What is a Low, Medium, or High Vacuum?

Vacuums come in all shapes and sizes - and I am not referring to vacuum cleaners! Any local reduction in air pressure significantly below standard atmospheric pressure (760 mm of mercury, 14.7 pounds per square inch) is termed a vacuum (except by your local weather person who talks about 'low pressure areas'). For convenience (and because there must have been a meeting of elder statesman with nothing better to do), the Torr in honor of some Italian named Torrecelli is used to designate a pressure of 1 mm of mercury I guess referring to 'Torrecellis' all the time would be too confusing. :-) The following dividing lines between low, medium, high, and ultra-high vacuums are somewhat arbitrary but will be convenient for discussion:
Biosphere: 1 atm (760 Torr) to .5 atm. At sea level, the average pressure is about 1 atm. The weather person will talk about 'inches of mercury' instead of 'mm of mercury' since most people in the U.S.A. at least haven't entered the metric age. :-) 29.92 inches = 760 mm. The wildest storm imaginable doesn't produce variations beyond a few percent of this nominal value. Mountain climbers have to endure reduced pressure and above about 10,000 feet, require breathing equipment. Anyone who has traveled by air knows the standard speech at the beginning of each flight "....should oxygen be needed, the compartments overhead....". This would also happen above about 10,000 feet. Astronauts on American spacecraft (at least they used to), breath unaided at a pressure of perhaps 1/5 of an atm because they breath nearly pure oxygen. Since in the normal atmosphere, oxygen is only about 18 percent of the total mixture (most of the rest is nitrogen with a little CO2 and inert gasses thrown in), the resulting biological activity (and the flammability of common materials, for that matter) is about the same but there is no need to carry the approximately 80% of useless other gasses and the stesses on the spacecraft structure (from the difference between the internal pressure and the vacuum outside) are reduced by 80% as well.

Low vacuum: 1 atm (760 Torr) to 100 Torr. This is something you may have dealt with - the suction of a vacuum cleaner, spark advance manifold on your automobile, a siphon, and so forth. None of these is anywhere near the bottom end of this range - all are probably better than .5 atm and usually much closer to 1 atm. All except the smallest incandescent light bulbs are filled with inert gas at a fraction of an atm as well. A low vacuum can be obtained by any number of simple mechanical means including fans and centrifugal blowers, piston and rotary pumps, aspirators, siphons, chemical combustion and other reactions (which use up the air), etc. Liquids boil at reduced temperature - often room temperature - in a modest vacuum but minimal or no precautions are needed to prepare surfaces and equipment since any outgassing is small compared to the remaining air.

Medium vacuum: 100 to .1 Torr. This is the range where most of the gas lasers operate. In addition, neon signs, fluorescent lamps, and other glow discharge tubes, distillation pumps, vacuum packing, and so forth require medium vacuums. A medium vacuum can be achieved with a high quaility mechanical pump.

High vacuum: .1 to 1E-6 Torr. Crooks radiometer (that thing with the black and silver vanes that spins in Sunlight), small light bulbs, thermos bottles, cold cathode (gas type) X-ray and Crooks tubes, mass spectrometers, etc. At the bottom end of this range true vacuum electronics technology becomes possible including: vacuum fluorescent display tubes, CRTs, modern hot cathode X-ray tubes; smaller particle accelerators like cyclotrons and betatrons; scanning and transmission electron microscopes.

Ultra-high vacuum: 1E-6 to 1E-14 Torr. The actual vacuum inside the CRT of your computer monitor or TV is probably at a level of 1E-9 or better. For many processes, the ultimate quality in terms of yield and performance can directly tied to the quality of the vacuum used in the manufacturing processes. To put a 1E-9 Torr vacuum into perspective: If all of the gas molecules remaining inside a typical 17 inch monitor CRT that had been manufactured at this level of vacuum were rounded up, captured, and returned to normal atmospheric pressure, they would occupy a volume of space less than 25 um on a side - roughly 1/10th the diameter of the dot in the explanation point at the end of this sentence or half the diameter of a human hair! Yet, inside the CRT, there would still be approximately 1,000,000,000,000 gas molecules remaining for unsuspecting electrons to run into!

Interstellar space: <1E-19 Torr. It is estimated that in the space between galaxies, there may only be a few molecules per cubic meter - which isn't much! Think of how much easier it would be to 'fill' CRTs with vacuum out between the galaxies! Of course, the transportation costs might eat up your profit margin. :-) You may also hear the term 'hard vacuum'. I don't know if there is a precise definition for this either but I would assume that anything with a low enough pressure to behave similarly to a perfect vacuum from the normal experiences point of view would qualify.