Activated alumina vs carbon

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

Activated alumina vs carbon

Postby chrismb » Sat Jan 01, 2011 9:05 pm

I've just assembled my 'new' vac chamber setup and it has been struggling to pull down well with the foreline trap in place. Hardly surprising, perhaps, as it is old alumina and I've not baked it out yet after its disassembly. But as it is old, I should put some new in anyhow.

That being said, my question here is whether I should put effort into getting alumina or whether cheap 'aquarium' activated carbon pellets will do. The objective, after all, is to avoid backstreaming. Alumina, as I understand it, should be used for acids and water vapour (etc..) whereas carbon for organics and water vapour. Clearly in the type of stuff we're doing, activated carbon will do (perhaps better, even?).

There again, maybe if it is just for back streaming, wire wool in the filter would do and will not cause the problem I'm just having at the moment (requiring a bake-out).

Thoughts, please. Cost- and availability-wise, activated carbon looks the best. But is regular aquarium carbon 'clean' enough (after a bakeout)? Should I just used some wire wool to maximise pumping speed?
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Re: Activated alumina vs carbon

Postby Doug Coulter » Sat Jan 01, 2011 11:00 pm

I thought you were using a turbo? So no trap needed in that case anyway? We're using SS wire wool in our diff pump system between the pumps, but nothing on the chamber side but a valve there. Tossed the old alumina out, it was filthy. Most of the guys using diff pumps are using DC 704 or 705 -- need no trap to the chamber, just need to keep mech pump oil out of it. We ran with diffoil-20, cheaper, no problems we had were attributed to that, it worked pretty well it seemed. With an oil type mech pump, the main issue I worry about is suckback, so just a big volume/reservoir between the pumps takes care of that one if it happens (but valves are better if you remember to do them when you should! In our case, we didn't really see any difference when we put in the wire wool, it was more of a safety first thing -- it was the big jug that really did the trick there, a few liters worth. It can hold all the mech pump oil in an accident, and then let it drain back into the pump via gravity. Seemed like a good idea.
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.
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Re: Activated alumina vs carbon

Postby johnf » Sat Jan 01, 2011 11:13 pm

Chris
if you have alumina bake it unless the smell drives you crazy

if you have a good oven / furnace, 400 degress celsius is good it gets rid of hydrocarbons (the smelly ones) and most other badies are also gone

carbon is used usually with liquid N2 or liquid air but it may work
but try to make the alumina work again first
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Re: Activated alumina vs carbon

Postby chrismb » Mon Jan 03, 2011 9:57 am

I took the trap out and it pulled down fine.

I thought you were using a turbo? So no trap needed in that case anyway?


Doug, I am not sure about not having one in place. I think it degrades the system over time if you don't, and there are always the unexpected events/human error that might pull through oil mist. Does anyone want to offer their 'last word' on this point? My only vac experience is with my own kit, and I am only doing what seems sensible to safeguard it as best I can.

So I have installed the spark plugs in the Al baseplate (as discussed in another thread). I have 8(!). I've not done a prolonged dwell at low vac yet am now getting a pressure rise of less than a micron in 5 minutes (starting at 1 micron). It seems to be getting better, as well. So the spark plug feedthrough solution seems to have really worked out. Very happy with that. I shall be posting on that soon.

Reason for the extra post here is that I did as John suggested and plumbed the trap back in and put on a heater element to bake it. After a few hours I opened the valve between trap and chamber (which was at a few 10's microns by then), but whilst the trap was still warm. The Pirnai gauge (on the PKR251) shot up to 4000 microns whilst the new inverted magnetron active gauge I have just bought shot down to 1e-5 torr (0.01microns). I switched on the turbo and it spooled up just fine (indicating sub 100 microns?), yet neither gauge then moved at all. Both read oddly until I turned everything off, vented to air, and started over. What on earth happened? What odd vapours were there in the activated AlO that could've boiled off and caused these strange readings? Have I just done something seriously not-good?
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Re: Activated alumina vs carbon

Postby Doug Coulter » Mon Jan 03, 2011 10:02 am

I'm afraid I don't know enough about how you've got this lashed up to say. Where is the trap? Usually you put one between forepump and the high vac pump. You only have one between high vac pump and chamber if the high vac pump is a diff pump of some sort. A schematic or picture of your plumbing would help someone know what to say. Obviously an oil trap wants to keep oil out of the turbo in the first place. You should have been holding the highest vacuum possible on any trap while baking it out especially, else you just generate oil fumes that get into everything.
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Re: Activated alumina vs carbon

Postby chrismb » Mon Jan 03, 2011 10:11 am

Base pump - trap - valve1 - turbo - valve 2 - chamber.

Valve 2 open throughout. Valve 1 closed. I baked the trap, opened valve 1, and the gauges on the chamber when mad.

Presumably this is simply down to volatiles flooding back from the trap when the valves opened, but what kinds of gases/contaminants have such a perverse effect on such gauges?
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Re: Activated alumina vs carbon

Postby Doug Coulter » Mon Jan 03, 2011 10:38 am

You probably should have had the turbo running when you opened that valve, and valve 2 shut. Probably the foreline pressure was elevated some due to junk evaporating off the trap, and you maybe should have waited longer as well. It's probably oil vapor and related junk (hopefully the trap didn't have old lewis acids from semiconductor fab in it). That will make all those gage types act crazy for awhile until it gets out of them again. I'd say "pump overnight and call in the morning"....you might get lucky. A little oil in the chamber hasn't been shown to mess things up too badly except if it gets on things that get hot -- which will take it out of the picture quickly except maybe for some carbon left behind if its a kind that decomposes under hot vacuum (likely). Mainly an issue for things with low ion currents (gages) and relatively low voltage gradients (some parts of electron microscopes). In your case, it's probably just going to be a matter of some patience required.

Worst case, you wind up taking the chamber back down for a re-cleaning of all the surfaces. The gages will more often than not take care of themselves if run in good vacuum for awhile. Awhile being a long time to wait if you wanted to do something today -- it may take a couple of days pumping to make it go away.
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