Back streaming is well understood in the scientific vacuum world. We are not in the scientific vacuum world. We are in the technical vacuum world, operationally. Thus, backstreaming is no issue to anyone here doing fusion provided they just obey the rules and operate their gear as directed and use a half way decent oil.
You are either operating your gear well or you are not.
I am a fan of small diff pumps as a horrid yet necessary evil to allow a backfill of D2 such that an operational 10 micron fusion pressure is mostly D2.
A person may personally seek to construct all manner of diff pumps from scratch if that is what you think is important to your purposes. For me, its fusion I'm after. The only reason I am after fusion is to get at the neutrons.
I don't care about fusion energy or fusion power or bettering the fusor in any way. It does just what I want it to do as long as I play by the rules.
Like any smart dog, we need to know the rope limits within our world. If we obey the masters of vacuum technology already developed, our rope limit on backstreaming is virtually infinite and thus a non-issue except for the extreme vacuum neophyte who hears that it is a great bug-a-boo lurking to ruin his vacuum.
If you have a fusor and it is displacing the D2 fuel with oil vapor in your chamber then you just don't know how to operate your own vacuum system.
No one here demands you have an ultra-clean 10e-11 torr vacuum nor will they admonish you if you can't get but an oil vapor laden 10e-4 torr vacuum as long as you are doing fusion. Both of the above systems will do fusion and probably without enough difference to notice, all other factors being equal.
If anyone thinks they can build a better diffusion pump, then good luck on refining what has certainly already been refined many times over.
This is a fusion site, not an in-depth vacuum enthusiast's site. The Bell jar is one of the best of those type sites, especially if you seek deep, clean vacuums where it is all about the vacuum in and of itself. This vacuum forum is a technical forum designed to get neophytes into a simply created, fusion capable, vacuum environment.
Richard Hull
Backstreaming
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Richard Hull
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Backstreaming
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
-
Doug Coulter
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Re: Backstreaming
All I'm going to say on this, is: Right on.
Anyone who imagines Richard and I disagree on any of this would be wrong.
It's about fusion, the vacuum stuff is a "necessary evil".
Edit:
I DID undertake an investigation of possible effects from running super high purity, which did require some fairly exotic vacuum technique and tooling to accomplish. It's fun to talk about and many people here do seem interested in it.
But to say it's required for fusors, even advancing their art, is not indicated by results I have so far.
Everything says "something else is more important -- and you don't yet know what it is".
Since I'm now set up to run pure, why not continue to do so? And add contaminants at will rather then accept some you don't control. Seems like good science technique to me. And there is actually slight evidence here that some contaminants might actually improve the output. Whether pump oil is one is open as of now. At present I have no proof whatever that it's better to run a turbo in a low base pressure system or a simple, small diff pump. It is easier, no question about that or proof needed, if one is getting into and out of the system a lot.
But this is a fusion site, as Richard points out.
My main point in telling people about any of this stuff is:
They ask. And I know and can save them endless effort and money at it (that I've spent, why should you have to as well?).
It *is* fun to go on about.
But yes, rather than go on about that, lets see some more fusor results and improvements on here.
I am finding some that I'm getting ready to report on, as soon as they replicate enough times to suit me and I do some further tests. In other words, I am avoiding going on about untested theories until I have some sort of reasonable proof of them, else it's all hot air (or high vacuum ;~). Those will of course get their own thread when and if.
Anyone who imagines Richard and I disagree on any of this would be wrong.
It's about fusion, the vacuum stuff is a "necessary evil".
Edit:
I DID undertake an investigation of possible effects from running super high purity, which did require some fairly exotic vacuum technique and tooling to accomplish. It's fun to talk about and many people here do seem interested in it.
But to say it's required for fusors, even advancing their art, is not indicated by results I have so far.
Everything says "something else is more important -- and you don't yet know what it is".
Since I'm now set up to run pure, why not continue to do so? And add contaminants at will rather then accept some you don't control. Seems like good science technique to me. And there is actually slight evidence here that some contaminants might actually improve the output. Whether pump oil is one is open as of now. At present I have no proof whatever that it's better to run a turbo in a low base pressure system or a simple, small diff pump. It is easier, no question about that or proof needed, if one is getting into and out of the system a lot.
