A QUESTION WAS BROUGHT UP ABOUT VACUUM AND HERE IT IS. IF 29.921 IN HG IS A PERFECT VACUUM HOW WERE THEY ABLE TO DETERMINE THAT AS PERFECT VAC. AND WHAT WOULD HAPPEN IF BY CHANCE IT WAS EXCEEDED TO SAY 30 OR 35 IN HG? I KNOW IT HAS SOMETHING TO DO WITH ATMOSPH... Login to read more.