Vacuum

Question 1

Please identify the factors that determine vacuum generation and vacuum performance

Answer 1

Number density
Mean free path
Flow
Particle flux
Throughput
Conductance
Pumping speed

Question 2

Outline the meaning of the main vacuum pump parameters: pumping speed, effective pumping speed, and conductance

Answer 2

Pumping speed - volumetric rate at which gas is transported across a plane, ratio of throughput of a given gas to the partial pressure of that gas a specific point neat the inlet of the pump, measure ability to remove gas from its inlet in some state time period
Effective - inlet speed of a particular pump only if pump is joined to the vessel 1/se=1/s+1/c
Conductance - ratio of throughput, under steady-state conservation conditions, to the pressure differential between two specified isobaric sections inside the pumping system

Question 3

How are vacuum pumps classified on the base of their operational principle? Give an example

Answer 3

Primary - low vacuum, compress gasses to atmospheric pressure - rotary, vane, diaphragm, scroll
Secondary - high vacuum, require primary pump to operate - oil, diffusion, terbomolecular

Question 4

Give and example of pumps that could be used to achieve low, medium, high and ultra-high vacuum

Answer 4

low - diaphragm
medium - rotary
high - diffusion
ultra-high - terbomolecular

Question 5

Briefly explain (using a diagram if necessary) the principle of operation of a rotary vane pump, diaphragm, scroll, oil diffusion and turbo-molecular pumps

Answer 5

Rotary vane - oil filled 
Diaphragm - dry operating
Scroll - oil free, no pump emissions
Oil diffusion - control of complex valve system and temperature
Turbomolecular - jet engine

Question 6

Please outline the methods used for vacuum measurement. Give examples

Answer 6

Physical - mercury, piezo-electric, dial gauges
Heat transfer - pirani, thermocouple
Ionisation - hot or cold cathode

Question 7

Give an example of vacuum gauges that could be used to measure low, medium, high and ultra-high vacuum

Answer 7

Low - Bourdon
Medium - thermocouple
High - Scultz Phelps
Ultra-high - Hot cathode

Question 8

Briefly explain (using a diagram if necessary) the principle of operation of a physical deflection (bourdon), capacitance diaphragm, pirani, hot and cold cathode ionisation vacuum gauges.

Answer 8

Bourdon - pointer actuated by evacuation process due to external atmospheric pressure
Capacitance diaphragm - change in capacitance proportional to change in pressure
Pirani - change in electrical resistivity measured
Hot cathode - measure current proportional to pressure
Cold cathode - magnet orbits electrons produced by electrodes