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jpsmith123
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Vacuum From Peristaltic Pumps
I'm wondering, has anyone ever used a peristaltic pump as a vacuum pump?
It seems to me that a peristaltic pump should be capable of achieving a fairly low ultimate pressure...albeit at a very low throughput.
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chromium
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I once tried to make one but i ran into problems which i was unable to solve.
Tubing did not stay in the place. It got streched and jammed in every possible way and this often led to losing vacuum. I still saw less than 10 torr
at my vacuum meter once or twice.
My pump was with rather good ball bearings as rolls but this did not help. It seems that tube must be very well fixed (there is great force applyed to
the tube in the axial as well as in the radial direction).
To be honest i have no idea what type of tubing should be used for good results. I did my tests with rubber tubing but, as i said, results were
disapointing.
If someone has seen such vacuum pump working, i would like very much to hear more about it.
[Edited on 14-1-2008 by chromium]
When all think alike, then no one is thinking. - Walter Lippmann
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Magpie
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We used a peristaltic pump to feed a brine where I once worked. Polyurethane hose gave the best life in our experience.
I have never heard of a peristaltic pump being used to pump gases. I'm surprised that you could develop a good vacuum with one.
The single most important condition for a successful synthesis is good mixing - Nicodem
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jpsmith123
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You tried to build the pump from scratch? I was thinking more along the lines of buying a used commercial pump on Ebay, or at least buying the pump
head.
It seems to me it should go well below 1 torr...if you're pumping a small volume.
Also, I'm thinking that maybe a multichannel pump could be connected to a manifold (i.e., in parallel) so that the throughput could be increased?
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microcosmicus
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As for ultimate pressure, the situation is going to be the same as with
any other mechanical pump --- you cannot obtain a vacuum less than a
millitorr because, at that point, the gas laws on which mechanical
pumps are based no longer hold. As for attaining that theoretical limit,
maybe try some sort of multistage arrangement like having the different
stages as different portions of the same hose.. If each stage reduces
the pressure by a factor of x, then n stages will reduce it by x^n so
even an inefficient pump can attain a low vacuum with enough stages.
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jpsmith123
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I'm not sure I see how 1 millitorr should be a theoretical limit for a mechanical pump. I know a mechanical pump with a molecular sieve trap can go
below 1 millitorr.
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microcosmicus
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A molecular sieve is not a mechanical pump --- it relies on
adsorption of individual molecules as opposed to compressing
gas in bulk.
Basically, the reason for the limit is that, once you get to a millitorr,
the gas is too dilute for there to be significant collisions between
molecules. Analyze how a mechanical pump works and you will
realize that the molecules in the molecules in the space you are
trying to evacuate bumping into each other and thereby being pushed
into the pump is important. Once the pressure becomes so low
that a molecule can travel across the length of the apparatus with
a good chance of not encountering another molecule, all that will
happen is that molecules will randomly move between the pump
and the bell jar with no net effect.
For this reason, pumps which make high vacuum, such as sorption
pumps, diffusion pumps, turbomolecular pumps, getters, etc. deal
with molecules on an individual basis rather than by compressing
gas in bulk.
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jpsmith123
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I didn't mean to imply that a mol. sieve trap "is a mechanical pump", I was merely relating my personal experience that a so-trapped mechanical pump
(with good oil) will go well below 1 millitorr; in part because of its ability to adsorb backstreaming oil which will occur when the system goes into
the molecular flow regime (or perhaps out of the "viscous flow regime"). (And yes, I know how a mol. sieve trap works).
You said that a mechanical pump will not go below *1 millitorr*; I was merely questioning that statement.
[Edited on by jpsmith123]
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microcosmicus
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Sorry about that -- I was merely pointing out some basic facts which
you obviously already know.
As for the millitorr, that was only meant as a round figure for when mean
free path becomes macroscopic (it is 5 cm long at that pressure and room
temperature) so one has molecular flow. I definitely wasn't trying to say that
the lower limit is precisely 1 militorr.
