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
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