Water from Air

• On a humid summer day, it's not hard to imagine where the water on the can or glass comes from--you can practically feel the moisture in the air. But there is always moisture in the air around us. Cold air holds less moisture than warm air. At the dew point temperature, the air is so cold that it can no longer hold all of its moisture. The excess has to go somewhere, and usually condenses into droplets. We see this happening when dew falls on grass overnight and sweat dribbles down the side of a cold glass. Both are examples of the air dumping its excess moisture as it cools down.
  
  

Liquid Oxygen Is Cold ... Really Cold

• Liquid oxygen has to be colder than minus-181 degrees Fahrenheit to remain liquid under any conditions and is usually stored below minus-250 degrees Fahrenheit. That's nearly 300 degrees colder than your refrigerator. The air around us--even on a cold winter day--is as hot to liquid oxygen as a cooking oven is to a person.
  
  

Ice from Water

• Because air is so much hotter than liquid oxygen, heat is constantly flowing out of the air, through the walls of a portable oxygen tank, and into the liquid oxygen. Portable oxygen tanks don't have enough liquid oxygen to cool off a large space of air, like a room or a building, but a thin layer of air around a liquid oxygen container is constantly having heat sucked out of it by the extra-cold oxygen. Eventually, this thin layer gets so cold that it falls below its dew point. Water condenses out of that air and builds up in the cold area. As more and more heat is sucked away from that thin layer--now covered in liquid water--the water can freeze and form ice.
  
  

Safety First

• Because portable oxygen containers carry such tremendously cold cargo, water can freeze on their surfaces. Parts of the containers that have little insulation, like connecting nozzles, are more susceptible to freezing because heat is more easily transferred away on those parts than around the more-insulated parts of the container. This ice can plug valves in liquid oxygen systems. More important, ice formed by liquid oxygen can cause serious, frostbite-like damage to skin and other tissues. Use caution when handling it, and follow the manufacturer's recommended safety procedures for an oxygen storage device.