But this is a fusion site, as Richard points out.
My main point in telling people about any of this stuff is:
They ask. And I know and can save them endless effort and money at it (that I've spent, why should you have to as well?).
It *is* fun to go on about.
But yes, rather than go on about that, lets see some more fusor results and improvements on here.
I am finding some that I'm getting ready to report on, as soon as they replicate enough times to suit me and I do some further tests. In other words, I am avoiding going on about untested theories until I have some sort of reasonable proof of them, else it's all hot air (or high vacuum ;~). Those will of course get their own thread when and if.
Why guess when you can know? Measure!
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George Schmermund
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Re: Backstreaming
I'm not sure if this new thread is designed to encourage or stifle the discussion of backstreaming. I have some potentially useful information to share about the subject, but I fear that I may post something that shouldn't even be broached on this forum anymore.
Anything obvious in high vacuum is probably wrong.
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Richard Hester
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Re: Backstreaming
Backstreaming with a diff pump should not be a problem for fusor type apps if using the proper fluid. A silicone fluid like DC-704 or 705 is fine, and has much more resistance to oxidation than cheap-a** Octoil or similar. One of the main attractions of using the silicone fluids is the low ultimate pressures obtainable without having to use a trap (the equates to lower backstreaming). The only people that really seem to hate the silicone fluids are the mass spectrometer crowd, that dislike all the odd species introduced when silicon is added.
Of more concern is cutting in the diff pump at the right time to avoid backstreamng from the roughing pump, which is a much bigger problem. O'Hanlon (seminal practical vacuum reference) recommends not roughing below around 75 microns before cutting in the diff pump. I've pumped down a chamber to a few microns using just a roughing pump, and the thing smelled of pump oil when I brought it back to air. A word to the wise...
Of more concern is cutting in the diff pump at the right time to avoid backstreamng from the roughing pump, which is a much bigger problem. O'Hanlon (seminal practical vacuum reference) recommends not roughing below around 75 microns before cutting in the diff pump. I've pumped down a chamber to a few microns using just a roughing pump, and the thing smelled of pump oil when I brought it back to air. A word to the wise...
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Chris Bradley
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Re: Backstreaming
Please go ahead George, I would like to hear.
Richard's comments are sound, but relate to correct operation. To my mind, if you know what 'incidents' might possibly occur in faulty operation then it may be possible to instigate procedures and hardware to 'fool-proof' the system. Anything that gives me a heads-up on what happens in 'wrong' operation is stuff I like to hear about.
(Well, what would you expect, seeing as I am a reliability engineer! My days are spent fixing and testing out problems that the design engineers didn't know could happen because they didn't ask and thought they knew enough that they expected everything to go fine! Design engineers design thing to work. Reliability engineers design things not to fail. It's a big difference in thought processes and often makes it difficult to get engineers to address questions *they* think are irrelevant.)
Richard's comments are sound, but relate to correct operation. To my mind, if you know what 'incidents' might possibly occur in faulty operation then it may be possible to instigate procedures and hardware to 'fool-proof' the system. Anything that gives me a heads-up on what happens in 'wrong' operation is stuff I like to hear about.
(Well, what would you expect, seeing as I am a reliability engineer! My days are spent fixing and testing out problems that the design engineers didn't know could happen because they didn't ask and thought they knew enough that they expected everything to go fine! Design engineers design thing to work. Reliability engineers design things not to fail. It's a big difference in thought processes and often makes it difficult to get engineers to address questions *they* think are irrelevant.)
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Richard Hull
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Re: Backstreaming
This was not a FAQ on backstreaming. Pretty much anything goes that contributes to the subject.
My intention in the original posting was to note that with properly chosen and proportioned commercial diff pumps, proper operation of same and using a good silicone or better oil, and no special cold trap, the amateur fusioneer has nothing to worry about related to backstreaming interfering with his fusor operations.
Most diff pump designs were frozen in stone years ago and were designed to limit backstreaming with octoil and other older, more volatile oils. In the ensuing years, the oils got better and better, thus making the pumps automatically better at their original job.
Some of the best ultra high vac oils like santovac, pentavac and others do require a bit higher boiler temps, but all will function in older pumps, providing phenominally low vacuums with no trap or special procedures.
