vacuum

 

  • Vacuum is primarily measured by its absolute pressure, but a complete characterization requires further parameters, such as temperature and chemical composition.

  • Vacuum fluctuations are an essential and ubiquitous part of quantum field theory.

  • 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.

  • An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure.

  • [33] As a result, QED vacuum contains vacuum fluctuations (virtual particles that hop into and out of existence), and a finite energy called vacuum energy.

  • [22][23] Some authors refer to this reference medium as classical vacuum,[22] a terminology intended to separate this concept from QED vacuum or QCD vacuum, where vacuum fluctuations
    can produce transient virtual particle densities and a relative permittivity and relative permeability that are not identically unity.

  • For example, a typical vacuum cleaner produces enough suction to reduce air pressure by around 20%.

  • [40] The kenotometer is a particular type of hydrostatic gauge, typically used in power plants using steam turbines.

  • Measuring instruments [edit] Many devices are used to measure the pressure in a vacuum, depending on what range of vacuum is needed.

  • Many inexpensive low vacuum gauges have a margin of error and may report a vacuum of 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) 1 torr.

  • [7] 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 technologies
    has since become available.

  • Ultra-high vacuum chambers, common in chemistry, physics, and engineering, operate below one trillionth of atmospheric pressure (100 nPa), and can reach around.

  • On the surface of Venus, where ground-level atmospheric pressure is much higher than on Earth, much higher relative vacuum readings would be possible.

  • Medieval thought experiments into the idea of a vacuum considered whether a vacuum was present, if only for an instant, between two flat plates when they were rapidly separated.

  • 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.

  • While outer space provides the most rarefied example of a naturally occurring partial vacuum, the heavens were originally thought to be seamlessly filled by a rigid indestructible
    material called aether.

  • • Perfect vacuum is an ideal state of no particles at all.

  • Even if all particles of matter were removed, there would still be photons and gravitons, as well as dark energy, virtual particles, and other aspects of the quantum vacuum.

  • Vacuum does not mean that the curvature of space-time is necessarily flat: the gravitational field can still produce curvature in a vacuum in the form of tidal forces and
    gravitational waves (technically, these phenomena are the components of the Weyl tensor).

  • But no vacuum is truly perfect, not even in interstellar space, where there are still a few hydrogen atoms per cubic meter.

  • The principle behind cold cathode version is the same, except that electrons are produced in a discharge created by a high voltage electrical discharge.

  • The quality of a partial vacuum refers to how closely it approaches a perfect vacuum.

  • Electromagnetism [edit] In classical electromagnetism, the vacuum of free space, or sometimes just free space or perfect vacuum, is a standard reference medium for electromagnetic
    effects.

  • Relative versus absolute measurement [edit] Vacuum is measured in units of pressure, typically as a subtraction relative to ambient atmospheric pressure on Earth.

  • [37] The drag here is low enough that it could theoretically be overcome by radiation pressure on solar sails, a proposed propulsion system for interplanetary travel.

  • Beyond this line, isotropic gas pressure rapidly becomes insignificant when compared to radiation pressure from the Sun and the dynamic pressure of the solar winds, so the
    definition of pressure becomes difficult to interpret.

  • The current temperature of this radiation is about 3 K. Measurement 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.

  • [10] He concluded that air’s volume can expand to fill available space, and therefore the concept of a perfect vacuum was incoherent.

  • Vacuum is often also measured on the barometric scale or as a percentage of atmospheric pressure in bars or atmospheres.

  • Similarly, much higher than normal relative vacuum readings are possible deep in the Earth’s ocean.

  • 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.

  • 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.

  • A number of electrical properties become observable at this vacuum level, which renewed interest in further research.

  • Thermal conductivity gauges rely on the fact that the ability of a gas to conduct heat decreases with pressure.

  • [5] Stars, planets, and moons keep their atmospheres by gravitational attraction, and as such, atmospheres have no clearly delineated boundary: the density of atmospheric
    gas simply decreases with distance from the object.

  • [34] Theoretically, in QCD multiple vacuum states can coexist.

  • Vacuum quality is subdivided into ranges according to the technology required to achieve it or measure it.

  • For theories obtained by quantization of a classical theory, each stationary point of the energy in the configuration space gives rise to a single vacuum.

  • This vacuum state is called high vacuum, and the study of fluid flows in this regime is called particle gas dynamics.

