CouchHatter
Hazard to Others
Posts: 146
Registered: 28-10-2017
Location: yes
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aspirator cold trap
I have a styrofoam box with a glass honey jar surrounded by ice to protect my pump from volatile stuff. Tubing runs from the aspirator through the jar
to the takeoff adapter. The jar lid leaks a little and I'm in search of a better DIY solution. I've considered buying more thick-walled vacuum flasks,
or using Mason jars, or just a copper coil, but prefer an actual trap to catch a good amount of condensate. Mason jars are preferred because I have
them, but I'm not sure how trustworthy they are at 28inHg which is what the jar I've used twice has withstood. The current jar was just a proof of
concept. What do you think? Vacuum flasks aren't too expensive. I couldn't find much talk of that on this forum, but from a search online:
| Quote: | | What people usually don't 'get' is that even a rough vacuum is almost the full 14.7 psi, and that getting to a perfect hard vacuum applies almost no
additional force on the container. |
Also, what's the proper way to report vacuum? I've seen other members on the site report their vacuum as 24 or 27. Is that the amount of vacuum, and
the atmospheric pressure in the system is the inverse of that? And a nomograph uses the actual system pressure on the logarithmic axis? It seems like
I have to subtract my gauge reading from 29.92 inHg to get boiling points to line up. Just curious that I would not see this written anywhere about
how to use a nomograph. Maybe it is supposed to be obvious, I guess I would just like to confirm that there is not some better explanation.
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Sulaiman
International Hazard
Posts: 3558
Registered: 8-2-2015
Location: 3rd rock from the sun
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Pressure is a simple concept,
made confusing by different methods and units of measurement.
look here https://en.wikipedia.org/wiki/Pressure_measurement
The quick answer to your question is that you are correct,
subtract the reading on your vacuum gauge from atmospheric pressure to get absolute/actual pressure,
which is what most nomographs and tables use.
A common simple 'pressure gauge' indicates the difference from atmospheric pressure, so-called 'gauge pressure',
this is the type that most of us use as they are cheap and convenient.
see 'Bourdon gauge' https://en.wikipedia.org/wiki/Pressure_measurement#Bourdon_g...
"Atmospheric Pressure" varies with altitude and weather, https://en.wikipedia.org/wiki/Atmospheric_pressure
so it is difficult to measure absolute pressure exactly when using a Bourdon gauge.
CAUTION : Hobby Chemist, not Professional or even Amateur
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