Sciencemadness Discussion Board

Looking for info on vacuum pump to buy, compiled short list of products Im interested in. Advice/Critiques/Suggestions...

Rocket89 - 18-1-2016 at 09:32

Hello, I'm a long time lurker, first time poster. First off I want to say thank you to everyone on here, I have learned a lot from just reading this board.

Over the last year I have started putting together a little garage lab with your standard distillation and filtration glassware, stands, clamps, etc. I'm at that point where I want to buy a decent vacuum pump. Mostly for vacuum filtration and desiccation purposes, but also for the occasional vacuum distillations, and sporadic automotive applications.

Now, I did UTFSE, but not extensively, and I might've missed something, I didn't find much information regarding suitable pumps for the garage chemist.

Here are the links to the pumps I have found and taken an interest in. This is by no means conclusive and if anyone has any other options (particularly cheaper), then I am all ears.

Cole-Palmer Vacuum/pressure diaphragm pumps, PTFE-coated wetted parts, 0.75 com, 115 VAC
Link: http://amzn.com/B003LYSFS6

Two Stage 12.0 CFM, 1 HP, Deep Vacuum Pump
by Western Pacific Machinery
Link: http://amzn.com/B00TRAB6NO

HFS Vacuum Pump Single Stage 110v - Inlet: SAE 1/4"-3/8"; 22.5micron, 1hp , 12cfm 3pa
Link: http://amzn.com/B00VF1EUYK

HFS (Tm) 12 CFM Vacuum Pump Double Stage 110v - Inlet 1/4 and 3/8 Inch SAE- 1 Hp - 340l/min
HardwarefactoryStore.com
Link: http://amzn.com/B00XFLA2MC

Diaphragm Vacuum Pump Oil Free Type 15L/m 75W 0.075Mpa 250mbar 110v OR 220v
by MX
Link: http://amzn.com/B00WB3ZZ5Y

I am leaning towards the Cole-Palmer as it is PTFE-coated, and with my limited understanding, would last the longest when filtering organic solvents, corrosives, etc. But then again, it is fairly expensive, and though affordability isn't exactly a sticking point, I am not opposed to spending less for a "comparable" product. Especially since I will be have a bubbler/trap set up anyway.

Any and all advice, suggestions, etc, will be greatly appreciated! Thanks, Rocket.

Heavy Walter - 18-1-2016 at 09:58

Hi Rocket89

Me too an old lurker and a new member.
I have a small high vacuum lab and planning some chemical synthesis.
Think of having two vacuum systems: a simple water jet for filtering and low requirements, easy (in case you have running water close to) and an elaborated system (two-stage rotary pump plus diffusion pump -air or water cooled) with high-vacuum reading.
The high vacuum system will be helpful for delicate syntheses.
Don't need to buy new ones: e-bay offers are interesting and cheap.
Water jets come in glass, plastic or metal, no too expensive.
High vacuum is tricky: leaks are common and pinpointing them not so easy. You will have to do your own practice.
Feel free to ask me any doubts.
Can send you some images.

Heavy Walter - 18-1-2016 at 10:18

See my synthesis line.
It is partially disassembled: diffusion cooling fan was removed and high vacuum gauge is missing.
The black instrument is for pressure readings of gas stored at the upper flask.

Linea de Sintesis_01.jpg - 156kB

Dr.Bob - 18-1-2016 at 10:45

The Cole parmer pump has a loq CFM (0.75) and a poor ultimate vacuum (Max vacuum: 23.2" Hg). The Western Pacific Machinery one is much higher flow (12 CFM) and a better ultimate vacuum, but is an oil pump. The others are also fine for some tasks, depends on what you want to do.

If you just want to filter stuff and do minimal vacuum work, then the Cole Parmer might work, but if you want to really have a high vacuum (boiling high BP compounds, drying materials under vacuum), then the second one is much better, but you should put a trap of some sort between the pump and the work to protect the pump, or even an acid filter (container of K2CO3 or the like to absorb acid vapors). Diaphragm pumps can rarely get a high vacuum or high throughput, but they are more resistant to chemicals. But you have to buy a bigger diaphragm pump if you get one of them, in order to do much with it, to match the suction of an oil pump, or even a direct drive rotary pump. Also, check the duty cycle, and make sure the pump can be left on continuously, if it only shows an hour or so, the pump will not go long before dying in real usage.

