Editing Vacuum (section)

=== Pumping and ambient air pressure ===
[[File:Hand pump.png|thumb|left|upright|Deep wells have the pump chamber down in the well close to the water surface, or in the water. A "sucker rod" extends from the handle down the center of the pipe deep into the well to operate the plunger. The pump handle acts as a heavy counterweight against both the sucker rod weight and the weight of the water column standing on the upper plunger up to ground level.]]

{{Main|Vacuum pump}}

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 (anatomy)|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 [[vacuum pump#Positive displacement pump|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.

[[File:Cut through turbomolecular pump.jpg|thumb|upright|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. [[vacuum pump#Momentum transfer pump|Momentum transfer pumps]], which bear some similarities to dynamic pumps used at higher pressures, can achieve much higher quality vacuums than positive displacement pumps. [[vacuum pump#Entrapment pump|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 {{convert|1e-13|torr|pPa}}.<ref>{{cite journal| author=Ishimaru, H | title= Ultimate Pressure of the Order of 10<sup>−13</sup> torr in an Aluminum Alloy Vacuum Chamber | journal= Journal of Vacuum Science and Technology | date=1989 | volume=7 | issue=3–II | pages= 2439–2442 | doi= 10.1116/1.575916 | bibcode= 1989JVSTA...7.2439I }}</ref>  However, pressures as low as {{convert|5e-17|torr|fPa}} have been indirectly measured in a {{convert|4|K|C F}} cryogenic vacuum system.<ref name=Gabrielse>{{cite journal | doi = 10.1103/PhysRevLett.65.1317| pmid = 10042233| title = Thousandfold improvement in the measured antiproton mass| journal = Physical Review Letters| volume = 65| issue = 11| pages = 1317–1320| year = 1990| last1 = Gabrielse | first1 = G.| last2 = Fei | first2 = X.| last3 = Orozco | first3 = L.| last4 = Tjoelker | first4 = R.| last5 = Haas | first5 = J.| last6 = Kalinowsky | first6 = H.| last7 = Trainor | first7 = T.| last8 = Kells | first8 = W.|bibcode = 1990PhRvL..65.1317G | url = https://cds.cern.ch/record/493773/files/cm-p00043126.pdf}}</ref> This corresponds to ≈100 particles/cm<sup>3</sup>.