Vacuum Fundamentals

High-Vacuum Technology Course

Week 3

Paul Nash

HE Subject Leader (Engineering)

Vacuum Fundamentals

• Final outstanding enrolments• Recap on last week

Measuring Vacuum

Gauge types, ranges and costs

Active and Passive Gauges

Location of gauges

Vacuum Technology

Vacuum Fundamentals

• 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

Learning Objectives

Vacuum Fundamentals

Measuring Vacuum

Vacuum Fundamentals

Vacuum GaugesThere are 3 phenomena used to measure

vacuum:

MechanicalDisplacement of materials

TransportMovement of gases

IonisationIon 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• 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

Schematic Circuit for a Pirani (hot wire) gauge

Vacuum Fundamentals

Pirani gauge

Vacuum Fundamentals

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

AdvantagesLow temp operating increases thermal stability by reducing thermal oxidisation of contaminantsSignificantly better resolution in the range above 100 TorrLong service lifeRobust and stable under normal working conditions

DisadvantagesCannot be used in high temp operations or located near heat sourcesResults can vary depending on the orientation of the gauge (preferred vertical)Can be easily contaminated particularly on installation

Pirani gauge

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

Vacuum Fundamentals

Vacuum Fundamentals

Penning (Cold cathode)

Gauge

Vacuum Fundamentals

Capacitance Manometer

Gauge head on chamber Controller and digital read-out

Vacuum Fundamentals

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

PassiveGauges Analogue

Digital

Vacuum Fundamentals

Vacuum Fundamentals

Active GaugesMade 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

Vacuum Fundamentals

ActiveGauges

Multiple gauge inputs

Vacuum Fundamentals

ActiveGauges

Multiple gauge inputs

Vacuum Fundamentals

http://www.edwardsvacuum.com/Products/View.aspx?sku=D14701000

Combined Gauges

Vacuum Fundamentals

Positioning of Gauge Heads

Vacuum Fundamentals

Positioning of Gauge Heads

Vacuum Fundamentals

Positioning of Gauge Heads