Sulaiman - 18-1-2016 at 14:03

For vacuum filtering I have found this http://www.ebay.co.uk/itm/DC12V-DC-Micro-Piston-Vacuum-Pump-...
small pump more than adequate.

I bought a 2-stage rotary vane vacuum pump, 0.3 pa ultimate pressure
a single stage, 5 pa would be just as useful for distillations etc.

With cheap Chinese rotary vane vacuum pumps costing less than the refurbishment cost of a 'proper' vacuum pump
I can not see the value of an expensive rotary vane oil sealed vacuum pump,
unless you have other uses for it.
e.g. http://www.ebay.co.uk/itm/3-CFM-1-4HP-Single-Stage-Rotary-Va...

maybe a chemically resistant diaphragm pump is overall best for chemistry?
( I want one ! )

elementcollector1 - 18-1-2016 at 15:52

Personally, I prefer https://www.amazon.com/gp/product/B012CFTYX4/ref=oh_aui_deta... for both price and efficacy.

gdflp - 18-1-2016 at 16:33

I personally own this pump, which I bought used on eBay for $60. Typically I see them around $100. The only issue I've had with it so far is that the diaphragm teared, and a replacement cost $66! Wasn't terribly happy about that, but it's the cost of a diaphragm pump(on the upside, it's oil-free). The pump is more than sufficient for vacuum filtrations and many vac distillations, but it only pulls around a 175 torr vacuum which doesn't lower the boiling point as far as I would like/potentially need it to for certain compounds.

I would recommend a KNF pump if you want a good diaphragm pump. They pull around a 20 torr vacuum and, like the pump I have and the Cole Parmer you linked, they have a Teflon or Viton coated head, a feature which I consider to be essential for use in organic chemistry(traps are a pain). The downside is the price, around $300 for a used pump. I couldn't justify this when I was buying one, but if you can fit that into your budget, then I would definitely go for one of those.

UN726FTP-Single-Head-Diaphragm-Vacuum-Pump.JPG - 21kB

[Edited on 1-19-2016 by gdflp]

XeonTheMGPony - 18-1-2016 at 16:51

Too powerful of a vacuum pump is a bad thing with out implementing a gas ballast (Way to admit air in between the stages) a 2 to 3 CFM dual stage is plenty for most jobs, secondly I'd suggest you build a cold trap for it as well (Will vastly increase the life of the pump and pump oil!)

DO NOT WASTE TIME WITH SINGLE STAGE UNITS! If you plan to do any serious vacuum stuff they are an utter waste of time, money, effort! All so avoid cheap name brands they will not last! Sadly you need to be care full with even Yellow jacket gear now days. Use a quality 3M or Nu-Calgon oil (http://www.nucalgon.com/products/oils/vacuum-pump-oil)

As said above have a smaller pump for the filtration and distillations then the higher powered one for dessication.

A Supco Vg46 (http://www.amazon.com/Supco-VG64-Digital-Accuracy-Connection...) vacuum gauge is a very nice mid range vac gauge that measures in Torr and MiliTorr, Microns, Inches of mercury.

I had mine tested at a laser fabrication factory and it will measure down to 1.0Militorr! (Awesome having such a place next door to me, just wish they'd sell me glass stock)

BTY you can make an ok cold trap using an old water cooler with some modifications!

For high vac system use all 1/2 Copper soft annealed, then terminate to your final size at the final point of utilization for best performance.

[Edited on 19-1-2016 by XeonTheMGPony]

Vacuum pumps on Elemental Scientific

tomholm - 18-1-2016 at 21:26

I have some used Schuco vacuum pumps that my customers like. Nice pumps for a reasonable price. Here's the link Schuco Pumps.

Tom

kt5000 - 18-1-2016 at 21:49

Quote: Originally posted by tomholm  
I have some used Schuco vacuum pumps that my customers like. Nice pumps for a reasonable price. Here's the link Schuco Pumps.

