Stable unit-sccm flow rates.

How to get to vacuum, what the classes are, and what is needed for what job.

Stable unit-sccm flow rates.

Postby chrismb » Tue Jan 11, 2011 7:11 pm

I have a leak valve I have been trying to use with my new chamber setup, and was really struggling to get a uniform flow.

...Then... two breakthroughs.

Firstly, I bought, initially as a spare, another active magnetron gauge. After testing it I didn't really have an immediate use for it so stuck it atop the intake port of the turbo.

Immediately I realised it was giving me a very accurate [relative, perhaps, rather than absolute] reading of flow rates in the system. At its full rated60l/s pumping speed and 1e-5 showing, for example, I then know I have 0.036 sccm flow through the system.

This gave me an immediate insight into the internal workings of the leak valve, and showed it must have a series of volumes, each of which was buffering one way then the other and making control hell.

I had been looking for capillary tubes for a while but, this being UK, I didn't find anything readily available. I found a supplier of 'blunt dispensing needles' [non-medical] and got some a few days back. The smallest, 0.21mm ID, still flowed too much gas but that was rectified by passing a 0.18mm wire down it.

This needle is inserted through two pieces of nitrile rubber I cut out and sandwiched between NW10 flanges.

The 0.21 ID needle (1" long) and the 0.18mm wire gave a flow rate of 0.5sccm from 1 bar to 0. Because it is just one single flow restriction and drops a full bar pressure, the vacuum pressure can vary 'considerably' but the flow rate doesn't - e.g. it could be 0.1 micron or 20 microns, the percentage change from one full bar is negligible, but the flow rate is roughly the same.

To alter the flow rate, I change the pressure on the needle 'intake'. To set the system up, I merely dial in the pressure I want in the chamber on the chamber-turbo vent valve, set the pressure on the needle (I have mapped this out tonight and 1 bar gives 0.5 sccm, while 3 bar gives 5 sccm - not sure why it isn't linear, but that's what the results said. The pressure gauge on my regulator is just a round, dry needle type on a scale and isn't calibrated, so that is the most likely source of error, if it should be linear). There is then a small change on the chamber pressure once the pressure is set and settles after a moment, and that chamber pressure can then be given a final fine adjusted. Or otherwise set the flow rate first, &c...

The whole thing is then dead stable. It will just sit at any pressure I choose, at any flow rate between 0.5 sccm and 5 sccm, with a 0.05 sccm resolution. I can now see why Carl W. has gone for a flow-orifice, as a single, big pressure drop is so much more controllable than a series of 'dead spaces' between valves or restrictions.

[notes; outgassing from the turbo-chamber line was <0.01sccm, and the chamber outgassing rate before and after these measurements was ~0.06sccm, so these are 'error margins' on the above figures.]
chrismb
 
Posts: 620
Joined: Thu Aug 05, 2010 6:32 pm

Re: Stable unit-sccm flow rates.

Postby Doug Coulter » Tue Jan 11, 2011 9:13 pm

I use readily available (here) capillary tubing for this (better than an orifice for a bunch of reasons, one of which is you can use a bigger hole size and longer tubing and it doesn't clog as easily, for same flow). I don't think I understand what you're doing though. Setting chamber pressure with "turbo vent valve" is confusing me -- are you speaking of the vent valve that lets in air behind the turbo? Picture? More words?

With a constant inflow rate, which is what you describe (and also what I get with a similar inflow "controller" - a leak driven by pressure regulated gas) -- how do you set precisely constant pump speed to get the "two integrators" to have a constant net difference (pressure)? I'm missing something here. Even with a "real" mass flow controller, you use feedback from a gage to keep the difference constant (and all "real" mass flow controllers are far too fast for fusors unless you like wasting gas). So, how exactly are you controlling pumping speed to match inflow rate?
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
User avatar
Doug Coulter
 
Posts: 3515
Joined: Wed Jul 14, 2010 7:05 pm
Location: Floyd county, VA, USA

Re: Stable unit-sccm flow rates.

Postby chrismb » Wed Jan 12, 2011 2:27 am

I have an NW40 bellows valve from the chamber, an NW40 pipe from the bellows valve to a tee, then the turbo [with a gauge, on the tee].

The flow rate of the valve is 40l/s max, and the pipe is prob 20l/s, so the turbo runs at essentially full speed locally at the tee all the time, unless I have higher pressure in the chamber and open the valve suddenly, which would force the turbo speed to dip momentarily. With the valve set so that I have a micron in the chamber and units 1e-6 torr above the turbo, the turbo is running at full speed. With that set and leaving the bellows valve alone, if I then apply a pressure to the needle, then the chamber pressure will rise to around 5 microns and the turbo to around 2e-4. I can then fine-tune the chamber pressure to taste, as the flow-rate is a largely independent function of chamber pressure and changes very slowly.

Pics later. Maybe some videos if I can get around to it.
chrismb
 
Posts: 620
Joined: Thu Aug 05, 2010 6:32 pm

Re: Stable unit-sccm flow rates.

Postby johnf » Wed Jan 12, 2011 2:57 pm

Chris
send me an email with your address i'll send you a metre length of 1mm tube with a 180 micron hole down the centre
johnf
 
Posts: 433
Joined: Sun Aug 08, 2010 3:51 pm
Location: Wellington New Zealand


Return to Vacuum Technique

Who is online

Users browsing this forum: No registered users and 4 guests