Why Is There Vacuum?(The sequel to Bill Cosby’s “Why Is There Air?”)
Matthew C. DeLongUniversity of Utah
OptoElectronic Materials Laboratory7 January 2008
Ranges of Vacuum
• Low: 1 atm to 1 Torr• Medium: 10-3 ( 1 m) to 1
Torr• High: 10-8 to 10-3 Torr
• Ultra High: 10-12 to 10-8 Torr• Extreme: < 10-12 Torr• Note: low vacuum ↔ high
pressure
• Drying, drinking straws• Sputtering
• Thermal evaporation, e-gun, SEM
• STEM, FIM, AES, SIMS• Anti-particle
accumulators, space simulation
Pressure: Units of Measure
• Pressure exerted by a column of fluid:• P ≡ F/A = mg/A = ghA/A = gh h • 1 Atm (mean sea level) = 760 Torr = 1013
mBar = 1.01x105 Pa = 101.3 kPa = 14.7 psi = 34 ft. water
• Average atmospheric pressure in SLC is about 635 Torr, 12.3 psi, 28.4 ft water…
“Kinds of Pressure”
• Gauge Pressure: measured with respect to ambient.
• Absolute pressure: measured with respect to vacuum
• Car tires, basketballs, boilers, LN2 tanks, JFB compressed air supply…
• Vacuum systems, cathode ray tubes, light bulbs, barometers
Measurement Techniques
• Low
• Medium• High
• Ultra High• Extreme
• Mechanical (Bourdon), Hg column, capacitance
• Thermocouple, Pirani• Ionization [hot and cold (Penning)
cathode]• Ionization (hot cathode: Bayard-Alpert)• Modulator Bayard-Alpert
Capacitance Manometer
• A = Annular electrode• D = Disk electrode• S = Substrate• G = Getter (in vacuum
space)• Differential capacitance
between annulus and disk depends on pressure difference between Test Chamber and “Getter”.
Heat Transfer of Gases
• Conductivity is linear in pressure over about 2 orders of magnitude.
• Molecular flow regime
• Pirani and thermocouple gauges
Ionization gauges
• Hot cathode: more sensitive; less forgiving• Cold cathode: less sensitive; more forgiving
Mean Free Path in Gases
With sufficient accuracy for approximate calculations we may take:
λ = 7 x 10-3/p mbar-cmλ = 5 x 10-3/p Torr-cmλ = 5/p μmHg-cm
Roughing pump comparisons: Oil Sealed Pumps
Type Advantages DisadvantagesRotary vane Low ultimate
pressure.Low costLong pump life.
Backstreams oil. Produces hazardous waste.
Rootes Lobe Very high pumping speed
Frequent maintenance. Requires a purge gas. Requires a backing pump. Must be absolutely horizontal.
Rotary piston High volumeLow cost
Noise. Vibration Safety Valve.
Roughing pump comparisons: Dry Roughing Pumps
Scroll Clean.Low "dry" ultimate pressure.Easily serviceableQuiet.Technology is well known.
Limited bearing life. Limited scroll life. Permeable to small gases. Not hermetically sealed. Clean applications only.
Diaphragm Low cost.Quiet.Easily serviced.
Low pumping speed. High ultimate pressure. Frequent service required.
Hook and Claw
No backstreaming.Low ultimate pressure
Expensive
Screw rotor Low ultimate vacuum.Less maintenance than hook & claw
Expensive
Dry piston Low ultimate pressure Expensive
Sorption Clean Requires LN2.
Rotary Vane Mechanical Pump
• Robust• Inexpensive• Operates to
ambient pressure
• Single stage and two stage
Sorption Pump
• Clean: no oil• Very inexpensive:
170,000 Torr-liters for $1000 + 8.5 l LN2
• Requires LN2• Air adsorbs onto
zeolite at 77K• 10-3 Torr capability
Oil Vapor Diffusion Pump
Vacuum system
• Robust (silicone oil!)
• Low maintenance: no moving parts
• Requires backing• 10-3 – 10-7 Torr
Turbomolecular Pump
• Requires backing: Operates only <1 Torr
• Clean: no oil• Expensive:
Approximately triple the cost of a rotary vane mechanical pump and oil diffusion pump
• Limited lifespan
Getter pump• Low maintenance:
no moving parts• 10-4 – 10-10 Torr• Requires backing• Clean: no oil• Based on
chemical reaction of “air” with very reactive metals
References• A. Chalmers, B.K. Fitch, and B. S. Halliday, Basic
Vacuum Technology, IOP Publishing, Bristol (1998). TJ/940/C45/1998.
• D. Hucknall, Vacuum Technology and Applications, Butterworth-Heinemann, Oxford (1991). TJ/940/H83 (1991).
• Vacuum Equipment, Granville-Phillips Co., Boulder CO. TJ/940/G7.
• R. R. LaPelle, Practical Vacuum Systems, McGraw-Hill, New York (1972).
• David Joy, “New Lecture 3” on course website.