Tom


Funny, I was getting ready to post a link to the Schuco 130A on your site.. I've started looking for a vacuum pump as well and figured the pump+trap design was handy. I've only used house vacuum in college lab, so I'm still figuring out what would be good for home. I'm planning to do some vacuum distillation and filtering. Nothing too fancy.

Schuco 130A: http://www.elementalscientific.net/store/scripts/prodView.as...

Rocket89 - 20-1-2016 at 22:48

Thank you everyone for all the input. Looks like I will be going with a 2 stage rotary pump.


Rocket

[Edited on 21-1-2016 by Rocket89]

XeonTheMGPony - 21-1-2016 at 06:06

Good choice, now learn about vapor pressures and their effects on substances, sublimation, triple points, and cold traps (Cold traps will be the most critical part on vacuum desiccation! (Well pretty much every thing).

The oil is the life blood of your vac pump! Keeping it clean and dry is critical.

Trick is warm up the pump first befor using it for the function (Helps keep the oil dry) When the oil is saturated it will look milky or foggy.

Change the oil early and often (Oil will be cheaper then the pump!) If doing acid distillation you will need very aggressive scrubber drying system leading to the pump.

Magpie - 21-1-2016 at 10:52

I bought a Harbor Freight one-stage vacuum pump that uses oil. It will pull down to 1mm Hg. Tell me why I would need a 2-stage.

XeonTheMGPony - 21-1-2016 at 16:21

Quote: Originally posted by Magpie  
I bought a Harbor Freight one-stage vacuum pump that uses oil. It will pull down to 1mm Hg. Tell me why I would need a 2-stage.


It will not pull that for long (Providing it can actually even get near that in real world usage) if you actually use it for any amount of time, all so it will take much longer to get even close to that rating!

When dealing with larger volumes the first stage is the rapid pull down, then once it gets down to a usable vacuum the second stage kicks in to get you to the final vacuum rating. At this point the first stage is still working!!! It is helping the 2nd stage pull a deeper vac by keeping the middle pressure zone at a much lower presser then atmospheric, this is why a single stage will never pull to a deep vac no matter their claims!

Real world usage only a two stage will get near its rated ultimate vacuum even after multiple usages! But keep in mind this is providing one gets a quality pump to begin with! Cheaper ones go down even worse, the 2 stage ones hold out a bit longer but cheap is cheap.

[Edited on 22-1-2016 by XeonTheMGPony]

Magpie - 21-1-2016 at 16:37

My question is based on "real world experience," ie, my own, on several occasions. I usually set up my vacuum distillations with 19/22 tapered glass, well greased joints with silicone vacuum grease. I think my pump is rated at 2.5cfm. I measure my vacuum with a Bennert mercury gage. This vacuum of 1mmHg is reached very quickly.

So far I have not abused my pump, ie, I've only used it on fatty oils and alcohols, etc, and have not had to use a trap. I've changed the oil once, I can't remember why, probably because I estimated it might contain enough fatty oil contamination to no longer remain a good lubricant.

[Edited on 22-1-2016 by Magpie]

[Edited on 22-1-2016 by Magpie]

[Edited on 22-1-2016 by Magpie]

[Edited on 22-1-2016 by Magpie]

XeonTheMGPony - 21-1-2016 at 16:47

On such a tiny system you may get away with it, but as time and age effect it they do not hold up. (We are talking about industrial type pumps right? as these are no where in the same league of a scientific built single stage unit!)

Admittedly I am coming from the perspective of my vac pump is a critical tool and used on refrigeration systems that contain huge volumes of gas by comparison, but seeing single stages side by side 2 stage units the single stages are not worth even looking at.

BTY try it on a digital vacuum gauge! My pump will get to 25microns (same as milliTorr) Ultimate vac (Analog gauges are useless to reading this level accurately)

All so oil should be changed after so many run hours regardless of how it looks!, oil is cheap, a good quality of vacuum pump is not!

Magpie - 21-1-2016 at 16:58

Quote: Originally posted by XeonTheMGPony  

Admittedly I am coming from the perspective of my vac pump is a critical tool and used on refrigeration systems that contain huge volumes of gas by comparison, but seeing single stages side by side 2 stage units the single stages are not worth even looking at.

