HENRY’S LAW

    Henry’s Law.    Henry’s Law has two parts.  Part one states that as pressure increases, solubility of gasses in liquids increases. Part two states that as temperature increases, solubility of gasses in liquids decreases.  Part one in English:  More pressure means more gas can be dissolved in a liquid. Decreasing pressure causes that gas to come out of the liquid.  Part two in English:  Colder liquids hold more gas than warmer liquids. As a liquid warms up, the gas starts to come out of solution.

     The classic example of Henry’s Law is the bottle of your carbonated beverage of choice. Before the bottle is opened, the contents of the bottle are under pressure. Because of this pressure, the carbon dioxide (the gas that makes the beverage carbonated) is soluble in the liquid portion of the beverage. When you open the cap, you release the pressure and the carbon dioxide becomes less soluble.  Because the carbon dioxide is less soluble, it can’t remain dissolved and comes out of solution in the form of bubbles.

     Part one. When a diver goes underwater and subjects his body to increased pressure, his tissues are able to absorb more gasses.  The oxygen is used up by cellular processes, but the nitrogen is inert and just packs into the tissues. The deeper the diver goes and the longer he stays, the more nitrogen packs into the tissues. The nitrogen itself is not a problem. Eventually, it would reach a state of equilibrium and stop on-gassing (packing in).  The problem begins when the diver ascends and reduces the pressure his body is under, making the nitrogen less soluble in his tissues. If the diver comes up too fast (releases the pressure too fast), the nitrogen comes out in the form of bubbles, just like the soda. This condition is known as decompression sickness (DCS), sometimes called the “bends”. DCS is discussed in more detail in the Diving Injuries section. So, to avoid decompression sickness, a diver must monitor his depth and time to limit the amount of nitrogen on-gassing and he must ascend slowly enough that the pressure is released slow enough to allow the nitrogen to leave the tissues without forming bubbles.

     Part two. The second part of Henry’s Law comes into play when diving in cold water.  Cold water makes the body on-gas faster, allowing shorter dive times or shallower depth than would be possible in warm water.  In addition, working hard or jumping in a hot tub right after a dive can heat up the tissues and make the nitrogen less soluble in the tissues which can result in DCS.