Diffusion pumps

[Doug Coulter]

I'll begin by saying help! I've used diffusion pumps only a little and I'm sure someone else here can say more and better than I am now, so please do.

I am going to assume you want to work at plasma pressures here -- say nothing below e-3 millibar. You want purity, so you want to be able to have a base pressure of 2-3 orders of magnitude below that to get all contaminants down to less than a percent or so. For this, a diffusion pump will do. There are some problems and issues, but they work and are reliable. For a small surface area, nearly sealed off system, most diffusion pumps are actually "too good" and need some kind of throttling to keep the gas flow at the desired pressure low enough that you're not wasting too much process gas. They also have a nasty tendency to backstream oil into your apparatus, so you have to apply them more carefully than you do a turbo. Further, an inrush accident can dump your nice, expensive diff pump oil into the backing pump, which costs you having to change the oil in both -- diff pump oil isn't a good lubricant in most cases.

There are two effective ways to throttle a diff pump. One is simply by putting a valve between it and the tank -- good move as this also cuts backstreaming down. The other is to heat it less, via a variac or lamp dimmer in the heater circuit which helps as most valves are a little twitchy near the fully closed position you're going to probably wind up using without some heat reduction.

Most surplus diffusion pumps are simply too big for the small science worker. You want probably 3" throat or less for a system that fits in a small lab, and if you can find one, an air cooled one is by far more convenient than a water cooled one. If you're not finding what you want surplus, keep looking as having the wrong pump for the job is more hassle and more expense by a lot.

Backstreaming of oil vapor into the tank can be a real problem but this depends more on what you are doing there. If you are making low energy charged particle optics, it's pure poison.
If on the other hand you're running a fusor, not so much -- it decomposes into carbon and hydrogen, with the carbon getting on the hot stuff, but not affecting how things work much
at the high currents and voltages involved. The ways to stop it are various traps, a "cold top" which is kept at room temperature or below, and by using a low back-streaming oil. Also a simply labyrinth trap will help lots if you remember to keep cleaning it out, which can be tough with some oils that cling tightly and don't respond to the normal solvents well. Nevertheless, you have to clean them too, as they "creep" along surfaces until they find a place to make trouble.

A further issue is that most oils if exposed to air when hot, either catch on fire or are decomposed into lighter fractions, ruining the oil, so with any diff pump system you'll be wanting to have a few more valves, or have to be really patient in letting things cool off before allowing air into the diff pump. Here's a picture of an old style experimental system that is valid today, except for one thing - most newer pumps are already multistage, so you'll only need one of them. Note that with this system, you can let your main tank up to STP while keeping good vacuum in the diffusion pumps(s) and leaving them hot. You can then use the forepump to rough pump the chamber, open all the right valves, and get going again quickly. There's another little trick in this picture I recommend highly, and that's having a discharge tube in the foreline someplace as a poor mans mass spectrometer. With a little hand held optical spectrometer and some practice, you can find out quickly what your gas composition looks like on a rough scale -- often all you need to know. Also helps in leak hunting when you are doing that. You simply use a leak probe gas or fluid that gives a different color glow discharge. Click the pic for a readable version.

From John Strong

The fusor we built for Bill Fain here uses almost exactly this design, it's a good one, except we used some good ball valves instead of the hose clamps for those parts. This really isn't rocket science, and the system works great.

Many agonize over the choice of diff pump oil. We kept it simple, just using Lesker's Diffoil-20 for both the diff pump and the backing pump, as it's rated for both. We never did put a really good ion gage on that system, but it gets well below where a fusor runs, and seems to have no troubles due to the cheap oil we used. His system is water cooled, which due to a lack of plumbing in my lab is done via a 6 gallon bucket of water, a fountain pump, and a bunch of "blue ice" so we can run long enough for it to be useful. It has a cold-top trap which sees the water while it's the coldest. We don't have evidence that backstreaming oil is a problem, but we are careful running it. This diff pump is also on a lamp dimmer. We run it full blast for about 10 minutes at initial pumpdown, then set it to about 1/3 power, which it cools down to in another 10 minutes or so and becomes stable there. We recently acquired a smaller air cooled diff pump and will be retrofitting that one to get rid of the inconvenient water system and save that space on the cart.

