Chapter 3 showed that at depth your body takes up inert gas: the nitrogen and helium in your breathing gas that your body does not use. Chapter 5 showed what happens if that gas comes out too fast. It forms bubbles, and the bubbles cause decompression sickness (DCS).
This chapter opens the part of the book on decompression models. It gives the whole picture first: what a model does, the ideas all models share, and the main kinds of model. The chapters after it explain each idea in detail.
The problem a model solves
Taking up inert gas is called on-gassing, and releasing it is called off-gassing. On the bottom you on-gas. On the way up the pressure around you falls, and the gas starts to leave. It leaves through your blood and your lungs, and that takes time.
If you come up faster than the gas can leave, your tissues hold more gas than the pressure around them can keep dissolved. They are supersaturated. A little supersaturation is normal on every ascent, and it is what drives the gas out. Too much, and bubbles grow. Think of it like opening a fizzy drink: release the pressure slowly and the gas leaves quietly; open it fast and it foams.
So before every dive, a diver needs to know how fast they can come up from it.
What a decompression model does
A decompression model, also called a decompression algorithm, is a set of rules that works this out. It does 3 things.
- It estimates how much inert gas is in your body at every moment of the dive, from your depth, your time and your breathing gas. Nobody can measure this during a dive, so the model calculates it.
- It sets a limit on how much extra gas your body may hold as the pressure drops. Past that limit, bubbles and DCS become more likely.
- It plans the ascent so you stay within that limit. It sets your ascent rate (how fast you come up), any decompression stops, and how long you can stay down before stops are needed.
Out of those 3 steps come the terms you will see on every dive plan and dive computer:
- The decompression ceiling is the shallowest depth you can safely go to at that moment. Above it, the gas in your body would pass the model’s limit. The ceiling deepens while you on-gas and rises as you off-gas.
- A decompression stop is a planned pause at a set depth on the way up, to let gas leave before you go shallower.
- Your decompression obligation is the stops you must make before you can surface. Once you have one, you cannot go straight up.
- The no-decompression limit (NDL) is the longest time you can stay at a depth and still ascend straight to the surface, at the normal ascent rate, with no stops.
The figure shows all 4 on a real plan. The dive is 30 m for 30 minutes on air (21 % oxygen, 79 % nitrogen).
Planned with DiveLogicPlan this dive
On the bottom, the ceiling deepens
On the way down and at the bottom you on-gas. For the first few minutes there is no ceiling: you could go straight back to the surface.
As the gas builds up, a ceiling appears and gets deeper. The shaded area above it is where you may not go. By the end of 30 minutes at 30 m on air, the ceiling is about 13 m. You now have a decompression obligation.
A direct ascent breaks the ceiling
The red line is what a diver without a plan might do: swim straight up at the normal ascent rate of 9 m/min.
That diver passes the ceiling at about 12 m, less than 2 minutes after leaving the bottom. From there to the surface, the gas in their body is past the model’s limit. This is the kind of ascent that leads to DCS.
The planned ascent stays below the ceiling
The planned ascent goes up at 9 m/min to a first stop at 15 m. Then it makes a decompression stop every 3 m. Each stop lasts until the ceiling has risen above the next stop, so you never go shallower than the ceiling.
The stops get longer as you go shallower, and the last one at 3 m is the longest. The whole dive takes about 71 minutes, of which 30 are on the bottom.
A dive within the no-decompression limit
A shorter, shallower dive may never get a ceiling below the surface. This dive is 15 m for 15 minutes on air. With the same safety margin as the dive above, the NDL at 15 m is 26 minutes.
15 minutes is well within it. The ceiling stays at the surface, so the dive needs no stops. Most recreational dives are planned like this, and most divers add an optional safety stop at 3 to 5 m anyway.
The ideas every model shares
Every model since John Scott Haldane’s in 1908 is built from the same few ideas. You will meet each of them again in the chapters that follow.
Tissue compartments. Your body is not one block that fills with gas at one speed. Blood and brain take up gas in minutes; fat and joints take hours. So a model divides the body into several compartments. A compartment is not a real organ. It stands for all the parts of the body that take up and release gas at about the same speed. Think of it like a row of buckets under one tap, each with a different-sized opening: some fill in minutes, some take hours.
Half-times. Each compartment has a half-time: the time it takes to close half of the gap between the gas it holds and the gas you breathe. A fast compartment has a half-time of a few minutes. A slow one has a half-time of several hours. On a short, deep dive the fast compartments fill up. On a long, shallow dive the slow ones do.
A limit for each compartment. Each compartment is allowed to hold a certain amount of extra gas at each depth. This limit is called the M-value (M for maximum). Fast compartments can hold more extra gas than slow ones. Chapter 12 explains M-values.
