**6. Conclusions**

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 validity of this solution for more than 3 years.