There are more expensive oils out there for special purposes and a lot of the fusor crowd swear by the DC-705 series, currently listed at $315 for half a liter at Lesker. It must work as they are getting good results. At the extreme you get into some very good but also very expensive oils that need no traps and backstream very little, for things like table top electron microscopes -- a very picky application compared to plasma kinds of things, and there is no need for that for most people. Lucky as it can cost half a grand for one fill of that stuff shown here. Unless you need that and know why, you don't need that. Things like low current low voltage electron microscopes or quadrupole mass spectrometers are really trashed by a thin layer of insulating oil on the electrodes. Glow discharge electrodes couldn't care less, they just burn it off.

With anything other than the cheap Diffoil-20, you have to begin to worry about cross contamination of oils. If you get too much diff pump oil in the mech pump, it will wear out quick. If you contaminate the other way, then your diff pump will stop working well and backstream more. Did I mention that it's a real PAIN to clean oil contamination off things, particularly silicone oil? A quick price comparison and hassle comparison will show most why we landed with what we did. But I am also sure that someone else knows more than I. I've run Bill's system, which I built, and some systems way back when I was in college (early '70s) and that's about what my experience is. We have seen many of the more successful plasma workers using the DC class pump oils in the air cooled pumps with great success. We here are lucky enough to have a couple of oil free turbo systems, and have basically never looked back. Sadly, the good turbo-drag pumps aren't hitting surplus yet, so if you're on a budget, this is probably what you'll wind up doing -- a mech pump/diff pump system.

[Tyler Christensen]

I've been running my fusor on a small 2" air cooled diffusion pump, it seems to work great. I'm running DC704 oil, I don't really have any comparisons to other oils, but I haven't noticed any catastrophic backstreaming. It's a trapless system with a baffle on top of the pump, never changed the diff oil after about a year and a half, and still no troubles with it, pretty maintenance free. I even bled it to about 1/8 atmospheric by accident once and it hissed a bit but quickly recovered and there wasn't any long term damage to the oil that I could observe.

I used to run the pump off of a variac to achieve the throttling I needed with no valve on top of it, but this ended up being the cause of all my instability issues, right on the fringe of operation where I needed it, the pumping rate was not constant, went all over the place, so it was nearly impossible to keep a stable pressure inside the fusion chamber. I now run it at full 110V and adapted a shutoff valve to become far more precise and use it as a throttle valve, it only has to open about 1/2 a millimeter on a 2.5" diameter seal o-ring, barely takes anything to get just the right fusor flow rate.

[lutzhoffman]

Hello:

Just to add to the excellent material on the subject of DP pumps, and fluids, that has already been posted:

Reference to DC-704, it is excellent stuff, in fact I prefered it to DC-705 when I was running silicon fluid, since it was cheaper, and faster than 705, and I never had backstreaming issues. My reasons for stopping have nothing to do with performance, and are entirely related to the niche application of potential drop accelerators. The concern here, which is also true for electron microscopes is: With silicon based fluids, the oil decompositions products under ion bombardment etc. can deposit an insulating layer of SiO2, which can affect the potential distribution in the column. A quartz coating is not easy to remove if it does happen, even though it is not likely. I could still use DC-704, but if "Mr Murphy" stops by, then I have an epic cleaning task, if major backsteaming were to occur. I would assume that for fusor application a tiny amount of SiO2, would not be a problem, especially given that you can reach the components inside if you had to. So for fusor application I would think that silicon fluid would be just fine. Another benifit of silicon oils is that they do not tend to form a lot of tars, and varnishes, which are a pain to remove from your pump, and which sometimes require heroic efforts like bead blasting etc.

