Understanding Gradient Factors Without a PhD
Understanding Gradient Factors Without a PhD
If you’ve spent any time around technical divers, you’ve heard arguments about gradient factors. GF 30/85 vs 45/85. Why low GF Lo for deep dives. Why some instructors run 20/80 on everything.
Most recreational divers ignore gradient factors entirely and dive on whatever their computer defaults to. That’s probably fine. But if you want to understand the tool you’re trusting your life to, here’s what’s actually happening.
Start Here: What Bühlmann Says
The Bühlmann ZHL-16C algorithm models 16 tissue compartments — think of them as 16 different theoretical tissues in your body, each with a different speed for absorbing and releasing nitrogen.
For each compartment, Bühlmann defines an M-value: the maximum tolerated dissolved gas pressure before bubble formation becomes a real risk. This is the hard ceiling. Go above it and you’re in trouble.
A dive computer running pure Bühlmann (GF 100/100) would let you ascend right up to those M-value ceilings at every stop. Technically within the model. Practically, not a lot of margin.
Gradient factors are a way of saying: don’t go all the way to the limit. Stay at X% of the maximum.
The Two Numbers
GF Low and GF High control different parts of your ascent.
GF Low sets the ceiling for your deepest deco stop — typically the stop where you switch from a travel gas to your first deco gas, or just where deco begins. A GF Lo of 30 means: only go up to 30% of Bühlmann’s M-value ceiling at the deepest stop. Lower = deeper first stop = longer, more conservative decompression.
GF High sets the ceiling at the surface — how close to the M-value limit you ascend at your shallowest stop (usually 6m/20ft or 3m/10ft). Most people set this between 80–90. Going higher (say, GF Hi 100) means you’re surfacing right at the theoretical limit. Going lower adds safety margin but increases your 6m stop time.
Between these two numbers, the computer interpolates linearly — drawing a straight line between GF Lo at depth and GF Hi at the surface, calculating a ceiling for each stop along the way.
What This Looks Like in Practice
Say you’re diving to 60m on trimix, running GF 30/85.
At your deepest stop (say 33m), your computer calculates 30% of the maximum tolerated pressure for the dominant tissue at that point. You stop there — deeper than a less conservative setting would require — and your tissues off-gas from a lower supersaturation state.
As you ascend through the stops, the allowed ceiling progressively increases. By the time you’re at 6m, you’re allowed up to 85% of the M-value. Your stop there may be longer, but you’re arriving at it with tissues that are already in better shape.
The result: more total decompression time, but a smoother ascent profile with less stress on fast tissues at depth.
Why GF Lo Matters More on Deep Dives
Here’s the part that matters for technical diving: the deeper you go, the more important GF Lo becomes.
Fast tissues (those with short half-times) load up quickly on deep dives. If you ascend too fast from depth, those tissues hit high supersaturation before the slow ones have even started loading. A low GF Lo forces a deeper first stop — giving fast tissues time to off-gas before you start ascending through the water column.
This is why the standard in technical diving tends to be GF Lo settings in the 20–45 range for deep dives, not the 80–100 you might see on recreational defaults.
Common Settings and When to Use Them
| Setting | Use Case |
|---|---|
| GF 100/100 | Don’t. Pure Bühlmann, no conservatism. |
| GF 80/100 | Shearwater “Norm” default. Fine for recreational no-deco diving. |
| GF 45/85 | Entry-level tech. Good starting point. |
| GF 35/85 | Intermediate tech. Deeper first stop, solid margin. |
| GF 30/85 | Common tech standard. Most courses use something close to this. |
| GF 20/80 | Conservative tech. Deep trimix, expedition diving, repetitive multi-day. |
These are starting points, not gospel. Your instructor, your training agency, and your own experience all factor into what’s appropriate for a given dive.
What GFs Don’t Tell You
Gradient factors are a conservatism modifier, not a complete safety system. They don’t account for:
- Exercise at depth — increases gas loading faster than the model predicts
- Cold — peripheral vasoconstriction slows off-gassing in extremities
- Dehydration — reduces blood volume, affects gas transport
- Patent foramen ovale (PFO) — a hole in the heart present in ~25% of people that allows venous bubbles to bypass the lungs; a real risk factor for DCS regardless of GF settings
- Repetitive diving over multiple days — slow tissue loading accumulates in ways single-dive models underestimate
GFs are a tool. They work best alongside good technique, conservative ascent rates, and an honest assessment of your physical condition on any given dive.
The Honest Take
There’s no single right answer for GFs. Most technical diving instructors will give you a starting point and have you adjust from there based on experience, dive conditions, and how you feel post-dive.
What matters is that you understand what the numbers mean — not just that you can type them into a computer. A diver who understands GFs can make an informed decision when conditions change. A diver who doesn’t is just hoping their defaults are appropriate.
Abyssi’s dive planner lets you set gradient factors and model decompression using Bühlmann ZHL-16C before you’re in the water.