Sloshing liquid into warmer regions of the container can cause sharp pressure rises. Before l50 and l75 liter industrial-type hydrogen dewars are moved, the internal pressure must be reduced to 5 psig or lower. The relief valve must be set at not more than 10 psig. Vents must be protected against icing and plugging. Liquid hydrogen is supplied only in double-vent vessels.
Liquid helium is supplied in both single and double-vent vessels.
The single-vent helium vessels have a special large diameter neck and are issued on special request. All helium and hydrogen dewars in use are checked every few days to ensure safe operation. Liquid hydrogen and liquid helium may be transferred only into approved systems and only by personnel experienced in the handling of these liquids. Operators must avoid physical contact with any liquefied gases because burns can occur. Face masks and gloves must be worn while transferring and handling cryogenic liquids.