As far as fluids go on one side of the spectrum you have Polyphenyl Ethers like Santovac, and Lion-S electron microscope DP oil from Japan, which as correctly stated can cost a fortune, the key word here is "can". Sometimes however you get real lucky, I traded a guy a nice new SS KF-50 high vacuum rated ball valve, for a pint of Santovac 5, which for me is essentially a lifetime supply. The decomp product of Santovac tend to be H2O, and CO2, and not varnishes. Santovac is so thick at room temp that you cannot even pour it, so it has to be heated to about 50-60C min. to even fill your pump. The only down side of using it is that you tend to not want to do any "dirty" pumping, so as to not contaminate your oil, which on a psychological level causes me to limit some "dirty" experiments, this is not good of course, since who knows what you might miss? Ref. Japanese Lion-S fluid, it is supposed to be really good stuff, but I have never seen any on ebay or anywhere else affordable for that matter.

On the other end of the spectrum you have Diff Oil 20, which will do everything that all the others will do, and even faster in many cases. It makes life real simple, which is always a good thing. Yes you have to practice good hygine and vacuum technique, but this should be the case with any setup period. Its so cheap, to the point where an oil change of 100ml is simply a non event, and if you have any concerns, you just swap the oil and your done, and back to business. Yes you should use a trap, but you do not have to have LN2 either, my friend just circulates ice water through a water baffle trap, with insulated tubes, via a cheap fountain pump, and a modified cooler. He has never has had a contamination problem. He does mainly mirrors, and optical coatings, which do require a clean vacuum. In his case the only preperation required, is checking the ice maker in the fridge : )

Octoil is also a pure hydrocarbon fluid which some folks swear by, personally I hate the stuff because it costs a lot more than diff oil 20, it delivers not much more than diff oil 20, plus its toxic, and a suspected carcinogen. My HS2 DP came with this crap in it, I do not like it and I never will. It does have one good use: Which is to make home made C-4, or Semtex, its a great inert plastic binder if you happen to have a pile of RDX, or PETN laying around, no thanks.

In between these are the fluids like DC-704, which can offer a some of both worlds, here you can relax a bit, and still not panic if air gets into your system while your DP is hot, etc. If something does happend then you can still afford an oil change, and you do not tend limit what you are doing, simply because you do not want to "mess up your oil". I was always happy with it when I used it, no complaints.

There are also very nice spec. fluorocarbon DP pump fluids, but these have some issue, where you are supposed to in addition also use a fluorocarbon fluid in your mechanical pump, Yuck!!! I do not clearly understand the reasons, since they make no sense to me, if you use a trap between the DP and the RP? Is this reality or just good marketing, of an extremely expensive product? Given their cost, and no clear advantage in non pure oxygen applications etc, I just avoid fluoro DP fluids period, the idea of spending 500 bucks just to fill my roughing pump also, in addition to the cleaning and disassembly of the RP to convert it, is a big turn off. For other apps, I really like fluorocarbon fluids like fluorinert, and Galden, but not for DP's.

So which oil is best? This question is like asking what's the best marinade for a good steak? The best oil is the one that works for you, and for what you are doing. One factor to consider however is the solvent that you use to clean a DP which has been used with "X" oil ? Here it is worth your time to look up the spec sheet on the oil, unless of course its Diff Oil 20, in which case just about anything non-polar will work, my friend used "Brake Clean" spray from the auto shop, which is simply Tetrachloroethylene, since he wanted a non flammable solvent to restore an his old used DP. Different solvents will wotk with different fluids, some can be very specific, especially with the fluorocarbon, and silicon fluids. One piece of good advice for fluids, do NOT be afraid to buy generic oils. Generic 704, and 705 is just as good as the Dow Corning version, in fact it probably is DC, it just came from a bigger barrel at some point.

For throttling a DP, I prefer a butterfly valve style system, with the DP running in its normal comfort range, not to cold, and not to hot, for reasons already stated. Doug had a super simple project solution to this issue on the fusor site, so that it does not have to cost a bundle either to build one in an afternoon. For me diffusion pumps offer an extremely effective almost nostalgic solution, to high vacuum, I just love the idea of "no moving parts". Unless you "F" up really bad, like with a major house fire, the DP can almost always be repaired, or restored to service with some effort. Would I like to have a nice turbo pump in my shop, well heck yes, who would not ! I would just love to have one spinning under my bench, but I am not willing to pay $1000+ for one, and this is the subject of a different thread.

