I come from a physics background where we talk about vacuum in absolute terms and we mostly use torr and in the range of 10E-3 to 10E-8.
I wanted to clarify what vacuum means to chemists and the standard units which are used.
The standard units seem to be mm Hg which are very similar to torr which I can handle but what are the ranges of low, medium and high vacuum.
Do chemists speak of vacuum in absolute terms or relative to atmospheric?
neptunium - 3-3-2013 at 17:03
i come from a physics background too and as far as chemistry is concerned 10E-3 torr is more than enough for most experiments.
Chemist deal with vacuum when distilling and filtration (mostly) and a few torr are usually enough to help along a low temp distillation.
and a coarse vacuum is more than enough for filtration
did i miss anything?BromicAcid - 3-3-2013 at 18:01
At my work our pumps are equipped with McLeod gauges which measure from 0.01 to 10 mmHg however you can 'eyeball' in the 10-20 range off the scale.
For lower vacuum we have digital manometers. I think that is where some of my ranges come from.
Quote:
The standard units seem to be mm Hg which are very similar to torr which I can handle but what are the ranges of low, medium and high vacuum.
I always thought it was amusing to talk to others about vacuum in this way. High vacuum to some is the same as saying low vacuum. I.e., at 0.001mmHg
you have 'lots of vacuum' and it would be considered high vacuum to some, but others would say the vacuum is really low because they use vacuum
interchangeably with pressure as a term. Writing this up I found myself make the mistake once of saying 'low vacuum' when I really meant low pressure
which would actually be be high vacuum. Although if you think on it too much you could think that high vacuum would be at the high end of the scale
and be bad vacuum and therefore low vacuum.
Anyway:
<0.07 mmHg is what I consider high vacuum for standard operations. That is toward the maximum vacuum measured by our McLeod gauges. Good for
final product distillations of materials that are high boiling.
3.0 to 20 mmHg would be a medium vacuum. Perfect for stripping off solvents of lower volatility like toluene or pyridine at or near room temperature.
100-200 mmHg would be low vacuum. Often used at the start of solvent strips or for stripping off highly volatile solvents like DCM, diethylether, and
even THF
95% of the time I find myself distilling in one of these butter zones in the process of completing my work. Always take into account 'how am I going
to capture these vapors'. If you distill DCM at full vacuum you might run into it filling your traps and making its way into your pump oil where it
will cause diminished pumping capacity.Houngan - 3-3-2013 at 21:28
Cheers Bromic,
I got thinking about this when I got a car vacuum guage to use on my setup which is in "in Hg" and the scale goes from 0 to 30. 0 being no vacuum and
30 being high vacuum.bfesser - 3-3-2013 at 21:39
