How is vacuum measured?
Vacuum level is usually measured in terms of inches of mercury (“Hg). This is the same term used when the weatherman reports the barometer reading “the barometer is 29.21 “Hg and falling”. The maximum vacuum that can be achieved on a standard day is 29.91999. Inches of water is another term used to measure vacuum where 1″Hg = 13.6″H20. Inches of water is normally used when measuring low vacuum or air pressure. Vacuum can also be measured in torr, microns, mbar, atmospheres – however, these terms are not usually used in woodworking. Your elevation from sea level affects the amount of vacuum that can be achieved. A rule of thumb is to subtract 1″Hg for each 1000′ above sea level. Thus, if a pump normally reaches 27″Hg at sea level, then in Denver (5000′ above sea level) it will only get 22 – 23″Hg.
