Mastering the Algorithm: A Practical Guide to Gradient Factors

Every modern dive computer has a “Conservatism” setting. For many divers, this feature remains a mystery—a setting left on default or adjusted based on a vague recommendation from a fellow diver. Changing this setting without understanding its function is like adjusting your car’s engine timing based on hearsay; you might be fine, or you might be creating a problem you don’t understand.

This guide is here to demystify one of the most powerful safety tools on your dive computer: Gradient Factors (GF). Using the widely-used Bühlmann ZHL-16C algorithm found in Shearwater computers as our example, we will translate complex decompression theory into a practical, decision-making framework.

Our goal is simple: by the end of this article, you will be able to confidently and intelligently set your computer’s conservatism to match your dive, your physiology, and your personal risk tolerance.

Chapter 1: Decompression 101 – The “Soda Bottle” on Your Back

To understand decompression, imagine you are a bottle of soda. At the surface, everything is stable. As you descend, the pressure around you increases. This is like shaking the soda bottle—it doesn’t bubble yet, but the potential is there. Under pressure, the inert gas you breathe (mostly nitrogen) dissolves into your body’s tissues. The deeper you go and the longer you stay, the more gas dissolves.

The danger comes during the ascent. If you ascend too quickly, it’s like opening that shaken soda bottle abruptly. The dissolved gas comes out of solution and forms bubbles in your tissues and bloodstream. This is Decompression Sickness (DCS), or “the bends.”

For decades, divers used tables to manage this risk. Tables are rigid, one-size-fits-all recipes for a square-profile dive. Modern computers use a dynamic algorithm, like Bühlmann ZHL-16C. This model imagines your body as 16 different types of tissue (like sponges of different sizes and densities) and constantly calculates the gas loading in each one. It defines an absolute limit for each tissue, a “Maximum Value” or M-value, which represents the theoretical line you must never cross on ascent. But in diving, we don’t want to just avoid disaster; we want to maximize safety. How do we create a buffer zone below that absolute limit?

Chapter 2: Introducing Gradient Factors – Your Personal Safety Margin

Gradient Factors are the tool that lets you define that buffer zone. They are represented by two numbers, GF Low and GF High (e.g., 40/85). They essentially tell your computer what percentage of the M-value limit you are willing to approach at different stages of your dive.

GF Low (The “Deep” Factor): This number controls the first part of your ascent, from the deepest point of your dive. It dictates the maximum gas loading you will allow while deep. A lower GF Low (e.g., 30) forces your computer to make you ascend slower or add deep stops, giving your “fast” tissues more time to off-gas before you reach shallower depths. Think of it as your budget for creating tiny, harmless “silent bubbles” while still under pressure.

GF High (The “Shallow” Factor): This number controls the final phase of your ascent as you near the surface. It represents the maximum total gas loading you are willing to have when you surface. This is your total “risk budget” for the entire dive. A lower GF High (e.g., 75 or 80) will result in longer, shallower safety or decompression stops, ensuring you have off-gassed more nitrogen before reaching the surface.

A setting of 40/85, a common default, means you are willing to reach 40% of the theoretical limit at your maximum depth, and you are willing to surface with a gas loading that is 85% of the theoretical limit. By lowering these numbers, you create a more conservative dive profile, increasing your safety margin.

Chapter 3: The Decision Framework – How to Choose Your Numbers

Setting your GFs is not about finding one “perfect” number. It’s about performing a personal risk assessment before every dive. The following decision tree is a structured thinking tool, not a medical prescription, designed to guide you through the key factors that influence DCS risk.

The Golden Rule: When in doubt, always choose a more conservative (lower) number. You can never be too safe.

The Gradient Factor Decision Tree

Step 1: Start with a Baseline * Default Baseline: 40/85 (Moderate, good for average recreational diving) * Conservative Baseline: 35/75 (A safer starting point for new or cautious divers)

Step 2: Evaluate The Dive Plan * Is this a single, no-stop dive? -> No change. * Is this a repetitive dive, or day 3+ of multi-day diving? -> Decrease GF High by 5-10. (e.g., from 85 to 80 or 75). Your body accumulates residual nitrogen over time. * Is the water cold (<21°C / 70°F)? -> Decrease both GF Low and High by 5-10. Cold impairs circulation and off-gassing. * Will the dive involve strenuous exercise (e.g., fighting current)? -> Decrease both GF Low and High by 5-10. High workload can increase gas uptake and bubble formation.

Step 3: Evaluate Your Personal Condition (Be Honest) * Are you tired, dehydrated, or stressed? -> Decrease GF High by 5-10. These factors are known to increase DCS susceptibility. * Are you over the age of 50 or have a higher than average BMI? -> Consider a permanently more conservative baseline (e.g., start from 35/75). These are statistical risk factors. * Are you feeling 100% physically fit and well-rested? -> No change needed, stick to the plan.

Step 4: Evaluate Post-Dive Logistics * Is immediate access to a recompression chamber difficult or far away? -> Decrease GF High by 10. The consequences of a DCS hit are higher, so your risk tolerance should be lower. * Are you flying within 24 hours? -> Use the most conservative setting practical.

Example Scenarios:

• Persona A (Vacation Diver): Sarah is on a liveaboard in the Caribbean (warm water). It’s day 4 of diving. She is doing a relaxed 18m/60ft dive.

  • Baseline: 40/85.
  • Dive Plan: Multi-day diving. -> GF High becomes 75.
  • Resulting Setting: 40/75.

• Persona B (Local Wreck Diver): Dave is doing a single, strenuous 30m/100ft wreck dive in 15°C/59°F water.

  • Baseline: 40/85.
  • Dive Plan: Cold water and strenuous. -> Decrease both by 10.
  • Resulting Setting: 30/75.

Disclaimer: This guide is for educational purposes to aid in understanding. It is not a substitute for proper training from a recognized diving agency. Always consult with your instructor for personalized advice.

Conclusion: From Passive Monitor to Active Tool

Gradient Factors transform your dive computer from a passive monitor that simply reports data into an active risk management tool that you control. The goal is not to learn how to be more aggressive or to shave minutes off a safety stop. The goal is to become a more informed, deliberate, and safer diver.

Start with a conservative setting. Understand the “why” behind each factor in the decision tree. As your experience and knowledge grow, you can make small, reasoned adjustments. Ultimately, your safety is your responsibility. Your computer, when you understand its language, is the most powerful tool you have to manage it.