Understanding Vacuum Measurement

When discussing vacuum for the laboratory, it is common for lab designers, engineers and scientists to use different measuring units. Engineers and designers commonly specify vacuum with “relative” units, such as psi, psig or inches or mercury (in. Hg), while scientists typically use absolute scales, such as millibar or Torr. While each of these approaches has its place, the different reference points make it difficult for lab designers and lab scientists to communicate about vacuum conditions needed in the laboratory.

Relative scales are often useful for pressures above atmospheric pressure, but they make it difficult for scientists to determine vacuum conditions accurately, since these values are measured relative to atmospheric pressure. For example, gauge values reported at sea level may reflect different conditions from the same values reported at higher elevations. Also, these relative scales may count upward or downward from a zero point at ambient pressures, leading to scientific uncertainty about the reported conditions unless the context (pressure above or below atmospheric pressure) is reported.

To address these issues, scientists typically use units like millibar and Torr that describe pressure on an “absolute” scale, that is, independent of ambient pressures. This is important for scientific endeavors, so that conditions are reproducible from day to day and place to place, and accurately relate to the vapor pressures of solvents in use. These scales start at zero corresponding to a theoretical perfect vacuum, and count up as the pressure rises from that point, irrespective of ambient conditions.

To assist with communications between scientists and designers during lab specification efforts, VACUUBRAND has developed a chart that compares Vacuum Scales used in discussing laboratory vacuum, relating the scales to one another. The chart also includes reference to the pressure levels needed for actual applications for which scientists employ vacuum, from filtration and aspiration, to evaporation and freeze drying.