Gradient Factors Explained
GF Low, GF High and Bühlmann conservatism explained for dive computer users
ℹ️ Read time: 8 minutes. This article is for divers wanting to understand the inner workings of their dive computer.
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The end of "I follow my computer"
Many modern dive computers use a Bühlmann ZH-L16-based decompression model. While most divers just turn on their device and follow the numbers on the screen, tech divers and the curious know there is an often-ignored menu: the Gradient Factors (GF) settings.
Not understanding your GFs is like driving a sports car without knowing what the gears are for. You are moving, but you don't truly control your safety. Let's explore how these two small numbers (often presented as 30/85 or 40/85) dictate your decompression profile.
What is a Gradient Factor?
To put it simply, the Bühlmann algorithm defines a theoretical limit of inert gas supersaturation (the "M-Value"). At 100% of this M-Value, the model has no additional gradient-factor margin. In practice, divers usually choose lower values to add conservatism and reduce decompression stress.
This is where Erik Baker steps in during the 90s. He invented Gradient Factors to force the computer to create stops before reaching this critical limit. GFs are therefore percentages of conservatism.
GF Low: Managing Deep Stops
The first number (the "Low") determines when your computer will trigger your first stop.
- A low GF Low (e.g., 30%): The computer will stop you early in your ascent, creating deep stops. This limits the growth of micro-bubbles formed at the bottom.
- A high GF Low (e.g., 80%): The computer will let you ascend much closer to the surface before requiring a stop.
For years, the trend was very deep stops (GF Low at 10 or 20%). Today, recent physiological studies show that a stop that is too deep saturates slow tissues. The current trend stabilizes around a GF Low of 30 to 50%.
GF High: Surface Safety
The second number (the "High") is arguably the most crucial for recreational diving. It defines your allowed saturation level when you break the surface.
- A GF High of 100%: You exit the water with zero safety margin compared to the pure algorithm. This is not recommended.
- A GF High of 80%: You exit with a 20% safety margin. Your computer will force longer stops at 3m and 6m to ensure you reach this rate.
- A GF High of 70%: Very conservative. Ideal for tired, older divers, or diving in very cold waters.
Which settings to choose?
There is no perfect setting, only a setting adapted to your physiology and your type of diving. Here is what is generally observed on boats:
- 30/85 or 40/85: The default standard on many tech computers. An excellent compromise for committed recreational air diving.
- 50/90: Less conservative, gives profiles that often resemble older tables or older generation Suunto RGBM computers.
- 30/70: Very conservative at the surface. Recommended if you have a risk profile, if the water is freezing, or if you do intense effort at the bottom.
Conclusion
Playing with your Gradient Factors is not trivial, but it's a sign of maturity. The next time you prep your gear, don't just check your air pressure. Go into your computer settings, look at your GFs, and ask yourself: "Is this conservatism adapted to the dive I'm about to do today?"
Use this with the tables and calculators
Gradient Factors make more sense when combined with the underlying Bühlmann ZH-L16C coefficients, the basics of gas laws for divers and practical planning tools.