**Thanks**

CO2

the helium is extremely pure. In fact *yj*

of the getter material extends.

82 Superfluids and Superconductors

nation of the H2

ified by extra-sensitive H2

**Acknowledgements**

Universidad de Zaragoza

**6. Conclusions**

unique function of the getter is to capture H2

impurities.

the validity of this solution for more than 3 years.

into the application instruments. The evaporated gas from non-H2

to the vapor pressure of the hydrogen at 4.2 K and 100 kPa (i.e., *yH*<sup>2</sup>

, etc.) except for the neon and hydrogen case (see **Figure 4**). The neon is not a problematic substance for the impedance clogging issue, since at liquid helium temperature (4.2 K at Patm), the vapor pressure is negligible; besides, if there exists a molecular concentration at higher temperatures, it is not affected by the getter material because it is a noble gas like helium. Therefore, the best place to put the hydrogen grabber is at the ATP output (**Figure 6**), when

The "Clean helium" gas produced by the Clean Helium Recovery Plant (**Figure 6**) is ultimately liquefied in a commercial ATL and transferred directly or by intermediate transport Dewars

could be initially filled with commercial non-"Clean" liquid (e.g., NMRs, MEGs, high field magnet cryostats, etc.), and can have a hydrogen quantity equal or below that corresponding

in the gas bag, compressed and injected again in the ATPs for purification and complete elimi-

The validity of the "Clean helium" plant concept is demonstrated by the fact that impedance blockages have been completely eliminated for more than 3 years, when the plant configuration was implemented in the Cryogenic Liquids Service at the University of Zaragoza [39]. Furthermore, the efficiency of the double purification method presented in this chapter was ver-

We have proposed a plausible mechanism for explaining the capillary blocking issue that occurs in many laboratories using helium evaporation cryostats. The unavoidable presence of traces of molecular hydrogen is at the heart of the problem. Molecular hydrogen condenses in the low-pressure side of the fine capillary tubes when helium is pumped to obtain temperatures below 4.2 K. Finally, we have found a solution to produce hydrogen-free liquid helium in two steps: (1) cryocondensation of the air components and (2) chemical adsorption of hydrogen molecular traces that are not solidified in step (1). We have already demonstrated

The authors are greatly appreciative and acknowledge the financial support from the Spanish Ministry of Economy and Competitiveness through the Project No. MAT2015- 64083-R and through the *Personal Técnico de Apoyo (PTA) 2016* program for personal funding. They would also like to acknowledge the use of Servicio General de Apoyo a la Investigación-SAI,

detection techniques presented in [2], for both gas and liquid phases.

< 10−14 for all the substances except for the H2; thus, the

. In this way, the process is optimized and the life

sensitive instruments, that

= 3.5 ∙ 10−10), is also collected

We want to acknowledge the collaboration and support of the technical team of the Servicio General de Apoyo a la Investigación-SAI, specifically to Mrs. M. Castrillo, Mr. D. Finol, Mr. F. Gómez, Dr. A. Arauzo, Mr. E. Guerrero and Mr. P. Tellez for their technical support. We thank Dr. Christoph Haberstroh from TU Dresden for additional discussion and very useful information.
