...it has something to do with the gravity of the fluid itself, if I'm not mistaken. If I read the instructions correctly, the "output" accuracy of the guage will be affected by different/higher levels of vacuum,so they ask you to calibrate it to each new vacuum source. I didn't really pay much attention to the explanation as to why when I did it... I just calibrated the thing and marched on. :) It's easy enough to do. The tool does not give you any sort of in/Hg reading...... it's just a comparative vacuum tool. :)
Last edited by SpazOnaZX; 04-12-2009 at 11:41 AM.