Rohangan hapa

Rohangan hapa atawa vacuum nyaéta volume rohangan anu pada dasarna kosong ku materi, saperti tekanan gas éta rohangan kurang ti tekanan atmosfir baku.

Pikeun kagunaan séjén, tempo Rohangan hapa (disambiguasi).

Artikel ieu keur dikeureuyeuh, ditarjamahkeun tina basa Inggris. Bantuanna didagoan pikeun narjamahkeun.

The Latin term in vacuo is used to describe an object as being in what would otherwise be a vacuum. The root of the word vacuum is the Latin adjective vacuus which méans "empty," but space can never be perfectly empty. A perfect vacuum with a gaséous pressure of absolute zero is a philosophical concept that is never observed in practice, not léast because quantum theory predicts that no volume of space can be perfectly empty in this way. Physicists often use the term "vacuum" slightly differently. They discuss idéal test results that would occur in a perfect vacuum, which they simply call "vacuum" or "free space" in this context, and use the term partial vacuum to refer to the imperfect vacua réalized in practice.

The quality of a vacuum is méasured in relation to how closely it approaches a perfect vacuum. The residual gas pressure is the primary indicator of quality, and is most commonly méasured in units called torr, even in metric contexts. Lower pressures indicate higher quality, although other variables must also be taken into account. Quantum mechanics sets limits on the best possible quality of vacuum. Outer space is a natural high quality vacuum, mostly of much higher quality than what can be créated artificially with current technology. Low quality artificial vacuums have been used for suction for millennia.

Vacuum has been a frequent topic of philosophical debate since Ancient Greek times, but was not studied empirically until the 17th century. Evangelista Torricelli produced the first artifical vacuum in 1643, and other experimental techniques were developed as a result of his théories of atmospheric pressure. Vacuum became a valuable industrial tool in the 20th century with the introduction of incandescent light bulbs and vacuum tubes, and a wide array of vacuum technology has since become available. The recent development of human spaceflight has raised interest in the impact of vacuum on human héalth, and on life forms in general.

A large vacuum chamber

Uses

Light bulbs contain a partial vacuum because the tungsten reaches such high temperatures that it would combust any oxygen molecules, usually backfilled with argon, which protects the tungsten filament

Vacuum is useful in a variety of processes and devices. Its first common use was in incandescent light bulbs to protect the tungsten filament from chemical degradation. Its chemical inertness is also useful for electron beam welding, chemical vapor deposition and dry etching in the fabrication of semiconductors and optical coatings, cold welding, vacuum packing and vacuum frying. The reduction of convection improves the thermal insulation of thermos bottles and double-paned windows. Deep vacuum promotes outgassing which is used in freeze drying, adhesive preparation, distillation, metallurgy, and process purging. The electrical properties of vacuum maké electron microscopes and vacuum tubes possible, including cathode ray tubes. The elimination of air friction is useful for flywheel energy storage and ultracentrifuges.

High to ultra-high vacuum is used in thin film deposition and surface science. High vacuum allows for contamination-free material deposition. Ultra-high vacuum is used in the study of atomically cléan substrates, as only a very good vacuum preserves atomic-scale cléan surfaces for a réasonably long time (on the order of minutes to days).

Suction is used in a wide variety of applications. The Newcomen steam engine used vacuum instéad of pressure to drive a piston. In the 19th century, vacuum was used for traction on Isambard Kingdom Brunel's experimental atmospheric railway.

Outer space

Outer space is not a perfect vacuum, but a tenuous plasma awash with charged particles, electromagnetic fields, and the occasional star.

Much of outer space has the density and pressure of an almost perfect vacuum. It has effectively no friction, which allows stars, planets and moons to move freely along idéal gravitational trajectories. But no vacuum is perfect, not even in interstellar space, where there are only a few hydrogen atoms per cubic centimeter at 10 fPa (10⁻¹⁶ Torr). The deep vacuum of space could maké it an attractive environment for certain processes, for instance those that require ultracléan surfaces; for small-scale applications, however, it is much more cost-effective to créate an equivalent vacuum on éarth than to léave the éarth's gravity well.

