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www.thinkSRS.com High Pressure Sampling with RGAs Application Note #8 The types of analysis performed by an RGA are useful in decades or more, the aperture size must be compromised to many applications other than vacuum systems. But, the RGA tolerate the pressure range. For example, consider a process is intrinsically a vacuum instrument that operates best near pressure that varies from 10-1 to 10 mbar. The aperture 10-6 mbar. Above 10-5, the response becomes non-linear, and would be designed to drop the pressure from 10 mbar to above 10-4 the filament will be shut off by the control 10-5 mbar. When the process pressure was at 10-1 mbar, the electronics. To sample gasses at higher pressures, a pressure pressure at the RGA would be 10-7 mbar. The noise floor of reduction system is needed. These systems are basically a the RGA does not depend on the process pressure: for a restriction and a vacuum pump package. Common restrictions Faraday cup detector it is about 10-10 mbar. Therefore, the are pinholes and capillaries, which can provide pressure dynamic range of the measurement varies from five decades at reductions of more than six decades. The vacuum pump high process pressure to only three decades at the low package consists of a turbomolecular pump and a backing pressures. For applications where the full dynamic range is not pump. In addition to achieving the desired pressure reduction, needed, operating the RGA at low pressure may be acceptable. the design of a system should provide for a fast response and If the full dynamic range is required over a variety of process high signal-to-background ratio. pressures, a variable reduction is required. Suitable variable leak valves are available but are significantly more expensive At pressures common to vacuum processes, a simple aperture- than a fixed aperture. based pressure reduction system is suitable. At atmospheric and higher pressures, a two stage reduction systembased on Another method of increasing the dynamic range and data a capillary and apertureis used. These two systems will be acquisition rate is to use an RGA with an electron multiplier. used to illustrate the design of pressure reduction systems for The electron multiplier provides gains from 102 to 106 and RGAs. lowers the noise floor to as low as 10-14 mbar. This lower