Richard Hull
My intention in the original posting was to note that with properly chosen and proportioned commercial diff pumps, proper operation of same and using a good silicone or better oil, and no special cold trap, the amateur fusioneer has nothing to worry about related to backstreaming interfering with his fusor operations.
Most diff pump designs were frozen in stone years ago and were designed to limit backstreaming with octoil and other older, more volatile oils. In the ensuing years, the oils got better and better, thus making the pumps automatically better at their original job.
Some of the best ultra high vac oils like santovac, pentavac and others do require a bit higher boiler temps, but all will function in older pumps, providing phenominally low vacuums with no trap or special procedures.
Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Re: Backstreaming
Come on George....Spill the beans......Most here are listening and interested.....
Re: Backstreaming
If the mass spec people don't like it, it nust be backstreaming, Richard......or have I missed something?
Re: Backstreaming
I couldn't have stated either point better myself, Chris.
I also tend to 'over engineer' everything.
I also tend to 'over engineer' everything.
Re: Backstreaming
Richard, while we don't need UHV for fusion, most of us want to pump our system down as far as we dare before introducing D so that the percentage of D in the chamber is as high as possible.
I will pump the chamber down, then purge with D before pumping down again, and going for neutrons.
(There is a league table now for efficiency)
I therefore need to understand the limitations of diff pumps and oils.
(Also, as others have said, we don't seem to get diff pumps on ebay this side of the pond, and therefore have to make our own.)
I will pump the chamber down, then purge with D before pumping down again, and going for neutrons.
(There is a league table now for efficiency)
I therefore need to understand the limitations of diff pumps and oils.
(Also, as others have said, we don't seem to get diff pumps on ebay this side of the pond, and therefore have to make our own.)
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Doug Coulter
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Re: Backstreaming
I would mention on this topic that backstreaming is a function of vapor pressure at the conditions in play.
This means that for a given oil and temperature, you'll have the same vapor pressure. You might be able to suck more gas through a larger orifice at that pressure, per second (eg a big pump vs a small one) but in either case, the pressure is the pressure until the oil is gone. Pressure doesn't signify how much is available at at that pressure. It's just a pressure. Like a volt is just a volt, it's not power unless you also have amperes.
So, the only way you'd see more molecules from a larger diff pump than a smaller one, is if it were plumbed in with a larger pipe, and if there was some other, better, pump on the tank to see it go out into. Otherwise, same oil, same temps, same tank oil partial pressure no matter the plumbing. It's an equilibrium thing -- when the pressure in the tank matches the pump vapor pressure, equal numbers of oil molecules are going either way and there is no net flow.
This is of course distinct from "suck back" when due to some weakness in the system, the tank vacuum actually pulls liquid oil from a pump, or more correctly, allows the higher air pressure outside to push it in.
As Lesker says, "vacuum doesn't suck".
This means that for a given oil and temperature, you'll have the same vapor pressure. You might be able to suck more gas through a larger orifice at that pressure, per second (eg a big pump vs a small one) but in either case, the pressure is the pressure until the oil is gone. Pressure doesn't signify how much is available at at that pressure. It's just a pressure. Like a volt is just a volt, it's not power unless you also have amperes.
So, the only way you'd see more molecules from a larger diff pump than a smaller one, is if it were plumbed in with a larger pipe, and if there was some other, better, pump on the tank to see it go out into. Otherwise, same oil, same temps, same tank oil partial pressure no matter the plumbing. It's an equilibrium thing -- when the pressure in the tank matches the pump vapor pressure, equal numbers of oil molecules are going either way and there is no net flow.
This is of course distinct from "suck back" when due to some weakness in the system, the tank vacuum actually pulls liquid oil from a pump, or more correctly, allows the higher air pressure outside to push it in.
As Lesker says, "vacuum doesn't suck".
Why guess when you can know? Measure!
Re: Backstreaming
The old VEECO operating manual gave similar advice to what Richard Hester mentions. They suggested 100 um for a mech. pump limit, and then by means of a small diameter line around the high vacuum valve, let the DP begin pumping from there. When the pressure gets to about 1 um, or a bit more, then the main high vac valve can be opened.
This also seems consistent with Doug's observations. The key point, is to realize that backstreaming is less of a concern with the low vapor pressure DP oil, than it is with the RP. DP oil has anywhere from 2 to 4 orders of magnitude lower vapor pressure at RT, than RP oil. This actually says it all.