[Edited on 13-1-2008 by microcosmicus]
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vulture
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Peristaltic pumps are designed for pumping liquids, they suck at vacuum production. Well, they don't suck well enough.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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jpsmith123
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I don't think there's any theoretical reason why they can't make a half-decent vacuum. Assuming that the rollers are completely sealing the tubing, I
think that for small volumes, the ultimate pressure attainable depends on the vacuum rating of the tubing used...IOW if the walls are too thin the
tubing will collapse.
Here's a really crude and cheap linear peristaltic pump that someone threw together and it apparently goes lower than 1 torr.
[Edited on by jpsmith123]
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chromium
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Theoretically it can indeed go to very low pressures but there are tehnical difficulties that must be overcome and it does not seem easy.
If you build this very simple apparatus from your link you will see that to close tube relialby you must apply strong force. This not only presses
walls of tubing together but also deforms it in great extent. Tubing starts to twist, to move chaotically in sideways and most important - along with
rollers. This axial force is so strong that it can stretch your tubing to double of its length if it is anything that can be stretched at all.
I know, there are some commercial vacuum pumps that use this technique. Unfortunately i have not seen one myself but i am sure this needs special sort
of tubing and there must be some nontrivial tricks to hold it in place and to release unnecessary tensions that otherwise will destroy the tubing or
tear it apart from connectors.
[Edited on 15-1-2008 by chromium]
[Edited on 16-1-2008 by chromium]
When all think alike, then no one is thinking. - Walter Lippmann
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vulture
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| Quote: |
I don't think there's any theoretical reason why they can't make a half-decent vacuum. Assuming that the rollers are completely sealing the tubing, I
think that for small volumes, the ultimate pressure attainable depends on the vacuum rating of the tubing used...IOW if the walls are too thin the
tubing will collapse. |
And that is exactly why it doesn't work very well. If the vacuum rating of the tubing is high, it is rather unflexible and won't seal completely when
pumping. Also it'll require considerable force.
A fridge compressor is much better as long as you keep nasty gasses out of it. Or build your own membrane pump.
One shouldn't accept or resort to the mutilation of science to appease the mentally impaired.
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jpsmith123
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So you agree with me that there are no theoretical obstacles, only engineering issues, right?
And those are the issues I'm exploring. They may turn out to be show-stoppers, and they may not.
This tubing caught my eye. I think it's intended for peristaltic pumps (although it doesn't say that specifically...so I'm going to ask them).
I'm interested in the 3/8" ID 5/8" OD tubing. Looks Like it's rated for full vacuum, yet the wall thickness is only 1/8". I know that "Masterflex"
makes pump heads that will take 3/8" ID tubing, but I don't know what the OD of the Masterflex tubing is.
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chromium
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I may be wrong but i am sceptical on this type of tubing. Mainly because it is too strechable and smoothness of inner surface may be downside as
stronger force might be needed to close it completely.
I think that one needs something trully special like soft silicon tubing which is armored with polyamide threads to make it nonstretchable and with
special soft inner surface so that it closes vacuum-tight even if walls are pressed together with only slight force.
I agree that problems are technical and if you can find type of tubing that works and lasts, you can go further than 0.1 torr. With ideal tubing
(which, of course, does not exist) and infinite amount of time you could eliminate even last gas molecules. In reality it is not easy way. Good luck!
[Edited on 16-1-2008 by chromium]
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microcosmicus
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This topic has gotten me interested enough that I am going to try to
build a peristaltic vacuum pump. Even though such a contraption may
be too inefficient for consideration as a practical vacuum pump, it at
least promises to be a fun project.
The first order of business for me was to determine the force necessary
to seal the tubing. To test this, I attached a metal beam to my workbench,
on which I placed the tubing. To simulate the roller in a pump, I placed a
piece of pipe on top of the tubing (9.5 mm diameter). Through this pipe
ran a string attached to a bucket underneath the beam. By pouring water
into the bucket, I gradually increased the force of the pipe on the tubing.
I blew into one end of the tubing and closed the other end with a bit of
soapy water; when the weight in the bucket had gotten to the point that
bubbles no longer formed, I weighed the water.
I had on hand both some regular aquarium tubing and silicone tubing.