The DP gives nearly anyone who can think, the ability to practice high vacuum, while the other forms of high vacuum pumps, tend to be pumps of opportunity for most folks, given their cost. One thought which has occured to me is the idea of having two diff. pumps in the shop. Not in the sense of one being a spare, instead as two working pumps, each with different fluids. One with a premium ultraclean fluid like Santovac, or DC-705, and one pump with diff oil 20 for other different applications. This would be the poor mans DP version of a turbo and DP pump combo. For now its just not worth the extra effort, just for the occasional "dirty" job, I just use some leftover LN2 from work, and activated charcoal, in an improvised sorbtion pump setup. This actually works pretty good, as long as I rough out the system well to begin with, and if I remember to vacuum de-gas the charcoal (forgot that once). One note: If your DP pump does not have the flange that you want, a used SS conical adapter fitting can often provide the flange that you want, and you can harvest it for your pump, if you can weld, or even silver braze. All for a much lower cost, than for an ASA to ISO adapter for example, which is harder to find, and must often must be purchased new.

Modern DP's are sooo much faster than their ancient versions, a modern Varian HS-2 for example will do 280 L/Sec. which is more capacity than most folks will ever need. One great, long time going deal on a new DP, is Dunaway Vacuum's: Innovac 2" pump, which they sell new for $500.00, it has been "on sale" for years now. This S.Steel pump will do 300L/S, with only 80ml of pump fluid, yes 300L/S is somewhat overkill, for most folks, but the price is right, they dropped the price for me on one, to well below this sale price : ) All around Varian, Veeco, and Edwards and many others are all of excellent design, with very low backstreaming. When you compare these pumps, look at the backstreaming figures, they say a lot about the design. I prefer Varian pumps in the 2" size, but others folks may like others designs better, due to features like air cooling, built in throttle valves, etc. For even a fast 2" pump like the HS-2, or an Edwards 60, no more than a 4.5CFM backing pump is really needed.

In commercial use, the DP has much been replaced by other more modern pumps,they however have some advantages like: What if a tiny screw rolls into the intake of a whirling turbo pump, or even if some fine sand like material from a coating chamber gets in? Even if it has a screen for protection, major damage is sometimes not far away. In the DP, the same small screw will just sit in the boiler, and do nothing to harm the pump, or to even spoil the vacuum. For compact DP cooling setups some folks are borrowing from the computer overclocking folks, via liquid CPU cooling systems. Complete with liquid cooling heat exchanger, fan, tube, and pump packages, that will handle a small DP, can now be had from many computer supply outlets, in the form of super CPU liquid cooling kits. Instal one of these on a small liquid cooled DP, or buy an air cooled one, and then you can say: Look Ma....No wall plumbing : )

DP's can be found in so many places, to the point where being picky is an option. On this subject I would look for the following features for making comparisons: Backstreaming Rate: In real life, not with an "optional" trap etc. Capacity in L/S: look for the 90-300 range. Oil volume: Less is better, this way you can use a premium fluid if you want, like if there is a great deal somewhere, and you have some special requirments etc. Parts Availability: What if you need a new heater? Pump Condition: Is it full of tar, does it have a missing cold cap etc. Special Features: Does the pump include a built in throttle valve, or something else that you would find handy.

Once you get a pump, you have to clean it well if it is used, this leads back to the prior mention of specific solvents. Stubborn tar in small amounts can often be handled by Methylene Chloride based paint remover, followed by rinsing, drying, and acetone. A pre-soak in engine cleaner can sometimes help also. Just watch for Aluminum internal parts, because some harsh cleaners like TSP, and other caustic stuff can eat it. You can also bead blast the inside of the barrel for real stubborn crap, the tree, or internal stack parts can be fragile, so treat these with extra care. The complete removal of any old oil residue, with a significant vapor pressure is essential. Some extra time spent with solvents will save you a lot of trouble later. When you first fire up the cleaned pump, expect some vapor release, this is normal. Some folks "bake" the cleaned pump without oil under rough vacuum with at least a Cu wool trap between the DP and the RP, this has to be done very carefully so as to not overheat the pump, the cooling water or air should still be on for safety, since some cooling coils are soft soldered to the pump. If all this has gone well, then charge the pump with fluid, connect the cooling, rough it out, and turn on the heat. This has some risk because without oil, there is not much heat transfer other than conduction from the boiler, so that it can be risky, it has to be very carefully done.