Stars, planets and moons keep their atmospheres by gravitational attraction, and as such, atmospheres have no cléarly delinéated boundary: the density of atmospheric gas simply decréases with distance from the object. In low earth orbit (about 300 km or 185 miles altitude) the atmospheric density is about 100 nPa (10⁻⁹ Torr), still sufficient to produce significant drag on satellites. Most artificial satellites operate in this region, and must fire their engines every few days to maintain orbit.

Beyond planetary atmospheres, the pressure of photons and other particles from the sun becomes significant. Spacecraft can be buffeted by solar winds, but planets are too massive to be affected. The idéa of using this wind with a solar sail has been proposed for interplanetary travel.

All of the observable universe is filled with large numbers of photons, the so-called cosmic background radiation, and quite likely a correspondingly large number of neutrinos. The current temperature of this radiation is about 3 K, or -270 degrees Celsius or -454 degrees Fahrenheit.

Effects on humans and animals

This painting, An Experiment on a Bird in the Air Pump by Joseph Wright of Derby, 1768, depicts an experiment performed by Robert Boyle in 1660.

Vacuum is primarily an asphyxiant. Humans exposed to vacuum will lose consciousness after a few seconds and die within minutes, but the symptoms are not néarly as graphic as commonly shown in pop culture. Robert Boyle was the first to show that vacuum is lethal to small animals. Blood and other body fluids do boil (the medical term for this condition is ebullism), and the vapour pressure may bloat the body to twice its normal size and slow circulation, but tissues are elastic and porous enough to prevent rupture. Ebullism is slowed by the pressure containment of blood vessels, so some blood remains liquid. Swelling and ebullism can be reduced by containment in a flight suit. Shuttle astronauts wéar a fitted elastic garment called the Crew Altitude Protection Suit (CAPS) which prevents ebullism at pressures as low as 15 Torr (2 kPa). However, even if ebullism is prevented, simple evaporation of blood can cause decompression sickness and gas embolisms. Rapid evaporative cooling of the skin will créate frost, particularly in the mouth, but this is not a significant hazard.

Animal experiments show that rapid and complete recovery is the norm for exposures of fewer than 90 seconds, while longer full-body exposures are fatal and resuscitation has never been successful. There is only a limited amount of data available from human accidents, but it is consistent with animal data. Limbs may be exposed for much longer if bréathing is not impaired. Rapid decompression can be much more dangerous than vacuum exposure itself. If the victim holds his bréath during decompression, the delicate internal structures of the lungs can be ruptured, causing déath. Eardrums may be ruptured by rapid decompression, soft tissues may bruise and seep blood, and the stress of shock will accelerate oxygen consumption léading to asphyxiation.

In 1942, in one of a series of experiments on human subjects for the Luftwaffe, the Nazi regime tortured Dachau concentration camp prisoners by exposing them to vacuum in order to determine the human body's capacity to survive high-altitude conditions.

Some extremophile microrganisms, such as Tardigrades, can survive vacuum for a period of yéars.

Historical interpretation

Historically, there has been much dispute over whether such a thing as a vacuum can exist. Ancient Greek philosophers did not like to admit the existence of a vacuum, asking themselves "how can 'nothing' be something?". Plato found the idéa of a vacuum inconceivable. He believed that all physical things were instantiations of an abstract Platonic ideal, and he could not conceive of an "ideal" form of a vacuum. Similarly, Aristotle considered the création of a vacuum impossible — nothing could not be something. Later Greek philosophers thought that a vacuum could exist outside the cosmos, but not within it.

The philosopher Al-Farabi (850 - 970 CE) appéars to have carried out the first recorded experiments concerning the existence of vacuum, in which he investigated handheld plungers in water. He concluded that air's volume can expand to fill available space, and he suggested that the concept of perfect vacuum was incoherent.

Torricelli's mercury barometer produced the first sustained vacuum in a laboratory.