BTY try it on a digital vacuum gauge! My pump will get to 25microns (same as milliTorr) Ultimate vac (Analog gauges are useless to reading this level accurately)



I am speaking from the standpoint of the home chemist which I assume includes the OP and most of the others here that are interested in buying a vacuum pump.

Have you done any home chemist vacuum distillations? I would think that 99% of them don't care about discrimination in the sub-torr range.

XeonTheMGPony - 21-1-2016 at 17:40

but he did specify vacuum desiccation, for that you want the best ultimate vac! So yes I did think of it, that is why I recommended against getting a 12cfm one! a 1.5 to 3 is tons.

Magpie - 21-1-2016 at 18:48

I think the amount of vacuum required for dessication would be application specific and would usually not be < 1mmHg. For example, this vacuum dessicator specifies a vacuum of 27"Hg - not much of a vacuum:

https://www.ece.umd.edu/MEMS/resources/sop/vacuum_dessicator...

For vacuum distillations at the home lab scale not much capacity is required either. If the system holds a vacuum it will suck down very fast with a 2.5cfm unit. If it doesn't hold a vacuum the system seal should be corrected.

AvBaeyer - 21-1-2016 at 21:13

I am in total agreement with Magpie on what is a sufficient vac pump for the home lab. I do many vac distillations with a "cheap" single stage pump using standard ground glass joint glassware. I use a digital gauge (which cost far more than the pump) and my pump will routinely pull 0.5mm or better on properly greased joints. Frequently, I have to have a bleed to raise the pressure. I NEVER use the pump for filtration or bulk solvent removal. For that I have a recirculating aspirator pump which pulls in the 20-25 mm range. It saves much on water usage - very important here in California. I got one a couple of years ago on Ebay for about $200- expensive but worth every cent. I really recommend such a unit if it is within your budget.

In my working days, we did a lot of freeze drying of aqueous samples using vacuum at 2- 3 mm, sometimes worse. At the end of the day, it all comes down to application and what is sufficient.

AvB

Sulaiman - 22-1-2016 at 03:39

A 'Cold Trap' may be trivial in a full laboratory but I have found it not practical at home
a small water-ice&salt etc trap does not cool common solvents or even water enough
and because here in UK dry ice is GBP 35 for a minimum order of 10 kg
which is more than the cost of the 3 cfm 5 pa rotary vane pump pointed to by elementcollector1 :o
These cheap rotary vacuum pumps are sold as devices to suck all of the water out of automotive air-conditioners,
so they should be fairly tolerant of a little water or solvent vapour.
I have not used mine enough yet to give any comments on long term reliability though.

If you do buy an expensive rotary vane vacuum pump, consider the cost of looking after it,
especially dry-ice cost and availability.



For filtration a continuous vacuum pump is not required as the filter flask 'holds' a lot of vacuum
so even a hand vacuum pump will work fine.
My little 12 Vdc, -80 kpa (150 mm Hg absolute ... on a good day) vacuum pump can pull filtrate of plant extracts through a 90 mm diameter 'slow' paper quickly.

A continuous pump would allow rapid air-drying of the Residue

[Edited on 22-1-2016 by Sulaiman]

Magpie - 22-1-2016 at 10:27

The dry ice situation is pretty good in the US apparently. Although I have not bought any for some time websites are claiming it can be had for $0.99/lb. This then would be a minor cost of most of my experiments.

Edit: I see there are minimum purchase amounts however. I'm not sure what these are yet.

Dry ice should be dirt cheap. I believe it is a by-product of the industries who extract O2 and N2 from the air. I see it in small (locked) freezers at Walmart for consumer use. In liquified form it is being delivered in tank trucks to restaurants and taverns for carbonation of beverages.

[Edited on 22-1-2016 by Magpie]

XeonTheMGPony - 22-1-2016 at 14:58

You do not need dry ice for most low vacuum set ups, a modified water cooler will do the job.

Unless you plan to separate out alcohol at 300 microns(MilliTorr) then you'd want -80c! Cascade system! (Easy to build it is the gasses that will kill your budget though!)

