05_vacuum

05_vacuum

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Unformatted text preview: e is the restriction is the – example in book has 100 l/s pump connected to tube 2.5 cm in diameter, 10 cm long, resulting in flow of 16 l/s – pump capacity diminished by factor of 6! • such that Q = (P1 P2) C Winter 2008 9 Winter 2008 10 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Tube Conductance Pump-down time • For air at 293 K: • In bulk behavior (> 100 mTorr): mTorr): • Longer than you wish – Viscous air removed quickly, then long slow process to remove rest – to go from pressure P0 to P , takes t = (V /S) ln( P 0/P ) – note logarithmic performance C = 180 P D4/L (liters per second) – D, the diameter, and L, the length are in cm; P in Torr – note the strong dependence on diameter! – example: 1 m long tube 5 cm in diameter at 1 Torr: • allows 1125 liters per second • In molecular behavior (< 100 mTorr): mTorr): C = 12 D3/L – now cube of D – same example, at 1 mTorr: • allows 0.1 liters per second (much reduced!) Winter 2008 Lecture 5 11 Winter 2008 12 3 Vacuum Systems 01/24/2008 UCSD: Physics 121; 2008 UCSD: Physics 121; 2008 Mechanical Pumps Lobe Injection Pumps • Form of “positive displacement pump” pump” • For “roughing,” or getting the roughing,” the bulk of the air out, one uses mechanical pumps • Can move air very rapidly • Often no oil seal • Compression ratio not as good – usually rotary oil-sealed pumps – these give out at ~ 1–10 mTorr • A blade sweeps along the sweeps walls of a cylin...
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This note was uploaded on 01/30/2014 for the course PHYS 121 taught by Professor Staff during the Winter '08 term at UCSD.

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