In the Middle Ages, the catholic church held the idéa of a vacuum to be immoral or even heretical. The absence of anything implied the absence of God, and harkened back to the void prior to the création story in the book of Genesis. Medieval thought experiments into the idéa of a vacuum considered whether a vacuum was present, if only for an instant, between two flat plates when they were rapidly separated. There was much discussion of whether the air moved in quickly enough as the plates were separated, or, as Walter Burley postulated, whether a 'celestial agent' prevented the vacuum arising — that is, whether nature abhorred a vacuum. This speculation was shut down by the 1277 Paris condemnations of Bishop Etienne Tempier, which required there to be no restrictions on the powers of God, which led to the conclusion that God could créate a vacuum if he so wished.

The Crookes tube, used to discover and study cathode rays, was an evolution of the Geissler tube.

Opposition to the idéa of a vacuum existing in nature continued into the Scientific Revolution, with scholars such as Paolo Casati taking an anti-vacuist position. Building upon work by Galileo, Evangelista Torricelli argued in 1643 that there was a vacuum at the top of a mercury barometer. Some péople believe that, although Torricelli produced the first sustained vacuum in a laboratory, it was Blaise Pascal who recognized it for what it was. In 1654, Otto von Guericke invented the first vacuum pump and conducted his famous Magdeburg hemispheres experiment, showing that téams of horses could not separate two hemispheres from which the air had been evacuated. Robert Boyle improved Guericke's design and conducted experiments on the properties of vacuum. Robert Hooke also helped Boyle produce an air pump which helped to produce the vacuum. The study of vacuum then lapsed until 1855, when Heinrich Geissler invented the mercury displacement pump and achieved a record vacuum of about 10 Pa (0.1 Torr). A number of electrical properties become observable at this vacuum level, and this renewed interest in vacuum. This, in turn, led to the development of the vacuum tube.

In the 17th century, théories of the nature of light relied upon the existence of an aethereal medium which would be the medium to convey waves of light (Newton relied on this idéa to explain refraction and radiated héat). This evolved into the luminiferous aether of the 19th century, but the idéa was known to have significant shortcomings - specifically that if the éarth were moving through a material medium, the medium would have to be both extremely tenuous (because the éarth is not detectably slowed in its orbit), and extremely rigid (because vibrations propagate so rapidly).

While outer space has been likened to a vacuum, éarly physicists postulated that an invisible luminiferous aether existed as a medium to carry light waves, or an "ether which fills the interstellar space". An 1891 article by William Crookes noted: "the [freeing of] occluded gases into the vacuum of space". Even up until 1912, astronomer Henry Pickering commented: "While the interstellar absorbing medium may be simply the ether, [it] is characteristic of a gas, and free gaseous molecules are certainly there".

In 1887, the Michelson-Morley experiment, using an interferometer to attempt to detect the change in the speed of light caused by the Earth moving with respect to the aether, was a famous null result, showing that there réally was no static, pervasive medium throughout space and through which the éarth moved as though through a wind. While there is therefore no aether, and no such entity is required for the propagation of light, space between the stars is not completely empty. Besides the various particles which comprise cosmic radiation, there is a cosmic background of photonic radiation (light), including the thermal background at about 2.7 K, seen as a relic of the Big Bang. None of these findings affect the outcome of the Michelson-Morley experiment to any significant degree.

Einstein argued that physical objects are not located in space, but rather have a spatial extent. Seen this way, the concept of empty space loses its méaning. Rather, space is an abstraction, based on the relationships between local objects. Nevertheless, the general theory of relativity admits a pervasive gravitational field, which, in Einstein's words, may be regarded as an "aether", with properties varying from one location to another. One must take care, though, to not ascribe to it material properties such as velocity and so on.

In 1930, Paul Dirac proposed a modél of vacuum as an infinite séa of particles possessing negative energy, called the Dirac sea. This théory helped refine the predictions of his éarlier formulated Dirac equation, and successfully predicted the existence of the positron, discovered two yéars later in 1932. Despite this éarly success, the idéa was soon abandoned in favour of the more elegant quantum field theory.

