Boyle's Law

[Capt Nemo]

In this thread, I'm going to talk about things that most dive instructors won't talk about for legal reasons. "You can't change the laws of physics, but lawyers love to try!" However, I will talk about ALL the implications of the law, and what it can allow.

Moderators, please sticky this thread. This is important for not only scuba divers, but for freedivers and mermaids as well.

Boyle's Law

If the temperature remains constant, the volume of a gas will vary inversly as the absolute pressure and density will vary directly.

Mathmatically

P1 * V1 = P2 * V2

P1 = Initial Pressure
V1 = Initial Volume
P2 = Ending Pressure
V2 = Ending Volume

Broken down this gives us three possibilities.

P1 > P2 and V1 < V2 Pressure decreases, volume increases.
P1 = P2 and V1 = V2 Pressure and volume stay the same.
P1 < P2 and V1 > V2 Pressure increases, volume decreases.

This law is very important to anyone who enters the water.
It is the cause of most diving related injuries.

At sea level, you are breathing air at 14.7 psi. 14.7 psi is considered as 1 atmosphere (ATM). As it's all around us, it is considered as ambiant pressure or absolute pressure. It only takes 33 feet of saltwater (34 freshwater) to equal 14.7 psi. So, for each 33 feet of depth, we gain 1 atmosphere of pressure. Hence, at 33 feet, we are at a total of 2 atmospheres ambiant pressure. This will continue as we decend.



From the chart, we can see how the pressure increases the deeper we go. We can also see what happens to a 100 cu/ft volume of air that started at 1 atmosphere. As we can see, the volume decreases until at 132 feet it is only 20 cu/ft or 20% of it's original volume. This volume change with pressure change is what causes the pressure related injuries. On descent, the air cavities in the human body will decrease in size as pressure increases. If the cavity, cannot be equalized, tissue damage will occur.

In the last column of the chart, we can see what happens to the same 100 cu/ft volume of air at 132 feet 5 ATM. Notice that as we ascend, the volume of air increases until at the surface, it now occupies 500 cu/ft. In the human body, this expansion creates the most serious injuries, and equalization becomes extremely important.

For snorkeling, the pressure change is the reason you don't see snorkel tubes longer than about 1 foot. On the surface, you have 14.7 pounds of pressure over every square inch of your chest. Altogether, that can equal hundreds of pounds! However, you have air in your lungs that's pushing back, and counteracts all that pressure. In the case of a long snorkel tube, the air in your lungs stays at 14.7 psi, but say at 4 feet of depth, there's an extra 2 psi of ambiant pressure for 16.7 psi. The pressures become unequal, and the extra 2 psi will feel like there's an elephant sitting on your chest. You could say the average human chest area is about 3 square feet. So at 14.7 psi, that's 6350 lbs, and at 16.7 psi, 7214 lbs, for a differance of 864 lbs. So you see why there are no long snorkel tubes!

For scuba, this pressure differance is why we must breathe compressed air on a regulator, or why mermaids breathe compressed air from hoses. We have to breathe air at ambiant pressure in order to breathe at all!

For freedivers, they too breathe compressed air! The air naturally compresses in their lungs due to Boyle's Law as they decend. However, they are not normally subject to ascent type injuries, as the volume they went down with is equal to what they surface with.

Divers and mermaids aren't so lucky! As they are submerged breathing air at increased pressure, they have to know the effects of Boyle's Law and how to prevent injury. Back at the table, look at the last column, and how a pressurized air volume expands as you ascend. A diver holding his breath and swimming to the surface from 132 feet would see that air expand to 5 times it's original volume. The differance in pressure would be 58.8 psi. Now the lungs can handle 2 psi before damage occurs, which is about the pressure differance of 4 feet of water. So our diver would most likely explode before reaching the surface. But before that, he would experience the worst injury that a diver could face, Air Embolism. Because our lungs are so fragile, divers came up with the first rule of scuba "NEVER HOLD YOUR BREATH."

However....

If we take a good look at Boyle's Law, we will find that it actually allows us to hold our breath while breathing compressed air. It is only in the first instance of P1 > P2 and V1 < V2 that we have expansion of the air volume in our lungs over that of what we initially took in. With P1 being greater than P2, the volume must increase. And therefore, if a diver holds his breath and ascends, he will suffer overexpansion of the lungs.

With the second case of P1 = P2, we begin to see how we can hold our breath. In this case, P1 stays equal to P2, and causes V1 to stay equal to V2. There is NO expansion, and thus, NO expansion type injury can occur. However, temprature can cause some expansion to occur, but it would be small. Our bodies are very adept at warming and humidifying the air we take into our lungs, and so by the time we have finished inhaling, that air is almost at body temprature. So there would be negligible thermal expansion. Heliox and trimix divers may be more suceptable to thermal expansion as the helium in the mixes can absorb more heat than air.

In the third case of P1 < P2, there is no expansion at all! The volume must contract because the pressure is increasing. No expansion injury is possible!

So for the last two cases it is actually possible to hold your breath while breathing compressed air. As long as P2 stays greater than or equal to P1 there can be no expansion greater than the lungs can handle.



Does this mean a diver can do it in all situations? NO!!!

Normal scuba diving is generally changing depths throughout the entire dive. Even when neutrally buoyant, a diver rises when he inhales, and sinks when he exhales. Swimming around, it's very hard to keep track of depth, unless it's a very controlled situation. For normal diving practice, stick with the first rule of scuba, NEVER HOLD YOUR BREATH.

So when can you hold your breath? For modeling and performing.

For modeling, the model is usually weighted so she will stay put at constant depth. This keeps her in a P1 = P2 situation. However, when the model is changing depth, like going from a prone position to a standing position, the model may change depth enough to get her into trouble. So therefore, models should be breathing when changing positions (changing positions is usually a rest time for the model anyway). Some shots may require a model to change depth. As long as she decends, she may hold, and may return to the depth where she took her last breath from. If the model has to ascend from a deeper starting point, she must begin exhaling, or she will go into overexpansion.

For performance, the mermaids at Weeki Wachee hold their breath regularly. They also use their lungs as a buoyancy compensator to remain neutrally buoyant. This means that they are holding less than full lung volume. This gives them the ability to do things that seem impossible to scuba divers. They have room for limited expansion to occur, but generally, they operate from the P1 = P2 area and P1 < P2 areas where volume remains the same or decreases. In a way, it's like freediving, only the surface is 15' down.


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