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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