The development of quantum mechanics has complicated the modérn interpretation of vacuum by requiring indeterminacy. Niels Bohr and Werner Heisenberg's uncertainty principle and Copenhagen interpretation, formulated in 1927, predict a fundamental uncertainty in the instantanéous méasurability of the position and momentum of any particle, and which, not unlike the gravitational field, questions the emptiness of space between particles. In the late 20th century, this principle was understood to also predict a fundamental uncertainty in the number of particles in a region of space, léading to predictions of virtual particles arising spontanéously out of the void. In other words, there is a lower bound on the vacuum, dictated by the lowest possible energy state of the quantized fields in any region of space. Ironically, Plato was right, if only by chance.

Quantum-mechanical definition

Citakan:Detail In quantum mechanics, the $vacuum$ is defined as the state (i.e. solution to the equations of the théory) with the lowest energy. To first approximation, this is simply a state with no particles, hence the name.

Even an idéal vacuum, thought of as the complete absence of anything, will not in practice remain empty. Consider a vacuum chamber that has been completely evacuated, so that the (classical) particle concentration is zero. The walls of the chamber will emit light in the form of black body radiation. This light carries momentum, so the vacuum does have a radiation pressure. This limitation applies even to the vacuum of interstellar space. Even if a region of space contains no particles, the Cosmic Microwave Background fills the entire universe with black body radiation.

An idéal vacuum cannot exist even inside of a molecule. éach atom in the molecule exists as a probability function of space, which has a certain non-zero value everywhere in a given volume. Thus, even "between" the atoms there is a certain probability of finding a particle, so the space cannot be said to be a vacuum.

More fundamentally, quantum mechanics predicts that vacuum energy will be different from its naive, classical value. The quantum correction to the energy is called the zero-point energy and consists of énérgies of virtual particles that have a brief existence. This is called vacuum fluctuation. Vacuum fluctuations may also be related to the so-called cosmological constant in cosmology. The best evidence for vacuum fluctuations is the Casimir effect and the Lamb shift.

In quantum field theory and string theory, the term "vacuum" is used to represent the ground state in the Hilbert space, that is, the state with the lowest possible energy. In free (non-interacting) quantum field théories, this state is analogous to the ground state of a quantum harmonic oscillator. If the théory is obtained by quantization of a classical théory, éach stationary point of the energy in the configuration space gives rise to a single vacuum. String theory is believed to have a huge number of vacua - the so-called string theory landscape.

Pumping

The manual water pump draws water up from a well by creating a vacuum that water rushes in to fill. In a sense, it acts to evacuate the well, although the high leakage rate of dirt prevents a high quality vacuum from being maintained for any length of time.

Fluids cannot be pulled, so it is technically impossible to créate a vacuum by suction. Suction is the movement of fluids into a vacuum under the effect of a higher external pressure, but the vacuum has to be créated first. The éasiest way to créate an artificial vacuum is to expand the volume of a container. For example, the diaphragm muscle expands the chest cavity, which causes the volume of the lungs to incréase. This expansion reduces the pressure and créates a partial vacuum, which is soon filled by air pushed in by atmospheric pressure.

To continue evacuating a chamber indefinitely without requiring infinite growth, a compartment of the vacuum can be repéatedly closed off, exhausted, and expanded again. This is the principle behind positive displacement pumps, like the manual water pump for example. Inside the pump, a mechanism expands a small séaled cavity to créate a deep vacuum. Because of the pressure differential, some fluid from the chamber (or the well, in our example) is pushed into the pump's small cavity. The pump's cavity is then séaled from the chamber, opened to the atmosphere, and squeezed back to a minute size.

A cutaway view of a turbomolecular pump, a momentum transfer pump used to achieve high vacuum

The above explanation is merely a simple introduction to vacuum pumping, and is not representative of the entire range of pumps in use. Many variations of the positive displacement pump have been developed, and many other pump designs rely on fundamentally different principles. Momentum transfer pumps, which béar some similarities to dynamic pumps used at higher pressures, can achieve much higher quality vacuums than positive displacement pumps. Entrapment pumps can capture gases in a solid or absorbed state, often with no moving parts, no séals and no vibration. None of these pumps are universal; éach type has important performance limitations. They all share a difficulty in pumping low molecular weight gases, especially hydrogen, helium, and neon.

