Outgassing

Evaporation and sublimation into a vacuum is called outgassing. All materials, solid or liquid, have a small vapour pressure, and their outgassing becomes important when the vacuum pressure falls below this vapour pressure. In man-made systems, outgassing has the same effect as a leak and can limit the achievable vacuum. Outgassing products may condense on nearby colder surfaces, which can be troublesome if they obscure optical instruments or react with other materials. This is of great concern to space missions, where an obscured telescope or solar cell can ruin an expensive mission.

The most prevalent outgassing product in man-made vacuum systems is water absorbed by chamber materials. It can be reduced by desiccating or baking the chamber, and removing absorbent materials. Outgassed water can condense in the oil of rotary vane pumps and reduce their net speed drastically if gas ballasting is not used. High vacuum systems must be clean and free of organic matter to minimize outgassing.

Ultra-high vacuum systems are usually baked, preferably under vacuum, to temporarily raise the vapour pressure of all outgassing materials and boil them off. Once the bulk of the outgassing materials are boiled off and evacuated, the system may be cooled to lower vapour pressures and minimize residual outgassing during actual operation. Some systems are cooled well below room temperature by liquid nitrogen to shut down residual outgassing and simultaneously cryopump the system.

Quality

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:^[30][31]

	pressure Torr	pressure (Pa)
Atmospheric pressure	760	101.3 kPa
Low vacuum	760 to 25	100 kPa to 3 kPa
Medium vacuum	25 to 1×10−3	3 kPa to 100 mPa
High vacuum	1×10−3 to 1×10−9	100 mPa to 100 nPa
Ultra high vacuum	1×10−9 to 1×10−12	100 nPa to 100 pPa
Extremely high vacuum	<1×10−12	10
Outer Space	1×10−6 to 3×10−17	100 µPa to 3fpa
Perfect vacuum	0	0 Pa

• Atmospheric pressure is variable but standardized at 101.325 kPa (760 Torr)
• Low vacuum, also called rough vacuum or coarse vacuum, is vacuum that can be achieved or measured with rudimentary equipment such as a vacuum cleaner and a liquid column manometer.
• Medium vacuum is vacuum that can be achieved with a single pump, but is too low to measure with a liquid or mechanical manometer. It can be measured with a McLeod gauge, thermal gauge or a capacitive gauge.
• High vacuum is vacuum where the MFP of residual gases is longer than the size of the chamber or of the object under test. High vacuum usually requires multi-stage pumping and ion gauge measurement. Some texts differentiate between high vacuum and very high vacuum.
• Ultra high vacuum requires baking the chamber to remove trace gases, and other special procedures. British and German standards define ultra high vacuum as pressures below 10⁻⁶ Pa (10⁻⁸ Torr).^[32][33]
• Deep space is generally much more empty than any artificial vacuum. It may or may not meet the definition of high vacuum above, depending on what region of space and astronomical bodies are being considered. For example, the MFP of interplanetary space is smaller than the size of the solar system, but larger than small planets and moons. As a result, solar winds exhibit continuum flow on the scale of the solar system, but must be considered as a bombardment of particles with respect to the Earth and Moon.
• Perfect vacuum is an ideal state that cannot be obtained in a laboratory, nor can it be found or obtained anywhere else in the universe, apart from possibly the singularity of a black hole, or the (potentially large) spaces between atoms in lesser vacuums.
• Hard vacuum and Soft vacuum are terms that are defined with a dividing line defined differently by different sources, such as 5 psia^[34], one Torr^[35], or 0.1 Torr^[36] the common denominator being that a hard vacuum is a higher vacuum than a soft one.

Examples

	pressure (Pa)	pressure (Torr)	mean free path	molecules per cm3
Vacuum cleaner	approximately 80 kPa	600	70 nm	1019
liquid ring vacuum pump	approximately 3.2 kPa	24
freeze drying	100 to 10 Pa	1 to 0.1	100 μm to 1 mm	1016 to 1015
rotary vane pump	100 Pa to 100 mPa	1 to 10−3	100 μm to 10 cm	1016 to 1013
Incandescent light bulb	10 to 1 Pa	0.1 to 0.01	1 mm to 1 cm	1015 to 1014
Thermos bottle	1 to 0.01 Pa[1]	10−2 to 10−4	1 cm to 1 m	1014 to 1012
Earth thermosphere	1 Pa to 100 nPa	10−2 to 10−9	1 cm to 100 km	1014 to 107
Vacuum tube	10 µPa to 10 nPa	10−7 to 10−10
Cryopumped MBE chamber	100 nPa to 1 nPa	10−9 to 10−11	100 to 10,000 km	107 to 105
Pressure on the Moon	approximately 1 nPa	10−11		4 X 105[37]
Interplanetary space				10[1]
Interstellar space				1[38]
Intergalactic space				10−6[1]

Measurement

Main article: Pressure measurement

Relative vs Absolute measurement

Vacuum is measured in units of pressure, typically as a subtraction relative to ambient atmospheric pressure on Earth. But the amount of relative measurable vacuum varies with local conditions. On the surface of Jupiter, where ground level atmospheric pressure is much higher than on Earth, much higher relative vacuum readings would be possible. On the surface of the moon with almost no atmosphere, it would be extremely difficult to create a measurable vacuum relative to the local environment.

