Transcript Vacuum Fundamentals

High-Vacuum Technology
Course
Week 3
Paul Nash
HE Subject Leader (Engineering)
Vacuum Fundamentals
Vacuum Technology
• Final outstanding enrolments
• Recap on last week
Measuring Vacuum
Gauge types, ranges and costs
Active and Passive Gauges
Location of gauges
Vacuum Fundamentals
Learning Objectives
• To be able to describe a variety of vacuum
measurement techniques
• To be able to select gauge types for a
given range of vacuum
• To explain the difference between Active &
Passive gauges
Vacuum Fundamentals
Measuring Vacuum
Vacuum Fundamentals
Vacuum Gauges
There are 3 phenomena used to measure
vacuum:
Mechanical
Displacement of materials
Transport
Movement of gases
Ionisation
Ion currents
Vacuum Fundamentals
Vacuum Gauges
• Many different types of
gauges are available
because each only covers a
limited range of pressures
Range of gauge utility
Vacuum Fundamentals
Pirani gauge
Schematic Circuit for a
Pirani (hot wire) gauge
• The Pirani is a
dedicated low vacuum
gauge device
• The resistance of the
hot wire changes with
the rate of heat loss
(conduction) to the gas
• The Wheatstone
bridge then measures
the change in
resistance of the hot
wire
• Pirani’s are rugged and
generally reliable and
rarely need attention
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Pirani gauge
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Pirani calibration
• The calibration of a
Pirani depends on
thermal conductivity
and so on the actual
gas in the system
Correction Curve for Pirani
Gauges
Vacuum Fundamentals
Pirani gauge
Advantages
Low temp operating increases thermal stability by reducing
thermal oxidisation of contaminants
Significantly better resolution in the range above 100 Torr
Long service life
Robust and stable under normal working conditions
Disadvantages
Cannot be used in high temp operations or located near heat
sources
Results can vary depending on the orientation of the gauge
(preferred vertical)
Can be easily contaminated particularly on installation
Vacuum Fundamentals
Penning (Cold cathode) Gauge
• A Penning gauge measures
the ion current flowing
from the cathode to the
anode
• The magnetic field
increases sensitivity by
making the ions spiral as
they travel to cause
secondary ionization
Penning gauges require
routine cleaning and
testing
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Penning (Cold
cathode) Gauge
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Capacitance
Manometer
Gauge head on chamber
Controller and digital read-out
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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”.
Vacuum Fundamentals
Ion gauges
(Hot cathode)
• Pressures lower than 10-5 Torr
can be measured with ion gauges
Vacuum Fundamentals
Interpreting the output
The gauge output must be interpreted and
converted into a vacuum measurement
This output may also be used to control
activation of:
Pumps
Other gauges
Valves and process control
Vacuum Fundamentals
Passive & Active Gauges
Vacuum Fundamentals
Passive
Gauges
Analogue
Digital
Vacuum Fundamentals
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Active Gauges
Made possible by Surface
Mount Technology on printed
circuit boards
The gauge has an ID resistor
to ‘tell’ the controller what
type of gauge it is
Can also be interfaced to a PC
Network for process control
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Active
Gauges
Multiple gauge inputs
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Active
Gauges
Multiple gauge inputs
Vacuum Fundamentals
Combined Gauges
http://www.edwardsvacuum.com/Products/View.aspx?sku=D14701000
Vacuum Fundamentals
Positioning of Gauge Heads
Vacuum Fundamentals
Positioning of Gauge Heads
Vacuum Fundamentals
Positioning of Gauge Heads
Vacuum Fundamentals