Fully reprinted from "Decompression Theory Every Diver Should Know (Part 1), Author: Lin Yu-ping"
Q: Decompression? That has nothing to do with me… I'm just a recreational diver…
Q: No worries… I just need to watch my dive computer and follow what it says — everything's OK!
The truth is, any kind of diving involves decompression — unless you never come back up. The moment you ascend from the bottom, you are decompressing.
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What are the parameters being set?
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A dive computer isn't injected into your arm — how does it know how much nitrogen is in your body?
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You sometimes hear about someone feeling unwell and ending up in a recompression chamber. If they were wearing a computer, how did they still get decompression sickness (DCS)?
Following GPS navigation alone still leads plenty of drivers astray. Only by building your own understanding can you develop the judgment to make the right calls. Knowledge is power — so let's all read this diving lesson carefully. It's worth every bit of that NT$4,500 course fee!
Decompression Theory: Ambient Pressure
To understand modern decompression research, we have to start with a gentleman named Haldane. In 1905, Haldane conducted live animal experiments using goats, observing the effects of varying depths, dive times, and ascent profiles on the animals' bodies. After sacrificing 85 goats — and personally subjecting himself to human trials — he arrived at one key conclusion: the safe decompression limit is an ambient pressure ratio of 2
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Goats inside a pressurization chamber
Let me walk through a few examples to illustrate this:
First, understand that given enough time, the nitrogen in our bodies will reach saturation equilibrium with the surrounding ambient pressure. Here's an everyday analogy: add a large spoonful of sugar to a glass of water — if you add too much, it won't dissolve, and you've hit a saturation point. But if you heat the water, its capacity to dissolve sugar increases, the undissolved sugar disappears, and you can keep adding more sugar until it, too, can no longer dissolve — reaching a new saturation point in "hot water."
Now pour that sugar water out and let it cool. Remarkably, sugar crystals will appear at the bottom of the glass — a new saturation state (cold water) has been reached. During the cooling process, the sugar water passes through a supersaturated state, which is what causes the sugar to precipitate out as crystals.
Now map this analogy onto diving: replace "heating" with "pressurizing," replace "water" with the human body, replace "cooling" with "decompression," and replace "sugar" with nitrogen. We can see that under different pressures, the human body holds different amounts of dissolved nitrogen.
Haldane's research found that after staying long enough at an ambient pressure of 2 atmospheres (10 m depth) to reach saturation, a diver can return to an ambient pressure of 1 atmosphere (the surface) without adverse effects.
Similarly, a diver who has been at an ambient pressure of 4 can ascend to an ambient pressure of 2 and remain there for a while (the 2
ratio), allowing the supersaturated body to reach a new equilibrium before ascending to the surface.But we know the culprit behind decompression sickness (DCS) is nitrogen. At an ambient pressure of 2 atmospheres, the partial pressure of nitrogen in the breathing air is 1.58 atm (since nitrogen makes up approximately 79% of air: 2 × 0.79 = 1.58). So after a long stay at 10 m, the saturated nitrogen partial pressure in the body is 1.58 atm. Returning to the surface at 1 atm puts the body in a supersaturated state, yet no adverse effects occur. Later research refined this finding to:
The human body can tolerate a supersaturated nitrogen partial pressure of up to 1.58 times the ambient pressure.
What Is the M-Value?
From this, we can clearly define the nitrogen tolerance threshold for the human body at any given ambient pressure — the deeper you go, the higher the tolerance. This threshold is defined as the M-value, and its magnitude depends on ambient pressure (depth). Because every dive ultimately ends at the surface, the lowest M-value applies at the surface. In simple terms: as long as the nitrogen partial pressure in my body does not exceed 1.58 atm, it is safe for me to surface. During a dive, as long as you keep your body's nitrogen partial pressure below 1.58 atm and maintain a safe ascent rate, you can head straight to the surface with no decompression stop required — that is the definition of a no-decompression dive.