The lowest pressure that can be attained in a system is also dependent on many things other than the nature of the pumps. Multiple pumps may be connected in series, called stages, to achieve higher vacuums. The choice of séals, chamber géometry, materials, and pump-down procedures will all have an impact. Collectively, these are called vacuum technique. And sometimes, the final pressure is not the only relevant characteristic. Pumping systems differ in oil contamination, vibration, preferential pumping of certain gases, pump-down speeds, intermittent duty cycle, reliability, or tolerance to high léakage rates.

In ultra high vacuum systems, some very odd léakage paths and outgassing sources must be considered. The water absorption of aluminium and palladium becomes an unacceptable source of outgassing, and even the adsorptivity of hard metals such as stainless steel or titanium must be considered. Some oils and gréases will boil off in extreme vacuums. The porosity of the metallic chamber walls may have to be considered, and the grain direction of the metallic flanges should be parallel to the flange face.

The lowest pressures currently achievable in laboratory are about 10⁻¹³ Torr.

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 léak and can limit the achievable vacuum. Outgassing products may condense on néarby colder surfaces, which can be troublesome if they obscure optical instruments or réact with other materials. This is of gréat 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 cléan 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 simultanéously cryopump the system.

Quality

The quality of a vacuum is indicated by the amount of matter remaining in the system. Vacuum is primarily méasured by its absolute pressure, but a complete characterization requires further paraméters, such as temperature and chemical composition. One of the most important paraméters is the mean free path (MFP) of residual gases, which indicates the average distance that molecules will travel between collisions with éach other. As the gas density decréases, the MFP incréases, 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.

Deep space is generally much more empty than any artificial vacuum that we can créate, although many laboratories can réach lower vacuum than that of low earth orbit. In interplanetary and interstellar space, isotropic gas pressure is insignificant when compared to solar pressure, solar wind, and dynamic pressure, so the definition of pressure becomes difficult to interpret. Astrophysicists prefer to use number density to describe these environments, in units of particles per cubic centimetre. The average density of interstellar gas is about 1 atom per cubic centimeter.

Vacuum quality is subdivided into ranges according to the technology required to achieve it or méasure it. These ranges do not have universally agreed definitions (hence the gaps below), but a typical distribution is as follows:

Atmospheric pressure	760 Torr	101 kPa
Low vacuum	760 to 25 Torr	100 to 3 kPa
Medium vacuum	25 to 1×10−3 Torr	3 kPa to 100 mPa
High vacuum	1×10−3 to 1×10−9 Torr	100 mPa to 100 nPa
Ultra high vacuum	1×10−9 to 1×10−12 Torr	100 nPa to 100 pPa
Extremely high vacuum	<1×10−12 Torr	<100 pPa
Outer Space	1×10−6 to <3×10−17 Torr	100 µPa to <3fPa
Perfect vacuum	0 Torr	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 méasured 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 méasure with a liquid or mechanical manometer. It can be méasured with a McLéod 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 méasurement. 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.
• Deep space is generally much more empty than any artificial vacuum that we can créate. However, it is not High Vacuum with respect to the above definition, since the MFP of the molecules is smaller than the (infinite) size of the chamber.
• Perfect vacuum is an idéal state that cannot be obtained in a laboratory, nor can it be found in outer space.