Last I really like Doug's slick idea of including a discharge tube in the foreline, this could be very handy for tracking down leaks etc. You should also be able to find a copy of the pump manual on the net, reading this helps a lot.

[johnf]

All good stuff guys
In the early 70's I learnt all about diff pumps their oils / mercury (santovac had not been invented at this stage). We used glass and metal ones both with mercury and DC704 /DC 705. At good vacuum all systems slowly got contaminated with the DC oil unless you used cold traps with LN2. As a trainee it was my job to top up all traps every morning. I went away from all this in the mid eighties and pursued a career as an RF engineer 2004 saw me at the present place and almost all was the same except for the diffstack, a different pump that swells in diameter (x2 approx) near the top. I was told that this was the latest in diff technology and was told /read that the swelling near the top is to stop backstreaming into the vacuum space above and even better, if Santovac is used a base pressure of 2 by ten to the minus eight millibar is achievable without LN2 coldtrap above.

I have seen many diff pumps of both types over the years that have had the oil crack the end result for all is a nasty black epoxy like substance that almost defies all solvents and requires substantial elbow grease to chip /sand out.
I have never seen diffoil 20 so the above relates to DC 704 /DC705, Santovac.

Santovac has a problem with pumps designed for the DC oils in that the critical stall pressure is lower than that for DC 704 /DC705.
Santovac needs 2 by ten to the minus 1 millibar to work while DC 704 /DC705 will work up to the very high ten to the minus 1 millibar.
So DC704 DC705 will work with a single stage or stuffed two stage pump but Santovac requires a two stage pump in good nick to back it.
The reason for this is Sanotvac runs at a higher temperature than DC 704 DC 705 and when the backing pressure is too high the heat loss from the diffpump rises through conduction, cooling the oil too much ---hence stall

[Doug Coulter]

I think we are doing a great job here "writing the book" for the eventual newbs when they find this. Instead of answering the same questions over and over, we could simply point them at this body of knowledge -- we can think of these really basic tech threads as FAQs if you like. Just written with more than one author, which I consider a strength -- between us, we know more.

My limited experience with oil back-streaming and creep was all in high power input systems (plasma pinch) where the oil would simply be blasted off things -- but! Not so much things that weren't the main show, as in ion gages or other sensitive stuff in the system -- they got in trouble long before you'd see or feel any oil on other things. I suspect a fusor fits in the same category, you'd see troubles on things other than the fusor first.

We've done well throttling these guys (with the low boiling hydrocarbon Diffoil-20) down to about 1/3 power, or in other terms, about 60-65% of the nominal heater voltage.
That alone is not enough to get to decently low process gas flow, so you need other help to do that -- both Tyler and I have found that out. But at least it helped here with the large gate valve we were using -- it went from full closed to open a crack in a very small knob angle turn, hard to get right, so this was a "fine tuning" thing. As Tyler found, you can only go so far with that but it brought up the important issue of "stalling" which John seems to know something about, maybe more than the vague statements I see elsewhere.

So I have a question -
For very low flow regimes, after one has hit base pressure and either wants to keep it there, or at some higher process pressure, but with low flow, has anyone successfully used intermittent running of the forepump in this kind of system, perhaps with a decent sized reservoir in the foreline?

