Sciencemadness Discussion Board

Suggestions on vacuum pump purchase

KoiosPhoebus - 10-3-2024 at 20:46

Hi everyone! I'm contemplating getting a vacuum pump for vacuum distillations/desiccation and working with solutions which readily oxidise in contact with air (e.g. certain iron(II)/manganese(II) compounds and some organics). My solvents will primarily be water, or various lower alcohols (and maybe acetone, for situations where I've washed a precipitate with acetone). My major concerns are excluding sufficient oxygen to prevent the dissolved compounds from oxidising (especially when heated - an example is anhydrous iron(II) chloride, which is reasonably stable once it has been isolated from solution but can oxidise readily upon contact with the air when dissolved) and lowering the boiling point to make evaporating the solvent easier.

I've been reading up on various vacuum pump types and I'd appreciate some advice on what I actually need for my use case. The types I've considered are as follows:

  1. Vacuum aspirator: I've heard great things about a vacuum aspirator for vacuum filtration but very mixed results on distillation/desiccation. In particular, I've heard that vacuum aspirators often don't generate enough vacuum to meaningfully lower the boiling point. This would be a concern when working with air-sensitive compounds/solutions, which may still oxidise if sufficient oxygen remains in the system. An aspirator does have the advantage of "scrubbing" any chemical vapours as it runs though, so the system is less susceptible to any vapour damage.


  2. Small, relatively cheap diaphragm vacuum pumps: There are some diaphragm vacuum pumps on Aliexpress which claim to pull -90kPa of vacuum for about ~40USD (e.g. the UV-U3 model). I've seen cheaper models which are rated for -60kPa, but I doubt -60kPa is sufficient for my use cases.

    Pros: Relatively cheap (can be easily replaced if broken), low price means that multiple can be connected in series to increase vacuum depth if I have reasonable results with the first.
    Being a diaphragm pump means that I don't have to worry as much about organic solvent vapours recondensing in the pump and damaging the innards.

    Cons: I will have to set up all the circuitry and housing myself (which I don't have experience in), and I'm unsure if it will actually pull the rated -90kPa. Additionally, I'm not entirely sure if -90kPa is sufficient for my use cases (in particular - would it allow for effective removal of water from a sample in a desiccator?).
    The low price does make me wonder how reliable it is (expected lifespan etc). Even if it survives organic solvent vapours, it might simply wear out quickly.


  3. Single-stage rotary vane pump: I specified single-stage as I'm not sure that a double-stage pump is really necessary for my needs. There's a few brand-new single-stage rotary-vane pumps going for 56-63 USD on eBay in my country (RS-1 model). They claim to be able to go to an ultimate vacuum of "5Pa/0.05mbar", which should be more than sufficient for my use cases. My main concerns with rotary vane pumps are mostly associated with the oil seal - I've been told that they cannot be used with distillation or even filtration of organic solvents lest the solvents recondense in the oil and destroy the pump.

    Pros: Will almost certainly achieve sufficient vacuum depth for my use case.
    Very reasonably-priced.

    Cons: Being a rotary vane pump, I don't know if it'll survive use with organic solvent vapours. I could buy a cold trap/inlet filter unit but my understanding is that a gas washing bottle would add another 10+ USD onto the total price which would cause the cost of such a pump to approach that of a low-end laboratory diaphragm vacuum pump (see next entry).
    Changing the oil is an ongoing cost, especially if I have to keep changing the oil whenever I draw a vacuum on samples containing organic solvents.
    I have read that some rotary vane vacuum pumps, especially the lower-end ones (like the kind listed here), emit oil fumes when running. If it is a problem, this would limit where I could use the vacuum pump. There is one review from February this year on the pump I'm considering which says that it does emit a "oil fog", although the seller claims that the pump has a filter to filter out such fumes.


