Vacuum is space devoid of matter. The word
stems from the Latin adjective vacuus for
"vacant" or "void". An approximation to such
vacuum is a region with a gaseous pressure
much less than atmospheric pressure. 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. In engineering and applied physics
on the other hand, vacuum refers to any space
in which the pressure is lower than atmospheric
pressure. The Latin term in vacuo is used
to describe an object that is surrounded by
a vacuum.
The quality of a partial vacuum refers to
how closely it approaches a perfect vacuum.
Other things equal, lower gas pressure means
higher-quality vacuum. For example, a typical
vacuum cleaner produces enough suction to
reduce air pressure by around 20%. Much higher-quality
vacuums are possible. Ultra-high vacuum chambers,
common in chemistry, physics, and engineering,
operate below one trillionth (10−12) of
atmospheric pressure (100 nPa), and can reach
around 100 particles/cm3. Outer space is an
even higher-quality vacuum, with the equivalent
of just a few hydrogen atoms per cubic meter
on average in intergalactic space. According
to modern understanding, even if all matter
could be removed from a volume, it would still
not be "empty" due to vacuum fluctuations,
dark energy, transiting gamma rays, cosmic
rays, neutrinos, and other phenomena in quantum
physics. In the study of electromagnetism
in the 19th century, vacuum was thought to
be filled with a medium called aether. In
modern particle physics, the vacuum state
is considered the ground state of a field.
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. A torricellian
vacuum is created by filling a tall glass
container closed at one end with mercury,
and then inverting it in a bowl to contain
the mercury (see below).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.
== Etymology ==
The word vacuum comes from Latin, meaning
'an empty space, void', noun use of neuter
of vacuus, meaning "empty", related to vacare,
meaning "be empty".
Vacuum is one of the few words in the English
language that contains two consecutive letters
'u'.
== Historical interpretation ==
Historically, there has been much dispute
over whether such a thing as a vacuum can
exist. Ancient Greek philosophers debated
the existence of a vacuum, or void, in the
context of atomism, which posited void and
atom as the fundamental explanatory elements
of physics. Following Plato, even the abstract
concept of a featureless void faced considerable
skepticism: it could not be apprehended by
the senses, it could not, itself, provide
additional explanatory power beyond the physical
volume with which it was commensurate and,
by definition, it was quite literally nothing
at all, which cannot rightly be said to exist.
Aristotle believed that no void could occur
naturally, because the denser surrounding
material continuum would immediately fill
any incipient rarity that might give rise
to a void.
In his Physics, book IV, Aristotle offered
numerous arguments against the void: for example,
that motion through a medium which offered
no impediment could continue ad infinitum,
there being no reason that something would
come to rest anywhere in particular. Although
Lucretius argued for the existence of vacuum
in the first century BC and Hero of Alexandria
tried unsuccessfully to create an artificial
vacuum in the first century AD, it was European
scholars such as Roger Bacon, Blasius of Parma
and Walter Burley in the 13th and 14th century
who focused considerable attention on these
issues. Eventually following Stoic physics
in this instance, scholars from the 14th century
onward increasingly departed from the Aristotelian
perspective in favor of a supernatural void
beyond the confines of the cosmos itself,
a conclusion widely acknowledged by the 17th
century, which helped to segregate natural
and theological concerns.Almost two thousand
years after Plato, René Descartes also proposed
a geometrically based alternative theory of
atomism, without the problematic nothing–everything
dichotomy of void and atom. 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. By the
ancient definition however, directional information
and magnitude were conceptually distinct.
In the medieval Middle Eastern world, the
physicist and Islamic scholar, 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, according
to Nader El-Bizri, the 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 mathematically demonstrated
that place (al-makan) is the imagined three-dimensional
void between the inner surfaces of a containing
body. According to Ahmad Dallal, Abū Rayhān
al-Bīrūnī also states that "there is no
observable evidence that rules out the possibility
of vacuum". The suction pump later appeared
in Europe from the 15th century.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. There was even speculation
that even God could not create a vacuum if
he wanted and 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.Jean
Buridan reported in the 14th century that
teams of ten horses could not pull open bellows
when the port was sealed.
The 17th century saw the first attempts to
quantify measurements of partial vacuum. Evangelista
Torricelli's mercury barometer of 1643 and
Blaise Pascal's experiments both demonstrated
a partial 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 partially evacuated.
Robert Boyle improved Guericke's design and
with the help of Robert Hooke further developed
vacuum pump technology. Thereafter, research
into the partial vacuum lapsed until 1850
when August Toepler invented the Toepler Pump
and Heinrich Geissler invented the mercury
displacement pump in 1855, achieving a partial
vacuum of about 10 Pa (0.1 Torr). A number
of electrical properties become observable
at this vacuum level, which renewed interest
in further research.