[johnf]

Doug
My SEM cycles the forepump as you cant get a pic around the 1um level with the fore pump running. there are three valves in the pump line one isolates the backing bufferspace about the size of the reciever in Jerrys quite pump project. the buffer spce is pumped down to 3 by ten to the minus 2 millibar and the isolation valve is closed. The pressure is allowed to rise to 2by ten to the minus 1 millibar when the backing pum is started and the pump vacuum is measured and the isolation valve is only opened again to the bufferspace when the mechanical pump has got down to the 3 by ten to the minus 2 millibar level. ( I presume Philips did it this way to stop mechanical pump oil backstreaming into the bufferspace). With this arrangement the backing pump is on for around 5 minutes every half hour. The third valve is to allow the mech pump to rough out the sample space with the gate valve above the diff pump shut and the ioslation valve shut so the bufferspace is backing the diffstack

Note the buffer tank on the left of pic

[lutzhoffman]

Diffusion Pump Operation Tips....

Here are some tips for optimum DP pump operation, intended for use with a DP system, which has been cleaned, filled with fluid, and otherwise prepped, and plumbed, properly.

For more specific information on any given model of pump, please check the manufactures web site, and download a copy of the pump manual when possible, this is very helpful. Varian for example, keeps all of its DP manuals available on line for download.

The following are just some concepts for optimization, applicable for your DP vacuum system, after you have already learned all of the basics about diffusion pumps, and about their basic operation. This information can be found from any the many sources on the net, regarding diffusion pumps, and their operation…….(Google Search, Etc.)

First question: Do you have all of the needed components in your system, for what you are trying to do? If you are only after a crude high vacuum, with no major issues, and if for example you can tolerate some DP oil back streaming etc, then much of this will be a mute point. If on the other hand you need a very clean vacuum for precision work etc, then you may find some of this information useful

This also depends somewhat on the DP fluid that you are using. If you are running ultra low VP fluids like Santovac 5, Pentavac 5, or possibly even DC 705, then for most applications you can get by without a trap between your DP, and your vacuum chamber. If however backstreaming would cause you major headache, like in an electron microscope, then you need to consider some additional steps. Some examples are listed below.

Even if your specs are fairly liberal, one should still consider having at least a butterfly style - throttle valve, or another type of valve, like a vacuum gate valve, between the DP, and your vacuum chamber, even with these fluids. This valve in any given vacuum run, will stay closed until your DP is running at near capacity, in order to avoid the potential of an initial “burp” of pump fluid vapor, while the pump is warming up, and while the pump vapor jets have not stabilized yet. It is also very helpful to have a vacuum gauge between the DP, and this valve, to monitor the DP warm up. If this is not an option, then 5 min or so, of extra warm up time can be allowed for, to ensure that the DP is up to speed, before cracking the valve to the main chamber, and beginning its high vacuum evacuation. When you do open it, do is slowly, while watching your foreline pressure, so that you do not overwhelm your RP, unless you have a lot of overcapacity on your RP, in which case you may want to read the notes below reference to having your DP outlet forepressure just right, and not to low. The same is true for shutdown: The main valve above the DP should be closed at the same time that the power to the DP heater is turned off

Do you have throttle valve between your RP and the DP? This can be a much cheaper brass, or SS, Teflon sealed ball valve, in addition to many of the more expensive dedicated vacuum valves. The reason that you DO want this, is in order to regulate the level of vacuum at the outlet side of the DP. The specific reason is: If the DP outlet pressure drops to below a certain vacuum level, then oil vapor from your RP can backsteam like crazy, towards your DP, and eventually into your high vacuum chamber. This becomes a big issue when the gas flow transition between the viscous-molecular flow region, and the optical flow region occurs. If your vacuum level in the foreline leading to your DP outlet drops to below 20-25 microns, then you will have FP oil making its way to the DP at over 10-20 times the level, of what it would be at 50-100 microns DP outlet vacuum level. Here a deeper vacuum is NOT always better, instead a “Just Right Vacuum” level is. I try to keep mine at 50-100 microns, for my HS-2. Check the specs for your pump, you do not want to be near the operation limit, but you also do not want to be 10-100 times lower either.

