*Published by DiveRobotix LLC · DiveCommand · DepthPlanner.com*
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Your Shearwater Perdix is running a piece of mathematics that was born in a Swiss physiology lab in the 1950s. The same equations that Albert Bühlmann refined over four decades of human pressure experiments now execute thousands of times per second on a chip strapped to your wrist — and on DiveCommand before you ever enter the water.
Here’s how we got here, why 16 compartments matter, and why your dive computer’s nitrogen tracking is fundamentally different from the pressure group letters you learned in your open water course.
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The Problem Haldane Left Behind
The story starts earlier than Bühlmann. In 1908, Scottish physiologist John Scott Haldane published the first mathematical model of nitrogen uptake and elimination in divers. His key insight: nitrogen doesn’t dissolve uniformly across the body. Different tissues — blood, fat, muscle, nerve — absorb and release gas at different rates. Haldane described this with a concept called **half-time**: the time it takes a tissue to reach halfway between its current nitrogen loading and the surrounding pressure.
Haldane used **5 tissue compartments** with half-times ranging from 5 to 75 minutes. His model generated the first staged decompression tables — the direct ancestors of the U.S. Navy tables still in use today.
The problem: Haldane’s model was conservative in the wrong direction for some divers and not conservative enough in others. It described dissolved gas well but didn’t fully account for the mechanics of bubble formation. Decompression sickness rates remained frustratingly unpredictable.
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Enter Albert Bühlmann
Albert A. Bühlmann was a Swiss physician and professor at the University Hospital Zurich. Between 1959 and his death in 1994, he conducted what remains the most comprehensive experimental program in the history of decompression research.
His subjects included commercial divers, military divers, and research volunteers. He ran pressure experiments in hyperbaric chambers at altitudes ranging from sea level to high-altitude lakes in the Swiss Alps — which is why Bühlmann’s tables have always included altitude correction as a native feature rather than an afterthought.
His first published model appeared in 1983: **ZH-L12** (Zurich, Limits, 12 compartments). By 1990, he had expanded and refined it to **ZHL-16** — 16 tissue compartments with half-times from 4 minutes to 635 minutes. The “L” stands for *Limits* — specifically, the tolerated supersaturation limits for each compartment.
His 1990 book *Decompression–Decompression Sickness* remains a primary reference. The algorithm it describes is what runs inside your dive computer today.
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What the 16 Compartments Actually Represent
Bühlmann’s 16 compartments don’t map perfectly to 16 specific anatomical structures. They’re mathematical abstractions — **theoretical tissue groups** characterized by how fast they absorb and release nitrogen under pressure changes.
The compartments are defined by their nitrogen half-times:
| Compartment | Half-Time (min) | What It Approximates |
|————-|—————–|———————-|
| 1 | 4.0 | Blood, highly perfused tissue |
| 2 | 8.0 | Fast muscle, organ tissue |
| 3 | 12.5 | Brain, spinal cord |
| 4 | 18.5 | Muscle (moderate) |
| 5 | 27.0 | Fat-lean mixed tissue |
| 6 | 38.3 | Moderate fat tissue |
| 7–10 | 54–146 | Fat, connective tissue |
| 11–14 | 187–390 | Dense connective tissue, bone |
| 15–16 | 498–635 | Avascular tissue, tendons |
Fast compartments (short half-times) dominate at depth. They load nitrogen quickly during a dive and clear it quickly on the surface. **Slow compartments** load gradually during a dive but take hours — sometimes days — to fully offgas. They’re the reason the 18-hour flying-after-diving rule exists, and why day 3 of a liveaboard is statistically the most dangerous for DCS.
Each compartment has its own tolerated supersaturation limit** — mathematically defined by the Bühlmann coefficients *a* and *b*. These coefficients determine how much nitrogen a compartment can hold above ambient pressure before bubble formation becomes clinically significant. The algorithm continuously checks all 16 compartments against these limits. Your NDL is the time remaining before any single compartment exceeds its limit.
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## Why Pressure Groups Are a Simplification — And Why That Mattered
The U.S. Navy tables and PADI’s RDP don’t use continuous tissue tracking. Instead, they use **pressure groups — letters (A through Z) that represent a diver’s approximate nitrogen loading after a dive. Surface interval tables, then map those letters to new groups that determine the adjusted NDLs for subsequent dives.
This was brilliant engineering for 1956. Pressure groups are:
– Calculable without a computer
– Simple enough to use on a slate underwater
– Conservative enough to protect a broad population of divers
But they’re also a **lossy compression** of what’s actually happening in your body. A pressure group letter collapses 16 independent tissue compartments into a single symbol. Two divers can have the same pressure group after very different dives and carry meaningfully different nitrogen loads — particularly in their slow compartments.