It seems that if you have a DPwith an oil sealed rough pump system, it is better to use the DP throttled, than to use the RP wide open, when operating in the 1 - 20 um range. This approach minimizes the actual amount of oil backstreaming into the chamber.
A cold trap obviously cleans up almost everything, collecting HC vapors and water.
The scroll pumps are nice, being oil free, but if our lab experience is anything normal, we seem to be rebuilding them about once every year or two. But ours do run 24/7 mostly. And of course, there's the $$.
Dave Cooper
This also seems consistent with Doug's observations. The key point, is to realize that backstreaming is less of a concern with the low vapor pressure DP oil, than it is with the RP. DP oil has anywhere from 2 to 4 orders of magnitude lower vapor pressure at RT, than RP oil. This actually says it all.
It seems that if you have a DPwith an oil sealed rough pump system, it is better to use the DP throttled, than to use the RP wide open, when operating in the 1 - 20 um range. This approach minimizes the actual amount of oil backstreaming into the chamber.
A cold trap obviously cleans up almost everything, collecting HC vapors and water.
The scroll pumps are nice, being oil free, but if our lab experience is anything normal, we seem to be rebuilding them about once every year or two. But ours do run 24/7 mostly. And of course, there's the $$.
Dave Cooper
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Jerry Biehler
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Re: Backstreaming
When I cleaned out my system I managed to find a pint of Vacoil-S on ebay. Great stuff. Once I baked out my system it will go so low that I cant even read the pressure with my ion gauge which bottoms out at 2x10^10. And thats with a brand new ion controller. I have another one, and old Ultek, that will go down to -11 but I am still hitting the x-ray limit.
So far I have seen zero evidence of backstreaming with that stuff. The expensive oils are worth it. And its not bad on a small pump. But my 6" diff takes that entire pint!
So far I have seen zero evidence of backstreaming with that stuff. The expensive oils are worth it. And its not bad on a small pump. But my 6" diff takes that entire pint!
Re: Backstreaming
C'mon George,
Dustin
Dustin
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John Futter
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Re: Backstreaming
I'm not going to comment on back streaming from the diff pump into the vacuum chamber as this site has little to worry with the pressures that fusors run at.
However some sense has prevailed as to what happens with the backing pump connected to the diff pump.
As the mechanical pump speed slows towards zero ie ultimate vacuum there has to be a reason
Ultimate vacuum for a backing pump is where backstreaming = pumping speed.
Backstreaming in this case is made up of many things
these include the vapour pressure of the backing pump oil ---minimal
and the leakage past shaft seals and rotor tips ---maximal.
I have said in earlier posts that a foreline trap is preferred to stop the back streaming of the backing pump oil vapour to the diff pump.
I have serviced many pumping systems over the years and any system without a foreline trap of activated alumina to stop the backing pump oil back streaming has polluted the diff pump oil to a point where ultimate vacuum has risen by a magnitude after 2-3 years of continuous running.
Now take my SEM @ work built in 1974 with an Edwards 300l/s diffstack. It has been cleaned once and the original SANTOVAC returned to it with a top up to bring the level up to the dip stick level. It still reaches 7 by 10 to the minus 8 millibar overnight after a sample change ( a few clicks above what this pump is rated at new). The microscope uses two tricks to obtain this sort of longevity to the oil
1./ a Foreline trap to the backing pump ( the beads are changed once a year).
2./ and a buffer vessel that is pumped down by the backing pump and then the backing pump is isolated and turned off. The backing pump is restarted @ 1 by ten to the minus 1 millibar the isolation valve opened and the buffer space is pumped back down to 2 by ten to minus 2 millibar where the buffer space is again isolated from the pump and the backing pump turned off.
Not bad for thirty five years continuous running.
Now for the portable pumping stations 2 with air cooled diff pumps one with a water cooled unit, all with no foreline traps. These are not used continuously maybe if they are lucky get 25% usage.
1./ the backing pumps are serviced once a year ie oil change and if not reaching below ten to minus 2 millibar they are kitted ( the same for all backing pumps)
2./ the santovac is changed (and the black epoxy like crap from inside is removed the hard way with scotchbrite etc.