  4. Oil-free vacuum pump: I'm not entirely sure if it's a piston or diaphragm vacuum pump, but it's definitely an oil-free model (BST-V100D). The manufacturer has rated it to achieve -98kPa of vacuum, which I think should be sufficient for my use cases. It's pricier than the options above at ~90 USD (including shipping).

    Pros: -98kPa of vacuum should achieve my desired goals (solvent removal, vacuum desiccation, minimising oxidation).
    Being an oil-free pump, the concerns above regarding oil fumes, solvent contamination, and the continued maintenance cost of oil changes are non-issues.

    Cons: More expensive than the other options. I would prefer not to spend the money if I don't need to, I'm just unsure if the additional cost is necessary given my use cases.
    Sold by the manufacturer on Alibaba. Everything on this list is probably made in China anyway, but I am a little cautious on whether the product page is a scam, or what the refund process might be if I get shipped a lemon/it gets damaged in transit. The seller has a good review, but it's from 2022 and they don't seem to have any reviews since then (link to seller profile: https://brostun.en.alibaba.com/company_profile.html?spm=a270...)
    Unsure at the moment if it comes with an inlet filter. If I have to buy an inlet filter myself, it might be better to just buy a rotary vane pump and buy a combined cold trap/inlet filter anyway? To be honest, I'm not even sure how useful the inlet filter would be for the lifespan of the pump.

Any advice would be greatly appreciated! I'd be interested in hearing about peoples' experiences with vacuum distillation/desiccation and with attempting to isolate compounds which either oxidise when heated in solution or otherwise refuse to crystallise upon heating. In particular, I'm unsure as to what kind of vacuum depth is sufficient for my use cases, and how much of an issue the oil thing is with rotary vane pumps.

Oh, and if anyone has suggestions on platforms I might be able to find vacuum pumps, that would also be helpful! Here's what I've used so far:
  • Amazon (not very useful - everything tends to be sold by the same sellers, just marked up)
  • ebay (This is where I found the rotary vane vacuum pump mentioned above. There's a few used ones on sale at the moment, but they're old + expensive enough that it makes no sense to get them over the new ones)
  • Aliexpress (Only usable for the cheap, low-end stuff. Middle to high end stuff is often found more cheaply on Alibaba)
  • Alibaba (This is where I found the oil-free vacuum pump. Filter by 'Ready to Ship' to get listings with shipping quotes)
  • Gumtree (Local ads, including used equipment. As with ebay, prices at the moment are high enough that I might as well buy a brand-new vacuum pump)
  • Techtrader/Machines4u (The platforms I've found so far for used equipment. Again, prices are high enough that I might as well buy a brand-new pump. In many cases, these are used high-end pumps, which means that even discounted, they are more expensive than the options listed above)

Sulaiman - 10-3-2024 at 23:49

I'm not expert but I think that a good combination may be
An aspirator and a bottle of argon gas (welding suppliers)

Rainwater - 11-3-2024 at 01:34

A water aspirator is the cheapest most reliable and weakest and doesnt require special machining skills to make(but skills help)

Chemically 'resistant' oil free vacs might be the most expensive. But are the easiest to setup and use in my opinion. Mild vacuum strength.

1 stage rotary pump have their own drawbacks. Giving strong vacuum
The cheap ones have a low duty factor ( run time / off rime ratio) usually 5-15 minutes on, 30 minutes off. Expensive ones can run continuously. And they must be protected from contamination. (Cold trap)
As for a non oxidizing gas source, I have found this product useful, it delayed me spending $400 on a larger tank. But is the most expensive way to obtain it( per pound )

Keras - 11-3-2024 at 02:39

I have a water aspirator, but while this is OK for, eg, vacuum distillation, it is an overkill for vacuum filtrations, and definitely very cumbersome to set up to just filter crystals. So, for vacuum filtrations I bought a small cheap diaphragm pump, and I’m pretty happy with it. Both pumps run on < 12 V DC (the vacuum aspirator pump is a rugged pump designed to sprinkle pesticides on crops, so it is pretty resistant to chemicals) and I power them with a basic DC power supply that can be controlled both in voltage (modulation of the vacuum) and current (protection against shorts).

