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Vacuum System Overview: Pressure, Mass-Flow and Conductance.


  A vacuum pump literally pulls (and pushes) gas molecules out of the vacuum chamber and then does not let them go back in.   In this way, it establishes a pressure gradient that causes gas molecules further away to diffuse toward the pump inlet.   One could almost say that the pump acts like a one-way road for gas molecules allowing them to travel out of the chamber but not back into it.   The earliest vacuum pumps did this by: 1] first allowing gas within the chamber to fill a volume inside the pump, 2] closing off that volume and squeezing it to raise the pressure, 3] followed by expelling that squeezed “high pressure” gas out of the pump to some other location.   Pushing volumes of gas from inside the chamber to the outside by compressing them along the way is what “rotary vane”, “rotary piston” and other mechanical pump designs still do today.   Given this method of pushing molecules out of a chamber, it is quite natural to define a pumps’ ability to remove gas in terms of its’ pumping “speed.”   (The units are liters/second or l/s.)   We would say that a pump has the ability to remove X liters of volume from the chamber every second if it is an “X-l/s pump.”   The pressure of the gas inside the chamber then determines how many gas molecules actually reside inside those X liters!   The pumping speed is usually denoted as Sp.   Figure 1 shows the pumping speed curves for 3 different pumps: a rotary vane pump, a dry pump and a turbo-molecular pump.   Note that the pump curves for the Turbo-molecular pump are rapidly decreasing as the pressure goes above about 10-2 mbar (7.6 mTorr).   Turbo-molecular pumps (TMPs) are not nearly so efficient at pumping when the pressure is large because the gas load slows down the pump vanes.   On the other hand, a rotary vane pump (RVP) with its oil sealing of the rotating vanes, has a good pumping speed all the way up to atmoshperic pressure.   It has difficulty pumping under...

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Chemistry. Anti Essays. Retrieved December 14, 2018, from the World Wide Web: http://parimatch-stavka7.com/free-essays/Chemistry-320995.html


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