Helium Penalty

Technical divers use helium to reduce gas density and narcosis.

The “Helium Penalty” is the theoretical idea that because helium is a smaller, faster molecule than nitrogen, it requires deeper stops and longer decompression times. Modern research suggests this is theoretically incorrect. The famous shearwater article described this inaccuracy and quickly added a disclaimer because of the adoption of techniques warranted it.

History

Historically, this belief became entrenched in “diving folklore” following early reports, such as those from the U.S. Navy in 1939 (Momsen), which stated that heliox diving required longer times at deeper stops. However, these early implementations were not based on direct, prospective experimental evidence comparing the two gases. Instead, the penalty manifested through a series of theoretical and algorithmic assumptions.

The penalty arose from theoretical considerations of the differences between the two gases, specifically, helium’s higher diffusivity compared to nitrogen. Decompression models were designed to reflect this perceived faster uptake. For example, the Bühlmann ZH-L16 algorithm fixed nitrogen half-times as exactly 2.65 times longer than those of helium.

Why exactly 2.65? This precise number comes directly from Graham’s Law, which states that the rate of effusion of a gas is inversely proportional to the square root of its molecular mass. Early decompression modelers applied this classical physics principle to the human body, assuming that gas transport across tissue membranes was driven strictly by this molecular weight differential.

The mathematical breakdown is straightforward.

Molecular weight of Nitrogen ~28.01 g/mol

Molecular weight of Helium~4.00 g/mol

If you look at the ratio of their weights and apply Graham’s Law:

Because helium is lighter, the math dictated it must move approx 2.65 times faster than nitrogen. Consequently, Bühlmann simply divided the established nitrogen tissue half-times by 2.65 to calculate the helium half-times (for example, a 4-minute nitrogen compartment became a 1.5-minute helium compartment). While this elegant math works perfectly when a gas is escaping through a tiny pinhole into a vacuum, the human body is vastly more complex.

The Bühlmann Paradox

To understand why we use a “wrong” model, we should look at an analogy – the Bühlmann algorithm itself. Strictly speaking, Bühlmann is also theoretically incorrect. It is a dissolved-gas model that assumes we are preventing bubbles from forming entirely. We know, however, through Doppler technology and VGE (Venous Gas Emboli) studies, that bubbles form on almost every dive.

Contrast this with Bubble Models (like VPM or RGBM). These models are theoretically smarter because they try to account for bubble dynamics and micro-bubbles. Yet, in practice, multiple studies have shown that bubble models can lead to higher rates of decompression sickness (DCS).

More deco reduces the risk of DCS most of the time. The Bühlmann helium penalty, while technically wrong, works in the real world.

Fixing the Problem

Once divers learn that the helium penalty isn’t real, the temptation is to find ways to eliminate it. This is a logical conclusion – why do more deco when there isn’t a benefit. This usually manifests in two common methods.

1. Omitting Helium
Some divers choose to program their gas as Air or Nitrox even when they are breathing Trimix (programming 21/00 when using 21/35). This will reduce the deco obligation if using trimix diluent because the computer doesn’t apply the helium penalty.

The Logic: If the helium penalty is fake, why let the computer add “fake” time?

The Reality: All you are doing is making your dive more aggressive. If you want less deco, you could achieve a similar result by changing your Gradient Factors. Most divers aren’t comfortable cranking up their GFs, yet they are happy to omit helium from the computer. It is an inconsistent approach.

2. Flush out Helium
On CCR, some divers at the 70-foot mark will plug an offboard deco gas of 50% in and change the computer’s diluent to reflect this change. They believe they are “flushing” the helium out. This will reduce the deco obligation if using trimix diluent (assuming you’re not also doing #1).

The Problem: “Flushing” helium isn’t a real physiological concept. Helium is less soluble and off-gases extremely fast on its own. By plugging in 50%, you aren’t helping the helium leave; instead, you are likely increasing the nitrogen load in your intermediate tissues.

The Risk: Manually plugging in gases on a rebreather introduces the risk of a wrong-gas switch. Depending on the bottom mix, you could also introduce IBCD issues or increase the partial pressure of N2 when you’re trying to offgas. This practice is widely used and while not explicitely dangerous, it shows a misunderstanding of the data. If helium offgases faster, why would you need to flush it out?

Physiology Over Electronics

The truth of decompression is that models are just models. A dive computer doesn’t know if you’re dehydrated or if you’re cold.

When we manipulate our computers to eliminate the helium penalty, we are removing a layer of conservatism that we might actually need. Being “inaccurate” to the computer doesn’t automatically mean you’ll get bent, but it does mean you are operating without the safety margins the algorithm was designed to provide. The result of the helium penalty is more deco, and it’s reasonable to address it as a depth penalty for the added deco risk that comes from deeper (trimix level) deco dives. Is it especially risky to use one of the above techniques to do less deco, especially with a conservative GF on a shallower dive? Probably not. If we pair very deep with less deco, then we’re adding risk.

Understanding that the helium penalty is theoretically wrong allows us to be better-informed divers.