[beheada]

As I read literature, I notice that alot of varying synthesis state using a vacuum at a very particular atmosphere, like 40mmHg. The question is simple:

In the lab at my school, we use water aspirators with no guage of the atmosphere. Aside from having a chart which states the varying boiling temperatures of water at varying atmospheres, how do scientists in the lab (that actually use mechanical vacuum pumps) regulate the pressure in their system so that it is at a certain atmosphere?

I am guessing that this is accomplished by some sort of pressure regulator system but I'm not sure specifically how it is accomplished.

Thanks

[Sam (NG)]

I've only ever used vacuum for vacuum distillation, and we had a monometer to read the vacuum level (using a water pump).  They are not very accurate at all.  Sorry i can't be more helpful, but that is my personal experience in the organic lab.  Never actually carried out a synthesis under reduced pressure (as far as i can remember).

[movies]

There are very accurate manometers out there!

If you want a specific pressure, there are vacuum regulators, some good, some bad.  In my experience, I know that the vacuum pump in my hood pulls a vacuum of ~1 torr, so I try to adjust to that and it works for most things.  I don't have a regulator so I just go with 1 torr for everything on my line.

If I need to get to ~50 torr, I use a different vacuum pump that has a (crappy) regulator on it.  A good aspirator will get to about 30 torr.  One bad thing about water aspirators is that the pressure they can achieve varies depending on the temperature of the water, which can change as the motor runs.  Annoying for long distillations!

[Sam (NG)]

There are very accurate manometers out there!

Sorry, i wrote that rather ambiguously, when i said "They are not very accurate at all" i was referring only to the ones we have in the undergraduate laboratories.

[movies]

Oh, I see.  Yeah, undergrad labs don't usually get the top-of-the-line equipment.