I have one similar to this: http://www.ebay.com/itm/Vacuum-Pump-2-6-CFM-R12-R22-R134a-41... (Mine is 1.5cfm)

That I use for most my vacuum distillations using ethanol based solvents with recovery of the ethanol. I do them under a deep enough vac I can boil the ethanol at room temp! To control vac I use a needle valve to bleed in air between the pump and apparatus for the distillation.

As I rebuild the system I'll be sure to put up pictures.

[Edited on 22-1-2016 by XeonTheMGPony]

Magpie - 22-1-2016 at 16:44

Quote: Originally posted by XeonTheMGPony  
... a modified water cooler will do the job.


Please explain.

XeonTheMGPony - 22-1-2016 at 17:00

removing the thermistat and lock it on or off, baffel the cold pot and pack with copper scrubbies, insulate the out side.

they will get down to apx -17 when left to run long enough.

Once down to lowest temp set your vac limmit to match. Not the prettiest but works.

Magpie - 22-1-2016 at 18:32

I'm not sure what you mean by a "water cooler." Is this an example:

http://www.walmart.com/ip/Ragalta-Thermo-Electric-Hot-and-Co...

XeonTheMGPony - 22-1-2016 at 19:03

yes the ones with the vapour compression cooling system.

PlatinumLab - 22-1-2016 at 19:12

Quote: Originally posted by XeonTheMGPony  
yes the ones with the vapour compression cooling system.


That's more along the lines of an AC-cooling system, in which case, that's personally how I cool my water in distillations.

I built a small desktop compression unit that basically blows the air from the hotside of the coils outside, while on the cold side I have a liquid/liquid heat exchanger which I pump my condenser water through.

I personally think it's a brilliant way to keep a condenser cold and can allow adjustable condenser temperature by adjusting the compression differential on the hot/cold sides. It can keep the condenser cool for as long as I have electricity.

XeonTheMGPony - 23-1-2016 at 05:35

That is a very common mod in the hobby liquid cooling field. I used to fabricate custom water chillers with up to 600w capacity at -20.

I preferred to run my servers at 0c and have 13,500btuh cooling capacity.

Some point I need to make a simple 2 stage cascade system for a good cold trap, I'll use a horizontal rotary system at 500btuH.

But as I say a fast n dirty but use full cold trap can be made by moding a water cooler, it is much easier to clean and dry then a air con, especially if one doesn't have the skill or tools to open the system and do a more proper mod of it for the application.

Most systems run on R410a now days too which is a very high pressure gas that is a near azeotrope, so if not recovered very well it will need the gas replaced. With out looking at my tables if I recall it is siting at 200psi at average room temp!

So be care full when playing with modern air cons!

highpower48 - 23-1-2016 at 15:56

Quote: Originally posted by elementcollector1  
Personally, I prefer https://www.amazon.com/gp/product/B012CFTYX4/ref=oh_aui_deta... for both price and efficacy.


What fittings do you need to make this work? 1/4" flare male fittings seem hard to get. That is 1/4" flare to a barb fitting...

Sulaiman - 24-1-2016 at 00:56

It seems that you can buy two Chinese rotary vane vacuum pumps
for the cost of a meter that can measure the pressure :(

I guess that for 'serious' vacuum work you need a costly gauge,
or an un-calibrated DIY one.

From the little knowledge that I have remembered,
. unless you have a short path distillation setup there is no point going for less than a few mm absolute due to length/diameter of equipment
. if you have a short path distillation setup, with gauges, traps ...
then maybe there is a difference between a 5 pa single stage rotary vane and a 0.3 pa dual stage like I bought,
but to me there is no difference as far as distillation goes, and both are overkill for filtration.
. other interests may influence your choice,
refrigeration, thermionics, vapour deposition, clothing storage ....

I intend to make a vacuum desiccator soon so maybe my dual stage will be worth the extra ... maybe not :P

your question is like " which is the best automobile to buy ? "

P.S. how to measure down to 0.3 pa with some known accuracy/error, cheaply ?

