Gradient Factors (GF) are coefficients (e.g., 90%, 80%…) that can only be used with Haldanian parameter sets (e.g., U.S. Navy, Bühlmann…) and that allow the reduction of M-Values to increase stop times (or reduce no-stop limits) and, depending on the case, their depth.
In other words, they reduce, in Workman’s notation, the M0 and ΔM values of the parameter set.
GF have been used in computers for over 30 years
Depending on the computer, GF are:
- Predefined (L0, L1; P0, P1…);
- left to the user’s choice (sometimes with predefined GF proposals).
Computers of this second category make it necessary to inform divers about the settings to adopt.
The choice of GF is arbitrary
This raises the question of the validity of the chosen values.
‘“100/100”’ means you are using the exact M-Values provided by the creator of the parameter set (e.g., Bühlmann).
»90/90 » means you reduce the calculated M-Values by 10%.
The GF setting follows different rules:
- For nitrogen dives (air, nitrox);
- For helium dives (heliox, trimix).
The CMAS recommendations are the following:
- Air/nitrox: GFlow=GFhigh=between 90% and 80% (e.g., 90/90; 85/85; 80/80);
- Helium (heliox, trimix): GFlow between 30 and 50, GFhigh between 70 and 80 (e.g., 50/80; 30/70).
It should be recalled that it is impossible to guarantee a safe decompression in 100% of cases for 100% of people and that 90% of decompression accident (DCS) cases occur despite following the dive computer’s instructions.
The GF is one way among others to prevent the risk of decompression accidents.
They are, by definition, arbitrary and offer no guarantee.
To learn more: https://www.cmas.org/resources/fact-sheets.html
Alain Foret, President of the CMAS Technical Committee.
Photos Nicolas Barraqué.