So whats the difference
all pumps pump similar vacuum spaces as per contamination.
The only difference is that the portable units do not have foreline traps as they also do not have valving to enable roughing out the vacuum space ie they are connected to the space to be evacuated and the pump station is started the diff and roughing pump come on together and the space is usually at backing pressure in 1-5 minutes waiting for the diff pump to heat and start pumping.
The reason for not using a foreline trap in these situations is that roughing through the system with a foreline trap will poison the foreline trap with water vapour and hang backing pressure at a point very close to the stall figure of the diff pump.
Now for the turbo systems
We have run new systems without foreline traps and the vacuum comes down to the mid 10 to the minus 8 millibar just the same as the systems with a foreline trap. However after a couple of weeks the ones without foreline traps the ultimate vacuum rises by an order of magnitude. Putting a foreline trap into one without sees the system slowly return to the ten to the minus eight range over a couple of weeks (the back streamed oil is very slow to evaporate )
Any way the upshot of all this is if you want a very clean system you will minimise the possibility of back streaming backing pump oil into your high vacuum pump.
High vacuum mantra
Cleanliness is next to Godliness
However some sense has prevailed as to what happens with the backing pump connected to the diff pump.
As the mechanical pump speed slows towards zero ie ultimate vacuum there has to be a reason
Ultimate vacuum for a backing pump is where backstreaming = pumping speed.
Backstreaming in this case is made up of many things
these include the vapour pressure of the backing pump oil ---minimal
and the leakage past shaft seals and rotor tips ---maximal.
I have said in earlier posts that a foreline trap is preferred to stop the back streaming of the backing pump oil vapour to the diff pump.
I have serviced many pumping systems over the years and any system without a foreline trap of activated alumina to stop the backing pump oil back streaming has polluted the diff pump oil to a point where ultimate vacuum has risen by a magnitude after 2-3 years of continuous running.
Now take my SEM @ work built in 1974 with an Edwards 300l/s diffstack. It has been cleaned once and the original SANTOVAC returned to it with a top up to bring the level up to the dip stick level. It still reaches 7 by 10 to the minus 8 millibar overnight after a sample change ( a few clicks above what this pump is rated at new). The microscope uses two tricks to obtain this sort of longevity to the oil
1./ a Foreline trap to the backing pump ( the beads are changed once a year).
2./ and a buffer vessel that is pumped down by the backing pump and then the backing pump is isolated and turned off. The backing pump is restarted @ 1 by ten to the minus 1 millibar the isolation valve opened and the buffer space is pumped back down to 2 by ten to minus 2 millibar where the buffer space is again isolated from the pump and the backing pump turned off.
Not bad for thirty five years continuous running.
Now for the portable pumping stations 2 with air cooled diff pumps one with a water cooled unit, all with no foreline traps. These are not used continuously maybe if they are lucky get 25% usage.
1./ the backing pumps are serviced once a year ie oil change and if not reaching below ten to minus 2 millibar they are kitted ( the same for all backing pumps)
2./ the santovac is changed (and the black epoxy like crap from inside is removed the hard way with scotchbrite etc.
So whats the difference
all pumps pump similar vacuum spaces as per contamination.
The only difference is that the portable units do not have foreline traps as they also do not have valving to enable roughing out the vacuum space ie they are connected to the space to be evacuated and the pump station is started the diff and roughing pump come on together and the space is usually at backing pressure in 1-5 minutes waiting for the diff pump to heat and start pumping.
The reason for not using a foreline trap in these situations is that roughing through the system with a foreline trap will poison the foreline trap with water vapour and hang backing pressure at a point very close to the stall figure of the diff pump.
Now for the turbo systems
We have run new systems without foreline traps and the vacuum comes down to the mid 10 to the minus 8 millibar just the same as the systems with a foreline trap. However after a couple of weeks the ones without foreline traps the ultimate vacuum rises by an order of magnitude. Putting a foreline trap into one without sees the system slowly return to the ten to the minus eight range over a couple of weeks (the back streamed oil is very slow to evaporate )
Any way the upshot of all this is if you want a very clean system you will minimise the possibility of back streaming backing pump oil into your high vacuum pump.