But… we can't exactly limit all our diving to within 10 m, can we? At depths beyond 10 m, if you stay too long, the nitrogen partial pressure in your body can exceed 1.58 atm — so what then? Understanding the rate at which nitrogen enters the body therefore becomes an essential area of study. The goal is simply to surface before the nitrogen in your body reaches that upper limit!
Haldane's Decompression Model
Haldane understood that the human body has a complex physiological makeup and could not be quantified in simple terms. So he made a bold assumption. Drawing on the physics concept of radioactive half-life, he proposed a half-saturation time model.
The idea is that over a specific period of time, a tissue reaches half (50%) of full saturation; after a second half-saturation period it gains another half of the remaining gap (reaching 75%); after a third it gains another half (87.5%) — and so on.
Imagine walking into an all-you-can-eat hot pot restaurant when you're starving. In the first 10 minutes you devour 4 plates of meat — your stomach is warming up, but you're only "half full." In the next 10 minutes you can only manage 2 plates — you're feeling fairly full. In the 10 minutes after that, you're just chatting and slowly finishing one plate.
The rate at which nitrogen enters the body follows a similar pattern: fast at first, then gradually slowing as the amount accumulated in the body increases and the gradient narrows.
Haldane hypothesized a series of theoretical compartments (not representing any actual organ) and assigned each a different half-saturation time — 5, 10, 20, 40, and 75 minutes — to represent fast and slow tissues. Each compartment absorbs and off-gasses nitrogen according to its set half-time. For example, the fast tissue with a 5-minute half-saturation time reaches 50% saturation after 5 minutes, then 75% after another 5 minutes. Through mathematical calculation, the nitrogen content of all five theoretical compartments can be determined, providing a range of estimates. The compartment closest to its M-value at any given moment serves as the controlling compartment.
| 5-min half-time | 10-min half-time | 20-min half-time | Absorption rate |
|---|---|---|---|
| Time elapsed | Time elapsed | Time elapsed | Total absorbed |
| 0 | 0 | 0 | 0% |
| 5 | 10 | 20 | 50% |
| 10 | 20 | 40 | 75% |
| 15 | 30 | 60 | 87.5% |
| 20 | 40 | 80 | 93.8% |
| 25 | 50 | 100 | 96.9% |
| 30 | 60 | 120 | 98.5% |
But! Here comes another "but." The key to this model isn't absorption — it's off-gassing.
If we only considered absorption, the fast tissues would always win the race. But every dive involves an ascent, which means decompression. From the very beginning, fast tissues were defined as absorbing quickly but also off-gassing quickly — earn a lot, spend a lot, end up with nothing at the end of the month. Take the fast tissue with a 5-minute half-time: if it has been absorbing for 10 minutes and reached 75% saturation, then after just 5 minutes of off-gassing back at the original pressure, it drops back to 37.5%.
The blue line, which absorbs fastest, also off-gasses fastest — it quickly drops to the lowest level.
If you use a Garmin dive computer, press the lower-left button at the surface to enter the surface time screen and view your oxygen toxicity information; press the next page and you'll easily see a bar graph of tissue nitrogen loading. As your surface interval increases, you'll notice that the peak shifts from fast tissues toward slow tissues. This is because the fast tissues quickly drop below the threshold line, while the medium and slow tissues — though they absorbed less — off-gas very little and so continue to accumulate. This effect becomes even more pronounced with repetitive dives: the medium and slow tissues that seemed unremarkable at first end up becoming the controlling tissues.
Back to history: Haldane used his half-saturation time decompression model to validate his experimental results and found that most outcomes matched the model. He then used it to design a decompression dive plan involving a 50 m dive with a 30-minute bottom time, and put it to the test with actual dives — ultimately proving the model's validity. In 1908, Haldane published his decompression model. More than a century later, we still rely on the foundations of his half-time theory as the basis of modern diving decompression models. After over a hundred years of validation and refinement, humanity has opened a door toward a deeper truth in dive safety.
Editor: Jenny Tsai
Further reading:
