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  • AutorenbildMichael Mutter

What are optimal gradient factors? Remarkable findings from the Belgian Navy.

Aktualisiert: 21. Apr.

Testing the default Gradient Factor settings of a commercially available dive computer (Shearwater Perdix™) by the Belgian Navy yielded remarkable results.

Aphrodite diving spot, Lake Walensee, Switzerland. photo courtesy Karin Aggeler
Quick take: Rethink your GF settings. Ascend as far as possible and take plenty of time for decompression at shallow depths.

The Bühlmann (and Keller!) decompression model is used in many dive computers. It calculates the maximum supersaturation tolerance for various tissues, which must not be exceeded during ascent in order to avoid decompression sickness (DCI). Divers can use gradient factors (GF) to determine how close they want to get to this limit when surfacing. A high GF means diving closer to the limit, while a low GF ensures a greater distance to it. Usually, two factors are selected: GF low (or GF lo) for the initial ascent from depth and GF high (or GF hi) for the shallow ascent at the end of the dive. A common setting is, for example, GF lo/GF hi 30/70, which means that a diver initially only ascends to 30% of the permissible depth (e.g. on a 40 m dive initially only to 23 m instead of directly to 6 m), carries out a deep stop there to avoid the formation of gas bubbles in the vital organs and then ascends more quickly at the end of the dive.

Is the paradigm true, that low GF protect better against decompression sickness?

Since the introduction of the GF, the paradigm has been that the deeper the GF lo is selected, the safer the dive will be in terms of decompression sickness, and therefore low GF lo and slightly higher GF hi must be selected.

The standard setting of the gradient factors on the Shearwater Perdix™ dive computer is GF lo/GF hi 30/70. The Belgian navy had this setting analysed for air dives up to 60 m without repetitions in order to provide their divers with optimal guidelines.

The US Navy's air decompression tables were used as references (among others) due to their excellent validation and reliability in predicting a DCI. The decompression model used in the Shearwater Perdix™ (Bühlmann ZH-L16C algorithm with GF) was programmed in Python. The GF settings were then optimised to create decompression plans for a variety of dives that matched the US Navy tables as closely as possible.

The default settings in the Shearwater Perdix™ do not result in less DCI - on the contrary.

It turned out that the reference profiles could only be approximated if the GF lo was 100 % and the GF hi was reduced to a maximum of 75 %. In plain language, this means that the Bühlmann model only had to be adjusted very slightly and deep stops had to be avoided altogether. No evidence was found that the use of the standard GF setting (30/70) leads to safer decompression during air dives to a depth of 60 metres in seawater - rather the opposite.

The disadvantage of low GF has been known for years.

Are these results surprising? Not really. The Perdix standard GF-setting yields deeper initial stops, during which the vital organs are desaturated, but also leads to greater saturation of the slower tissues, which on balance results in a higher risk for DCI. This disadvantage of GF has been well known for several years (e.g. Doolette), but is still widely ignored.

The study suggests an actual paradigm shift for the choice of gradient factors.

However, the results of this study suggest a real paradigm shift. Choosing a GF lo which is higher than the GF hi diametrically opposes common practice and dive computers do not allow such a setting (e.g. GF 100/70). According to this study, the best approximation would be a symmetrical GF setting, in which the GF would be set identically between GF 75/75 and 95/95. This emphasises that very low GF lo should be avoided.

As a consequence of this study, Belgian Navy divers have been advised against using the standard GF settings, but to adopt symmetrical GF settings instead.

For diving practice, this means: Make the first deco stop as close to the surface as possible, but take plenty of time for the dive out. Choose high GF!

N.B. Mixed gas and repetitive dives were not analysed in this study.

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