vacuum pumps

[eddy]

hi!...i was curious to know .....i have a 24mmHG vacuum pump and was wondering if that would be enuff for the "bromination for beginners" (p230) it says at least use a 10mm vacuum..isn't 24mm better?
thnx..eddy

[starlight]

The lower the mm of mercury, the better the vacuum. Atmospheric pressure is around 760mm of mercury at sea level. A vacuum equivalent to 1/10 of atmospheric pressure is 76mm of mercury, 100th of atmospheric pressure is 7.6mm of mercury etc.

[ApprenticeCook]

Lower number in mmHg = greater vacuum

Well if yours is a 24mmHg and you need at least a 10mmHg that would suggest to me that its not good enough according to that description.

What is this bromo for beginners on p230 your on about? is it strikes book or something? (which i dont own)

-AC

[OcoteaCymbarum]

What you could do to verify hhow your vacuum really is, just boil a know substance under reduced pressure, note the temp it comes over at.
Take a look at this post for a link to a vapor pressure applet that will tell you your real vacuum.

Post 522600

(OcoteaCymbarum: "Online vapor pressure applet", Newbee Forum)

Then you could figure out how much you can really pull, as it never gets(IMHO) to what the manufacturer's claims in most bees labs.

[armageddon]

Let's assume your pump is labelled/specified correctly with having 24mmHg/torr..

I would say that if a vacuum of 10mmHg is desirable, you could get along very well with 24mmHg, too...

If your pump can really pull that much air from your system (which I doubt, as all pump manufacturers seem to exaggerate a bit when specifying their product capabilities), it will be probably good enough for your purposes - just make sure that you DON't apply more heat because the original procedure says "heat until distillate comes over at a rate of 1-2 drops per second" or something like that - just stick to lower temps and prepare for slooooow distillation.

I assume you need strong vacuum because u want to distill fragile compounds?

Greetz A

[ApprenticeCook]

I assume you need strong vacuum because u want to distill fragile compounds?

My assumption is that the 100mmHg is required as the distillation temp at any less of a vacuum would be too high for the desired compound....
Just from the way the initial question was worded...
"a minimum of 10mmHg"
But i guess you could use the lesser vacuum provided the compound your distilling wont decomp or poly at the disto temp at that vacuum.

It would be easier if you could tell us what compound your distilling? then the answer would be swift and easy.

-AC

[armageddon]

Hm, this seems to be exactly the right thread:

Does anybee know Edwards "Speedivac" rotary vane vac pumps (high vacuum)? Because I just was asked if I would like to buy one... - the guy said it was capable of pulling 0.001 torr!

WHOA - now THAT's a high vacuum, isn't it? Anyway, my question is: does anyone have experience with this type/manufacturer and can tell me if these pumps really can pull that much?

BTW the label sez: "in a gas-free, vacuum-suited system, this "Speedivac" pump achieves a vacuum of less than 0.001 torr (mmHg) without gas pressure, and 0.05 torr (mmHg) with full gas pressure, measured with a McLeod vacuum gauge." - and has a valve for regulating the amount of gas to be pulled per min., i.e. you can regulate whether you want to have a quick or a strong vacuum...

Sounds nice - what do you think? Should I buy it?

THX A

[Osmium]

> WHOA - now THAT's a high vacuum, isn't it?

No, that's a common rating for oil pumps. Nothing outstanding really.

> Anyway, my question is: does anyone have experience with this type/manufacturer
> and can tell me if these pumps really can pull that much?

Yes they CAN in principle, but you won't be able to achieve such a vacuum when you try to distill stuff.

[ApprenticeCook]

The biggest vacuum i have seen was a massive unit at university where it was permanently conected to its own container, able to pull to 1x10^-24Torr (something like that....)

0.001Torr is a standard limit for those types of pumps... but edwards are still usually pretty expensive. Worth every cent though.

-AC

[armageddon]

Osmium:

No, that's a common rating for oil pumps. Nothing outstanding really.

