded its history, all that I saw was an entry from MKS doing the initial testing (it passed). It also had default settings before I switched it over to D2. Furthermore, it has its seal intact so it hasn’t been opened by anyone. Also, no marks showing it ever have being mounted to anything.

If it has been over pressurized, would it seal up so well once I give it the close (0v input) command?

-Matt
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Re: MKS MFC Problem

Post by John Futter »

Matt
Warm-up Time (to within 0.2% of Full Scale
of steady state performance) =30 minutes

how long have you had the unit powered on before doing your tests??
Matt_Gibson
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Re: MKS MFC Problem

Post by Matt_Gibson »

John,

It stays powered on 24/7 waiting for some action.

-Matt
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Anze A Ursic
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Re: MKS MFC Problem

Post by Anze A Ursic »

I tried another MFC and I got better results but still not ideal. Instead of 7E-4 like with the GM50A I get around 2-3E-4. Granted, that again was with it being blanked off so...trying nitrogen tomorrow!
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Re: MKS MFC Problem

Post by Chris Seyfert »

Matt, to check proper operation of your MFC, if you leave the inlet open to atmosphere with the other end at vacuum, can you command various flow rates and have the MFC readout change accordingly? The chamber pressure should also track with MFC flow, obviously. If that doesn't happen, then you may have a malfunctioning MFC rather than a leak problem.

You mention that your MFC inlet has a KF25 adapter, can you post a picture? Adapters are often a leak failure point. Have you checked all sealing surfaces, e.g. VCO o-rings clean and seated properly, VCR gaskets properly torqued (preferably replaced), Swagelok ferrules not damaged, etc.


Anze (and Matt) when you have tried step 4) to purge air from the gas lines before the MFC, how long have you let the flow continue? Try at least 15 minutes. If there's no flow decrease (and corresponding chamber pressure decrease) then there would definitely be a gross leak somewhere.

Pictures are worth a thousand words, if you can post some pictures of your gas system that might save some back-and-forth on the forums.
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Re: MKS MFC Problem

Post by Matt_Gibson »

Chris,

I usually abort after a minute or so. By that point, it looks clear that I won’t be pulling a vacuum.

Here is my connection. The other end usually goes to a gas syringe. More recently I tried connecting to the regulator that you see.

I am able to control the flow rate very precisely with the inlet connected to the gas syringe. I’ll try it with the inlet open to atmosphere when I get home today, but remember my early testing seeming to indicate that it worked the same. Mine has the ability to connect via Ethernet and display a plot map. I can see that as I gradually increase the input voltage, the control current and flow rate also increases accordingly.
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Anze A Ursic
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Re: MKS MFC Problem

Post by Anze A Ursic »

Here is my setup. Currently I'm just running a blank and this model does not seem to leak. My pressure is at 3.4E-5 right now after less than 10min of opening it. So your advice, John, works for this model, not for the GM50A though. That may be because it's damaged or whatever. But anyway, I have a manual valve inlet and it goes like this

GAS -> LARGE REGULATOR -> SMALL REGULATOR -> MFC -> CHAMBER.

The reason why I currently have 2 regulators is because I'm burning Nitrogen. Once I use deuterium I will only use the smaller regulator.
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Re: MKS MFC Problem

Post by Anze A Ursic »

Sorry for two posts, but one last thing - the way I am powering this thing up is using the MKS 647B (I am using the -15, +15, Power Ground and chassis ground from it), and then the Pins 8 and 12 (set point input and signal common, respectively), go to another cable that goes outside of the lab to the remote station where I have a 0-5VDC power supply which allows me to manually regulate the valve. Is it the best solution? No. Does it work? Yep!
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Re: MKS MFC Problem

Post by Chris Seyfert »

Matt, one minute is not nearly long enough. There's easily 10+ cubic centimeters of volume on the high pressure side of your MFC and you're only flowing 5 sccm. Try a minimum of 15 minutes like I suggested.

I'm "reading between the lines" a bit here, but it sounds like leaks both across the MFC as well as into the MFC are being discussed simultaneously, which is confusing. Let's try to be a bit more precise about what sort we're discussing. As to leaks "across" the MFC, most MFCs are not designed to completely shut off flow. Some are better than others, but you cannot expect "zero". The GM50A only guarantees less than 1% of full scale at pressure delta of 25 psid.
GM50A spec.png

Anze, since you have a Swagelok system, while you are running nitrogen you should just remove the second regulator. I'm not sure it will function well with such low pressure on the inlet and it will just complicate diagnostics as you try to figure out the MFC system.
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Re: MKS MFC Problem

Post by Matt_Gibson »

I’ll give it another shot tonight and let it run much longer.

I think we’re both seeing the same issue. I’ll define “through” as meaning that I’ve given the MFC a command to open with my regulator attached to its inlet. This is what I think we’re both having trouble with.

If we give our MFC the close command or a 0v input, we’re able to pull down a decent vacuum.

-Matt
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Re: MKS MFC Problem

Post by Anze A Ursic »

I think Matt and I had an issue where air was leaking into the MFC (like a seal was broken). I have now gotten another MFC that holds vacuum much much better.
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Re: MKS MFC Problem

Post by Matt_Gibson »

Okay so I hooked the other end of the tubing up to a valve, shut it off, and I let it go for 30mins, just the roughing pump. Pressure went from 9 microns to 1 torr. Not good. Giving the close command took it back to 9 microns in just a few minutes.

I tried swapping out the pvc tubing with some Viton tubing and hose clamps. Same results.

I remember checking the vcr4 connection and tightening it pretty well. Maybe it wasn’t well enough? That’s my only other thought to try. Seems like it would be leaking from both ends if that was the case.
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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.
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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
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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
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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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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.
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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
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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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