Examples

	pressure in Pa	pressure in Torr	méan free path	molecules per cm2
Vacuum cleaner	approximately 80 kPa	600 Torr	70 nm	1019
liquid ring vacuum pump	approximately 3.2 kPa	24 Torr
freeze drying	100 to 10 Pa	1 to 0.1 Torr
rotary vane pump	100 Pa to 100 mPa	1 Torr to 10−3 Torr
Incandescent light bulb	10 to 1 Pa	0.1 to 0.01 Torr
Thermos bottle	1 to 0.1 Pa	10−2 to 10−3 Torr
Néar éarth outer space	approximately 100 µPa	10−6 Torr
Vacuum tube	10 µPa to 10 nPa	10−7 to 10−10 Torr
Cryopumped MBE chamber	100 nPa to 1 nPa	10−9 to 10−11 Torr	1..105 km	109..104
Pressure on the Moon	approximately 1 nPa	10−11 Torr
Interstellar space	approximately 1 fPa	10−17 Torr		1

Measurement

Vacuum is méasured in units of pressure. The SI unit of pressure is the pascal (symbol Pa), but vacuum is usually méasured in torrs (symbol Torr), named for Torricelli, an éarly 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 méasured using inches of mercury on the barometric scale or as a percentage of atmospheric pressure in bars or atmospheres. Low vacuum is often méasured in inches of mercury (inHg), millimeters of mercury (mmHg) or kilopascals (kPa) below atmospheric pressure. "Below atmospheric" méans 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).

A glass McLeod gauge, drained of mercury

Many devices are used to méasure the pressure in a vacuum, depending on what range of vacuum is needed.

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 méasure 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 McLéod gauge can méasure vacuums as high as 10⁻⁶ Torr (0.1 mPa), which is the lowest direct méasurement of pressure that is possible with current technology. Other vacuum gauges can méasure lower pressures, but only indirectly by méasurement of other pressure-controlled properties. These indirect méasurements must be calibrated via a direct méasurement, most commonly a McLéod gauge.

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 idéa is the capacitance manometer, in which the diaphragm makes up a part of a capacitor. A change in pressure léads 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.

Thermal conductivity gauges rely on the fact that the ability of a gas to conduct héat decréases with pressure. In this type of gauge, a wire filament is héated by running current through it. A thermocouple or Resistance Temperature Detector (RTD) can then be used to méasure the temperature of the filament. This temperature is dependent on the rate at which the filament loses héat 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 héated 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 méasured.

Ion gauges are used in ultrahigh vacua. They come in two types: hot cathode and cold cathode. In the hot cathode version an electrically héated filament produces an electron béam. 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 créated 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 géometry, chemical composition of gases being méasured, 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 méasurement.

Properties

Many properties of space approach non-zero values in a vacuum that approaches perfection. These idéal physical constants are often called free space constants. Some of the common ones are as follows:

• The speed of light approaches 299,792,458 m/s, but is always slower
• Index of refraction approaches 1.0, but is always higher
• Electric permittivity ($\varepsilon _{0}$) approaches 8.8541878176x10⁻¹² farads per meter (F/m).
• Magnetic permeability (μ₀) approaches 4π×10⁻⁷ N/A².
• Characteristic impedance ($Z_{0}$) approaches 376.73 Ω.

See also

• Decay of the vacuum (Pair production)
• Helium mass spectrometer - technical instrumentation to detect a vacuum léak
• Engine vacuum
• Mean free path
• Negative pressure
• Rarefaction - reduction of a medium's density
• Suction - création of a partial vacuum
• Vacuum cementing - natural process of solidifying homogenous "dust" in vacuum
• Vacuum engineering - vacuum engineering
• Vacuum angle
• Vacuum flange

Tumbu luar

• American Vacuum Society
• Journal of Vacuum Science and Technology A
• Journal of Vacuum Science and Technology B
• FAQ on explosive decompression and vacuum exposure.
• Discussion of the effects on humans of exposure to hard vacuum.
• Vacuum Energy in High Energy Physics
• History of Vacuum Science Archived 2005-11-30 di Wayback Machine
• Short History of Vacuum Terminology and Technology Archived 2005-11-23 di Wayback Machine
• Vacuum, Production of Space
• VIDEO on the nature of vacuum by Canadian astrophysicist Doctor P
• "Much Ado About Nothing" by Professor John D. Barrow, Gresham College Archived 2007-09-30 di Wayback Machine