  • [1] Physicists often discuss ideal test results that would occur in a perfect vacuum, which they sometimes simply call “vacuum” or free space, and use the term partial vacuum
    to refer to an actual imperfect vacuum as one might have in a laboratory or in space.

  • Other things equal, lower gas pressure means higher-quality vacuum.

  • 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.

  • In engineering and applied physics on the other hand, vacuum refers to any space in which the pressure is considerably lower than atmospheric pressure.

  • Although Descartes agreed with the contemporary position, that a vacuum does not occur in nature, the success of his namesake coordinate system and more implicitly, the spatial–corporeal
    component of his metaphysics would come to define the philosophically modern notion of empty space as a quantified extension of volume.

  • Classical field theories The strictest criterion to define a vacuum is a region of space and time where all the components of the stress–energy tensor are zero.

  • [41] 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.

  • This means that this region is devoid of energy and momentum, and by consequence, it must be empty of particles and other physical fields (such as electromagnetism) that contain
    energy and momentum.

  • The kenotometer measures the vacuum in the steam space of the condenser, that is, the exhaust of the last stage of the turbine.

  • [38] 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 McLeod gauge can measure vacuums as high as (0.1 mPa), which is the lowest direct measurement of pressure that is possible with current technology.

  • While the Earth does, in fact, move through a relatively dense medium in comparison to that of interstellar space, the drag is so minuscule that it could not be detected.

  • [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.

  • 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.

  • The gas may bloat the body to twice its normal size and slow circulation, but tissues are elastic and porous enough to prevent rupture.

  • This shallow water well pump reduces atmospheric air pressure inside the pump chamber.

  • Main article: Vacuum pump Fluids cannot generally be pulled, so a vacuum cannot be created by suction.

  • High vacuum systems must be clean and free of organic matter to minimize outgassing.

  • This expansion reduces the pressure and creates a partial vacuum, which is soon filled by air pushed in by atmospheric pressure.

  • [50][51] Cold or oxygen-rich atmospheres can sustain life at pressures much lower than atmospheric, as long as the density of oxygen is similar to that of standard sea-level
    atmosphere.

  • The reduction in pressure lowers the temperature at which blood and other body fluids boil, but the elastic pressure of blood vessels ensures that this boiling point remains
    above the internal body temperature of 37 °C.

  • Atmospheric pressure extends down into the well, and forces water up the pipe into the pump to balance the reduced pressure.

  • Ultra-high vacuum is used in the study of atomically clean substrates, as only a very good vacuum preserves atomic-scale clean surfaces for a reasonably long time (on the
    order of minutes to days).

  • Momentum transfer pumps, which bear some similarities to dynamic pumps used at higher pressures, can achieve much higher quality vacuums than positive displacement pumps.

  • Multiple pumps may be connected in series, called stages, to achieve higher vacuums.

  • Some aircraft instruments (Attitude Indicator (AI) and the Heading Indicator (HI)) are typically vacuum-powered, as protection against loss of all (electrically powered) instruments,
    since early aircraft often did not have electrical systems, and since there are two readily available sources of vacuum on a moving aircraft, the engine and an external venturi.

  • Deep vacuum lowers the boiling point of liquids and promotes low temperature outgassing which is used in freeze drying, adhesive preparation, distillation, metallurgy, and
    process purging.

  • This is the principle behind positive displacement pumps, like the manual water pump for example.

  • An experiment indicates that plants are able to survive in a low pressure environment (1.5 kPa) for about 30 minutes.

  • Entrapment pumps can capture gases in a solid or absorbed state, often with no moving parts, no seals and no vibration.

  • Some systems are cooled well below room temperature by liquid nitrogen to shut down residual outgassing and simultaneously cryopump the system.

  • [44] Although the blood will not boil, the formation of gas bubbles in bodily fluids at reduced pressures, known as ebullism, is still a concern.

  • Pumping and ambient air pressure [edit] Deep wells have the pump chamber down in the well close to the water surface, or in the water.

  • Suction can spread and dilute a vacuum by letting a higher pressure push fluids into it, but the vacuum has to be created first before suction can occur.

  • The most prevalent outgassing product in vacuum systems is water absorbed by chamber materials.

  • Ultra-high vacuum systems are usually baked, preferably under vacuum, to temporarily raise the vapour pressure of all outgassing materials and boil them off.

  • Above-ground pump chambers are only effective to a depth of approximately 9 meters due to the water column weight balancing the atmospheric pressure.

  • [46] Rapid boiling will cool the skin and create frost, particularly in the mouth, but this is not a significant hazard.

  • 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.

 

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