Exposure of a cryogenic storage dewar to direct sunlight can cause which outcome?

• A. Lower boil-off
• B. Stabilized temperature
• C. Reduced venting
• D. Increased boil-off and pressure

Answer: (D)

Direct sunlight adds heat to a cryogenic storage dewar, causing more of the stored liquid to boil away. As the liquid boils, it produces vapor that raises the pressure inside the headspace. The vent system is designed to release this gas to keep the pressure in a safe range, so with increased heat input from sunlight you get both more boil-off and higher pressure. The other possibilities—lower boil-off, a stabilized temperature, or reduced venting—don't occur because the added heat cannot be suppressed by the dewar’s insulation; it instead drives more energy into the liquid, leading to more boiling and more gas to vent.