Sulaiman
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Testing of small dc pumps for vacuum filtration
I have a bunch of small dc motor vacuum pumps that I want to compare with each other in a video.
I could test pressure with a Bourdon gauge or Hg U-tube,
I could time how long it takes to evacuate a given volume to a given pressure,
but I want to test how long it takes to filter stuff.
I have a 90mm Buchner for filter papers
and two 60ml sintered filters (15-40) and (2-5) um
what I don't have are 'reference' solid:liquid suspensions/slurries etc.
ie what should I filter to get near consistent results?
Any suggestions ?
PS cheap, non-toxic and a colour/visual change would be nice
[Edited on 15-8-2025 by Sulaiman]
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RU_KLO
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This is how I would proceed (brainstorming)
the main idea is to check:
1) how good was the filtering ( filter/filtration effectiveness by turbidity or by drying)
2) how long it takes (by volume)
3) how the other two relates to different substances.
Procedure:
1)if not graduated, adding a centimeter to the funel.
2)if not graduated, adding a centimeter to the receiving flask. (maybe not needed)
3) making a makeshift "turbid/color-imeter" with a flashlight, black box, and cellphone(+ any colorimeter/photometer app that could be used)
(attached a picture of what I meant)
4) using an excelsheet:
starting conditions:
substance:
weight:
volumen:
Note: (for example: it was stirred for 10 seconds)
a-timestamp
b-starting funel/ending funel meassure. If you want to only answer: "how much time takes for X ml), this is not necesary, but if you want a graph of
filtering against time (because of clogging) this information is needed.
c- receiving flask meassure. (maybe not needed)
d- starting turbidity of sample (using the meter)
e- turbidity of the filtered solution. (once finished, it should be stirred, because of decantation. (filter/filtration effectiveness)
another test, is to take the filtered solution, and redo the experiment (without cleanning the filter), to check how good is to make a second
filtration (this is used often in chemistry, so I think is good to know) or maybe a third.
(also if you have time, you could remove water by boiling and check weight of the substance against the starting condition (measssures with more
presicion the filter/filtration effectiveness)
and will try different reagents (CaSO4 should be used, because is found a lot in chemistry).
Water should be your first reagent as to have a known standard, and compare the rest to water.
You should try a reagent that settles fast, maybe a lead salt, an a reagent that makes a colloid.
You could make a solution and let it stand (decantation) and more or less check how it clears in a graduated cylinder.
if you want to go pro (or maybe take some ideas):
https://assets.thermofisher.com/TFS-Assets/LPD/Application-N...
ASTM D7315-17
Standard Test Method for Determination of Turbidity Above 1 Turbidity Unit (TU) in Static Mode
https://store.astm.org/d7315-17.html
If you proceed, please post your results.
Go SAFE, because stupidity and bad Luck exist.
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Sulaiman
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I see the value in your ideas but in this case I only want to compare the effectiveness of different vacuum pumps.
which filter catches what particles etc. is a much bigger topic,
I just want to test maybe 5 pumps vs 3 filters vs one or more test suspension/slurry samples.
Calcium sulphate is a good candidate,
I have some vermillion, that might be good
maybe just (coloured) water or glycerol would be enough to see the filtering performance of each pump ?
[Edited on 16-8-2025 by Sulaiman]
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AvBaeyer
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Sulaiman,
How about a slurry of diatomaceous earth (filter aid). Filtering such a mixture is a common lab procedure.
AvB
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Sulaiman
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Quote: Originally posted by AvBaeyer  | | ...How about a slurry of diatomaceous earth (filter aid). Filtering such a mixture is a common lab procedure... |
Excellent !
I could start a given mass of fresh diatomaceous earth for each run.
Now I'd like a coloured liquid that will be clear when filtered.
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bnull
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A slurry with glycerol as the liquid phase and a dye would do. The particles would take longer ro settle and you wouldn't need to shake the solution
time and again.
