  Helium has the lowest boiling point of any element in nature at normal
atmospheric pressure. Therefore, any unwanted substance like impurities present
in liquid helium will be frozen and will be in solid form. Even if these solid
impurities can be easily eliminated by filtering, liquid helium may contain a
non-negligible quantity of molecular hydrogen. These traces of molecular
hydrogen are the causes of a known problem worldwide: the blocking of
fine-capillary tubes used as flow impedances in helium evaporation cryostats to
achieve temperatures below 4,2K. This problem seriously affects a wide range of
cryogenic equipment used in low-temperature physics research and leads to a
dramatic loss of time and costs due to the high price of helium. Here, we
present first the measurement of molecular hydrogen content in helium gas.
Three measures to decrease this molecular hydrogen are afterward proposed; (i)
improving the helium quality, (ii) release of helium gas in the atmosphere
during purge time for the regeneration cycle of the helium liquefier's internal
purifier, and (iii) installation of two catalytic converters in a closed helium
circuit. These actions have eliminated our low-temperature impedance blockage
occurrences now for more than two years.