XeonTheMGPony - 24-1-2016 at 01:32

Ebay you can usually find a Supco VG46 vacuum guage for apx 100 or less. They used a fixed thermistor reference so hold calibration very well.

http://www.ebay.com/itm/Supco-VG64-Digital-Vacuum-Gauge-/172... < First hit.

http://www.ebay.com/itm/Supco-VG-60-Vacuum-gauge-/1519592630... < Much lower end one but even cheaper.

http://www.ebay.com/itm/Yellow-Jacket-69075-SuperEvac-LCD-Va... < Different brand

For fittings you want the largest practical diameter leading up to the point of use.

The deeper the vac the better the dehydration, for refrigeration water is toxic to the system, it will turn most newer oils into acids, break down the refrigerant and block the metering device, So the standard is 500microns vacuum that holds, at this level you are assured there is no trace of water in the system.

I like to break with OFN (Oxygen free nitrogen) then pull one more vac to be certain, for the real hard core guy for dessication you can invest in a small cylinder of nitrogen fairly cheaply! all so use full if you want to experiment with vacuum freeze drying (One of my goals to get around too, need to build the system for it)

With out accurate guages you may well be pissing into the wind in the pitch black!


[Edited on 24-1-2016 by XeonTheMGPony]

Sulaiman - 24-1-2016 at 04:31

I have a bid on a VG-64 on ebay UK at the moment !

Rocket89 - 25-1-2016 at 13:00

What are everyone's opinion of the Chemglass CG-4800-03 vacuum pump? Found a couple used on eBay in the $200-$300 price range.

Rocket89 - 30-1-2016 at 08:24

Bought a Chemglass CG-4800-03 :)


That is all.

Rocket89 - 30-1-2016 at 09:12

Bought a Chemglass CG-4800-03 :)


That is all.

Axismundi000 - 30-1-2016 at 15:52

I'm new here so hopefully the below is relevant and useful.

I got one of the the ghast vacuum pumps (24"Hg) from Cole Palmer last summer, it had a lot of good reviews re reliability. Applying vacuum to a distillation train after the system has settled down you definately need to close off the system with a stopcock after about 10 minutes. If not vacuum will attenuate with continued pump operation and also you cannot switch off the pump and maintain the partial pressure.

So for vacuum distillation it is great, once the system is settled close the valve and disconnect the pump. Also it is not that noisy and it is a diaphragm pump so no oil needed.

For continued usage where you cannot close off the system once vacuum established and you need to maintain highest vacuum this would not suite ( it wasn't a cheap pump).

So the main question then is perhaps the specific needs that the pump must satisfy. For vacuum distillation the one I got is fine because I just use it for distillation.

Has anyone tried the Ventuuri vacuum valves? I considered those because of potentially much better vacuum but I was concerned that thermal decomposition might occur before heating of the boiling flask established partial pressure in the system. If thermal decomposition at low temperature is not an issue they are a fraction of the cost.


[Edited on 30-1-2016 by Axismundi000]

EugeneOregon - 7-9-2017 at 07:29

Quote: Originally posted by Heavy Walter  
Hi Rocket89

Me too an old lurker and a new member.
I have a small high vacuum lab and planning some chemical synthesis.
Think of having two vacuum systems: a simple water jet for filtering and low requirements, easy (in case you have running water close to) and an elaborated system (two-stage rotary pump plus diffusion pump -air or water cooled) with high-vacuum reading.
The high vacuum system will be helpful for delicate syntheses.
Don't need to buy new ones: e-bay offers are interesting and cheap.
Water jets come in glass, plastic or metal, no too expensive.
High vacuum is tricky: leaks are common and pinpointing them not so easy. You will have to do your own practice.
Feel free to ask me any doubts.
Can send you some images.


I am from Oregon and in my retirement from disability I have adopted a great hobby of purifying cannabinoids out of the available and legal marijuanna extracts from dispensaries.

Step One is Dry Column Vacuum Chromatography. (DCVC), which is my adaptation of the DCVC method demonstrated by the University of Coppenhagen.
My video showing this is here:
https://www.youtube.com/watch?v=BIsiCfcLBL4.

The only good pump for DCVC is a diaphragm pump and I chose a PTFE economy pump for the vacuum through the column during DCVC which draws a lot of fume because the column is drawn dry between solvent gradients. When I used my dual vane cheap rotary pump it pukes out a vapor that fills the room which is a fine aerosol of oil mist attaching to fume as the fume passes through the oil.