Sulaiman - 11-3-2024 at 03:16

If the equipment volume is <1 litre,
and you have some patience,
Peristaltic pumps can produce a surprisingly good vacuum.
eg my little 12v vacuum pump gave -0.8atm when new, and -0.7atm after much abuse.
My peristaltic pumps move the needle on my cheap gauge to almost the same indication as my dual-stage rotary.
https://www.youtube.com/watch?v=gGtYlS1ihbE

fx-991ex - 11-3-2024 at 05:06

If you have a 3d printer you can print a vacuum aspirator for free :). worth giving it a try.

Ubya - 11-3-2024 at 05:32

I'd say go for the oil-free vacuum pump.
In theory you could use any of those pumps for what you want to do, if you are ok with the various caveats.
I used cheap chinese diaphragm "vacuum" pumps for filtration, they are not chemically resistant, (acetone vapours killed mine). Even if they are cheap, you are going to run through a few of them quickly.

I used a single stage rotary vane vacuum pump, the same-ish model you pointed out. I didn't change the oil enough, maybe twice in 6 years (with light use...) and recently I had the pleasure to dismantle the whole thing and clean rust from inside the pump. I'm now using it only for my physics experiments, no more water vapour or solvents xD

Right now for filtrations i'm using a fidge compressor to which i changed the original moisture sensitive oil with normal mineral compressor oil. I don't use it often, and this oil is much cheaper than rotary vane vacuum oil.

For distillations I use a water aspirator with its own closed water loop. if you keep the water cold, you can achieve pretty good vacuum, the limit is the vapour pressure of the water. But i don't use it often anymore because it is way too loud for my tastes.

If you want to work with oxygen sensitive compounds, just a vacuum won't be enough. A rotary vane vacuum pump that can remove 99.999% of air will do, but instead of buying multiple pumps you can buy a single lab diaphragm pump, and just flush whatever air sensitive apparatus you have with a bit of argon or nitrogen. If you pull a vacuum, fill the apparatus with argon/nitrogen and repeat this steps 2-3 times, you can be sure there won't be any oxygen left, and you won't actually use that much inert gas, so a bottle might last you years. Cheaper than having to buy multiple pumps, and having to deal with oil changes whenever you use the rotary vane pump.



[Edited on 11-3-2024 by Ubya]

KoiosPhoebus - 13-3-2024 at 13:27

Thanks for everyone's advice!
Quote: Originally posted by Sulaiman  
If the equipment volume is <1 litre,
and you have some patience,
Peristaltic pumps can produce a surprisingly good vacuum.
eg my little 12v vacuum pump gave -0.8atm when new, and -0.7atm after much abuse.
My peristaltic pumps move the needle on my cheap gauge to almost the same indication as my dual-stage rotary.
https://www.youtube.com/watch?v=gGtYlS1ihbE

Thanks for the suggestion Sulaiman. What model + brand of peristaltic pump is that? My concern with the peristaltic pumps is that, as far as I can tell, they're more meant for dosing than for vacuum work, so most peristaltic pump listings don't provide a rated vacuum pressure. My understanding from this thread is that whether the peristaltic pump can pull a decent vacuum is quite dependent on the tubing used - hence I'd probably have to get something with similar tubing material/quality as the ones you used in that video.

Also, the 12V vacuum pump you mentioned is different from the peristaltic pumps in the video, right? Because I have a pump like that - it reaches -70kPa, but I've found that this isn't sufficient for vacuum desiccation or avoiding oxidation of the compounds I'm working with.
Quote: Originally posted by fx-991ex  
If you have a 3d printer you can print a vacuum aspirator for free :). worth giving it a try.

