Testing vacuum pull power...

[Mercury]

An oil vacuum pump, brand new, but without a vacuum gauge can be like working in the dark. Would it be reasonable to test the vacuum power of a pump by setting up a distillation system to distill plain old water? Then, once the first drop of water distills over, you can record the tempturature and use a namograph to calculate the vaccum in mm/Hg.

[mr_pyrex]

Yep you sure could do that and it would give relatively accurate readings too.  But if you have a pool supply store near you...you could probably aquire an vaccum gauge from them for under $10.00.  Good Luck.  Pyrex out of vaccum oil ;-)

[Rhodium]

You would get a more accurate reading with for example toluene compared to water - the latter does not behave thatr well in a nomograph due to it's abnormally strong hydrogen bonds.

[Osmium]

Wrong, wrong, you are all wrong!  

A brand new oil pump will (should!) produce such a strong vacuum that you can even 'distill' frozen water. This is known as freeze drying. The final vacuum value you determine by distilling water will be too high. Most likely you won't be able to distill the water anyway, The vapors will simply be sucked through the pump.
When distilling toluene the same will happen, except that the tolly will dissolve in your pump oil which means you won't get the full vacuum again until you change the oil.
The new pump will most likely make sassy boil around 60°C. That temp is too low for 100% vapor condensation without a very efficient cold trap, so some of the more volatile components (and also some safrole) will end up in your pump oil.

All organic solvents and compounds will also dissolve in the rubber tubing you are using, and will slowly gas out of it even when they are not present during later distillations. That's one of the reasons why rubber tubing is not advisable if you want a better vacuum than 1mbar or so.

A $10 vacuum gauge is completely useless, all it can show you is whether the pump is running or not. These devices simply lack the necessary resolution at the lower end of the vacuum, you can guesstimate pressures like 200mbar or 700mbar with them, but they won't be able to tell you the difference between 0.2, 2 and 20mbar.

The final pressre of the pump is strongly dependent on what kind of equipment you are using, how clean it is, and how it is connected to the pump. Even the most expensive pump will not be able to reach its final rating as long as highly volatile (low boiling) stuff is present, the tubing used will let gasses diffuse through it, the diameter of the tubing used is low, and it's not given enough time to reach its final vacuum (which might take hours). It might reach a vacuum suitable to your purpose, but that value might differ greatly depending on exactly what you are doing and the condition it runs under.

[gruns]

OK Mr. Smarty-Osmium, what WOULD be a good substance with which to test the power of a vacuum for a particular setup? (And don't say a decent vacuum gauge, that's too easy... err.. expensive.)

[Osmium]

MDP2P!  

[mr_pyrex]

Technically Os, he didn't go into 100% detail as to what his distillation setup would or would not have been comprised of. Therefore, his setup could have included a capillary air inlet or even a manostat. But the reverse could also be true, neither present.  If he is someone that is rather knowledgable in the field of chemistry he might have known about those factors that you brought up and put in the necessary capillary inlet tube or manostat so that distillation would have occured.  He also didn't mention what he was going to use for cooling in the condensor...maybe it would have been acetone and C02.  So maybe it might have been more accurate to say that his setup wouldn't have worked unless he included the necessary capillary inlet tube or manostat.  This way he would have been made aware of what he needed to do if in fact he didn't already know.  And as for vacuum, he didn't state at what range he needed to have precise details, it might have been at the upper or lower end or even in the middle.  But the guage I speak of is not worthless....in fact it reads from 0-30 inches of vacuum in increments of 1 and has an accuracy of 2%...true it is not suitable for accurate high vacuum reading.  But it could give him a baseline reading so he would have known if on following distillations his pump performance suddenly dropped off.  Anyways to clarify my statement earlier...you could distill water at reduced pressure and then figure out the pump performance from the temp details, but you would need to include a capillary inlet tube or manostat in your distillation setup.  Well that about covers it.  Pyrex out of explanations ;-)