Understanding vacuum bag pressure

[ColeyS1]

So last night I chucked a couple bits in my vacuum bag. I pinned the bits in place to stop things sliding/moving.
The pressure gauge at the bag read 0.65 and seemed to pull everything together. I then tried finding what minimum reading I needed for the vacuum bag to work, I couldn't find it.
What would 0.65 be in comparison to weight ? I've got loads of 56 pound weights I could have used to squish it up but the vacuum bag is soooooo much lighter it was also a fairly large area - 1.0m x1.8m ish.
I'm just curious as to how much weight I effectively had by throwing it in the vac bag.


Coley

 

[bugbear]

You might want to check the equations that relate area, pressure and weight.

The clue in the units of pressure - e.g. "lbs per square inch".

BugBear

 

[RobinBHM]

About 8.5 tonnes per square meter assuming the vacuum is running at 85% vacuum.

 

[Argus]

0.65 what? Millimetres Hg, inches Hg or millibar………

There have been numerous units used to measure a high vacuum since Torricelli discovered the effects of air pressure, starting with the one named after him, the TORR (now no longer used, but still in the memories of us old Refrigeration Engineers).

The basic unit usually revolves around the ability of the atmosphere at mean sea level to support something in a sealed tube – usually mercury and has been measured over the decades in inches Hg, millimetres, then in air pressure fractions of an Atmosphere, or a Bar (cumbersome in numbers), latterly in millibars.

If you’re using the pressure to clamp wood, then do a bit of wikki-ing to look at the comparative weight loads at corresponding air pressures. For this purpose you do not, in my opinion, need as powerful vacuum pump as you would, for example to dehydrate a critical sealed system. The size of the bag is immaterial as the resulting pressure is equal over the entire surface.

At the pressures you are contemplating for clamping wood, I’d advise avoid over-powerful single or 2 stage vacuum pumps designed for other purposes (Refrigeration systems, for example) as one effect of a deep vacuum, typically below about 25 mb, is to rapidly remove residual moisture as a vapour at ambient temperatures from both the wood and glues.

Again, oil-less pumps may be better as it eliminates the effects of backward oil contamination of your work if the pump and check-valves fail and pressures equalise.
Here’s a site full of Vacuum pressure comparisons and calculators, ancient and modern, starting with pressure/temperature.

http://produkte.gea-wiegand.de/GEA/GEAC ... ex_en.html

Vacuum is a complex subject in its own right, but don’t get too bogged down outside the basics for your purposes.

 

[ColeyS1]

0.65 what? Millimetres Hg, inches Hg or millibar………

Sorry about the lack of info. I was lay in bed just wondering if using the 56ers would have had more pressure.
Just had a look on the gauge.

Looks like it measures in bar. I think it's a venturi valve that just plugs into the compressor. In my head I was trying to compare it with household water pressure for some reason. 0.65 bar didn't sound alot ?

Coley

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[Steve Maskery]

Well if it is 0.65 of full vacuum, and that sounds reasonable for a system like that, then that equates to just over 9lbs per square inch, plenty for any gluing operation, I'd have thought.
Over your whole bag I think that is equivalent of having about one and a half tonnes, or thereabouts.

 

[ColeyS1]

Thanks Steve. Crikey o riley,that much weight !!!! That makes the vac bag a much better choice then the lumpy weights any day. Im quite amazed by that

Coley