Millibar and Torr are units of pressure that are on an “absolute” scale. That is, they begin at zero pressure and get larger as pressure builds – all pressure is positive. Because they have an absolute reference point, these scales are favored in many scientific applications. When using these absolute scales, “vacuum” means simply pressure that is below normal atmospheric conditions. “Inches of mercury” are widely used in North America to represent vacuum in the construction professions (architecture and engineering). This is a relative scale where the zero point is atmospheric pressure as represented on a gauge. It is often reported as “in. Hg gauge”. The rising numbers on this scale correspond to decreasing pressure (increasing vacuum). Since scales have different zero points and run in opposite directions, conversion between these scales for communication between scientific and engineering specialists can be complicated. Adding to the confusion is that local atmospheric pressure differs from sea level to mountain locations, with the result that there can be a 15% difference in the absolute pressure represented by the zero point on a gauge scale. As a result, gauge pressures are not reliable indicators of actual conditions for scientific reporting. Tools like our handy vacuum unit conversion calculator and our unit conversion slide chart exist to help with converting among the wide range of standard vacuum units. If you’d like to receive a free vacuum unit conversion tool, please contact us to request one.