It's an intriguing idea, but my main concerns with the aspirator route would be whether it could generate sufficient vacuum depth, as well as obtaining and setting up the necessary associated equipment (water pump, power supply, tubing etc). In particular, I'm not sure if an aspirator would generate sufficiently strong vacuum that it could be used for vacuum desiccation - one thing I'd like to use my vacuum system for would be to isolate stubbornly-deliquescent compounds from solution (e.g. monopotassium glutamate).
Quote: Originally posted by Ubya  

If you want to work with oxygen sensitive compounds, just a vacuum won't be enough. A rotary vane vacuum pump that can remove 99.999% of air will do, but instead of buying multiple pumps you can buy a single lab diaphragm pump, and just flush whatever air sensitive apparatus you have with a bit of argon or nitrogen. If you pull a vacuum, fill the apparatus with argon/nitrogen and repeat this steps 2-3 times, you can be sure there won't be any oxygen left, and you won't actually use that much inert gas, so a bottle might last you years. Cheaper than having to buy multiple pumps, and having to deal with oil changes whenever you use the rotary vane pump.
[Edited on 11-3-2024 by Ubya]

Thanks for the suggestion on dealing with air-sensitive compounds. I probably should have been more specific regarding the exact type of procedures I'm interested in performing, but I'm thinking of things like this video from NurdRage on synthesising anhydrous ferrous chloride. For stuff like that, my impression is that the hydrogen gas + solvent vapour displaces most of the remaining oxygen in the system

For metal compounds, I also plan to have some of the elemental metal in the solution while the distillation/desiccation is ongoing to help reduce any oxidation. From experience, this process is too slow (especially at basic pH) to block oxidation altogether during a concentrating/drying process exposed to air, but if most of the oxygen is removed, I think it might be sufficient to keep the metal ions in the system reduced.

I have actually performed the anhydrous ferrous chloride synthesis using a vacuum distillation, and obtained a product with reasonably high iron(II) content (permanganometric assay finds 94.5% to 99.6% of theoretical). I couldn't find any specs on the vacuum pump used in that though as the vacuum was generated from an inlet/tap attached to a fume hood, and I have no idea where the actual pump is.

I'll probably use the suggestions on purging the setup with argon when working with oxygen-sensitive organic compounds, so thanks for that!

fx-991ex - 13-3-2024 at 14:37

Quote: Originally posted by KoiosPhoebus  

Quote: Originally posted by fx-991ex  
If you have a 3d printer you can print a vacuum aspirator for free :). worth giving it a try.

It's an intriguing idea, but my main concerns with the aspirator route would be whether it could generate sufficient vacuum depth, as well as obtaining and setting up the necessary associated equipment (water pump, power supply, tubing etc). In particular, I'm not sure if an aspirator would generate sufficiently strong vacuum that it could be used for vacuum desiccation - one thing I'd like to use my vacuum system for would be to isolate stubbornly-deliquescent compounds from solution (e.g. monopotassium glutamate).

I use the one from the link below, i just plug it to the cold tap water, it do 650 mmHg or torr. make sure to add a tube to the outlet if you give it a try, something like 6 inch or more.
https://www.printables.com/en/model/253828-aspirator-water-p...

[Edited on 13-3-2024 by fx-991ex]

Dr.Bob - 14-3-2024 at 07:00

I have used aspirators, and they are fine for occasional use, but for long periods, the water use can be a pain. Also, it will be hard to maintain a oxygen free atmosphere, given the low vacuum. There are also water pump recirculating ones, which are nice if you can find a used one cheap. Adding some ice to the water will inprove the vacuum, as it is dependent on the temp.

Diaphragm pumps are great for rotovaps, filtrations, and moderate vacuum distillatoins, they are cheap and work well. I use them daily at work, but have a limit to both the flow rate and ultimate vacuum achieved.

Rotary vane oil pumps are great for high vacuum, and I have found them to be very reliable. But I am only using Welch type good ones. Cheap ones vary a lot, but HVAC pumps (for evacuating A/C units, about $100 at Harbor Freight Tools) and old refrigerator compressors have been used by people here to do similar work, but may require some skill and patience to make work.

