What does the micron rating mean? How does it relate to inch Hg vacuum or absolute pressure?
Atmospheric pressure is about 29.92 inch Hg. Any pressure below 29.9 in Hg is considered to be under vacuum. So, absolute vacuum is 0 in Hg, or 29.92 in Hg vacuum! At very low pressure under vacuum, micron is used. One micron is defined as 1 micrometer Hg in absolute pressure. One inch Hg has about 25,400 microns in pressure. 25 micron will be about 29.92 in Hg vacuum! Deep vacuum pumps are rated by their vacuum levels they can achieve in microns. So, the lower the micron rating, better is the pump, deeper the vacuum it can reach.