No-decompression limit and gradient factors: GF low or GF high?

Anyone who dives using the Bühlmann model—which is practically all technically oriented divers using most modern computers—cannot avoid gradient factors (GF). But the question keeps coming up: Should no-decompression limit be calculated using GF low or GF high?

This is where GF high plays a role. Foto: Karin Aggeler

What does no-decompression limit actually mean?

No-decompression limit means that during a dive, you can ascend directly from depth to the surface without exceeding the permissible supersaturation in any tissue compartment.

Or more precisely: no-decompression limit occurs when the supersaturation tolerance is not exceeded in any tissue during a direct ascent to the surface.

What is supersaturation?

Supersaturation is nothing more than an excess pressure of inert gas in the tissue compared to the ambient pressure. During the dive, nitrogen (or helium) dissolves in the tissue. During the ascent, the ambient pressure drops faster than the inert gas can be eliminated. This creates a pressure gradient: the inert gas pressure in the tissue is higher than the ambient pressure.

A certain degree of supersaturation is tolerated (supersaturation tolerance). However, if it becomes too high, the risk of gas bubble formation increases – the basis of decompression sickness.

Historical background

As early as the end of the 19th century, Paul Bert, among others, showed that excessive pressure relief can lead to bubble formation. John Scott Haldane later recognized that a certain amount of pressure reduction is tolerated without symptoms.

The key finding: limited supersaturation is physiologically tolerable. Modern decompression models are based on this.

The Bühlmann model

The Bühlmann model calculates the maximum tolerable supersaturation for several tissue compartments. For each tissue, it determines how far the ambient pressure may be reduced during ascent without exceeding the modeled limit.

Where do the gradient factors come into play?

Gradient factors are safety multipliers on this modeled limit.

• GF 100 → exactly at the calculated tolerance limit

• GF 70 → 70% of the maximum permissible supersaturation (calculated as a percentage based on the maximum permissible supersaturation in relation to the partial pressure of the inhaled gas)

• GF 30 → significantly more conservative

Two values are commonly used:

• GF low → for the initial ascent from depth

• GF high → for the final ascent near the surface

A classic setting is, for example, 30/70. The lower the factors are selected, the greater the safety margin relative to the maximum supersaturation.

Why (if at all) GF high is crucial

In no-decompression diving, there is only one key question: Will the permissible supersaturation be exceeded in any tissue during a direct ascent to the surface? And it is precisely at the surface that GF high is applied in the gradient factor model.

So when we calculate how long we can stay in order to be able to ascend directly, we check the situation at the surface.

The GF high determines whether a full ascent is permitted.

This makes it clear that, if gradient factors are used, the no-decompression limit is limited by GF high. It defines how much supersaturation is tolerated at the end of the dive.

Why GF low has nothing to do with no-decompression limit

GF low defines the depth of the first decompression stop after leaving the maximum depth. Technically, this is also done by reducing the permissible supersaturation and thus shifting it to a higher ambient pressure – i.e., a greater depth. However, this mechanism serves exclusively to modulate the decompression profile after the start of the ascent. Conceptually, however, GF low has nothing to do with no-decompression limit.

GF low modulates the dive profile.

Where does the confusion arise?

Some computers or planning programs use internal safety margins, modify GF interpolation, or combine gradient factors with additional heuristics. These are black-box algorithms where users do not know what is actually being calculated. This can give the impression that GF low influences no-decompression limit. In addition, planning software often offers the option of determining no-decompression limit based on either GF low or high. However, this is not correct in terms of model theory. GF low defines the profile during decompression – not the no-decompression limit.

Conclusion

When gradient factors are used, GF high determines the no-decompression limit.

GF low influences the depth of the first stop after leaving the depth and modulates the dive profile. Conceptually, it has nothing to do with the no-decompression limit.