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

Charles’ Law

If you have read my first article in this series, An Introduction to Scuba Gas Laws – Part I: Boyle’s Law, you will already be familiar with Boyle’s Law which studies the relation between pressure and volume of a gas while temperature remains constant.  Charles’ law also studies volume of gas however taking into consideration the effects of temperature on the volume and Pressure of the gas.

Jacques Alexander Charles was a French scientist, mathematician, inventor and a balloonist who first studied the effects of temperature on the volumes of a gas and formulated Charles’ Law in 1787. The law states that “At constant pressure, the volume of a given mass is of an ideal gas increases or decreases by the same factor as its temperature increases or decreases.”

Mathematically Charles’ Law is expressed as:

P1 x V1     P2 x V2
______ = ________
T1               T2

Where’ T’ is temperature,’ P’ is pressure and ‘V’ volume.  ‘1’ indicates the initial temperature and volume while the’ 2’ indicates the final temperature and volume.

According to Charles’ Law, if we were to take a balloon filled with air and increase the temperature of the air inside, the volume of air would increase causing the balloon to expand.  This is caused by the heating of the molecules of air inside the balloon causing them to move rapidly. In the same manner if we cooled the balloon in a freezer, the volume of air decrease, making the balloon look partially deflated.

In the same manner a Scuba tank filled to 3000 PSI if heated, would increase the pressure in the tank as the volume of the air inside the tank is at a constant.

So how is this applicable to Scuba Diving? For starters, Charles’ law helps divers understand the hazards of leaving scuba tanks out in the hot sun, or why we should never leave tanks in the trunk of a hot car. The gas under pressure subjected to heat can cause the tank to explode. A scuba tank filled to capacity with compressed air at 3000 psi could just as easily go up to 3400-3500 psi if heated. Proper storage of air tanks on the dive boat too is crucial to ensure the tanks aren’t left to bake in the sun.

The law explains why the pressure in a scuba tank recently filled may appear to drop or increase slightly depending on whether the tanks were filled outdoors, or indoors. The law also explains why scuba tanks when being filled with compressed air, get hot.

I’ll admit knowledge of Charles’ Law isn’t as useful to us as perhaps Boyle’s Law, nevertheless it helps us understand everyday phenomenon and helps us better understand the remaining gas laws as well.

The reason for writing an article on Gas Laws is simply because I, like so many other divers, constantly forget the Scuba Gas laws taught to us in our Open Water course. We all tend to understand the concepts and application while diving, but when asked why bleed air from your BCD while ascending, I know that the volume of gas is expanding as pressure decreases, but what was the name of that law again?  Was it Charles’, Boyle’s, Dalton’s or Fred’s? Hopefully this series will help us better understand  and remember these scuba gas laws.

Boyle’s Law

Robert Boyle was a chemist and a physicist who published this law in 1662. The law describes the inversely proportional relationship between the absolute pressure and volume of a gas, if the temperature is kept constant within a closed system. In simple English: If you increase pressure, the volume of the gas decreases, and if you increase the volume of the gas, the pressure decreases.

Mathematically Boyle’s Law is expressed as

P1V1=P2V2, where

P1 = Starting Pressure (expressed in absolute pressure, i.e. atmospheres)

V1 = Starting Volume (liters)

P2 = Ending Pressure (expressed in absolute pressure, i.e. atmospheres)

V2 = Ending Volume (liters)

Let’s try and understand Boyle’s law using a simple example. At the surface we are subjected to 1 ATM (atmosphere) of pressure. At 33ft underwater, we are subjected to 2 ATM; i.e. 1 ATM of Air pressure and 1 ATM  of water pressure.

So if we take a 1 liter Coke bottle filled with air faced down with no cap on, to 33ft (10m) underwater, we would see that the volume of air decreases to around ½ a liter of air, and water would begin filling into the bottle without any of the air escaping. Because at 33ft the pressure has increased of 2 ATM or has doubled, thereby halving the volume of the air.  If we take the bottle down to 66ft (20m), we would be at 3 atmospheres of pressure and the air in the bottle would be 1/3 of a liter and so on.

Now assume we add air into the coke bottle from our scuba tanks at the depth of 33ft (10m) topping off the half full bottle, cap the bottle tightly, then begin to ascend.(remember the air in our scuba tank is also being subjected to Boyle’s law ) As we rise, the pressure decreases, causing the already compressed air to expand. At the surface the volume of the air in the 1 liter bottle would have doubled to 2 liters probably causing the bottle to burst on the way up.

This is the reason why we bleed out air from our BCD’s as we ascend to prevent us from rising too fast as the air present inside the BCD expands with decrease in pressure.

Boyle’s Law helps us understand the importance of compensating and balancing air in our BCD’s, masks, lungs, ears, and wherever air is contained. Mask squeeze as we descend, or the need to blip a little air into our BCD’s as we descend deeper to maintain neutral buoyancy is also attributed to this law.

The need to equalize is also attributed to the small air spaces inside our ears, which causes our ear drums to bend as the air behind the ear drums decreases in volume.

The law also explains the reason why we are instructed to exhale deeply and continuously while ascending, and also why we are told never to hold your breath while ascending, as the air inside your lungs can over expand, causing severe internal injury.

photos by riandreu and Mik Canavan