Definitions

Vacuum

[vak-yoom, -yoo-uhm, -yuhm]
This vacuum means "absence of matter" or "an empty area or space"; for the cleaning appliance, see vacuum cleaner. For the vacuum used in defining the MKS system of units, see free space.

A vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty," but in reality, no volume of space can ever be perfectly empty. A perfect vacuum with a gaseous pressure of absolute zero is a philosophical concept that is never observed in practice. Physicists often discuss ideal 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 real vacuum. The Latin term in vacuo is also used to describe an object as being in what would otherwise be a vacuum.

The quality of a vacuum refers to how closely it approaches a perfect vacuum. The residual gas pressure is the primary indicator of quality, and is most commonly measured in units called torr, even in metric contexts. Lower pressures indicate higher quality, although other variables must also be taken into account. Quantum theory sets limits for the best possible quality of vacuum, predicting that no volume of space can be perfectly empty. Outer space is a natural high quality vacuum, mostly of much higher quality than can be created artificially with current technology. Low quality artificial vacuums have been used for suction for many years.

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 laboratory vacuum in 1643, and other experimental techniques were developed as a result of his theories 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 health, and on life forms in general.

Uses

Vacuum is useful in a variety of processes and devices. Its first widespread use was in the incandescent light bulb to protect the filament from chemical degradation. Its chemical inertness is also useful for electron beam welding, cold welding, vacuum packing and vacuum frying. 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). High to ultra-high vacuum removes the obstruction of air, allowing particle beams to deposit or remove materials without contamination. This is the principle behind chemical vapor deposition, physical vapor deposition, and dry etching which are essential to the fabrication of semiconductors and optical coatings, and to surface science. The reduction of convection provides the thermal insulation of thermos bottles. Deep vacuum promotes outgassing which is used in freeze drying, adhesive preparation, distillation, metallurgy, and process purging. The electrical properties of vacuum make electron microscopes and vacuum tubes possible, including cathode ray tubes. The elimination of air friction is useful for flywheel energy storage and ultracentrifuges.

Vacuums are commonly used to produce suction, which has an even wider variety of applications. The Newcomen steam engine used vacuum instead 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 has very low density and pressure, and is the closest physical approximation of a perfect vacuum. It has effectively no friction, allowing stars, planets and moons to move freely along ideal gravitational trajectories. But no vacuum is truly perfect, not even in interstellar space where there are still a few hydrogen atoms per cubic centimeter. The deep vacuum of space could make it an attractive environment for certain industrial processes, for instance those that require ultraclean surfaces; however, it is much less costly to create an equivalent vacuum on Earth than to leave the Earth's gravity well.

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. The Earth's atmospheric pressure drops to about 1 Pa (10-3 Torr) at 100 km of altitude, the Kármán line which is a common definition of the boundary with outer space. Beyond this line, isotropic gas pressure rapidly becomes insignificant when compared to radiation pressure from the sun and the dynamic pressure of the solar wind, so the definition of pressure becomes difficult to interpret. The thermosphere in this range has large gradients of pressure, temperature and composition, and varies greatly due to space weather. Astrophysicists prefer to use number density to describe these environments, in units of particles per cubic centimetre.

But although it meets the definition of outer space, the atmospheric density within the first few hundred kilometers above the Kármán line is still sufficient to produce significant drag on satellites. Most artificial satellites operate in this region called low earth orbit and must fire their engines every few days to maintain orbit. 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. Planets are too massive for their trajectories to be affected by these forces, although their atmospheres are eroded by the solar winds.

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

Humans and animals exposed to vacuum will lose consciousness after a few seconds and die of hypoxia within minutes, but the symptoms are not nearly as graphic as commonly shown in pop culture. Blood and other body fluids do boil when their pressure drops below 6.3 kPa, (47 Torr,) the vapour pressure of water at body temperature. This condition is called ebullism. The steam 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 restrained by containment in a flight suit. Shuttle astronauts wear a fitted elastic garment called the Crew Altitude Protection Suit (CAPS) which prevents ebullism at pressures as low as 2 kPa (15 Torr). Rapid evaporative cooling of the skin will create frost, particularly in the mouth, but this is not a significant hazard.

Animal experiments show that rapid and complete recovery is normal for exposures shorter 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 breathing is not impaired. Robert Boyle was the first to show in 1660 that vacuum is lethal to small animals. 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.

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 colder air temperatures found at altitudes of up to 3 km generally compensate for the lower pressures there. Above this altitude, oxygen enrichment is necessary to prevent altitude sickness, and spacesuits are necessary to prevent ebullism above 19 km. Most spacesuits use only 20 kPa (150 Torr) of pure oxygen, just enough to sustain full consciousness. This pressure is high enough to prevent ebullism, but simple evaporation of blood can still cause decompression sickness and gas embolisms if not managed.

Rapid decompression can be much more dangerous than vacuum exposure itself. Even if the victim does not hold his breath, venting through the windpipe may be too slow to prevent the fatal rupture of the delicate alveoli of the lungs. Eardrums and sinuses may be ruptured by rapid decompression, soft tissues may bruise and seep blood, and the stress of shock will accelerate oxygen consumption leading to hypoxia. Injuries caused by rapid decompression are called barotrauma. A pressure drop as small as 100 Torr, (13 kPa,) which produces no symptoms if it is gradual, may be fatal if occurs suddenly.

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

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 idea 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 creation 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. Hero of Alexandria was the first to challenge this belief in the first century AD, but his attempts to create an artificial vacuum failed.

