DIVING & WELLNESS: Dive decompression theory I
Gas partial pressure‚ solubility and relation to atmospheric pressure
How does your dive computer know when you need to ascend?
“Based on my dive profile it determines my no decompression limits.”
What determines no decompression limits?
“There are depth-time algorithms that calculate that”
What is that algorithm based on?
This was how I got into studying decompression theory. It all starts with one question‚ that leads just to more questions‚ until you start reasoning whether it will be your slow or fast tissue compartments determining your deep stop during your dive...
I would like to take you through this wonderful journey of understanding dive decompression theory with me. I want to take you through this process one step at a time‚ giving our brain cells a bit of time to let all that information settle down. I’m convinced that understanding more of dive decompression theory will give you more confidence and control during your dives.
“Joy in looking and comprehending is nature’s most beautiful gift.” Albert Einstein
Let’s start at the beginning. Our dive decompression theory journey starts with Paul Bert. In his work “La Pression Barometrique”‚ published in 1878‚ he described his studies on the physiological effect of air-pressure‚ below and above atmospheric pressure.
Paul Bert’s experiments showed that changes in ambient pressure resulted in changes in proportions of oxygen in the blood; where low pressures resulted in oxygen deprivation and high pressures resulted in oxygen overloading and poisoning. His important finding on the toxic effect of oxygen on the Central Nervous System (CNS) is today still known as the “Paul Bert Effect”. This concept is very relevant to scuba diving as well‚ especially Technical Scuba Diving‚ but more about this in a future blog.
For now‚ let’s keep our concentration on dive decompression theory‚ where nitrogen is the gas we want to concentrate on. What Paul Bert found during his experiments‚ were effects of nitrogen under high pressure.
In one of his experiments‚ he put 24 dogs under 7-9 ATA‚ followed by a rapid decompression of 1-4 minutes to 1 ATA. The result was that 21 dogs died‚ 2 had severe morbidity and only 1 seemed to be fine. Dogs that were exposed for moderate periods and/or were relieved from atmospheric pressure more gradually seemed to have less to no symptoms.
Now‚ I know‚ every animal lover like me‚ has a cramp in their stomach while reading about such experiments‚ but… this did lead Paul Bert to an interesting finding that laid the fundaments of dive decompression theory. He determined that an increase in partial pressure of nitrogen caused nitrogen to become dissolved in the body tissues and a decrease in partial pressure of nitrogen caused nitrogen to come out of solution and form bubbles.
Paul Bert concluded that decompressing slowly during diving‚ resulted in a slower release of nitrogen into the blood and a better chance to escape from the body without forming bubbles.
Let’s imagine how this works with a little bit of science‚ using two important formulas in dive decompression theory.
According to Henry’s Law of physics‚ at constant temperature‚ the solubility of a gas is directly proportional to the pressure that the gas exerts on the solution.
Solubility = (Henry’s constant) * (Partial Pressure Gas)
As we dive to depth‚ the ambient pressure increases. With an increase in ambient pressure‚ the partial pressure of the gasses we breathe during our dive increases as well‚ according to Dalton’s law of physics. This law states that the total pressure of a gas mixture is the sum of all gas partial pressures.
Total pressure = partial pressure gas 1 + partial pressure gas 2 + etc. (including all gasses)
Let’s see in an example what that means for nitrogen uptake in our body.
Before I start my dive‚ I’m at the surface at 1 ATA. Let’s start by finding what our partial pressure of nitrogen is at the surface using Dalton’s law.
Having 1 ATA ambient pressure at the surface means that the pressure of the air I’m breathing is 1bar. The air I breathe is composed of roughly 21% O2 and 79% N2.
Given these proportions‚ that means that O2 exerts a pressure 21% of 1bar‚ which is 0.21 bar. This is the partial pressure of O2. The partial pressure of N2 is 79% of 1 ATA -> 0.79 bar.
1 ATA (Total pressure) = 21% of 1 ATA (O2) + 79% of 1 ATA (N2)
= 0.21 * 1 ATA + 0.79 * 1 ATA
= 0.21 bar (ppO2) + 0.79 bar (ppN2)
Let’s have a look what that means for the solubility of nitrogen at the surface using Henry’s Law and considering Henry’s constant for nitrogen to be 6.1 * 10-4M/bar.
Solubility N2 at surface (1ATA) = 6.1 * 10-4M/bar * 0.79 bar = 4.8 * 10-4M
I start my dive and descend to 30m depth. The ambient pressure is 4 ATA. This means that the pressure of the air I’m breathing is 4bar.
Let’s fill these changes into our formula:
4 ATA (Total pressure) = 21% of 4 ATA (O2) + 79% of 4 ATA (N2)
= 0.21 * 4 ATA + 0.79 * 4 ATA
= 0.84 bar (ppO2) + 3.16 bar (ppN2)
So the partial pressure of nitrogen increased from 0.79 bar at the surface‚ to 3.16 bar at depth. Now‚ let’s see what the effect of that higher partial pressure of nitrogen is on the solubility of this gas in our body tissues. Looking at Henry’s law again and filling in the data we have:
Solubility N2 at 30m depth (4ATA) = 6.1 * 10-4M/bar * 3.16 bar = 19.3 * 10-4M
The solubility of N2 in our tissues increased from 4.8 * 10-4M to 19.3 * 10-4M‚ comparable to the increase in ambient pressure of 4 times higher diving from the surface to 30m depth.
Pardon my rough calculations (considering equal temperature in all situations‚ considering saturation‚ etc.)‚ but these are for the sake of simplicity and better understanding.
So‚ we have actually calculated ourselves that indeed‚ nitrogen solubility increases at depth during a dive. And a higher solubility means a higher nitrogen uptake into the tissues.
The contrary is also true of course‚ where that solubility of nitrogen decreases when surfacing from depth during a dive‚ so more nitrogen offloading of tissues.
Looking at our dive‚ we can conclude that diving to deeper depths we onload nitrogen into our body tissue; surfacing to shallower depths during our dive we offload nitrogen from our tissues.
I think that’s enough for dive decompression theory lesson 1. We’ll dive further in depth in the next diving decompression theory lessons‚ but knowing these basics described here is fundamental to understand what’s coming next. Don’t worry‚ the further we advance‚ the easier it actually gets!
... A presto!
Esther
