General > General Technical Chat
Making a high pressure vacuum chamber
james_s:
--- Quote from: No_Shorty on March 10, 2023, 10:11:15 am ---It was only as acrylic is used for a lot of DIY chamber 'guides' you find online that I had been considering it (and of course the ability to see whats going on inside) - do you think there is no safe level of vacuum in which acrylic can be used?
--- End quote ---
You keep going back and forth between pressure and vacuum. Vacuum is really just the absence of pressure, 1 atmosphere is about 15 psi absolute, anything under that is a vacuum, a perfect vacuum is 0 psi absolute, but vacuums are typically measured in inches of mercury for partial vacuums, and microns of mercury for deep vacuums. The difference in mechanical stress on a vessel between the sort of vacuum you can get from a cheap single stage rotary pump or even a hand pump, and a deep vacuum as you'd find inside a CRT is actually pretty small.
jpanhalt:
Yes, please clarify whether you mean vacuum or pressure. In a recent post, you mention using it to degas epoxy. That is usually done with vacuum. You can also try sparging with helium, which has a low solubility, but vacuum is far simpler and cheaper.
( https://en.wikipedia.org/wiki/Sparging_(chemistry) )
No_Shorty:
I am at this stage not sure why there is confusion between how I am referencing vacuum and pressure. I did look back at my posts but I am unsure why they are still causing confusion.
For the debubble machines - like the one linked in my OP - I now understand these work at high pressures - not vacuums. I am still interested in these machines, but I do not believe I can achieve what they do via a DIY route.
I am now concentrating on the idea of trying to remove bubbles from LOCA (the liquid/glue, not the OCA film) with a vacuum chamber.
The only point I have referred to pressure again was in describing the "pressure cooker style vacuum chambers" - by this I mean this kind of thing, I am aware it is not a pressure vessel, just that it looks like a "Pressure cooker": https://www.amazon.co.uk/BACOENG-Stainless-Stabilizing-Degassing-Silicones/dp/B07GR5FTQJ/ref=sr_1_5
I hope that makes sense.
I am broadly a little confused by the use of the word 'pressure' when describing vacuum - the word "pressure" comes up frequently when looking into vacuums, and if I google what units vacuums should be measured in a link like this: https://sens4.com/vacuumunits.html - talks repeatedly about pressure and measurement in mbar. I expect I am missing something simple here, so I have been trying to avoid the use of the word pressure when referencing a vacuum.
edit: Ian.m - thanks for that suggestion, I have a sous vide machine so the thought had crossed my mind (not to use the soud vide machine itself, but bagging as a form of creating a vacuum). It guess it would also provide some pressure (right word this time?! :-// ) to the screen as the bag constricts around it.
james_s:
--- Quote from: No_Shorty on March 10, 2023, 07:39:00 pm ---I am broadly a little confused by the use of the word 'pressure' when describing vacuum - the word "pressure" comes up frequently when looking into vacuums, and if I google what units vacuums should be measured in a link like this: https://sens4.com/vacuumunits.html - talks repeatedly about pressure and measurement in mbar. I expect I am missing something simple here, so I have been trying to avoid the use of the word pressure when referencing a vacuum.
--- End quote ---
Because pressure and vacuum are like heat and cold. Cold is not really a thing itself, it is the absence of heat. Vacuum likewise is the absence of pressure. There are two ways to discuss pressure, one is absolute pressure, which is relative to a perfect vacuum. The other is gauge pressure (psig) which is relative to atmospheric pressure. Atmospheric pressure is around 15 psi absolute, and it is by definition 0 psig. A vacuum is negative psig, but positive absolute, you cannot go below 0 psi absolute just like you cannot get colder than absolute zero, because absolute zero in both cases is the complete absence of heat or pressure.
In the real world an absolutely perfect vacuum is impossible to achieve, there will always be some pressure, which is created by some number of remaining gas molecules flying around and bouncing off the walls of the vessel. A vacuum is by definition lower than atmospheric pressure, so there will also always be pressure on the outside of the vessel caused by air molecules exerting force on it. With a vessel at atmospheric pressure you have an equal amount of pressure inside and out so the net pressure is zero.
TimFox:
With vacuum and high pressure, there are two measurements:
"Absolute pressure" is the actual pressure inside the chamber. It must be positive.
"Gauge pressure" is the difference in pressure between the chamber and the local atmosphere. It can be positive or negative, but the most negative value equals the absolute pressure of the ambient air.
You may see a gauge calibrated in "PSIG" for positive gauge pressure (pounds per square inch gauge, pronounced pigs' eye) and "Inch Hg" (inches of mercury) for negative gauge pressure (vacuum).
Vacuum gauges for "high vacuum" are usually for absolute pressure, with an interesting collection of units:
"mm Hg" = "Torr", where 760 mm is standard air pressure, based on traditional units for mercury barometers
"microns" = micrometer Hg, defined similarly
"mbar", roughly equal to "mm Hg", since 1 bar = standard air pressure = 760 mm Hg.
In physics labs, I found the ultra-high-vacuum gauges calibrated in mm Hg to be amusing, since we could obtain a very good vacuum of 10-10 Torr, and a mercury atom (300 pm diameter) is much larger than 10-10 mm = 0.1 pm.
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