Rotary vane vac pump only pulls 26 Hg

[flanajb]

I have an airpress vac pump and even if I put my thumb over the end of the pipe it only pulls 26"hg which equates to 0.2 bar. A full vac is 0 bar, and we are only 200ft above sea level. Is it common for these pumps not to be able to pull a full vacuum, or should they be perfectly capable of doing so?

Trying to degas some silicone that I using to make a rubber mold, but as the vac pump did not pull down enough, it failed to degas.

Really annoying

 

[Steve Maskery]

You will never get a perfect vacuum, but I used to get about a 90% vacuum with mine. It was fine for veneering and laminating but I've never tried degassing silicone. I don't have it anymore, unfortunately, but I was very happy with the results when I did use it.

S

 

[Chrispy]

Are you sure about your numbers? I thought 1bar was something like 760mm of Mercury so 26" would be quite good, and surely a vacuum is minus one bar?

 

[Steve Maskery]

Chrispy, a perfect vacuum is zero bar.
S

 

[monkeybiter]

Depends if you're using BarG [guage] or Bar absolute. In BarG 0 is atmospheric pressure, in absolute 0 is hard vac. [or would be if achievable].

 

[Chrispy]

My view is that we use a vacuum as a force, a vacuum on it's own is not a force you need atmospheric pressure to act against to achieve a force, therfore the force achieved using a perfect vacuum is that of atmospheric pressure is. 1 bar. This may not be technically correct but it's the way my brain works.

 

[geordie]

Im lucky if i get 22 at my vac chuck and it holds bowls just fine never done degasing but it maybe a problem with your chamber not being 100% air tight

Geordie

 

[flanajb]

Peter at Airpress was very helpful. Apparently, the dry vane pumps will pull a max of 26" Hg where as to degas silicone or other rubber mold making material you need 29" Hg. Only the oil type vac pumps are capable of doing that. Can't really justify buying an oil vac pump for a single project.