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. Borrowing somewhat
from the pneuma of Stoic physics, aether came
to be regarded as the rarefied air from which
it took its name, (see Aether (mythology)).
Early theories of light posited a ubiquitous
terrestrial and celestial medium through which
light propagated. Additionally, the concept
informed Isaac Newton's explanations of both
refraction and of radiant heat. 19th century
experiments into this luminiferous aether
attempted to detect a minute drag on the Earth's
orbit. 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.
In 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".Later, in 1930, Paul Dirac proposed
a model of the 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, confirmed two years later.
Werner Heisenberg's uncertainty principle
formulated in 1927, predict a fundamental
limit within which instantaneous position
and momentum, or energy and time can be measured.
This has far reaching consequences on the
"emptiness" of space between particles. In
the late 20th century, so-called virtual particles
that arise spontaneously from empty space
were confirmed.
== 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. 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.
=== Gravity ===
In general relativity, a vanishing stress–energy
tensor implies, through Einstein field equations,
the vanishing of all the components of the
Ricci tensor. 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). The black hole (with zero electric
charge) is an elegant example of a region
completely "filled" with vacuum, but still
showing a strong curvature.
=== Electromagnetism ===
In classical electromagnetism, the vacuum
of free space, or sometimes just free space
or perfect vacuum, is a standard reference
medium for electromagnetic effects. Some authors
refer to this reference medium as classical
vacuum, 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.In the theory of
classical electromagnetism, free space has
the following properties:
Electromagnetic radiation travels, when unobstructed,
at the speed of light, the defined value 299,792,458
m/s in SI units.
The superposition principle is always exactly
true. For example, the electric potential
generated by two charges is the simple addition
of the potentials generated by each charge
in isolation. The value of the electric field
at any point around these two charges is found
by calculating the vector sum of the two electric
fields from each of the charges acting alone.
The permittivity and permeability are exactly
the electric constant ε0 and magnetic constant
μ0, respectively (in SI units), or exactly
1 (in Gaussian units).
The characteristic impedance (η) equals the
impedance of free space Z0 ≈ 376.73 Ω.The
vacuum of classical electromagnetism can be
viewed as an idealized electromagnetic medium
with the constitutive relations in SI units:
D
(
r
,
t
)
=
ε
0
E
(
r
,
t
)
{\displaystyle {\boldsymbol {D}}({\boldsymbol
{r}},\ t)=\varepsilon _{0}{\boldsymbol {E}}({\boldsymbol
{r}},\ t)\,}
H
(
r
,
t
)
=
1
μ
0
B
(
r
,
t
)
{\displaystyle {\boldsymbol {H}}({\boldsymbol
{r}},\ t)={\frac {1}{\mu _{0}}}{\boldsymbol
{B}}({\boldsymbol {r}},\ t)\,}
relating the electric displacement field D
to the electric field E and the magnetic field
or H-field H to the magnetic induction or
B-field B. Here r is a spatial location and
t is time.
== Quantum mechanics ==
In quantum mechanics and quantum field theory,
the vacuum is defined as the state (that is,
the solution to the equations of the theory)
with the lowest possible energy (the ground
state of the Hilbert space). In quantum electrodynamics
this vacuum is referred to as 'QED vacuum'
to distinguish it from the vacuum of quantum
chromodynamics, denoted as QCD vacuum. QED
vacuum is a state with no matter particles
(hence the name), and also no photons. As
described above, this state is impossible
to achieve experimentally. (Even if every
matter particle could somehow be removed from
a volume, it would be impossible to eliminate
all the blackbody photons.) Nonetheless, it
provides a good model for realizable vacuum,
and agrees with a number of experimental observations
as described next.
QED vacuum has interesting and complex properties.
In QED vacuum, the electric and magnetic fields
have zero average values, but their variances
are not zero. As a result, QED vacuum contains
vacuum fluctuations (virtual particles that
hop into and out of existence), and a finite
energy called vacuum energy. Vacuum fluctuations
are an essential and ubiquitous part of quantum
field theory. Some experimentally verified
effects of vacuum fluctuations include spontaneous
emission and the Lamb shift. Coulomb's law
and the electric potential in vacuum near
an electric charge are modified.Theoretically,
in QCD multiple vacuum states can coexist.
The starting and ending of cosmological inflation
is thought to have arisen from transitions
between different vacuum states. 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. String theory is believed to have
a huge number of vacua – the so-called string
theory landscape.
== Outer space ==
Outer space has very low density and pressure,
and is the closest physical approximation
of a perfect vacuum. But no vacuum is truly
perfect, not even in interstellar space, where
there are still a few hydrogen atoms per cubic
meter.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
3.2×10−2 Pa at 100 kilometres (62 mi) 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 winds, 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 couple of
weeks or a few times a year (depending on
solar activity). 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 significantly 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.