For foreline plumbing many materials will work just fine, even wire reinforced PVC hose will work, so much of this is preference. Rosin core soldered, or epoxy bonded clean copper pipe works great for DP foreline plumbing by the way. I just do not like plastic, it’s a personal thing. With soldered copper, a solvent flush, and some heating is also possible for any potential serious cleaning.

Yes, a good foreline trap between the RP, and the DP, will largely prevent the migration of RP oil, but the trap has to be CLEAN, and if it is not, then RP oil vapor contamination can spread real fast, and how the heck do you really know, when the trap medium function is exhausted? Regulating the foreline vacuum level can reduce this problem by an astounding 10-20 times, which is worth a little “valve play” in my mind. Traps are the subject of another thread, but for here, any reasonable volume trap stuffed with cheap cleaned copper wool scrub pads will do just fine, just wash them with dish soap, rinse them well, dry them, and finally, degrease them with any clean, good solvent like clean acetone, and oven dry them at about 300deg. F for a few min. (When they are dry, and free of acetone, so that you do not blow up the dam oven) imagine trying to explain that one to the wife, or your SO! This process will need to be repeated as they reload up with an oil film during use. I clean mine at every 4-5 runs, but to be honest I have no idea how long you could push this, since there is no magic number of runs, so I try to be conservative. Some others may rightfully laugh at me for doing this, but oh well it’s my time, and no one has educated me further on this. If you run the same fluid like diffoil 20, in both the RP, and in the DP, then much of this is less of an issue. It is still an however an issue, because as the DP operates, the more volatile fractions, and oil cracking products etc, tend to migrate to the RP, where they can build up. Alumina filled traps, either bake out, or disposable do work well, but you have to be careful because alumina is corundum, a well known abrasive, if its filtered out well, then no problem, but if it gets into your RP, then it will wear out very fast. So I prefer old style Cu wool. The new micromaze traps from Lesker, are really cool, but they are not cheap either.

If you look at a schematic for a given commercial DP pump system, you will normally see a by-pass roughing line, this is there so that you can rough out your chamber, with the DP to chamber valve closed, this line T’s off with its own valve right after the RP trap, (Seen from the RP) and before the DP outlet. This line is also used in reverse, to re-admit air into the entire system. In part so that the tree inside of the DP, does not get slammed into the main valve / main chamber if it is loose, which can happen if air is re-admitted quickly via the DP foreline, which can cause the rushing air literally launch the inner works, or the tree, inside of the DP, towards the main chamber. Also, this way the air flow is always: From your chamber to the RP: If the valve for re-pressurization is put near where the roughing line enters the chamber, with another T fitting, then the gas flow will be better. At this same location a very small “gas leak” can also be built in. In addition, if you have over capacity on your RP, a small amount of air, or other gas, can be introduced via an adjustable leak needle valve, to keep your DP outlet foreline pressure in the optimum range for your DP. This small gas flow can also directly help to prevent RP oil from migrating out of the RP and into your system. I like to use this same valve to back fill my system with dry CO2 from a bottle, if my system is going to sit for a while with no use. This way all the fluids stay dry, and you can avoid mold etc, if you live in a humid areas.

Reference Fluids: I recently learned from another post, that apperently I just got lucky while using silicon based fluids, and that they can indeed form some very nasty hard to remove epoxy like deposits, I do think that the conditions required to form these, are more severe than those with classic hydrocarbon fluids like diff oil, and octoil. Reference Santovac however: I have had a total of four "Holy Crap!!!" events due to glass breakage etc. where air rushed into my DP at full temp: The result each time was as follows: My Panic Response, System cool down while pacing, and a small fluid sample drawn from outlet of the DP, and finally: A look down the inlet of the DP, and a sense of relief. No tar, no visible deposits, and clear fluid the color of light salad oil, which is normal. Thus if there is some way to abuse Santovac to the point where it forms tars and deposits, then it would take the most abusive envionment on the planet, the industial setting with no one watching, or not enough workers : )

