🎚️ Gradient Factors

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.

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.

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.

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.

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 formalized his observation into a method:

1. Calculate your conventional decompression schedule
2. First deep stop = halfway between max depth and first required deco stop
3. Stay 2-3 minutes
4. If the gap to the next stop is still >9m (30ft), add another halfway stop

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.

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.

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.

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

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!

The problem with deep stops is subtle but important:

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

Reference table courtesy of Jirka Hovorka.

CMAS published a 2025 fact sheet summarising current evidence on gradient factors. Its advice differs from some of the values in the reference table above — worth knowing both perspectives before you pick your settings.

1. For air/nitrox, use symmetric GFs

CMAS recommends GF Low = GF High on nitrogen dives — typically 90/90, dropping to 85/85 or 80/80 when risk factors apply (age, fatigue, cold, exertion). Asymmetric GFs force deep stops, and multiple independent studies found this increases DCS incidence on air/nitrox dives:

• French Navy (2005) — deep stops raised DCS risk on air deco dives
• U.S. Navy NEDU (2011) — "redistribution of stop time from shallow to deep stops increases incidence of DCS in air decompression dives"
• Belgian Military (2023) — default 30/70 preset was unsafe for 60 m air dives

2. For helium (trimix/heliox), asymmetric GFs are still justified

On helium mixes, forcing some deeper stops does help (inner-ear DCS, fast helium tissues). CMAS suggests GF Low 30–50, GF High 70–80 (e.g. 30/70, 50/80). This aligns with the "Deep dive" row of the table above.

3. On a no-deco dive, GF Low doesn't matter

Non-obvious but important: GF Low only kicks in at the first deco stop depth. If your dive has no mandatory stops, setting 45/95 behaves exactly like 95/95 — because there is no "first stop" for GF Low to anchor. Don't expect conservative GF Low to help you on a pure recreational profile.

4. Check your computer's default preset

Many dive computers ship with GF 30/70 out of the box — a preset chosen for tech/trimix diving. CMAS argues the same preset is not right for air/nitrox.

📄 Read the CMAS fact sheet →

• 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