The 5.2 g/L Rule: Why Gas Density May Be the Most Important Number in Deep Diving

CO₂ Is Not Just “Discomfort” — It Can Cause Unconsciousness

In his Rebreather Forum 3 speach, Dr. Simon J. Mitchell reminds us of something that is easy to overlook:

“Carbon dioxide (CO₂) elimination is particularly important in diving as unconsciousness can result from excessive arterial CO₂ tension (PCO₂), known as hypercapnia.”

He explicitly lists gas density as one of the key factors controlling CO₂ elimination. That is a powerful statement.

Hypercapnia is not only about scrubber failure. Very often, it is about inadequate ventilation relative to CO₂ production. And dense gas makes adequate ventilation progressively harder.

What Actually Happens When Gas Gets Dense

RF4 (2024) sharpened this discussion significantly.

Gas flow in rebreather loops and large airways is turbulent, and turbulent flow depends on gas density. The pressure required to move that gas rises sharply with both density and flow rate

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In practical terms:

The deeper you go, the harder you work, the more breathing effort increases. And if CO₂ production exceeds your ability to ventilate, respiratory failure becomes inevitable

RF4 summarizes this bluntly:

Gas density poses one of the most important depth constraints… avoidance of diluents with densities exceeding 5.2 g·L⁻¹

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That is not phrased as a suggestion. It is described as a depth constraint.

Why Planning by Gas Density Is Smarter Than Planning by END Alone

Here’s the insight many divers miss.

If you plan primarily by END, you are managing narcosis. But you may still be breathing gas that is physiologically heavy.

If you plan by gas density and keep it below 5.2 g/L, you must increase helium.

And what happens when you increase helium?

• Gas density decreases
• Work of breathing decreases
• CO₂ retention risk decreases
• Narcotic potential decreases

Which means:

When you plan according to density, your END automatically becomes more conservative.

But the reverse is not true.

You can have an “acceptable END” and still exceed 5.2 g/L.

Planning by density is therefore more physiologically robust. It protects both ventilation and cognition.

The Dangerous Part: You Don’t Feel Density Rising

One of the most important observations from RF4 is this:

“Gas density is not something adequately perceived by divers. Increasing gas density will compromise diver performance far before it will be recognized.”

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That’s unsettling. You don’t feel density. You feel anxiety. You feel effort. You may feel narcosis. CO₂ further amplifies narcosis and cognitive slowing. At extreme depths, gas densities in certain trimix configurations rise far beyond recommended thresholds, and experts concluded that excessive work of breathing contributed to David Shaw’s fatal dive

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This is not theoretical modeling. It has happened.

CO₂ Retention Is Not Just a Scrubber Problem

Mitchell emphasized that CO₂ toxicity can occur due to retention even when the rebreather scrubber is functioning.

Rebreather Forum 3 lists contributing factors such as:

• Increased work of breathing
• Increased gas density
• Resistance within the breathing loop
• Respiratory muscle fatigue

Even at 30 meters on air, maximum ventilation capacity is already reduced to roughly half of what it is at the surface. Your ventilatory reserve shrinks faster than most divers realize. And once that reserve is gone, adding effort only produces more CO₂ — which worsens the problem.

The 5.2 g/L Rule Is Not Arbitrary

The repeated reference to 5.2 g/L in RF4 reflects accumulated physiological evidence and operational experience.

It represents a point where:

• Work of breathing escalates
• Ventilatory capacity becomes limited
• Safety margin begins to erode

It is not a magic cliff. But it is a well-supported ceiling.

Technology Is Finally Catching Up

This shift in thinking is no longer confined to conference proceedings.

The upcoming CCR Liberty firmware 2.19.0 will integrate density into dive planning logic and Issue warnings when density approaches unsafe levels. This directly reflects RF4’s recommendation that gas density should be visible in real time to remind divers of a hazard that is otherwise easy to miss. Gas density is becoming an active safety parameter — not just a planning spreadsheet value.

And It Doesn’t Stop at the Unit. The upcoming update of Divesoft.app will include:

• A full gas planner
• A gas blending assistant
• Gas density calculations

When adjusting helium fractions, divers will immediately see not only MOD and END, but also density — and observe how lowering density improves both breathing comfort and END. At the same time, density will be the determining parameter, so by setting the density, you can reverse calculate the composition of the mixture and its END.

That is the practical integration of rebreather science into everyday dive planning.

The Bigger Picture

If your mix is: Within MOD; Within acceptable END; But above 5.2 g/L you may already be operating with reduced ventilatory margin.

If your mix respects the 5.2 g/L density limit:

• Work of breathing is lower
• CO₂ clearance is more efficient
• END is inherently more conservative
• Cognitive margin improves

Gas density planning does not replace MOD or END. It completes them.

And increasingly, it may be the most physiologically honest constraint we have in deep diving.

In deep diving, the question is no longer just:

“How narcotic is my mix?”

It’s:

“How hard will it be to breathe?”

• Jakub Šimánek, Divesoft Factory IT