chemrox - 29-4-2007 at 18:09
I have a used pump that worksd well but I feel should perform better. I callibrated it with a liquid that boils at 209 and only managed to get it to
boil at 45.. indicating a vac of around 0.7mm. I thought I had itr boiling at 30 deg but that could have been an impurity that got boiled off. Or it
could have been closer to optimum performance. This is an older Seargent Welch two stage pump. I believe these achieve less than 0.1 mm when new.
I've run a lot of oil through it but crap still comes out and there's rust inside. Are there things I should try before tearing it down? Should I
tear it down? assuming I can get replacement seals and gaskets first? I'd like to hear from someone with a lot of experience tuning vacuum systems.
Sergei_Eisenstein - 30-4-2007 at 10:50
Do I read this correctly? You have a pump in your backyard shed that pulls 0.7 mmHg but you feel unhappy? You always have small leaks, and I honestly
think that if you really can pull 0.7 mmHg in an amateur setting, you should be satisfied.
Rust in your pump may indicate corrosion. Did you use a cold trap? Properly maintained pumps can go for years, but you'll have to use a cold trap.
bio2 - 30-4-2007 at 12:05
The rated pressure of vacuum pumps is the blank off
pressure which means the guage is mounted directly
on the pump body. This is done by the manufacturer
with a brand new pump and called the ultimate vacuum
attainable for the model tested.
As the previous poster mentioned you should be happy
with your results as the so called ultimate vacuum
is not readily (if at all) achievable in field (lab) use.
chemrox - 30-4-2007 at 22:58
Thanks both!
I need a decent cold trap .. I've been improvising with cold baths and a less than ideal Ehrlenmeyer (sp?) setup. I'm trolling LabX and ebay for a
dewar and proper trap. Meanwhile I change oil a lot. Would flushing oil help? The corrosion predates my ownership and I want to keep it from
getting worse.
Liquid N2 is a little beyond us right now but we can get dry ice.
Sergei- "backyard shed?" I actually have business cards, accounts, clients, I even have shoes!
Furch - 1-5-2007 at 01:37
I use a trap with 13X molecular sieves mixed with blue Drierite... Though I don't use my vacuum pumps to evaporate solvents, for that I use the water
aspirator.
... Also worth mentioning, my old pump was so rusty that it sucked worse than a <insert nationality> hooker on a bad day. The way to go to get
the pump back to pristine state again is without a doubt an ultrasonic bath... When I did that, it produced a less than 0.5 mmHg vacuum again.
Amazing! I suggest you do the same if you need a better vacuum.
chemrox - 1-5-2007 at 06:38
Please tell me more- was that a service you found? where? what kind of business does that?
bio2 - 1-5-2007 at 08:45
......Would flushing oil help? ........
Yes, as a first step, using vacuum pump oil, run the pump until the oil is hot and drain completely. Repeat until the oil is clean.
Many vacuum pumps have a replaceable "cartridge" which
houses the 2 rotors. The clearances of the lobes and
sliding vane are very miniscule and it is usually not advised
to attempt to disassemble them.
The pump Joe uses has a rebuild kit available which
includes new seals and gaskets along with a factory
rebuilt rotor cartridge element. The old cartridge
is exchanged when the kit is purchased.
chemrox - 1-5-2007 at 09:21
I just recorded a pressure of 0.06 mmHg based on bp of g-butyrolactone. I speculated the lactone taking up CO2 which I then outgassed but I brought
the stuff to a hard boil at 19 degC anmd held it there. I don't like to run this stuff very long because my improvised trap is not very good and the
oil takes this crap up like nobody's business.
[Edited on 1-5-2007 by chemrox]
Boomer - 9-5-2007 at 08:10
You are lucky you know? That's orders of magnitude below what most of us reach in the kitche/shed. The only solvents that boils at room temp with my
crappy fridge compressor are ether, acetone, MeOH and EtOH and IPA on a hot day. They condens in the pump, sputter out and take the oil with them, and
it needs more oil than electricity. I'll have to get an aa pirator some day.
The only way to get my GBL boilling is to add solid NaOH pettets. 
Furch - 12-5-2007 at 05:06
Chemrox, when I enter those values (b.p. of GBL @ 760 mmHg = 202 °C, and your observed boiling point = 19 °C) I get a resulting pressure of 0.1758
mmHg. If the pressure would've been 0.06 mmHg, the GBL would start coming over at about 8 °C.
About the ultrasonic bath... It was a friend of mine who had access to one. I don't know how to get your hands on one other than that...
bio2 - 12-5-2007 at 18:48
Extrapolation of boiling point using say a nomograph
gives approximate temperatures and may be several
degrees off depending which curve you select.
This is especially so when the corresponding points
are over 100 degrees apart.
Has anyone experienced a high boiing substance
being distilled at a rapid rate without a ripple on the surface with no visible boiling turbulence? It's quite an errie sight?