Yeah sure, it is quite common that you can distill P2P for example (bp 214°C/760 torr) at below room temp, isn't it? All oil pumps can do that, surely - very common...

Do you try to show off with your own amazing vac pump?    I at least know several models of several different manufacturers (Duerr, Heidolph, KNF to name a few) being "only" able to pull something like between 20.00 to 0.01 torr in theory - but 1*10^-3 torr with these smaller models was new to me, and even if it can't pull that much, it is still 10 times a stronger vacuum than I could theoretically achieve with my old pump...

edwards are still usually pretty expensive. Worth every cent though

The Edwards pump I'm talking of has a little valve for regulating the pumping speed, at the cost of less vacuum - and the specs say it should still be able to pull 0.05torr with this valve fully opened, i.e. with full gas ballast...

Oh, did I mention the pump was too used in a university until now? And the price is astonishing low...

THX for your replies anyway. Think I will buy it, as it isn't really expensive and should do good work...

Greetz A

[hest]

Almost all two stage pums are raded at 0,001torr. but that is with the pirani mounted on top on the wacuumpump.
In a more realistic setup (vacuumline with two coldtraps ect. you wont have a vacuum below 0,01torr).
One of the typicals error are that peaple mesure the vacuum werye close to the wacuum pump and not at the destilation setup ect.

[ApprenticeCook]

designed to sell the pumps so they are taken in the ideal spot in the ideal conditions with the ideal extras hooked up....
ie not representative of levels obtained through normal useage conditions....

But yeah armageddon almost all 2 stage oil vane pumps for lab use (ie not HIVAC) are rated at 0.001torr or thereabouts, rate is the variable factor.

-AC

[Osmium]

You can get close to 0.001 (but will never be able to reach it) if you use He-leak tested wide gauge all metal parts which are heated out to volatize every organic molecule (fingerprints, oil films etc) present, when your chamber to be evacuated is small and the vacuum lines are short and when you don't mind that reaching that kind of vacuum will take several hours.
As soon as you use a standard glass setup the vacuum will reach down to 0.1 torr at best, and once you use rubber tubing you can pretty much forget about high vacuum since you won't be able to get much lower than 1mbar. It's common sense.

The main limiting factor for the pump rating is vapor pressure of the pump oil, just like water vapour pressure is the limiting factor when using aspirators, UNDER OTHERWISE OPTIMUM CONDITIONS!
This minimum pressure pump rating is not very useful when it comes to determining the max vacuum you will be able to achieve under real world everyday use since so many other factors come into play. A small pump with a 0.001 torr max pressure rating might achieve a worse ultimate vacuum with a given setup performing a certain task than a big pump with a max pressure rating which is only 0.01 or even 0.1 torr. It all depends on what exactly you want to do and what the conditions are.

[bio]

My experience certainly agrees with Osmiums comments however in  normal everyday use with a 24/40 setup with a few adaptors and a 6 or 10 liter boiler about 1mm at the reciever measured with a closed end manometer is about the best it will do with a fresh seal kit. This is a refrigeration 3CFM pump also rated 1 micron which will never be achieved in real use. The braided stainless rubber lined refrigeration hoses work very well to about 50 microns or less but you definetly need the  floating piston double o-ring type manifold and quality guages. Problem with these though is they are only 3/16 ID but work OK in this application.

I would say that the free air rating is more important than the absolute vacuum rating because you will be pumping against  minute leaks. And don't forget to use the high vacuum silicone grease and change the oil every few hours or after every procedure.

[eddy]

i'm distilling over 1,3-bromobenzodioxane from strikes notes in "total synthesis" p230 thats why i'm curious how low i need the vacuum....so as not to scorch anything


i have to dream my own sassy as it's a nightmare over here to imagine

[Rhodium]

5-Bromo-1,3-benzodioxole boiling points at different pressures:

Post 360218 (missing)

(Rhodium: "3,4-Methylenedioxy-1-bromobenzene Synthesis", Newbee Forum)