Direct drive, oil free pumps are nice for clean vacuum use (like running a MS or low solvent vacuum system), but are easily destroyed in harsh use, and break easier than most other choices. They are also pricey for a good one.

If you could find a used but working pump on Ebay or elsewhere cheap, there are some good deals out there.

KoiosPhoebus - 14-3-2024 at 16:40

Quote: Originally posted by fx-991ex  
Quote: Originally posted by KoiosPhoebus  

Quote: Originally posted by fx-991ex  
If you have a 3d printer you can print a vacuum aspirator for free :). worth giving it a try.

It's an intriguing idea, but my main concerns with the aspirator route would be whether it could generate sufficient vacuum depth, as well as obtaining and setting up the necessary associated equipment (water pump, power supply, tubing etc). In particular, I'm not sure if an aspirator would generate sufficiently strong vacuum that it could be used for vacuum desiccation - one thing I'd like to use my vacuum system for would be to isolate stubbornly-deliquescent compounds from solution (e.g. monopotassium glutamate).

I use the one from the link below, i just plug it to the cold tap water, it do 650 mmHg or torr. make sure to add a tube to the outlet if you give it a try, something like 6 inch or more.
https://www.printables.com/en/model/253828-aspirator-water-p...
[Edited on 13-3-2024 by fx-991ex]

Quote: Originally posted by Dr.Bob  
I have used aspirators, and they are fine for occasional use, but for long periods, the water use can be a pain. Also, it will be hard to maintain a oxygen free atmosphere, given the low vacuum. There are also water pump recirculating ones, which are nice if you can find a used one cheap. Adding some ice to the water will inprove the vacuum, as it is dependent on the temp.


Thanks for giving me all of this info on aspirators. I probably won't go with aspirators for this project as there does seem to be quite a bit of variability and uncertainty on whether the vacuum depth generated would be sufficient. I'll definitely keep it in mind for filtrations or distillations where oxidation/high vacuum isn't a concern.
Quote: Originally posted by Dr.Bob  
Diaphragm pumps are great for rotovaps, filtrations, and moderate vacuum distillatoins, they are cheap and work well. I use them daily at work, but have a limit to both the flow rate and ultimate vacuum achieved.

The seller for the oil-free vacuum pump did get back to me, and apparently the pump I'm looking at is a piston vacuum pump. I understand that they use a piston instead of a diaphragm to displace air, but I'm not too sure what are the pros and cons of a piston vacuum pump versus a diaphragm vacuum pump.
Quote: Originally posted by Dr.Bob  
Rotary vane oil pumps are great for high vacuum, and I have found them to be very reliable. But I am only using Welch type good ones. Cheap ones vary a lot, but HVAC pumps (for evacuating A/C units, about $100 at Harbor Freight Tools) and old refrigerator compressors have been used by people here to do similar work, but may require some skill and patience to make work.

Good to know. My main concerns with a rotary vane vacuum pump are the maintenance issues related to the oil and how often I need to change it (especially when I've done vacuum work with a polar organic solvent, even with a cold trap). If I have to change the oil frequently, it might just be more economic to buy the oil-free piston pump.
Quote: Originally posted by Dr.Bob  
If you could find a used but working pump on Ebay or elsewhere cheap, there are some good deals out there.

Currently, the cheapest used vacuum pump on my country's eBay is going for a minimum bid of ~57 USD (when shipping is factored in). It's a model called a JAVAC Shark, and as far as I can tell, the only major benefit it would offer over a brand-new rotary vane pump is that it has a gas ballast.

I've heard that a gas ballast can help prevent the recondensation of vapours in the pump, but I don't know if it would serve as an effective replacement for a cold trap.

Sulaiman - 18-3-2024 at 10:06

I don't recommend using a peristaltic pump for vacuum,
but if it's all you've got....

