Henry’s Law

William Henry was an English physician and chemist who in 1803 proposed what is now called Henry’s law, which states that “At a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid. “

The Law essentially has two parts; one states that as pressure increases, solubility of gasses in liquids increases. The Second part states that as temperature increases, solubility of gasses in liquids decreases.

In simplistic terms:

• Under more pressure, a greater quantity of gas can be absorbed by a liquid
• Secondly, the cooler the liquid, more gas can be absorbed by it, as a liquid warms up, the gas escapes from it.

Mathematically the Law is expressed as:

P=KC     Where

P = the partial pressure of the gas

C = Concentration of the gas

K= Henry’s Law Constant

When trying to understand Henry’s Law, it helps to draw parallels to a bottle of Soda. Before the bottle is opened, its contents are under pressure, which causes the carbon dioxide in it to be soluble in the soda. As soon as you open the cap, you release the pressure causing the carbon dioxide gas to lose its solubility and escape in the form of bubbles or fizz.

In the same manner, as a diver descends, nitrogen inhaled has nowhere to escape and under pressure gets soluble in the bloodstream, muscles and tissues. This is no problem, until the diver begins his ascent.  As a diver ascends, the pressure is released and like the soda bottle, the nitrogen in the body tries to escape and may form bubbles if the diver ascends too quickly causing DCS or Decompression Sickness also known as the bends. This is the reason why divers ascend gradually, to allow the nitrogen to dissipate rather than form bubbles.

Henry’s Law also explains the reasons why divers are asked to not to take hot baths after a dive, or asked to abstain from doing strenuous activities or exercise. Based on the second portion of Henry’s Law, the increase in temperature caused by the exercise or hot bath may cause the nitrogen to become less soluble and increase the off-gassing possibly cause DCS or Decompression Sickness.

In the same manner, while diving in colder water, the on-gassing/absorption of nitrogen is greater, which should be taken into consideration, as it will allow the diver shorter dive times, and shallower dives.

Also Read: An Introduction to Scuba Gas Laws – Part 1 : Boyle’s Law
An Introduction to Scuba Gas Laws – Part 2: Charles’ Law

Photos Gaetan Lee, azteca90