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www.thinkSRS.com 1
Bayard-Alpert Ionization Gauges
This application note attempts to explain the principles of operation of the Bayard-Alpert Ionization
Gauge, or BAG, outline its fundamental limitations and describe the ion gauge types that have
successfully surmounted some of them. A few practical tips are also provided along the way. The
emphasis has been placed on gauges that are commercially available.
In This Application Note
Principle of Operation 3 Mechanical Construction 25
Introduction 3 Glass tubulated gauges 25
Gauge Principles 3 Nude gauges 28
Gauge Sensitivity 4 High-accuracy gauges 31
Definition 4 Tiny Gauges 32
Pressure Dependence 6
Gas Dependence 7
Filament Considerations 34
Electrode Geometry Dependence 9 Filament Materials 34
Bias Voltage and Emission Filament Reactions 36
Current Dependence 11 Emission of ions and neutrals 37
Gauge Envelope Dependence 12 Accuracy and Stability 38
Temperature Dependence 14 Reproducibility 38
Magnetic Field Dependence 14 Stability 38
History Dependence 15
Degassing 41
Limiting Factors for Low Pressure
Operation 17 Safety and Health Considerations 43
X-ray Limit 17 Electric Shock 43
Gauge design 18 Thoria Alpha Emission 43
Electrode Surface conditions 19 Glass breakage 43
Emission Current 19 Burns 43
Envelope Bias (Forward vs. Reverse X-ray X-rays 44
Effect) 19
References 45
Electron-Stimulated Desorption (ESD) 20
Leakage Currents 21
Outgassing 21
Gauge Pumping 22
Filament reactions and outgassing 24
Gas Permeation 24
(408)744-9040 Stanford Research Systems
www.thinkSRS.com
2 Bayard-Alpert Ionization Gauges
Stanford Research Systems (408)744-9040
www.thinkSRS.com
Principle of Operation 3
Principle of Operation
Introduction
The Bayard-Alpert ionization gauge (BAG) was first described in 19501. Modern
versions of the gauge have preserved most of the basic elements of its original
implementation. Standardization of the BAG design has made it possible for vacuum
equipment manufacturers to produce generic ion gauge controllers, such as the IGC100,
capable of controlling BAGs from many different manufacturers.
BAGs are not perfect, and the user who believes their pressure indications without a
basic understanding of their operation is likely to be fooled.
This application note attempts to explain the principles of operation of the BAG, outline
its fundamental limitations and describe the ion gauge types that have successfully
surmounted some of them. A few practical tips are also provided along the way. The
emphasis has been placed on gauges that are commercially available.
Since it is not possible to cover this complex gauge in a short note, a comprehensive list
of references is provided at the end that should allow the reader to find answers to most
problems.
Gauge Principles
Figure A-1 describes a prototypical BAG design. Electrons boil from the hot filament
(30Vdc) and are accelerated towards the anode grid (180Vdc). As the current (0.1-10 mA
typical) of highly energetic (150eV) electrons traverse the inner volume of the grid cage,
they ionize some of the gas molecules they encounter in their path. Electrons that do not
encounter any obstacles in their path, exit the grid and are immediately directed back into
its inner volume by the electrostatic field, resulting in a multiple-pass ionization path that
ultimately ends by collision with a grid wire. The ions formed inside the anode grid are
efficiently collected by the grounded (0Vdc) collector wire that is located along the axis
of the cylindrical grid and connected to the controller's electrometer. If the electron
emission current and the temperature of the gas are constant, then the ion current is
proportional to the number density and the pressure of the gas. The positive ion current
provides an indirect measurement of the gas pressure.
(408)744-9040 Stanford Research Systems
www.thinkSRS.com
4 Principle of Operation
Figure A-1. Typical Bayard-Alpert configuration (glass-tubulated design)
Gauge Sensitivity
Definition
The number of ions formed inside the anode grid, and therefore the current measured by
the electrometer of Figure A-1, is a function of
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