The 30-minute surface interval that looks adequate on a Navy table may leave significant residual nitrogen in your 54-minute and 77-minute compartments. Those compartments don’t appear in any pressure group calculation. They do appear in your Shearwater’s display.
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Gradient Factors: The Modern Conservative Layer
Bühlmann’s original model calculates NDLs at 100% of the theoretical tolerated supersaturation limit. In practice, most dive medicine professionals consider this too aggressive — it leaves no margin for individual variation, exercise, cold, dehydration, or the simple fact that laboratory subjects are not representative of every diver in every condition.
In the 1990s, Erik Baker formalized the concept of **gradient factors** as a way to add conservatism to Bühlmann without abandoning the algorithm. Two numbers — GF Low and GF High — scale the allowable supersaturation as a percentage of the theoretical limit.
**GF 40/85** — the factory default on Shearwater computers and the setting DiveCommand uses — means:
– At depth: only 40% of the theoretical ceiling is allowed (more conservative)
– At the surface: 85% of the theoretical ceiling is allowed
This creates a **conservative ascent profile** while allowing more bottom time than extremely low GF settings. It’s been validated across hundreds of thousands of dives and is the most widely deployed setting in the recreational and technical diving community.
When DiveCommand calculates your NDL at GF 40/85, it’s running the same calculation your Shearwater runs. Not an approximation. Not a table lookup. The same iterative tissue simulation, the same 16 compartments, the same coefficients.
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What 16-Compartment Tracking Means for Multi-Day Diving
This is where the difference between pressure groups and compartment tracking becomes most significant.
After a 60ft/35min dive, your fast compartments (4–18 min half-times) are heavily loaded. After 90 minutes on the surface, they’ve largely cleared. Your pressure group may look benign. But your slow compartments — particularly the 54, 77, and 109-minute half-time tissues — are still carrying meaningful nitrogen load that won’t appear in any Navy table calculation.
On day 2 of a liveaboard, those slow compartments start already elevated. By day 3, if you’re diving 3–4 times per day, your 146-minute and 187-minute compartments may be running at 60–70% of their tolerated limit before you enter the water for your first morning dive.
A Navy table with pressure groups doesn’t track this. Your Shearwater does — and now, DiveCommand does too.
The multi-day nitrogen tracking feature in DiveCommand lets you log previous dive days with their actual depths and times. The algorithm runs overnight, offgassing calculations for all 16 compartments, accounting for the actual surface interval between dive days. When you open the planner on day 3, your starting tissue state reflects what actually happened to your body over the previous 48 hours — not a reset to zero.
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The Number on Your Computer Screen
When your Shearwater shows a percentage in the nitrogen status bar, it’s showing you the **leading compartment saturation** — how loaded the most saturated of your 16 compartments is, expressed as a percentage of its GF-adjusted ceiling.
DiveCommand shows the same number, calculated the same way. Not a pressure group. Not a letter. A direct, honest representation of where your tissues are relative to their limit.
At 97%, you have very little margin. At 45%, you have a comfortable buffer. The number means the same thing in the water as it does on the surface — because it’s the same math.
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Bühlmann’s Legacy
Albert Bühlmann died in 1994. His algorithm has since been implemented in virtually every recreational and technical dive computer sold globally. Shearwater, Garmin, Suunto, SCUBAPRO, Mares, Oceanic — all run ZHL-16C variants, most at gradient factor settings in the 40/85 range.
The U.S. Navy continues to use its own tables for operational diving, but even the Navy’s most recent Rev 7 decompression tables incorporate research influenced by Bühlmann’s work.
For recreational divers, the practical consequence is this: the safest planning tool available to you is one that uses the same algorithm as your dive computer, run at the same settings, before you enter the water. That’s what DiveCommand was built to do.
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Try It
DiveCommand is free to try at [depthplanner.com/planner](https://depthplanner.com/planner). It runs the full Bühlmann ZHL-16C algorithm at GF 40/85 — the same as your Shearwater, Garmin, or Suunto factory default — with full 16-compartment visualization, multi-day nitrogen tracking, and a surface-interval timer that shows your tissue-state clearing in real time.
No account required. Works 100% offline after installation on any device.
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*DiveRobotix LLC · Gardnerville / Lake Tahoe, NV · Built by Master Scuba Instructors, Commercial Divers & U.S. Navy Divers*
*Educational reference only. Not a substitute for certified dive training or a calibrated dive computer. Always dive within your training limits. Never dive alone.*