DECO GASES AND HELIUM
Introduction
This block is about gases that contain less nitrogen than air – and how this changes decompression, narcosis, and planning. When does Nitrox make sense, what are its limits, and why is high-percentage Nitrox so efficient in decompression? When does helium become useful or necessary – and what are the typical trade-offs?
This section does not replace practical training in technical diving. The goal is to better understand decompression in this area of recreational diving, avoid common misconceptions, and make gas decisions comprehensible.
Before we start with Nitrox as a deco gas – have you already completed a regular Nitrox course? We won’t completely repeat that knowledge here. However, you can first test your knowledge and, if in doubt, refresh it before continuing.
Nitrox: Oxygen Sets Limits
Oxygen is vital – but at high partial pressures, it can be toxic. For recreational divers, the danger of CNS oxygen toxicity is particularly relevant: a seizure underwater is usually fatal. At the same time: Within the recognized limits, the risk is very low – so we are working with conservative safety margins here.
From your Nitrox course, you surely remember that you must pay close attention to the MOD, the maximum operating depth. More than 1.4 bar pO2 during bottom time and 1.6 bar pO2 during deco should be avoided.
Perhaps not much more has stuck. However, when it comes to deco gases, you should revisit the topic of oxygen.
You can read about this in your Nitrox course or in our materials. Please familiarize yourself with the terms MOD and Best Mix before continuing with this text, and take another look at “CNS Clock,” CNS toxicity, and pulmonary toxicity.
SSI Nitrox online
You can complete the entire Nitrox course, or simply review what you want to refresh.
Blog: O2 Limits - New Recommendations
Blog article on current recommendations for oxygen limits. The “CNS Clock” is also explained there.
Nitrox as Deco Gas: Why it’s so efficient
As a deco gas, high-percentage Nitrox plays a very important role. This is not about “more oxygen = better,” but about less inert gas in the breathing gas and thus a stronger gradient that drives inert gas out of the body. Deco gases reduce the inert gas partial pressure in the lungs – this facilitates desaturation.
Using Nitrox as a Deco gas has major advantages. We already saw when choosing gradient factors that a low GF Low forces deeper stops. And at these deeper stops, medium and slow tissues continue to on-gas, while only the faster ones are already off-gassing.
So, when choosing the first stop, we have a fundamental problem: desaturation becomes more efficient if we have a higher gradient between inert gas pressure in the tissues and inert gas pressure in the lungs. With only one gas, we can only achieve this by ascending further.
At the same time, the risk of bubble formation increases with increasing overpressure in the tissue. With only one gas, one has to accept this, but with a deco gas, both can be decoupled.
With less nitrogen in the breathing gas—and therefore in the lungs—the off-gassing gradient increases immediately. Medium tissues also stop on-gassing, or at least on-gas less. But you’re still at a depth where bubble formation is less likely.
This is what’s special about a deco gas: you can separate the inert gas pressure gradient from the ambient pressure.
In the basic block, you already learned about a very simple didactic dive planner. Now, deco gases come into play.
Here you can plan deeper dives with up to two deco gases. Before discussing which gases are suitable for which dive, take a look at a few profiles with different deco gases. What changes during the ascent? Pay attention to the length of the decompression, but also to which tissues desaturate after the gas switch that would otherwise continue to load.
Choosing the Best Deco Gas
In the planner, two gases are preset: EAN50 and pure oxygen. This is a very common combination for deeper dives on air or normoxic Trimix (more on that in a moment). Together, they provide very efficient decompression. The first gas switch can already take place at 21 m, and on the kinds of dives typically done in this range, the gradient factor is still very low there.
However, if you only use one deco gas, you have to decide what is more sensible: EAN50, pure oxygen, or something in between?
You will have to make this decision yourself. To have all relevant information for this, try the following with the planner:
- First, plan a simple dive to 40m with air, with a bottom time of 25 minutes, initially without deco gas.
- Use the gradient factors you consider appropriate.