[Edited on 16-8-2025 by bnull]
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MrDoctor
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if you want an utterly horrendous, maximum pore-clogging sample, try mixing copper sulfate solution with something emulating hard calcium rich tap
water, to presumably produce a very specific calcium sulfate particulate. that stuff blocked filter paper while barely forming a layer that could be
felt.
I feel like thats probably a worse case scenario calcium sulfate precipitation that could be replicated, that people are also probably likely to
encounter at some point or another.
Also a suggestion: include a peristaltic pump if you care to source one sizeable enough for the task. peri pumps may be slow, but they can pull a
vacuum past -600mmHg, or at least thats what i observed with one once hooked directly to a gauge, albeit this is static pressure. the caveat with them
is the flow rate is very low and it will probably struggle to dry anything on the filter using a substantial flow of air sucking through. The benefit
however is that you can filter literally anything, no suck back, and in the rare event you actually destroy the tube, just replace it with some more.
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AvBaeyer
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"Now I'd like a coloured liquid that will be clear when filtered."
A dilute solution of copper sulfate should suffice. The diatomaceous earth will not absorb the copper sulfate. I meant to include this suggestion with
my previous comment.
By the way, I do not recommend using sintered funnels unless you first lay down a paper filter. My experience is that filter aid clogs these types of
funnels which then become very difficult to impossible to clean.
AvB
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Sulaiman
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@MrDoctor: why do you want to make my life difficult ? 
calcium sulphate aka gypsum plaster is one of my least favourite things to filter.
(my least favourite is hot saturated solutions of anything)
I have two banks of three peristaltic pumps ex-dairy
after replacing the silicone tubing each one got to -1atm on a cheap Bourdon gauge,
maybe a tiny bit more pressure than my dual-stage rotary.
very very very slow pumping down though.
https://youtu.be/gGtYlS1ihbE?si=fSSyno99yzMYdBap
___________________________________________
@AvBaeyer: thanks for the diatomaceous earth vs frit tip.
____________________________________________
My new vacuum pump arrived yesterday - with a crack in its plastic body.
I've claimed refund already but this will delay these experiments.
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MrDoctor
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my point was that it IS unpleasant to filter, and its a contaminant that can occasionally pop up unexpectedly.
Also AvBaeyer, filtering the copper sulfate without the insoluble hard-water precipitate renders it meaningless. something as innocent as trying to
recrystalize copper sulfate accidentally using tap water, can render it unfilterable, which is especially problematic given you want to filter it
quickly while its hot, that first time.
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Sulaiman
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| Quote: Originally posted by MrDoctor | my point was that it IS unpleasant to filter, and its a contaminant that can occasionally pop up unexpectedly.
Also AvBaeyer, filtering the copper sulfate without the insoluble hard-water precipitate renders it meaningless. something as innocent as trying to
recrystalize copper sulfate accidentally using tap water, can render it unfilterable, which is especially problematic given you want to filter it
quickly while its hot, that first time. |
isn't it amazing how something as seemingly simple as filtering
stuff
is not simple at all.
Like everything, the more you learn the more you realise you know very little
__________________________
I may try CuSO4 with added calcium ions,
it is a good representative test,
I could filter a cloudy suspension to transparent blue/green - nice.
but not hot and saturated 
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Sulaiman
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for sale at the moment
Because the new dc motor vacuum pump that I ordered arrived broken
I got a refund and looked again for "dc motor vacuum pump"
Just out of curiosity I looked for the cheapest:
For free postage (and spares) I ordered 3 for equiv.USD4 total
just arrived and tested .....
better than -60kPa at 3.7V 330mA
a USD1.33 pump, some tubing plus a lithium cell and you're ready to go.
more than enough for vacuum filtration - I expect ......tbd
[Edited on 21-8-2025 by Sulaiman]
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MrDoctor
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i have those exact ones, they are nifty little suckers. i have always wondered if there wasnt some sort of counter-solvent i could introduce so those
rubber diaphragm pumps would stay safe. like say, via a submerged bleeder valve introducing water, or vegetable oil or something like that, so it
always sputters out a bit and hopefully causes any absorbed solvent to diffuse onto the bleeder-solvent keeping the rubber relatively pure and safe.
ive seen this implemented for peri pumps but moreso because i think they work better with a little something to actually pump so the hose isnt just
fully collapsed under vacuum, or being slick with a lubricant just helps i guess.