The regular tubing required 10 kg (100 N force) to pinch it off, whilst the
silicone tubing required 3 kg (30 N force) to pinch off.
I plan to go with the silicone tubing in building the pump. For the record,
this tubing has an inner diameter of 4 mm and 1 mm thick walls. A quick
test with a roller (a crude version of the linear pump in the link given
above) shows that this tubing should be suitable for making a
peristaltic vacuum pump.
To be safe, I will throw in a safety factor and apply a force more like
50 N in the actual pump. Also, 1 cm might be too small a roller size,
so I will repeat my measurement with a bigger pipe to see what force
would be needed with a larger roller. One of these days, it might make
for an entertaining exercise in continuum mechanics to compute the force
for pinching off tubing in terms of elastic constants but, for the purpose
of this project, the measured values are all that is needed.
As for the problem of stretching and tubing moving around, I am quite
sure that can be overcome by intelligent friction management with no
need for threads or similar recourses. As for sealing the inside well, I
suspect oiling the inside of tubing should help. Even if some of the
gas does leak past the seal, as long as the rate of pumping exceeds
the rate of leakage by a reasonable factor, that difficulty should be
surmountable by using multiple stages.
Next, I will repeat my measurement with a different size pipe as
mentioned above, measure friction coefficients and try out my plan
for dealing with stretching. Hopefully, I'll have some more results
to share tommorow or so.
[Edited on 17-1-2008 by microcosmicus]
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jpsmith123
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I sent some emails to a few companies asking a few questions about the ratings of their tubing and suitability for use in peristaltic pumps.
I see a peristaltic pump as a potentially very handy, general purpose pump that could be used not only to pump liquids, but to pull vacuum for
filtration, distillation, CVD experiments using methanol-water vapor, or anything else where you don't want to worry about condensable vapors
contaminating the pump, or pump oil contaminating anything, etc.
If it seems possible to use tubing that's 3/8" ID or larger, and rated for "full" vacuum, I may start looking for a pump on Ebay.
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microcosmicus
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Filtration and distillation sound like a poor match for a peristaltic vacuum
pump to me. Those applications don't need much of a vacuum but do
require high throughput rates to keep pulling out vapor being boiled off
and the like. By contrast, the discussion above leads me to expect that a
peristaltic pump would have the opposite characteristics --- high vacuum
at low throughput. For filtering and distilling, I would rather use the peristaltic
pump to feed liquid through an aspirator than directly pull vacuum.
When you hear back from the companies, please let me know what they
say about the ratings of tubing. For the time being, I am fine with the
silicone tubing I already have and don't want to spend money on this
project yet. However, if I get a prototype working reasonably well, then I
would certainly be interested in seeing whether it could be improved by
choice of tubing.
I've thought about and puttered around a little more with this project with the
last post. As chromium pointed out, for a peristaltic pump to pull good vacuum,
one is going to need to apply enough force to seal the tubing well. Based on
my measurements, the force required is not inordinately large, but it would
be good to minimize it. Therefore, I am thinking of small rollers rather than
the big brayer in the link above or the usual pump rollers. Last night, I made
a quick measurement and noticed that, by going to a roller with a diameter of
3 mm, I could cut down the force to 20 N.
So what I am now thinking of is an odd looking roller in the form of a shaft which
goes through the inner holes on two ball bearings at either end. As time permits,
I will make more careful measurements, look into keeping the hose from
slipping and stretching, and try repeating the demo in the link above with
this new sort of roller.
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jpsmith123
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Some Correspondence
Here's an email exchange I had with Tech Support at Masterflex/Cole-Parmer:
I'm wondering whether any of your peristaltic pumps can be equipped with tubing rated for full vacuum, i.e., 29.9" Hg or better? I'm
particularly interested in the larger size tubing, e.g., 3/8" ID.
Thank you for your recent inquiry and interest in Cole-Parmer.
Unfortunately all the tubing that is soft enough to be occluded in the peristaltic pumps is not strong enough to withstand full vacuum. The tubing
will collapse.