The pumps were bought used, I replaced the tubing with common food grade silicone tubing,
the wall thickness must match the rollers,
id determines od and ml/revolution.
I don't think the tubing would last a very long time but it's cheap - I think I bought 5m :)

The little 12V - 0.7kPa pump is great for vacuum filtration only.

I still think an aspirator and a bottle of argon is the cheapest suitable route.

Alternatively, low budget
https://www.youtube.com/watch?v=viJ3T-1KZqY&list=PLahna3...
Or for the extravagant
https://www.youtube.com/shorts/aFJ1ThX8XHU

Sir_Gawain - 18-3-2024 at 19:08

Quote: Originally posted by Sulaiman  

Alternatively, low budget
https://www.youtube.com/watch?v=viJ3T-1KZqY&list=PLahna3...

Only low budget if you have several kilograms of mercury lying around! Also, isn’t a Sprengel pump similar to a water aspirator?

Sulaiman - 19-3-2024 at 09:11

With recirculating pump <1kg, but still expensive if I had to buy it.
(according to one of my theoretical projects that may or may not get built one day)

Theoretically different - venturi effect not present in a Sprengel pump.
Judging by various comments of members using aspirators (no exit tube/long exit tube/exit tube under water etc) there may be more than just the venturi effect involved.

fx-991ex - 19-3-2024 at 11:09

Quote: Originally posted by Sulaiman  

Judging by various comments of members using aspirators (no exit tube/long exit tube/exit tube under water etc) there may be more than just the venturi effect involved.


Without a long enough exit tube, vacuum get diverted from the vacuum port to the water outlet port, i know its confusing but thats what i observed.

When i built mine i used too short exit tube and i had like zero vacuum, but if i pinched the tube with my hands it made some kind of turbulence in the water blocking the air/vacuum path and the vacuum started strongly on the vacuum port, having a longer exit tube fixed this problem.

If there is a path for air to suck back form an exit tube thats too short then theres barely any vacuum.

I use it for vacuum filtration and if i put my hand on top of the buchner funnel it do suck pretty strongly.


[Edited on 19-3-2024 by fx-991ex]

Catslovechemistytoo - 8-8-2024 at 20:29

I have never managed to pull a vacuum higher than -50kPa with an aspirator like this . I've tried to hook it onto a water pump which delivers more pressure than water tap but it performed even worse. There seems to be an issue with these aspirators. Maybe reducing the size of nozzle or chaning it's geometry can improve it.

Precipitates - 9-8-2024 at 01:15

Quote: Originally posted by Catslovechemistytoo  
I have never managed to pull a vacuum higher than -50kPa with an aspirator like this . I've tried to hook it onto a water pump which delivers more pressure than water tap but it performed even worse. There seems to be an issue with these aspirators. Maybe reducing the size of nozzle or chaning it's geometry can improve it.


I've managed to get down to around 20 kPa using this style of aspirator connected to a water tap. Creeps up to around 30 kPa during fast distillations though.

As the water temperature is around 30°C here, and the seal isn't exactly great - a monstrosity of couplings to connect the aspirator to the tap, 20-30 kPa probably ain't bad.

I bought a diaphragm water pump, but the noise was terrible (like a blender), and I live in a small apartment so was worried the neighbours would complain. The sound of a running tap for a couple hours is more explainable. The biggest reason I wanted to use it was to lower the temperature of the water passing through the aspirator (and it would be nice not to waste so much water).

And, yes I also have a feeling that if the amount of water passing through the aspirator is too great it actually decreases the performance. I should play around really, as perhaps I could optimise the vacuum by reducing the flow from the tap slightly - rather than just turning the tap on full every time.

jackchem2001 - 14-8-2024 at 22:08

I found that when measuring the vacuum pressure with a gauge, I had to coat the thread in silicone grease to get any kind of sensible reading (like 0.1MPa -> 0.8MPa vacuum). I would suggest being very liberal with grease so you can rule out leaks as affecting your vacuum.