doi:10.1103/PhysRevApplied.6.024017engGabal, M.Arauzo, A.Camón, A.Castrillo, M.Guerrero, E.Lozano, M.P.Pina, M.P.Sesé, J.Spagna, S.Diederichs, J.Rayner, G.Sloan, J.Galli, F.Van, Der GeestHaberstroh, C.Dittmar, N.Oca, A.Grau, F.Fernandes, A.Rillo, C.Hydrogen-Free Liquid-Helium Recovery Plants: The Solution for Low-Temperature Flow Impedance BlockingART-2016-97028The blocking of fine-capillary tubes used as flow impedances in He4 evaporation cryostats to achieve temperatures below 4.2 K is generally attributed to nitrogen or air impurities entering these tubes from the main bath. The failure of even the most rigorous low-temperature laboratory best practices aimed at eliminating the problem by maintaining the cleanliness of the helium bath and preventing impurities from entering the capillary tubes suggests that a different cause is responsible for the inexplicable reduction of impedance flow. Many low-temperature research laboratories around the world have suffered this nuisance at a considerable financial cost due to the fact that the affected systems have to be warmed to room temperature in order to recover their normal low-temperature operation performance. Here, we propose an underlying physical mechanism responsible for the blockages based upon the freezing of molecular H2 traces present in the liquid-helium bath. Solid H2 accumulates at the impedance low-pressure side, and, after some time, it produces a total impedance blockage. The presence of H2 traces is unavoidable due its occurrence in the natural gas wells where helium is harvested, forcing gas suppliers to specify a lower bound for impurity levels at about 100 ppb even in high-grade helium. In this paper, we present a simple apparatus to detect hydrogen traces present in liquid helium and easily check the quality of the liquid. Finally, we propose a solution to eliminate the hydrogen impurities in small- and large-scale helium recovery plants. The solution has been implemented in several laboratories that previously experienced a chronic occurrence of blocking, eliminating similar occurrences for more than one year.2016http://zaguan.unizar.es/record/5777910.1103/PhysRevApplied.6.024017http://zaguan.unizar.es/record/57779oai:zaguan.unizar.es:57779info:eu-repo/grantAgreement/ES/MINECO/IPT-2012-0442-420000info:eu-repo/grantAgreement/ES/MINECO/MAT2015-64083-RPhysical review applied 6, 2 (2016), 024017 [11 pp]byhttp://creativecommons.org/licenses/by/3.0/es/info:eu-repo/semantics/openAccess