High vacuum mantra
Cleanliness is next to Godliness
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George Schmermund
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- Real name: George Schmermund
- Location: Carlsbad, CA
Re: Backstreaming
"Thus, backstreaming is no issue to anyone here doing fusion provided they just obey the rules and operate their gear as directed and use a half way decent oil." - Richard Hull
Rather than jump right into "backstreaming", I thought I'd offer up an anecdotal account from my tender youth.
The parents of a friend of mine had purchased a new Ford station wagon. This was back in the late '50s. His mother volunteered to transport him and me to some destination, wherever that was, in the new car. We were traveling on a back country highway and the speed limit was probably 50 mph. I didn't know much about cars or engines at that innocent age, but I could still tell that there was something wrong.
The first hint was that she would accelerate to 50 mph and then coast back down to 30 mph. Then do it again, and again, and again. I was sure she must be having a fit, or the like, because my old man never drove like this even when he was drunk, which was most of the time. The second clue was that the engine was revving up each time until it had the pitch of a model airplane engine at full throttle. I was in the rear seat and being buffeted about and trying to maintain control over what might have been a tuna fish sandwich I had for lunch. At one point I was able to latch onto the back of the front seat long enough to see that the automatic transmission lever at the base of the steering column was set to D1 or D2 instead of just D. My folks had a Ford of the same vintage and I did take some early mental notes about which letters were most often used. After some desperate cajoling I was able to get her to explore the effects of the different D choices and she finally agreed that plain old D did lend a more pleasing sound to the engine.
However, It was hopeless to get her to stop accelerating from 30 to 50 and back again because she told me that the car dealer (a high authority) had instructed her that the engine needed to be broken-in by driving between 30 mph and 50 mph for the first 1000 miles. She was insistent that she had to operate the car as directed! The rules she was given were favorably inclined toward a good outcome, but they were totally inscrutable to her. As I recall, that car spent a lot of time in the shop.
This was one of my early epiphanies that adults other than my old man could have serious cognitive impairments. I'm also confident that there was "halfway decent oil" in the crankcase.
The simplest moral to this story (if it must be explained) is that rote operation of a somewhat involved system can often lead to grossly misunderstood sets of rules. Truly understanding the reasons for any procedure will always win out in the end, though each operator must decide how much effort they are willing to invest in any attempt to grok a set of ideas.
I'll return to backstreaming when the spirits move me.
Rather than jump right into "backstreaming", I thought I'd offer up an anecdotal account from my tender youth.
The parents of a friend of mine had purchased a new Ford station wagon. This was back in the late '50s. His mother volunteered to transport him and me to some destination, wherever that was, in the new car. We were traveling on a back country highway and the speed limit was probably 50 mph. I didn't know much about cars or engines at that innocent age, but I could still tell that there was something wrong.
The first hint was that she would accelerate to 50 mph and then coast back down to 30 mph. Then do it again, and again, and again. I was sure she must be having a fit, or the like, because my old man never drove like this even when he was drunk, which was most of the time. The second clue was that the engine was revving up each time until it had the pitch of a model airplane engine at full throttle. I was in the rear seat and being buffeted about and trying to maintain control over what might have been a tuna fish sandwich I had for lunch. At one point I was able to latch onto the back of the front seat long enough to see that the automatic transmission lever at the base of the steering column was set to D1 or D2 instead of just D. My folks had a Ford of the same vintage and I did take some early mental notes about which letters were most often used. After some desperate cajoling I was able to get her to explore the effects of the different D choices and she finally agreed that plain old D did lend a more pleasing sound to the engine.
However, It was hopeless to get her to stop accelerating from 30 to 50 and back again because she told me that the car dealer (a high authority) had instructed her that the engine needed to be broken-in by driving between 30 mph and 50 mph for the first 1000 miles. She was insistent that she had to operate the car as directed! The rules she was given were favorably inclined toward a good outcome, but they were totally inscrutable to her. As I recall, that car spent a lot of time in the shop.
This was one of my early epiphanies that adults other than my old man could have serious cognitive impairments. I'm also confident that there was "halfway decent oil" in the crankcase.
The simplest moral to this story (if it must be explained) is that rote operation of a somewhat involved system can often lead to grossly misunderstood sets of rules. Truly understanding the reasons for any procedure will always win out in the end, though each operator must decide how much effort they are willing to invest in any attempt to grok a set of ideas.
