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www.thinkSRS.com 1 PG105 vs. Thermocouple Gauges The two most common thermal conductivity gauge technologies used in modern vacuum applications are Pirani gauges and thermocouple gauges. This application note is designed to help vacuum users choose between the two competing gauge technologies and decide when a pressure measurement setup based on TC gauges should be upgraded to PG105 convection gauges. In This Application Note Introduction 3 Pressure Range Considerations 4 Response Times 5 Ion Gauge Auto Start 5 Remote Sensing 5 Controller/Gauge Interchangeability 6 Contamination Resistance 6 UHV Compatibility 6 Price/Performance ratio 7 Freeze-Drying Processes 7 Leak Testing 7 (408)744-9040 Stanford Research Systems www.thinkSRS.com 2 PG105 vs. Thermocouple Gauges Stanford Research Systems (408)744-9040 www.thinkSRS.com PG105 vs. Thermocouple Gauges 3 Introduction Pressure measurement in a thermal conductivity gauge is based on the transfer of heat from a hot wire, located inside the sensor, to the surrounding gases. Since gauge output depends on the thermal conductivity of the gases as well as their pressure, all thermal conductivity gauges provide indirect, gas-dependent, pressure readings. The two most common thermal conductivity gauge technologies used in modern vacuum applications are: Pirani Gauges In the Pirani gauge (PG) the voltage required to maintain the hot wire at a constant temperature is used as a non-linear, gas-dependent, function of pressure. Traditional Pirani gauges provide useful pressure readings between 10-3 and 10 Torr. In convection-enhanced Pirani gauges, the upper range is extended upward to 1000 Torr by taking advantage of thermal convection currents created at the higher pressures. Thermocouple Gauges In the thermocouple gauge (TC) the pressure is indicated by measuring the small voltage of a thermocouple spot welded directly onto the hot wire. The wire is fed with a constant current and its temperature depends on the thermal conductivity and pressure of the gases present. TC gauges display useful pressures between 10-3 and 1 Torr. TC gauges have long been regarded a cost-effective means to (1) monitor the foreline pressures of pumping stations and (2) as crossover gauges for vacuum systems in general.