If anyone should choose to make the compromise on one of those big cheap plastic open piston-pumps, something important to keep in mind is they dont
use any sort, i guess cam-bearing? basically that pison is directly hanging on an unlubricated metal rod with no bearing or bushing, and 3 minutes
under load melts that rod right out.
even if you submerge it in lubricant, which is a merit to the design, it will still melt without a strict, low duty cycle. whatever the intended
application is, it is weirdly common, or at least was at some time, so be cautious about that, just because it uses a replaceable O-ring as a piston
seal and can be lubricated under oil to control wearing and sealing. big waste of money.
[Edited on 21-8-2025 by MrDoctor]
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bnull
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@MrDoctor: Please scale down the picture.
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MrDoctor
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sorry, i dont know how. i tried html code but it doesnt seem to do anything
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bnull
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Right, I forgot that HTML is off and [img] has no size option.
Edit: Some sites store images in different sizes. The one above has a 640x640 available (https://cdn11.bigcommerce.com/s-g34rj6mm5p/images/stencil/64...)
This is one workaround that occasionally works. The other is resizing the picture in a phone or computer application.
[Edited on 22-8-2025 by bnull]
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Sulaiman
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just testing
I just downloaded the photo, cropped it a tiny bit and uploaded it.
The differences must be in our OS and/or browser
[Edited on 22-8-2025 by Sulaiman]
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bnull
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The difference is in the way the picture is added to the post. Using [img] puts the picture in the post without uploading it to the forum server; that
has the problem of leaving the picture with its real size and the possibility of link rot. Uploading the picture to the forum server avoids link rot
and keeps its size under control but makes it necessary to download and manipulate the pic before posting.
About @MrDoctor's no-bushing plastic piston pump, one can repurpose a sleeve bushing from another motor. It shouldn't be hard to find an appropriate
bushing and widen the piston shaft hole to fit it.
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Sulaiman
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| Quote: Originally posted by bnull | | About @MrDoctor's no-bushing plastic piston pump, one can repurpose a sleeve bushing from another motor. It shouldn't be hard to find an appropriate
bushing and widen the piston shaft hole to fit it. |
ten years ago I thought so https://www.sciencemadness.org/talk/viewthread.php?tid=62587
I still have one pump with a melted bearing
my other one still runs but it is very noisy and it struggles to pull -70kPa now.
it is ok for filtration but the noise and vibration is too much for me,
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MrDoctor
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its all thermoplastic so, its hard to say if the bushing, which will have to be a thin metal sleeve essentially, will have much impact. it doesnt have
to get that hot to melt out, also, the big damage is done under vacuum moreso than compression since under vacuum its pulling, not pushing into
something able to offer resistance.
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Sulaiman
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possibly repairable but not worth the effort
the orange/brown is due to my melting the ends back together with a soldering iron - a poor choice
I kept it as spares for the working pump
but soon both of the pump sections will be in the garbage bin
and I will add the two motors to my collection of very useful but never actually used motors
[Edited on 23-8-2025 by Sulaiman]
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MrDoctor
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will vibrate like crazy,any 3D printable solutions would probably not hold up reliably.
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macckone
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I use this pump to run a generic plastic venturi aspirator.
Pump: https://a.co/d/3SMzd4Z
Aspitator: https://a.co/d/ipWIlvF
You can find cheaper versions and I didn't pay $40 for the aspirator.
Some tubing a fitting and a gallon water jug make a system.
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