The quality of a vacuum is indicated by the amount of matter remaining in the system, so that a high quality vacuum is one with very little matter left in it. Vacuum is primarily measured by its absolute pressure, but a complete characterization requires further parameters, such as temperature and chemical composition. One of the most important parameters is the mean free path (MFP) of residual gases, which indicates the average distance that molecules will travel between collisions with each other. As the gas density decreases, the MFP increases, and when the MFP is longer than the chamber, pump, spacecraft, or other objects present, the continuum assumptions of fluid mechanics do not apply. This vacuum state is called high vacuum, and the study of fluid flows in this regime is called particle gas dynamics. The MFP of air at atmospheric pressure is very short, 70 nm, but at 100 mPa (~1×10−3 Torr) the MFP of room temperature air is roughly 100 mm, which is on the order of everyday objects such as vacuum tubes. The Crookes radiometer turns when the MFP is larger than the size of the vanes.
Vacuum quality is subdivided into ranges according to the technology required to achieve it or measure it. These ranges do not have universally agreed definitions, but a typical distribution is as follows:
|Type||Pressure [Torr]||Pressure [Pa]|
|Low vacuum||760 to 25||1×105 to 3.3×103|
|Medium vacuum||25 to 1×10−3||3.3×103 to 133×10-3|
|High vacuum||1×10−3 to 1×10−9||133×10-3 to 133×10-9|
|Ultra high vacuum||1×10−9 to 1×10−12||133×10-9 to 133×10-12|
|Extremely high vacuum||<1×10−12||<133×10-12|
|Outer Space||1×10−6 to < 3×10−17||133×10-6 to < 400×10-17|
Read more at Wikipedia.