Some of the more rigid tubing, like Masterflex Norprene 06404-35 will handle some vacuum (hard to determine the actual amount), but definitely not
full vacuum.
Thanks for your reply.
I wonder if I may ask you one more question: Most 3/8" ID plastic vacuum tubing I've seen has very thick walls; however, Saint-Gobain part #ABW00029
(as per the below link) seems to be rated at "full" vacuum (i.e., 29.9" Hg in this case). Is it possible that this tubing would work in any of your
pumps?
Thank you for your continued interest.
Masterflex has BioPharm tubing very similar to the Tygon 3350.
96420-35 is BioPharm tubing that will work with our Masterflex L/S line of peristaltic pumps and according to St. Gobain, may be able to handle full
vacuum.
I do not have any data on this, however, we do trust the specifications from St. Gobain for their tubing.
#####################################
So I suppose that sounds somewhat promising.
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microcosmicus
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| Quote: |
So I suppose that sounds somewhat promising.
|
Another promising fact which showed up in my measurements was that the
pressure needed to seal off even the soft silicone tubing was several times
atmospheric pressure. Using the value of 20 N for a 3mm diameter roller
given above and the fact that the tubing is 8mm wide gives a quick estimate
of 20/(3x8) = 0.8 N/mm^2 = 8 atm.
To be sure, my silicone tube does not do well under vacuum --- it collapses.
However, looking more closely at the matter, one notices that collapsed
does not mean completely shut --- rather, while the tubing is flattened, near
the ends, where it is most bent, there remain small channels:
O===============================O
Of course, the cross-sectional area in this case is ridiculously small so,
while this would work in a pump, it would also be preposterously
inefficient. Rather, one needs tubing with a much thicker wall relative
to the inner diameter..
When I get a chance. I'll have to sit down and do some elasticity theory
to work out how the cross-sectional area depends on applied pressure,
in particular look at the critical pressure where the tubing shuts off.
Basically, there are two pressures involved here, the pressure of the
rollers and the atmospheric pressure. The former pressure, of course,
should be set equal to or slightly larger than the critical pressure. Then
the question becomes what is a good wall thickness for a given tubing
material so that the area of the tubing under atmospheric pressure is
reasonably big while the critical pressure is not too large.
[Edited on 23-1-2008 by microcosmicus]
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jpsmith123
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Here's an interesting pump design:
http://www.eccentricpumps.com/index.html
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unionised
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http://www.tinkerhack.com/vac7.htm
gives a design for a peristaltic vac pump.
Incidentally, the last time I looked, turbomolecular pumps were mechanical and could get well below 1mTorr
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indigofuzzy
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Today, I successfully built and used my first peristaltic vacuum pump. I used aquarium air hoses and an aquarium check valve, routing the output gas
into a tub of mineral oil. I also put some mineral oil in the main pump tube to aid in sealing. I was able to pull enough vacuum on small glass tubes
(about the size of miniature holiday tree lights, actually, that was the first tube I pulled a vacuum on) to see purple plasma arcing around with a
few kV thrown in. I'll post some pictures when I can convince my roommate to play cameraman. I've been using a tiny solid state tesla coil as a power
source, and need my fingers to be on the glass to pull the arcs, so I really don't feel like holding my digital camera in one hand and a few kV in the
other 
Trouble is, the tubing collapses under the weight of the atmosphere before I can get down to a low enough pressure to see a discharge without touching
the glass.
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ShadowWarrior4444
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| Quote: | Originally posted by indigofuzzy
Trouble is, the tubing collapses under the weight of the atmosphere before I can get down to a low enough pressure to see a discharge without touching
the glass. |
Thick, braided low pressure tubing--should be available from your nearest large hardware store (Lowes, Home Depot, for those in the US.) I seem to
recall the clear vinyl ones being quite useful, though you may want something more resistant if you’re using them near plasma. (The clear ones are
perhaps... vinyl acetate? I didn't dig too deeply.)
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indigofuzzy
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Rarefied air makes such lovely discharges!
And a closeup of the discharge:
[edit] and now, I'm officially a hazard to others! yippee!
[Edited on 5.17.2008 by indigofuzzy]
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