Similarly, much higher than normal relative vacuum readings are possible deep in the Earth's ocean. A submarine maintaining an internal pressure of 1 atmosphere submerged to a depth of 10 atmospheres (98 meters; a 9.8 meter column of seawater has the equivalent weight of 1 ATM) is effectively a vacuum chamber keeping out the crushing exterior water pressures, though the 1 ATM inside the submarine would not normally be considered a vacuum.

Therefore to properly understand the following discussions of vacuum measurement, it is important that the reader assumes the relative measurements are being done on Earth at sea level, at exactly 1 atmosphere of ambient atmospheric pressure.

Vacuum measurements relative to 1 ATM

The SI unit of pressure is the pascal (symbol Pa), but vacuum is usually measured in torrs, named for Torricelli, an early Italian physicist (1608 - 1647). A torr is equal to the displacement of a millimeter of mercury (mmHg) in a manometer with 1 torr equaling 133.3223684 pascals above absolute zero pressure. Vacuum is often also measured using inches of mercury on the barometric scale or as a percentage of atmospheric pressure in bars or atmospheres. Low vacuum is often measured in inches of mercury (inHg), millimeters of mercury (mmHg) or kilopascals (kPa) below atmospheric pressure. "Below atmospheric" means that the absolute pressure is equal to the current atmospheric pressure (e.g. 29.92 inHg) minus the vacuum pressure in the same units. Thus a vacuum of 26 inHg is equivalent to an absolute pressure of 4 inHg (29.92 inHg − 26 inHg).

In other words, most low vacuum gauges that read, for example, −28 inHg at full vacuum are actually reporting 2 inHg, or 50.79 Torr. Many inexpensive low vacuum gauges have a margin of error and may report a vacuum of −30 inHg, or 0 Torr but in practice this generally requires a two stage rotary vane or other medium type of vacuum pump to go much beyond (lower than) 25 torr.

A glass McLeod gauge, drained of mercury

Many devices are used to measure the pressure in a vacuum, depending on what range of vacuum is needed.^[39]

Hydrostatic gauges (such as the mercury column manometer) consist of a vertical column of liquid in a tube whose ends are exposed to different pressures. The column will rise or fall until its weight is in equilibrium with the pressure differential between the two ends of the tube. The simplest design is a closed-end U-shaped tube, one side of which is connected to the region of interest. Any fluid can be used, but mercury is preferred for its high density and low vapour pressure. Simple hydrostatic gauges can measure pressures ranging from 1 torr (100 Pa) to above atmospheric. An important variation is the McLeod gauge which isolates a known volume of vacuum and compresses it to multiply the height variation of the liquid column. The McLeod gauge can measure vacuums as high as 10⁻⁶ torr (0.1 mPa), which is the lowest direct measurement of pressure that is possible with current technology. Other vacuum gauges can measure lower pressures, but only indirectly by measurement of other pressure-controlled properties. These indirect measurements must be calibrated via a direct measurement, most commonly a McLeod gauge.^[40]

Mechanical or elastic gauges depend on a Bourdon tube, diaphragm, or capsule, usually made of metal, which will change shape in response to the pressure of the region in question. A variation on this idea is the capacitance manometer, in which the diaphragm makes up a part of a capacitor. A change in pressure leads to the flexure of the diaphragm, which results in a change in capacitance. These gauges are effective from 10⁺³ torr to 10⁻⁴ torr, and beyond.

Thermal conductivity gauges rely on the fact that the ability of a gas to conduct heat decreases with pressure. In this type of gauge, a wire filament is heated by running current through it. A thermocouple or Resistance Temperature Detector (RTD) can then be used to measure the temperature of the filament. This temperature is dependent on the rate at which the filament loses heat to the surrounding gas, and therefore on the thermal conductivity. A common variant is the Pirani gauge which uses a single platimum filament as both the heated element and RTD. These gauges are accurate from 10 torr to 10⁻³ torr, but they are sensitive to the chemical composition of the gases being measured.

Ion gauges are used in ultrahigh vacuum. They come in two types: hot cathode and cold cathode. In the hot cathode version an electrically heated filament produces an electron beam. The electrons travel through the gauge and ionize gas molecules around them. The resulting ions are collected at a negative electrode. The current depends on the number of ions, which depends on the pressure in the gauge. Hot cathode gauges are accurate from 10⁻³ torr to 10⁻¹⁰ torr. The principle behind cold cathode version is the same, except that electrons are produced in a discharge created by a high voltage electrical discharge. Cold cathode gauges are accurate from 10⁻² torr to 10⁻⁹ torr. Ionization gauge calibration is very sensitive to construction geometry, chemical composition of gases being measured, corrosion and surface deposits. Their calibration can be invalidated by activation at atmospheric pressure or low vacuum. The composition of gases at high vacuums will usually be unpredictable, so a mass spectrometer must be used in conjunction with the ionization gauge for accurate measurement.