Detecting Hydrogen in Helium Streams

Three different approaches to detecting hydrogen in helium streams were assessed: thermal conductivity, atmospheric sampling using a mass spectrometer, and permeation through a palladium/silver membrane. The thermal conductivity detection system utilized an assembly of four cells fitted with filaments wired to make a Wheatstone bridge. Pure helium flowed through two of the cells. Helium containing hydrogen passed through the remaining two cells. The filament temperature, which is determined by the thermal conductivity of the gas surrounding the filaments, depends on the hydrogen concentration in the helium carrier. In the atmospheric sampling approach, a slip-stream is drawn from the carrier via two differentially pumped orifices and injected into a vacuum system. The vacuum system is fitted with a mass spectrometer tuned to hydrogen. In the permeation approach, the helium stream is passed over a hot palladium/silver (Pd/Ag) membrane. Hydrogen permeates through the membrane to increase the pressure in a vacuum system. The change in the vacuum pressure is directly related to the hydrogen concentration in the carrier on the upstream side of the membrane. The Pd/Ag permeation approach proved to be the most sensitive hydrogen detection technique. This approach was used to investigate the performance of a cryotrap. Several interesting phenomena were detected when evaluating the cryotrap. A large but brief breakthrough occurs when the column reaches capacity, and hydrogen elution can occur when the full, cold column is purged with helium.