🌫️ The Grey Zone: Why M-Values Aren't Enough
Silent Bubbles
Doppler ultrasound studies have revealed something surprising: bubbles form in divers after most dives, even when following conservative profiles with no DCS symptoms.
These "silent bubbles" (venous gas emboli) are sub-clinical — they don't cause symptoms, but they prove that M-values don't represent a bubble-free boundary. They represent a tolerable level of bubble formation.
This means the M-value line on our pressure-pressure diagram isn't a sharp "safe/unsafe" boundary. It's more like a probability gradient — the closer you get, the more bubbles form, and the higher your risk.
Individual Variability
DCS risk varies enormously between individuals and even between dives for the same person. Factors that increase risk include:
- Dehydration
- Poor physical fitness
- Age
- Body fat percentage
- Cold exposure
- Heavy workload
- Patent Foramen Ovale (PFO)
- Alcohol consumption
The term "undeserved hit" describes cases where divers followed all the rules but still got DCS. This happens because we're dealing with probabilistic biology, not precise physics.
🐟 Richard Pyle's Discovery
The Fish Collector's Observation
In the 1980s, Richard Pyle was a marine biologist collecting fish specimens from the "twilight zone" — depths between 50-70 meters. He noticed something curious about his post-dive fatigue.
The pattern: On dives where he caught fish, he had to stop at intermediate depths to vent their swim bladders (preventing them from bursting during ascent). After these dives, he felt great. On dives with no fish — and therefore no intermediate stops — he felt fatigued.
Pyle had accidentally discovered that brief stops at depths deeper than required by his decompression schedule made a noticeable difference in how he felt.
Pyle Stops
Pyle formalized his observation into a method:
- Calculate your conventional decompression schedule
- First deep stop = halfway between max depth and first required deco stop
- Stay 2-3 minutes
- If the gap to the next stop is still >9m (30ft), add another halfway stop
Example: 60m dive, first deco stop at 15m
Interestingly, Pyle later discovered that his empirical pattern closely matched the Varying Permeability Model (VPM) — a bubble-based decompression algorithm that independently predicted similar deep stop profiles.
Bridging Pyle's Discovery to Bühlmann
So Pyle showed that deep stops help, and VPM (a bubble model) explains why. But what if you want to keep using Bühlmann's dissolved-gas model — the algorithm in most dive computers — while still getting those beneficial deep stops?
That's exactly what Gradient Factors provide. They're a modification to Bühlmann that lets you dial in more conservative profiles, including deeper first stops, without switching to a completely different algorithm.
📐 Gradient Factors: The Tool
What Are Gradient Factors?
Gradient Factors (GF) are a way to add conservatism to Bühlmann's algorithm by reducing the allowed supersaturation. Instead of allowing tissues to reach 100% of the M-value, you set a lower percentage.
A gradient factor of GF 80% means you only allow the tissue to reach 80% of the way from ambient pressure to the M-value.
GF Low and GF High
Dive computers and planning software use two gradient factors:
The algorithm interpolates between these values during the ascent:
- At your deepest required stop → GF Low applies
- At the surface → GF High applies
- In between → linear interpolation
Lower GF Low = deeper first stop (similar to Pyle stops)
Lower GF High = more time at shallow stops
📈 Visual: GF 100/100 vs GF 50/80
Compare the same dive with different gradient factors. Notice how lower GF creates a tighter "allowed zone" on the M-value chart and affects the dive profile.
Example 1: GF 100/100 (No conservatism)
Raw Bühlmann limits — tissue can reach 100% of M-value.
Example 2: GF 50/80 (Conservative)
Tissue only allowed to 50-80% of M-value — notice the GF line below the M-value.
⚠️ The Deep Stops Controversy
The NEDU Study That Changed Everything
For years, deep stops were considered beneficial based on Pyle's observations and bubble model theory. Then came rigorous testing.
The U.S. Navy Experimental Diving Unit (NEDU) conducted a large comparative study: same dive (170 fsw/30 min), same total decompression time, but different profiles:
Deep stops resulted in 3.6× more DCS for the same dive!
Why Deep Stops Can Backfire
The problem with deep stops is subtle but important:
At deep stop depths, slow tissues are still on-gassing. While you're trying to let fast tissues off-gas, you're loading up the slow tissues that will control your later decompression.
Deep stops reduce the pressure gradient for off-gassing. The deeper you stay, the smaller the difference between tissue pressure and ambient — and that gradient is what drives gas elimination.
The Nuanced Reality
This doesn't mean all deep stops are bad:
- Very brief stops (Pyle's original 2-3 minutes) may still be beneficial
- The NEDU study used extended deep stops with significant deco time shifted deep
- Extremely aggressive deep stop profiles (GF 30/85) are now discouraged
- Moderate conservatism (GF 70/85) adds safety without excessive deep stops
🎯 Practical Guidance
Common GF Settings
| Setting | Conservatism | Typical Use |
|---|---|---|
| GF 100/100 | None | Raw Bühlmann — NOT recommended |
| GF 85/85 | Mild | Light recreational conservatism |
| GF 70/85 | Moderate | Popular balanced setting |
| GF 55/70 | Conservative | Technical diving, personal risk factors |
| GF 30/85 | Aggressive deep | Now discouraged (excessive deep stops) |
The Ratio Rule
Decompression researcher Dr. David Doolette suggests a practical guideline:
GF Low should be approximately 83% of GF High
This ratio matches the average Bühlmann 'b' parameter and avoids
over-emphasizing deep stops.
Examples following the ~83% rule:
The Bottom Line
- ✅ Some conservatism is good — GF 100/100 is too aggressive
- ✅ GF 70/85 is a sensible starting point for most divers
- ⚠️ Very low GF Low (like 30%) can be counterproductive
- ⚠️ More conservatism isn't always better — it's about balance
- 🎯 Adjust based on personal factors, dive conditions, and experience
🫧 Bubble Models (Advanced)
Beyond Dissolved Gas
Bühlmann's algorithm only tracks dissolved gas. It doesn't model bubble formation or growth. Alternative "bubble models" attempt to address this.
VPM (Varying Permeability Model)
VPM assumes that microscopic bubble nuclei always exist in tissues. During decompression, nuclei larger than a "critical radius" (determined by max depth) will grow.
VPM aims to minimize total bubble volume by keeping ambient pressure high enough to limit bubble growth — naturally producing deeper stops.
RGBM (Reduced Gradient Bubble Model)
RGBM models how bubbles grow by acquiring gas from surrounding saturated tissue. Like VPM, it tends to produce deeper stops than Bühlmann.
The Reality Check
Despite different theoretical foundations, all these algorithms produce surprisingly similar schedules — because they all must match the same empirical data from real dives.
"It is far too easy to look at the complexity of any calculation, and the number of decimal places that a modern computer will display, and get the impression that the result is extremely accurate when in fact it is not."