During vaporization of LH2, is a change in pressure as LH2 is converted to GH2? Does air enter as part of the process (whether in the tank or as it is vaporized)?

Vaporization of a trapped volume of LH2 will lead to significant increase in pressure due to the very large expansion ratio as the liquid converts to gas. Relief devices are required since the pressure increase is likely to be far in excess of the pressure rating of the system. When vaporized as part of a flowing process, pressure will not increase. As the LH2 is warmed, it undergoes a phase transformation from liquid to gas (progressively more bubbles until 100%) at a constant temperature and pressure as it boils. After it is 100% gas, it will continue to be warmed until it reaches the desired temperature, usually ambient. The vaporization occurs within a closed system and only hydrogen is present. No air enters the system and the volume of liquid removed from the LH2 tank is replaced with lower density gaseous hydrogen.