I'll return to backstreaming when the spirits move me.
Anything obvious in high vacuum is probably wrong.
Re: Backstreaming
To John - Many thanks for that very well done perspective. You actually explained a "mystery" for me, concerning a large old evap system, that a former boss had acquired from a technical swap meet. It was roughed by a Welch 1397 rotary, and had a 10" Diff pump... not sure whose. It had a vacuum reservoir, and a mole-sieve type foreline trap.
Whatever its original use, the big 36" dia. SS belljar, was outfitted with water cooled RF feedthroughs, windows, some spare blanked ports, and the whole bell jar was water cooled. Wonderful piece of gear.
I wheedled to get permission to put it back in running condition. Other than needing some paint on the control panel, and an updated pressure monitoring box... (it had an old Veeco tube type electrometer), the thing hummed and ticked away... hitting below 10-6 in about 15 min, after turn on.
Initial pumpdown was fun. We had a shipping clerk who liked to fritter away his time on the internet, so when we did a pumpdown from atm, I enjoyed watching his reaction as the big 1397 started up like a Harley Davidson in the plant!
But I could never figure out why it needed a vacuum tank!! Thanks John... seems to fit.
George.... just reading your description of the ride from the back seat view... was giving my stomach a churn , too.
Dave Cooper
Whatever its original use, the big 36" dia. SS belljar, was outfitted with water cooled RF feedthroughs, windows, some spare blanked ports, and the whole bell jar was water cooled. Wonderful piece of gear.
I wheedled to get permission to put it back in running condition. Other than needing some paint on the control panel, and an updated pressure monitoring box... (it had an old Veeco tube type electrometer), the thing hummed and ticked away... hitting below 10-6 in about 15 min, after turn on.
Initial pumpdown was fun. We had a shipping clerk who liked to fritter away his time on the internet, so when we did a pumpdown from atm, I enjoyed watching his reaction as the big 1397 started up like a Harley Davidson in the plant!
But I could never figure out why it needed a vacuum tank!! Thanks John... seems to fit.
George.... just reading your description of the ride from the back seat view... was giving my stomach a churn , too.
Dave Cooper
Re: Backstreaming
Modern Vacuum Practice by Nigel Harris (former head of training at Edwards) is a very informative book, with a large chapter on diff pumps, and quite a bit on backstreaming. It even differentiates between backstreaming and back migration.
"Backstreaming is the direct flight of vapour molecules from the pump nozzle (particularly that of the top stage) towards the mouth of the pump.
Back-migration is the transfer of vapour to the high vacuum side by the re-evaporation of the fluid molecules which cling to surfaces within the pump, particularly the pump body adjacent to the top stage. Back-migration is thus temperature dependant and can be reduced by arranging the cold water inlet feed to supply this region first."
So the water inlet should always be at the top. It goes on to say that backstreaming can be reduced by a factor of 800 by utilizing baffles and cryo-traps.
It also covers critical backing pressure and 'stalling', as well as oils.
"Backstreaming is the direct flight of vapour molecules from the pump nozzle (particularly that of the top stage) towards the mouth of the pump.
Back-migration is the transfer of vapour to the high vacuum side by the re-evaporation of the fluid molecules which cling to surfaces within the pump, particularly the pump body adjacent to the top stage. Back-migration is thus temperature dependant and can be reduced by arranging the cold water inlet feed to supply this region first."
So the water inlet should always be at the top. It goes on to say that backstreaming can be reduced by a factor of 800 by utilizing baffles and cryo-traps.
It also covers critical backing pressure and 'stalling', as well as oils.
Re: Backstreaming
Ash Small wrote:
>..."
>
> So the water inlet should always be at the top. It goes on to say that backstreaming can be reduced by a factor of 800 by utilizing baffles and cryo-traps.
>
> It also covers critical backing pressure and 'stalling', as well as oils.
So its best to a diff pump with a built in water cooled inlet baffle such as an edwards Diffstak
rather than a basic diff pump?
>..."
>
> So the water inlet should always be at the top. It goes on to say that backstreaming can be reduced by a factor of 800 by utilizing baffles and cryo-traps.
>
> It also covers critical backing pressure and 'stalling', as well as oils.