Step two is heated solvent purge under low vacuum provided by the PTFE pump. The fumes pass through the oiless daphragm pump and into a collection flask. Pump is unaffected by Hexanes, Ethyl Acetate, and alcohols used.

Step three is molecular distillation. This is the big one. You must be able to pull a vacuum down to 20 microns or less IF you hope to distill delta 9 THC and the other medicinal cannabinoids at temperatures listed in scientific journals. Pubchem lists THC (Dronabil) boiling point as 200C and .02 mm Hg (Torr). This is also stated as 20 microns. Very deep vacuum was achieved in my home hobby lab with my Edwards EM 28 outfitted with stainless steel bellows and fittings. I got the EM 28 from craiglist for $1000 and two oil changes and running the ballast wide open has returned the unit to factory specifications. I cannot emphasize the importance of being able to maintain a deep vacuum for molecular distillation. I have demonstrated in the video link below a technique for measuring the boiling points of cannabinoids. First drip into collection bulb noted at 140C on flask thermometer suspended one inch above puddle.

Using a single point conversion chart to make a crude estimate of likely boiling points reveals from this information that the temperature limit on my spherical cloth heating mantle of 400C will easily be exceeded at just 75 microns (.075 mm Hg). The heating mantle is just 60 watts and this is way more than enough IF you can pull down to this level and hold it. I believe this video is the first on youtube demonstrating decent precision measuring cannabinoid boiling temp and pressure data.

https://www.youtube.com/watch?v=rD5tIdnDNl0

Short answer to your question is I use three pumps, each well suited for their niche use but nearly useless outside their niche. Using the deep vacuum pump to attempt solvent purge means you will suck a lot of compound up into the uptake tube and purging rig from violent bumping. Then the pump oil needs decontamination procedures. The roughing pump, a $250 dual vane pump, is perfect for my one quart vacuum chamber used to purge trace solvents from medicine under low heat and high vacuum. Oil must be changed in the roughing pump after each use as it has no gas ballast but the pump pulls down to 75 microns which easily purges trace solvents and has the advantage that those solvents do not decontaminate the deep vacuum pump, hose, or pirani sensor used in it and brand new replacement is just a few hundred dollars.

You must read and understand the technical specifications on the pump and you must know what vacuum you need to achieve and what type of vapor load you plan to see. Ultimate vacuum is listed and rate of evacuation in cubic feet per minute will be listed. Ultimate vacuum is how much vacuum it pulls when the inlet line is blanked off. CFM is a bit trickier to explain because of (among other things) the time (integral) factor involved. As the vacuum becomes deeper the flow from the furthest part of the vaccum system towards the pump chamber become slower and then there is no "flow" at all of the compound being molecularly distilled. Gas does not "flow" as a fluid under conditions that I create during molecular distillation at one micro or less. In that state when the phase transition occurs the gas molecule takes off in a random brownian direction and continues in a straight line until it hits something. This repeats so that eventually the molecule finds its way out of the boiling vessel.

Leakage at molecular distillation vacuum levels is a factor and represents that which "flows" in a system under deep vacuum. ANY polymere seal has permeability and leakage. Any glass joint has leakage despite grease. I do not know how to compute the way to select the right CFM under these conditions precisely except to say that it can be expressed as "tonnage" which roughly translates into work capability. Tonnage is estimated by CFM squared. So a .9 CFM pump creates .81 tonnes of work while my 26 CFM Edwards creates 676 tonnes.

In the marijuanna industry here in Oregon refineries that use a common Welch pump rated at .9 CFM pump, at .81 tonnes, typically run up to 100C higher in their online videos than my experimental rig that runs a pump with the exact same ultimate vacuum rating as a practical matter but at 26 CFM which creates 676 tonnes of work to pass the leakage out of the system. This suggests that a curve could be plotted if more tests with different pump tonnage ratings were conducted but I have not seen any definitive table of data doing this and regarding the relationship between CFM and target vacuum level during molecular distillation as carried out at or near the ultimate vacuum of the pump which is under one micron (during which the only "flow" out of the system is leakage).

Forgive my long winded answer but I never got good at stopping at pne sentence...

Good luck