== 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. 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 shown in the following table. As we travel
into orbit, outer space and ultimately intergalactic
space, the pressure varies by several orders
of magnitude.
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 the pressure 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. 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
a bombardment of particles with respect to
the Earth and Moon.
Perfect vacuum is an ideal state of no particles
at all. It cannot be achieved in a laboratory,
although there may be small volumes which,
for a brief moment, happen to have no particles
of matter in them. 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.
Hard vacuum and soft vacuum are terms that
are defined with a dividing line defined differently
by different sources, such as 1 Torr, or 0.1
Torr, the common denominator being that a
hard vacuum is a higher vacuum than a soft
one.
=== Relative versus 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 metres; a 9.8 metre
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.
=== Measurements relative to 1 atm ===
The SI unit of pressure is the pascal (symbol
Pa), but vacuum is often 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 on the barometric scale or as a percentage
of atmospheric pressure in bars or atmospheres.
Low vacuum is often measured in millimeters
of mercury (mmHg) or pascals (Pa) below standard
atmospheric pressure. "Below atmospheric"
means that the absolute pressure is equal
to the current atmospheric pressure.
In other words, most low vacuum gauges that
read, for example 50.79 Torr. 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.
=== Measuring instruments ===
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.The kenotometer is a particular type
of hydrostatic gauge, typically used in power
plants using steam turbines. The kenotometer
measures the vacuum in the steam space of
the condenser, that is, the exhaust of the
last stage of the turbine.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 103 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 platinum
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.
Ionization 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.
== 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. The
chemical inertness produced by a vacuum 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 lowers the
boiling point of liquids and promotes low
temperature 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. Vacuum interrupters are used in
electrical switchgear. Vacuum arc processes
are industrially important for production
of certain grades of steel or high purity
materials. The elimination of air friction
is useful for flywheel energy storage and
ultracentrifuges.
=== Vacuum-driven machines ===
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. Vacuum brakes were once widely used
on trains in the UK but, except on heritage
railways, they have been replaced by air brakes.
Manifold vacuum can be used to drive accessories
on automobiles. The best-known application
is the vacuum servo, used to provide power
assistance for the brakes. Obsolete applications
include vacuum-driven windscreen wipers and
Autovac fuel pumps. 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.
Vacuum induction melting uses electromagnetic
induction within a vacuum.
Maintaining a vacuum in the Condenser is an
important aspect of the efficient operation
of steam turbines. A steam jet ejector or
liquid ring vacuum pump is used for this purpose.
The typical vacuum maintained in the Condenser
steam space at the exhaust of the turbine
(also called Condenser Backpressure) is in
the range 5 to 15 kPa (absolute), depending
on the type of condenser and the ambient conditions.
=== 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.
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 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.
=== Pumping and ambient air pressure ===
Fluids cannot generally be pulled, so a vacuum
cannot be created 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 (13 pPa).
However, pressures as low as 5×10−17 Torr
(6.7 fPa) have been indirectly measured in
a 4 K cryogenic vacuum system. This corresponds
to ≈100 particles/cm3.
== 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 depicted
in media and popular culture. 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. Although
the blood will not boil, the formation of
gas bubbles in bodily fluids at reduced pressures,
known as ebullism, is still a concern. The
gas may bloat the body to twice its normal
size and slow circulation, but tissues are
elastic and porous enough to prevent rupture.
Swelling and ebullism can be restrained by
containment in a flight suit. Shuttle astronauts
wore a fitted elastic garment called the Crew
Altitude Protection Suit (CAPS) which prevents
ebullism at pressures as low as 2 kPa (15
Torr). Rapid boiling will cool the skin and
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. A study by NASA on eight chimpanzees
found all of them survived two and a half
minute exposures to vacuum. 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.
An experiment indicates that plants are able
to survive in a low pressure environment (1.5
kPa) for about 30 minutes.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 in
humans that did not undergo prior acclimatization,
and spacesuits are necessary to prevent ebullism
above 19 km. Most spacesuits use only 20 kPa
(150 Torr) of pure oxygen. This pressure is
high enough to prevent ebullism, but decompression
sickness and gas embolisms can still occur
if decompression rates are not managed.
Rapid decompression can be much more dangerous
than vacuum exposure itself. Even if the victim
does not hold his or her 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 of 13 kPa
(100 Torr), which produces no symptoms if
it is gradual, may be fatal if it occurs suddenly.Some
extremophile microorganisms, such as tardigrades,
can survive vacuum conditions for periods
of days or weeks.
== Examples ==
== See also