So its best to a diff pump with a built in water cooled inlet baffle such as an edwards Diffstak
rather than a basic diff pump?
Re: Backstreaming
Yes, Derek, but most diffstacks seem to have a cryo-trap fitted, and most of us don't have liquid nitrogen on tap, although I've been reading up on some refrigerated ones, using fridge/freezer components and freon. It's possible to get to -50 centigrade, or lower. This isn't as good as around -200 for liquid nitrogen, but is still an improvement.
Re: Backstreaming
But isnt a water cooled baffle adequate for most purposes as the oil in question has a very low vapour pressure at 25 Deg C. (DC704 - 25°C: 2 x 10-8 Torr)? Isnt it only when you want to go below this pressure you need a lower trap temp and hence LN2 ?
Re: Backstreaming
From what I read, liquid nitrogen (or possibly liquid helium, which I understand is colder) is required to reduce backstreaming by a factor of 800, but water at 25 C will certainly reduce it, but not by as much.
I've tried to get a link to an article on using Freon 22, to get down to nearly -50 C, but the website appears to be 'down' at the moment. I will try again later.
EDIT: the link is below
http://lerch.no-ip.com/atm/
I've tried to get a link to an article on using Freon 22, to get down to nearly -50 C, but the website appears to be 'down' at the moment. I will try again later.
EDIT: the link is below
http://lerch.no-ip.com/atm/
Re: Backstreaming
Ash Small wrote:
> From what I read, liquid nitrogen (or possibly liquid helium, which I understand is colder) is required to reduce backstreaming by a factor of 800, but water at 25 C will certainly reduce it, but not by as much.
>
> I've tried to get a link to an article on using Freon 22, to get down to nearly -50 C, but the website appears to be 'down' at the moment. I will try again later.
>
> EDIT: the link is below
>
> http://lerch.no-ip.com/atm/
from the DC704 information attached one can calculate the vapour pressure
at 0C is 4.2E-10 torr
at -10C is 7.0 E-11 torr
at -20C is 1.0 E-11 torr
so a peltier device is sufficient. but LN2 is more fun
> From what I read, liquid nitrogen (or possibly liquid helium, which I understand is colder) is required to reduce backstreaming by a factor of 800, but water at 25 C will certainly reduce it, but not by as much.
>
> I've tried to get a link to an article on using Freon 22, to get down to nearly -50 C, but the website appears to be 'down' at the moment. I will try again later.
>
> EDIT: the link is below
>
> http://lerch.no-ip.com/atm/
from the DC704 information attached one can calculate the vapour pressure
at 0C is 4.2E-10 torr
at -10C is 7.0 E-11 torr
at -20C is 1.0 E-11 torr
so a peltier device is sufficient. but LN2 is more fun
- Attachments
-
- dc704.pdf
- (30.94 KiB) Downloaded 332 times
Re: Backstreaming
Derek, there is still some backstreaming, according to Dow Corning, but they describe it as 'negligible'.
You are also likely to have some backstreaming from the backing pump, unless it is a dry pump.
This will contaminate the diff pump oil, and will result in backstreaming into the chamber
Sales literature only ever gives one side of the story.
You are also likely to have some backstreaming from the backing pump, unless it is a dry pump.
This will contaminate the diff pump oil, and will result in backstreaming into the chamber
Sales literature only ever gives one side of the story.
Re: Backstreaming
Ash Small wrote:
> Derek, there is still some backstreaming, according to Dow Corning, but they describe it as 'negligible'.
>
> You are also likely to have some backstreaming from the backing pump, unless it is a dry pump.
>
> This will contaminate the diff pump oil, and will result in backstreaming into the chamber
>
> Sales literature only ever gives one side of the story.
Hence the peltier trap on the backing pump and the LN2 trap plus the diffstak ...
but then I go in for belt and braces.
> Derek, there is still some backstreaming, according to Dow Corning, but they describe it as 'negligible'.
>
> You are also likely to have some backstreaming from the backing pump, unless it is a dry pump.
>
> This will contaminate the diff pump oil, and will result in backstreaming into the chamber
>
> Sales literature only ever gives one side of the story.
Hence the peltier trap on the backing pump and the LN2 trap plus the diffstak ...
but then I go in for belt and braces.