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.

中文翻译：

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降低液氦中的氢含量

在正常大气压下，氦的沸点是自然界中任何元素的最低沸点。因此，液态氦中存在的任何不需要的物质（如杂质）都将被冻结并呈固态。即使这些固体杂质可以通过过滤轻松去除，液氦也可能含有不可忽视的分子氢。这些分子氢的痕迹是世界范围内一个已知问题的原因：在氦蒸发低温恒温器中用作流动阻抗的细毛细管堵塞，以达到低于 4,2K 的温度。这个问题严重影响了低温物理研究中使用的各种低温设备，并由于氦气的高价而导致时间和成本的巨大损失。在这里，我们首先介绍氦气中分子氢含量的测量。随后提出了三种减少这种分子氢的措施；(i) 改善氦气质量，(ii) 在氦气液化器内部净化器的再生循环吹扫期间释放大气中的氦气，以及 (iii) 在封闭的氦气回路中安装两个催化转化器。这些措施已经消除了我们两年多来发生的低温阻抗阻塞事件。