The leading compartment. At any moment, one compartment is closest to its limit. It is the leading compartment, and it sets the ceiling. Early in the ascent a fast compartment usually leads. Later, a slower one takes over. This is why the stops get longer near the surface.
Chapter 10, next, shows how the compartments load and unload gas during a dive.
The main kinds of model
Models differ in what they assume happens to the gas in your body. There are 2 main kinds.
Dissolved-gas models track the inert gas dissolved in each compartment, and limit how far it may exceed the pressure around you. They treat the gas as staying dissolved while you stay within the limit.
- Chapter 11: John Scott Haldane’s 1908 model, the first with compartments and staged stops.
- Chapter 12: Robert Workman’s M-values, a separate limit for each compartment at each depth.
- Chapter 13: Albert Bühlmann’s ZHL-16, with 16 compartments. Most dive computers use a version of it, and it is DiveLogic’s default model (ZHL-16C).
- Chapter 14: Edward Thalmann’s model for the US Navy, in which gas leaves the body more slowly than it came in.
Bubble models assume that tiny bubbles are always present, and try to keep them from growing. They tend to put the first stops deeper. Chapter 15 covers them, and what a large US Navy trial in 2011 found about deep stops.
Most divers do not dive a model at its raw limit. Gradient factors set how close to the M-value a plan is allowed to go, as a percentage. Think of it like driving below the speed limit: the limit is still there, but you keep a margin. The plans in this chapter use gradient factors of 30/70. Chapter 16 explains them.
Models also check one risk that has nothing to do with the ascent. Switching from one breathing gas to another at depth can cause bubbles on its own. This is isobaric counterdiffusion, covered in chapter 17.
What a model cannot do
A model is an estimate for an average diver. It is not a measurement of your body. The compartments, half-times and limits were fitted to the results of many test dives, and they describe groups of divers, not one person.
- DCS can happen within the limits. Diving inside a model’s limits makes DCS unlikely, not impossible. Some divers are bent on dives that followed the plan exactly.
- Your body changes the risk. Dehydration, hard exercise, being cold during decompression, age and a patent foramen ovale (PFO, a small opening between the upper chambers of the heart) can all raise it. The model does not know about any of them. Chapter 5 lists the risk factors.
- A model is only as good as the dive you give it. If you go deeper, stay longer or come up faster than the plan, the plan no longer applies.
So divers follow their training, keep a margin with conservative settings, and plan and dive within what they have been trained to do. Chapter 23 shows how DiveLogic puts a model to work in a full dive plan.
Try it
Choose a depth and a bottom time on air. DiveLogic finds the NDL at that depth and plans the dive with gradient factors 30/70. Stay within the NDL and the plan has no stops. Go past it and stops appear.
Near the limit, a shallow ceiling of 1 or 2 m can appear. The last 3 m of every ascent is planned at 1 m/min, slowly enough to stay below it, so no stop is needed.
- Air
- Ceiling
Planned with DiveLogicOpen the NDL calculator
What to remember
- A decompression model estimates the inert gas in your body, sets a limit on how much extra gas it may hold, and plans an ascent that stays within that limit.
- The ceiling is the shallowest depth you may go to at that moment. Decompression stops hold you below it until it rises. Within the NDL you need no stops.
- Every model divides the body into compartments with fast and slow half-times, each with its own limit, the M-value. The leading compartment sets the ceiling.
- Models differ in what they assume: dissolved-gas models limit dissolved gas, bubble models limit bubble growth, and gradient factors add a margin below the limit.
- A model is an estimate for an average diver. DCS can still happen within its limits, so keep a margin and dive within your training.
Models in the planner
Every DiveLogic plan shows the ceiling, the stops and the runtime, as in the figures above. Plan the 30 m dive from this chapter, then change the bottom time or the gradient factors and watch the stops change.
Sources
- Vann R. D., Butler F. K., Mitchell S. J., Moon R. E. (2011). Decompression illness. The Lancet 377(9760): 153-164.
- Boycott A. E., Damant G. C. C., Haldane J. S. (1908). The prevention of compressed-air illness. Journal of Hygiene 8(3): 342-443.
- Workman R. D. (1965). Calculation of decompression schedules for nitrogen-oxygen and helium-oxygen dives. Research Report 6-65, US Navy Experimental Diving Unit.
- Bühlmann A. A. (1984). Decompression-Decompression Sickness. Springer, Berlin.
- Baker E. C. (1998). Understanding M-values. Immersed 3(3).
- Doolette D. J., Gerth W. A., Gault K. A. (2011). Redistribution of decompression stop time from shallow to deep stops increases incidence of decompression sickness in air decompression dives. NEDU TR 11-06. Navy Experimental Diving Unit, Panama City, FL.
- Naval Sea Systems Command (2016). U.S. Navy Diving Manual, Revision 7. SS521-AG-PRO-010.