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CHAPTER 4 — UNDERSTANDING GRADIENT FACTORS

Martin Gomez   Aug 11, 2026

CHAPTER 4 — UNDERSTANDING GRADIENT FACTORS

Understanding Your Dive Computer — Part 4

 

If your dive computer uses Bühlmann ZHL-16C, you may have seen settings such as:

GF 30/70

GF 40/85

GF 45/95

But what do these numbers actually mean?

GF stands for Gradient Factor.

Gradient Factors are a way of applying additional conservatism to the Bühlmann model by controlling how closely the calculated tissue inert-gas pressure is allowed to approach the model's M-value limits during decompression.

A Gradient Factor is expressed as a percentage.

1. FIRST, REMEMBER M-VALUES

In Chapter 3, we learned that Bühlmann uses mathematical limits called:

M-VALUES

An M-value represents, in simplified terms, the maximum inert-gas pressure that the model permits in a theoretical tissue compartment at a particular ambient pressure.

Gradient Factors allow us to operate at a selected fraction of the gradient between ambient pressure and that Bühlmann limit.

Think of it conceptually as:

Ambient Pressure → GF Limit → Bühlmann M-Value

A lower GF generally means staying farther away from the Bühlmann limit.

2. WHY ARE THERE TWO NUMBERS?

Gradient Factors are normally written as two numbers:

GF LOW / GF HIGH

For example:

40 / 85

These numbers perform different jobs during a decompression ascent.

3. WHAT IS GF LOW?

GF LOW primarily influences where decompression begins.

For a decompression dive, GF Low helps determine the pressure/depth at which the first decompression ceiling appears.

In general:

LOWER GF LOW

→ First stop may begin deeper.

HIGHER GF LOW

→ First stop may begin shallower.

For example:

GF 30/85

will generally begin decompression deeper than:

GF 50/85

for the same modeled profile.

But remember: GF Low should not simply be interpreted as:

“Lower number = safer.”

Decompression physiology is more complicated than that.

4. WHAT IS GF HIGH?

GF HIGH primarily influences how closely the model approaches the Bühlmann M-value near the surface.

This makes GF High especially important for the final portion of the ascent and overall decompression obligation.

In general:

LOWER GF HIGH

→ More conservative relative to the Bühlmann limit.

HIGHER GF HIGH

→ Closer to the Bühlmann limit.

For example:

GF 40/70

is more conservative relative to the Bühlmann M-value near the surface than:

GF 40/95

for the same modeled exposure.

5. LET'S COMPARE THREE SETTINGS

Consider:

 

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