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Why is a Vacuum Needed? To move a particle in a (straight) line over a large distance.

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Presentazione sul tema: "Why is a Vacuum Needed? To move a particle in a (straight) line over a large distance."— Transcript della presentazione:

1 Why is a Vacuum Needed? To move a particle in a (straight) line over a large distance

2 Why is a Vacuum Needed? Contamination (usually water) Clean surface Atmosphere (High)Vacuum To provide a clean surface

3 PressioneNumero molecole/cm 3 un pacco di caffè imballato sotto vuoto10 4 Pa2.7 x un tubo catodico in un televisore10 -4 Pa2.7 x un acceleratore di particelle in fisica nucleare10 -8 Pa2.7 x 10 6 una camera con il miglior vuoto che attualmente si può produrre in laboratorio Pa2.7 x 10 2 nostra galassia Pa1-10 spazio intergalattico ?1 al m 3 Tabella 1a: La pressione in alcune tipiche applicazioni AltitudinePressione Al livello del mare Pa Sulla vetta del Monte Bianco50000 Pa Alla quota di crociera di un Jumbo-Jet (20000 m)5000 Pa Su un satellite artificiale alla quota di km2 x Pa Sulla superficie della luna5 x Pa Tabella 1b: Cambiamento della pressione in funzione dellaltitudine

4 HOW DO WE CREATE A VACUUM?

5 VACUUM PUMPING METHODS

6 BAROMETER WATER MERCURY 760 mm Mercury: times heavier than water: Column is x shorter : mm/13.58=760 mm (= 760 Torr) mm 29,9 in (Page 12 manual)

7 PRESSURE OF 1 STANDARD ATMOSPHERE: 760 TORR, 1013 mbar AT SEA LEVEL, 0 O C AND 45 O LATITUDE

8 Pressure Equivalents Atmospheric Pressure (Standard) = , , gauge pressure (psig) pounds per square inch (psia) inches of mercury millimeter of mercury torr millitorr or microns pascal bar millibar

9 THE ATMOSPHERE IS A MIXTURE OF GASES PARTIAL PRESSURES OF GASES CORRESPOND TO THEIR RELATIVE VOLUMES GASSYMBOL PERCENT BY VOLUME PARTIAL PRESSURE TORR PASCAL Nitrogen Oxygen Argon Carbon Dioxide Neon Helium Krypton Hydrogen Xenon Water N 2 O 2 A CO 2 Ne He Kr H 2 X H 2 O Variable x x x x x to 50 79,000 21, x x x x to 6650 (Page 13 manual)

10 VAPOR PRESSURE OF WATER AT VARIOUS TEMPERATURES T ( O C) P (mbar) x (BOILING) (FREEZING) (DRY ICE) (LIQUID NITROGEN) (Page 14 manual)

11 (Page 15 manual)

12 Vapor Pressure of some Solids (Page 15 manual)

13 PRESSURE RANGES RANGE ROUGH (LOW) VACUUM HIGH VACUUM ULTRA HIGH VACUUM PRESSURE 759 TO 1 x (mbar) 1 x TO 1 x (mbar) LESS THAN 1 x (mbar) (Page 17 manual)

14 Viscous and Molecular Flow

15 FLOW REGIMES Viscous Flow: Distance between molecules is small; collisions between molecules dominate; flow through momentum transfer; generally P greater than 0.1 mbar Transition Flow: Region between viscous and molecular flow Molecular Flow: Distance between molecules is large; collisions between molecules and wall dominate; flow through random motion; generally P smaller than 10 mbar -3 (Page 25 manual)

16 MEAN FREE PATH MOLECULAR DENSITY AND MEAN FREE PATH 1013 mbar (atm)1 x mbar1 x mbar # mol/cm 3 MFP 3 x (30 million trillion) 4 x (40 trillion) 4 x 10 7 (40 million) 2.5 x in 6.4 x mm 2 inches 5.1 cm 31 miles 50 km Il libero cammino medio è inversamente proporzionale alla pressione ed alla sezione durto della molecola di gas

17 A A P1P1 P2P2 Flusso P 1 > P 2 Portata: Q è costante lungo il tubo e pertanto Conduttanza:

18 Conduttanza in parallelo: C1C1 C2C2 P1P1 P2P2 Q1Q1 Q2Q2 Conduttanza in serie: C1C1 C2C2 P1P1 P2P2 Q P3P3 Flusso totale = somma dei flussi Flusso costante:

19 VELOCITA DI POMPAGGIO DI UNA POMPA C P Camera P pompa Q Camera Pompa VELOCITA EFFETTIVA DI POMPAGGIO DI UN SISTEMA: Leffetto della conduttanza è quello di ridurre la velocità di pompaggio efficace Rispetto alla velocità di pompaggio allimbocco della pompa

20 FLOW REGIMES Mean Free Path Characteristic Dimension Viscous Flow: is less than 0.01 Mean Free Path Characteristic Dimension Molecular Flow: is greater than 1 Mean Free Path Characteristic Dimension Transition Flow: is between 0.01 and 1

21 Conductance in Viscous Flow Under viscous flow conditions doubling the pipe diameter increases the conductance sixteen times. The conductance is INVERSELY related to the pipe length (Page 28 manual) d = diameter of tube in cm l = length of tube in cm P 1 = inlet pressure in torr P 2 = exit pressure in torr EXAMPLE: d = 4 cmP 1 = 2 torr l = 100 cmP 2 = 1 torr C=530 l/s

22 Conductance in Molecular Flow Under molecular flow conditions doubling the pipe diameter increases the conductance eight times. The conductance is INVERSELY related to the pipe length. d = diameter of tube in cm l = length of tube in cm T = temperature (K) M = A.M.U. EXAMPLE: T = 295 K (22 O C) d = 4 cm M = 28 (nitrogen) l = 100 cm C=7.9 l/s

23

24 GAS LOAD Outgassing Leaks Virtual Real Backstreaming Diffusion Permeation GAS LOAD (Q) IS EXPRESSED IN: mbar liters per second

25 Pumpdown Curve Pressure (mbar) Time (sec) Volume Surface Desorption Diffusion Permeation

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