Overall diffusion pumps are very easy to use on a basic level, but they become somewhat more complicated if you are trying to optimize everything. I like the plane analogy: A modern turbo pump with its controller, is like a modern plane on auto-pilot, while the DP is more like actually flying the plane, where you have to keep an eye on the gauges, and make an occasional small correction, even for level flight. Maybe I am getting nostalgic, but I actually enjoy the process of “piloting” the high vacuum system. It is not a fast paced crazy process; instead it provides me with something to do, besides just watching the face of the gauges during pump down. If you hate steam trains for example, and you do not like this kind of fiddly thing, then maybe hold out for a Turbo pump instead. If on the other hand it does not bother you, then the results are awesome, you can even play in the ultra high vacuum region, if you just put in a little effort. The best part about the DP in my mind is, that it will never wear out, and it requires only basic care, these things are tough as nails.

[Jerry]

Some good info in there!

The stuff I am using in my DP (6" Tecnics) is called VacOil S. It is supposed to be immune to breaking down when exposed to air at working temp. Dont know yet, dont want to try!

Seems to be good stuff though, I can pull into the 8's without using the cold trap.

[lutzhoffman]

Hello:

Johnf posted something here which I believe is important, and useful enough, to warrant some elaboration. Specifically when he talked about the DP backing vacuum reservoir tank. This volume if sufficient provides a major buffer between the DP and the roughing pump. This is very useful for two reasons, and even better it can even allow you to run a DP with a roughing pump, which is normally a little bit to small for the DP.

The first reason is that with a reasonable volume vacuum reservoir, which is equipped with at least an inlet shut off valve, installed between the RP trap, and the DP inlet, the following is possible: You rough evacuate the entire system, and the reservoir, to within the comfortable operating range of the DP, and switch it on: If your system is sealed well with no significant leaks, the level of vacuum will increase more and more. After some time the total volume of gas moved by the DP will become less, and less. You will reach a point where you can now isolate the vacuum reservoir from the RP, by closing the valve between it and the RP. With a low system gas load, you can now shut off the RP entirely, and simply allow the DP to pump into the reservoir. According to some vacuum articles in Scientific Americans Armature Scientist column, they claim that in a tight system: You can allow the DP to simply pump into this reservoir for many hours! They used a volume of 2 gal or about 8 liters. For fusor use, or any other application which introduces a gas, this time could of course be much shorter, but for an electron system with no gas introduction, many hours are possible, imagine just the power savings, not to mention the reduced wear on the RP, which are realized if you can shut it off for hours at a time.

The second benefit of having a vacuum reservoir volume in this location is, that it will act as a buffer. So if you have a burst of some gas for example: Go from your chamber towards your DP, the reservoir will accommodate the extra gas, without an intolerable spike of higher pressure in your DP foreline pressure. Even if you do not have a bust of gas, like in normal operation, it will make the foreline pressure much more stable. Resulting in far fewer adjustments being needed, during your high vacuum run. This will = far less work turning, and adjusting, valves to keep things in the "sweet spot" reference the ideal backing pressure for your DP. For just this benefit, the reservoir volume would not have to be as large as before.

The last benefit is realized if your RP is a little bit to small, by having the reservoir in place, it can compensate somewhat for your having a smaller pump. As long as your your reservoir is well evacuated, and the total gas throughput is not to high, then you could get away with using a smaller RP than normally would be required. All in all a very useful item, thanks.

PS: It would also provide the same benefits for a Turbo-molecular pump system as well : )

[johnf]

Lutz

you have put in words what I should have
MY SEM has around 40 viton gaskets to deal with, so the leak rate is right up there ( the Backing pup is an Edwards 12).
We have noticed the reduced power load @ work as we replace diffstacks with turbos. Every pump has its own backing pump

I'd love to put in buffer spaces on all and use one or two backing pumps for the whole lot, and another system the same for roughing chambers
one day I'll get my way, the power savings would be immense as any type of high vac system (except ion pumps) the backing pump draws the most power

PS thanks for the link to the Galilleo pumps some time back 5 arrived @ work a couple of days ago(consolidated cargo) all 12 litre units with 0.75hp 3 phase motors.
The ion pumps note gives me an idea for another post----more later