- Plan this dive with EAN50 as deco gas, and observe the difference.
- Plan it again with O2 as deco gas – what changes now?
- Do you remember where your first stop was in your plan without deco gas?
- Find the gas that is the best mix for a pO2 of 1.6 at this first stop.
- Plan the dive again with this gas as deco gas.
- Then consider: Which of the three options seems best to you?
Most of the time, you will use the deco gas that is simply available. Pure oxygen offers the most efficient desaturation but can only be used from 6m. Depending on the dive profile, a gas with a slightly lower oxygen content might be the better decision.
Helium & Trimix: Why, When, What For?
Helium is used to reduce narcosis and lower gas density. This can decrease breathing effort and improve mental performance. At the same time, helium is significantly more expensive, and its specific properties must also be considered in deco planning.
For deeper dives, air as a breathing gas presents two problems (which are physiologically closely linked): firstly, nitrogen has a narcotic effect at increasing depths, and secondly, the increased gas density makes breathing more difficult.
While the so-called “nitrogen narcosis” is a phenomenon about which there is little truly unambiguous knowledge – what exactly triggers the narcotic effect is still not clearly understood – the physiology of increased breathing effort is known.
If the gas density becomes too high, every breath is more strenuous than at the surface – and the maximum possible ventilation of the lungs decreases. With great exertion at depths that are truly no longer dived with air today, it could happen that one simply cannot breathe enough.
But even long before that, another effect can occur: the CO2 level in the body can become higher than normal. Normally, the body reacts to an increased pCO2 with increased breathing to exhale it. Underwater, however, some individuals, the “CO2 retainers,” experience an opposite reaction: their body tolerates a higher CO2 content to reduce the more strenuous breathing effort. Since CO2 is highly narcotic, this effect is suspected of being responsible for a significant part of the unpleasant effects of nitrogen narcosis.
Limits for Gas Density and Narcosis
What gas density is still acceptable, and what mixing ratio is optimal?
When determining the best mixture for the desired maximum depth, two factors are relevant: the equivalent narcotic depth and the gas density.
To determine the equivalent narcotic depth, one calculates – actually, similar to the equivalent air depth for Nitrox – at what depth air would have the same pN2 as the Trimix one intends to use at the maximum depth.
The gas density at this depth is calculated from the individual components oxygen, nitrogen, and helium.
Recommended Limits
END (Equivalent Narcotic Depth): 30m
Gas density: 5.2g/l in rebreather
up to 6.2g/l open circuit
Decompression with Trimix
Helium is a much lighter gas than nitrogen and should therefore diffuse into the various body tissues significantly faster. The Bühlmann model assumes that the half-times for helium are 2.65 times faster than for nitrogen. Decompression must therefore be calculated differently.
Those who start diving with Trimix often think that decompression should be shorter: the lighter gas leaves the body faster. However, this is a fallacy; deco would usually be longer – which can be well compensated for in practice with deco gases.
Why is that? The reason is the deeper stops. Since helium also enters the tissues faster, supersaturation is greater during ascent. To desaturate the faster tissues again, one must stop early. In doing so, medium and slow tissues continue to load further and must then, in turn, desaturate at the shallower stops.
With the planner, you can observe tissue saturation and desaturation in three heatmaps: the top one shows only nitrogen, the second only helium, and the third then the combination of both inert gas partial pressures.
If you have reached this point, you will have long been using a different program for real planning. Please stick to that; this is really only for illustration!
Here, however, you can experiment with different gases and their effects on runtime, and thereby better understand how the helium content in the breathing gas influences decompression.
What you can try with the planner
To systematically examine what changes in decompression with Trimix, try the following:
- First, plan a simple dive to 60m without deco gases. Optionally use air, Trimix 18/40, or even Heliox 18/82 and observe the deco.
- Then add a deco gas that you can breathe from the first stop. What changes?
SOMETHING MORE, IF THE PLANNER CALCULATES CORRECTLY!