In the medieval Islamic world, the Muslim physicist and philosopher, Al-Farabi (Alpharabius, 872-950), conducted a small experiment 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. However, the Muslim physicist Ibn al-Haytham (Alhazen, 965-1039) and the Mu'tazili theologians disagreed with Aristotle and Al-Farabi, and they supported the existence of a void. Using geometry, Ibn al-Haytham mathematially demonstrated that place (al-makan) is the imagined three-dimensional void between the inner surfaces of a containing body. Abū Rayhān al-Bīrūnī also states that "there is no observable evidence that rules out the possibility of vacuum". The first suction pump was invented in 1206 by the Muslim engineer and inventor, Al-Jazari. The suction pump later appeared in Europe from the 15th century. Taqi al-Din's six-cylinder 'Monobloc' pump, invented in 1551, could also create a partial vacuum, which was formed "as the lead weight moves upwards, it pulls the piston with it, creating vacuum which sucks the water through a non return clack valve into the piston cylinder."

In medieval Europe, the Catholic Church held the idea 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 creation story in the book of Genesis. 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. 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. The commonly held view that nature abhorred a vacuum was called horror vacui. 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 create a vacuum if he so wished. René Descartes also argued against the existence of a vacuum, arguing along the following lines:“Space is identical with extension, but extension is connected with bodies; thus there is no space without bodies and hence no empty space (vacuum)”. In spite of this, opposition to the idea of a vacuum existing in nature continued into the Scientific Revolution, with scholars such as Paolo Casati taking an anti-vacuist position. Jean Buridan reported in the 14th century that teams of ten horses could not pull open bellows when the port was sealed, apparently because of horror vacui.

The belief in horror vacui was overthrown in the 17th century. Water pump designs had improved by then to the point that they produced measurable vacuums, but this was not immediately understood. What was known was that suction pumps could not pull water beyond a certain height: 18 Florentine yards according to a measurement taken around 1635. (The conversion to metres is uncertain, but it would be about 9 or 10 metres.) This limit was a concern to irrigation projects, mine drainage, and decorative water fountains planned by the Duke of Tuscany, so the Duke commissioned Galileo to investigate the problem. Galileo advertised the puzzle to other scientists, including Gasparo Berti who replicated it by building the first water barometer in Rome in 1639. Berti's barometer produced a vacuum above the water column, but he could not explain it. The breakthrough was made by Evangelista Torricelli in 1643. Building upon Galileo's notes, he built the first mercury barometer and wrote a convincing argument that the space at the top was a vacuum. The height of the column was then limited to the maximum weight that atmospheric pressure could support. Some people believe that although Torricelli's experiment was crucial, it was Blaise Pascal's experiments that proved the top space really contained vacuum.

In 1654, Otto von Guericke invented the first vacuum pump and conducted his famous Magdeburg hemispheres experiment, showing that teams 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.

While outer space has been likened to a vacuum, early theories of the nature of light relied upon the existence of an invisible, aetherial medium which would convey waves of light. (Isaac Newton relied on this idea to explain refraction and radiated heat). This evolved into the luminiferous aether of the 19th century, but the idea was known to have significant shortcomings - specifically, that if the Earth were moving through a material medium, the medium would have to be both extremely tenuous (because the Earth is not detectably slowed in its orbit), and extremely rigid (because vibrations propagate so rapidly). 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 really was no static, pervasive medium throughout space and through which the Earth 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 meaning. 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 model of vacuum as an infinite sea of particles possessing negative energy, called the Dirac sea. This theory helped refine the predictions of his earlier formulated Dirac equation, and successfully predicted the existence of the positron, discovered two years later in 1932. Despite this early success, the idea was soon abandoned in favour of the more elegant quantum field theory.

The development of quantum mechanics has complicated the modern 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 instantaneous measurability 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, leading to predictions of virtual particles arising spontaneously 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.

Quantum-mechanical definition

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

Even an ideal 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 ideal vacuum cannot exist even inside of a molecule. Each 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 energies 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 theories, this state is analogous to the ground state of a quantum harmonic oscillator. If the theory is obtained by quantization of a classical theory, each 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

Fluids cannot be pulled, so it is technically impossible to create a vacuum by suction. 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 easiest way to create 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 increase. This expansion reduces the pressure and creates 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 repeatedly 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 sealed cavity to create a 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 sealed from the chamber, opened to the atmosphere, and squeezed back to a minute size.

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 bear 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 seals and no vibration. None of these pumps are universal; each 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 seals, chamber geometry, 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 leakage rates.

In ultra high vacuum systems, some very odd leakage 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 greases will boil off in extreme vacuums. The permeability 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. However, pressures as low as 5×10-17 Torr have been indirectly measured in a 4 K cryogenic vacuum system.

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:

Atmospheric pressure 760 Torr 101.3 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 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).
• Deep space is generally much more empty than any artificial vacuum that we can create. 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 in outer space.

Examples

pressure in Pa pressure in 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 100μm 1016
rotary vane pump 100 Pa to 100 mPa 1 to 10−3 100μm to 10cm 1016-1013
Incandescent light bulb 10 to 1 Pa 0.1 to 0.01 1mm to 1cm 1014
Thermos bottle 1 to 0.01 Pa 10−2 to 10−4 1cm to 1m 1012
Earth thermosphere 1 Pa to 100 nPa 10−3 to 10−10 1cm to 1000 km 1014 to 106
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 1..105 km 109-104
Pressure on the Moon approximately 1 nPa 10−11 4 X 105
Interplanetary space     10
Interstellar space     1
Intergalactic space     10-6

Measurement

Vacuum is measured in units of pressure. The SI unit of pressure is the pascal (symbol Pa), but vacuum is usually measured in torrs (symbol Torr), 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).

Many devices are used to measure 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 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.

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.

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.

Properties

As a vacuum approaches perfection, several properties of space approach non-zero values. The ideal values which would be attained in an ideal vacuum are called free space constants. Some common ones are as follows: