The Physics: Why Nitrogen Matters
Standard compressed air is roughly 79% nitrogen and 21% oxygen. At the surface, nitrogen is inert — your body absorbs and releases it without effect. But at depth, increased pressure forces more nitrogen into your blood and tissues. The deeper you go and the longer you stay, the more nitrogen saturates your body.
The danger comes during ascent. As pressure drops, dissolved nitrogen wants to come out of solution — just like carbon dioxide bubbling out of a soda when you open the cap. If you ascend slowly enough, the nitrogen exits through your lungs harmlessly. Ascend too fast, and the nitrogen forms bubbles in your tissues and bloodstream, causing decompression sickness (DCS). Symptoms range from joint pain and skin rash to paralysis and, in extreme cases, death.
A dive computer's entire purpose is to model this process and keep you within safe limits.
Tissue Compartments: The Body as a Model
Your body is not a single uniform substance — it is made up of tissues that absorb and release nitrogen at very different rates. Blood absorbs nitrogen quickly. Muscle absorbs it more slowly. Bone, cartilage, and fat absorb it very slowly. Decompression algorithms model this by dividing the body into theoretical "tissue compartments," each with its own absorption rate.
The Bühlmann ZHL-16C algorithm — the most commonly used model in dive computers today — uses 16 compartments with half-times ranging from about 4 minutes (fast tissues like blood) to 635 minutes (very slow tissues like bone marrow). A half-time is the time it takes a tissue to absorb or release 50% of the difference between its current nitrogen level and the ambient nitrogen pressure.
During your dive, the computer calculates the nitrogen level in all 16 compartments simultaneously. Your NDL is determined by whichever compartment is closest to its safe maximum — the "leading tissue." As you change depth, different compartments may take turns as the limiting factor.
No-Decompression Limits Explained
The NDL on your screen is the answer to a simple question: how many more minutes can you stay at your current depth before any tissue compartment reaches its maximum allowed nitrogen level?
Key facts about NDL:
NDL decreases with depth. At 18 meters (60 feet), a typical NDL on air might be 50+ minutes. At 30 meters (100 feet), it drops to around 15–20 minutes depending on the algorithm and conservatism setting.
NDL is dynamic, not static. Unlike a dive table that gives one fixed number, your computer recalculates continuously. If you ascend to a shallower depth, your NDL may increase because nitrogen absorption slows and your tissues begin off-gassing.
Running out of NDL is not an emergency — but it changes your ascent. If your NDL hits zero, you have entered mandatory decompression. Your computer will display required stop depths and times. You must complete these stops before surfacing. For recreational divers, the goal is to always surface with NDL remaining.
Comparative NDLs at 30 meters (air, first dive of the day):
Shearwater Peregrine 2 (Bühlmann, default GF): ~16 minutes
Garmin Descent Mk3i (Bühlmann-based): ~13 minutes
Suunto D5 (Fused RGBM): ~14 minutes
These numbers are approximate and vary with conservatism settings, water temperature, and other factors. The point is that different algorithms and configurations produce meaningfully different NDLs.
Bühlmann ZHL-16C in Depth
Developed by Swiss physician Albert A. Bühlmann at the University of Zurich, the ZHL-16C (Zurich Limits, 16 compartments, version C) was published in 1990 and has become the open standard for decompression modeling. It is used by Shearwater, Garmin (with proprietary enhancements), and the newer Mares Puck 4.
Gradient Factors (GF)
The raw Bühlmann algorithm defines a maximum nitrogen limit — called the M-value — for each tissue compartment at each depth. Gradient factors allow you to set your computer to use a percentage of that maximum rather than the full limit. They are expressed as two numbers: GF Low and GF High.
GF Low controls how much nitrogen the leading tissue is allowed to hold at the deepest point of the dive. A lower number means the computer will force you into deco stops earlier.
GF High controls how much nitrogen is allowed when you reach the surface. A lower number means you will spend more time on shallow stops.
Common settings:
Recreational default: 45/95 or 40/85 — moderately conservative, appropriate for warm-water no-deco diving.
Technical standard: 30/70 — requires deeper and longer stops, used by experienced decompression divers.
Aggressive: 50/80 or higher — sometimes used in specific technical scenarios but reduces safety margins.
If your computer supports gradient factors and you are a recreational diver, the factory default is usually fine. Do not change GF settings unless you understand their implications.
RGBM and Fused RGBM
The Reduced Gradient Bubble Model was developed by Dr. Bruce Wienke and is used in various forms by Suunto, Cressi, and older Mares computers. Unlike Bühlmann, which focuses on dissolved gas, RGBM also models free-phase microbubbles — tiny gas bubbles that can form in your bloodstream even within no-deco limits.
How RGBM Differs in Practice
More conservative by default. RGBM typically gives shorter NDLs than Bühlmann at the same depth, particularly on repetitive dives. Suunto's Fused RGBM variant adapts to your recent dive history, automatically adding conservatism after consecutive days of diving or after particularly deep or long profiles.
Penalizes violations more heavily. If you ascend too fast or miss a safety stop, an RGBM computer may significantly reduce your NDL on subsequent dives as a safety penalty. Bühlmann computers do not typically apply behavioral penalties.
Not user-adjustable in the same way. RGBM computers generally offer conservatism presets (low/medium/high) rather than specific gradient factors. You choose a level; the algorithm handles the rest.
Which Algorithm Is Safer?
Both algorithms are safe when used correctly. RGBM provides wider safety margins by default, which some divers appreciate and others find frustrating. Bühlmann gives the diver more control and transparency, which experienced divers prefer. No large-scale clinical study has proven one algorithm results in fewer DCS incidents than the other. The most important safety factor is not the algorithm — it is the diver following the computer's recommendations.
Ascent Rate and Safety Stops
A dive computer does not just track how much nitrogen you absorb — it also monitors how you come back up.
Ascent Rate
Most computers recommend 9–10 meters per minute (about 30 feet per minute), though some technical computers allow slower configurable rates. If you exceed the recommended rate, your computer will alarm — usually with a combination of audible beeps, screen flashes, and a visual ascent-rate bar. Ascending too fast is one of the most common triggers for DCS.
Safety Stops
Nearly all computers recommend a three-minute stop at 5 meters (15 feet) at the end of every dive. This is not a mandatory decompression stop — it is an additional precaution. However, if you entered deco during the dive, your computer will display mandatory stops that you must complete before surfacing.
Deep Stops (Pyle Stops)
Some older guidance recommended brief stops during deep ascents at half your maximum depth. Most current research and the major training agencies no longer recommend deep stops for recreational diving, and many computers have removed or de-emphasized them. Follow your computer's displayed stop recommendations.
Multi-Dive and Repetitive Diving
After your first dive, your body is still off-gassing residual nitrogen. When you dive again, you start with a higher nitrogen load. Your computer tracks this "residual nitrogen time" during your surface interval and adjusts your next dive's NDLs accordingly.
Do not turn off your computer between dives. It must stay on to track your surface interval and residual nitrogen. Most computers automatically enter surface mode and display desaturation time.
Do not switch computers between dives. If you use computer A for dive one and computer B for dive two, computer B has no record of your first dive's nitrogen loading and will overestimate your safe limits.
Fly-after-diving guidelines. Most computers display a "no-fly" timer — typically 12–24 hours after your last dive. Cabin altitude at 8,000 feet creates a pressure difference that can trigger DCS if you have residual nitrogen. Follow your computer's displayed no-fly time.
Algorithms are tools, not religions. Bühlmann and RGBM both keep divers safe. What matters is using one computer consistently, following its recommendations, and never exceeding the limits of your training.