The Search For The Perfect Gas
Take a deep breath. You just filled your lungs with air, the common name for a gas mix of approximately 21 percent oxygen and 79 percent nitrogen (give or take less than 1 percent for trace gases like argon and helium). Without it, you’d keel over and die. But when it comes to diving, “Nobody ever said air was the best gas to breathe”
Any diver worthy of his C-card understands why–all that inert nitrogen does funny things in your body under pressure. Much of what you learned in open-water training is designed to mitigate the accumulation of nitrogen in your tissues to prevent decompression illness (DCI). And on dives below 100 feet, nitrogen starts to produce a narcotic haze that can become quite debilitating as you go deeper. Somewhere between 150 and 180 feet, most divers will be so narced that they are incapacitated.
Go beyond 180, and oxygen starts to be a problem. Somewhere between 190 and 220 feet, oxygen becomes toxic, resulting in sensory distortions and seizures that can be fatal under water.
Clearly, air has its limitations as a diving gas, particularly for divers who want to stay longer or go deeper than the traditional recreational diving limits. Which is why tech divers have long been experimenting with alternative blends in the search for a better diving gas.
The logic behind nitrox is simple: Replace some of the nitrogen with more oxygen. Less nitrogen in the tank means less nitrogen in the diver and fewer problems with DCS and narcosis.
There’s just one catch: When diving with enriched air, you have to monitor your oxygen exposure to avoid toxicity. There are two crucial factors to consider. The first is the relative percentage of oxygen (or PPO2) in your tank. The second factor is the time of exposure. The combination of the two tells us the total oxygen dose. An oxygen dose of 1.6 PPO2 for 45 minutes is recognized as the maximum safe limit for divers who aren’t working strenuously.
Fortunately for recreational divers, this dose limitation gives us a wide latitude to dive in the 60- to 130-foot range using gas mixtures from 32 to 55 percent oxygen. With proper planning, you can significantly increase the duration of your dives without any increased risk of decompression illness or narcosis and with an extremely low risk of oxygen toxicity.
To go beyond traditional recreational depths, technical divers employ trimix, the general term for gas blends that replace much of the nitrogen and some of the oxygen with more benign inert gases, such as argon and helium.
Based on current research and practical experience, helium is the inert gas of choice. Its narcotic properties are negligible in comparison to nitrogen and it’s a thinner, more compressible gas that helps regulators work more efficiently at extreme depths. Helium-based mixtures allow properly trained and equipped divers to routinely go to 400 feet (and beyond) with a remarkable safety record.
Trimix divers custom blend their breathing gas to suit each dive, allowing them to more precisely control oxygen limits and more dramatically reduce narcosis. For example, let’s look at a 240-foot dive using 17/50 trimix. That’s 17 percent oxygen, 50 percent helium and 33 percent nitrogen. (When expressing percentages in a trimix gas, the oxygen is always stated first, followed by the helium. The balance of the gas is assumed to be nitrogen unless otherwise stated.) This dive yields a conservative PPO2 of 1.4 with an equivalent narcosis depth of only 57 feet. In other words, the diver would experience the same level of narcosis on this trimix dive as he would on an air dive to 57 feet.
The Cost of Deeper Diving
Of course, these benefits do not come without cost. The deeper the dive, the more complicated the dive plan and the gear configuration become. The first thing most recreational divers notice is all that redundant gear, particularly the twin back-mounted cylinders coupled together with an isolation manifold. The manifold allows the gases from the two tanks to “communicate” so that the diver can use the regulator attached to either tank while consuming the gas supply equally from both tanks. On deeper trimix dives, the oxygen content in the main cylinders may be too low to safely breathe at shallow depths. In these cases, the diver must also carry separate gas mixtures in additional tanks–one cylinder of travel gas (typically a nitrox blend of 32 to 40 percent oxygen) to breathe while passing through traditional recreational diving depths and another cylinder of decompression gas (either 80 percent or 100 percent oxygen) for shallow stops.
Decompression and Helium
While helium is extremely useful in combating the ill effects of nitrogen and oxygen at extreme depths, it’s not without its problems. Because it is a lighter, faster gas, divers load and unload helium more quickly than nitrogen. For some profiles, this requires slower ascents or deeper decompression stops to keep the helium from off-gassing too rapidly and causing DCI.
Finally, the major drawback to helium-based gases is cost. Helium supplies are tightly regulated, making the gas very expensive. The cost of filling a set of doubles with a trimix will typically range anywhere from $30 to $120 in Florida, depending on the amount of helium used.
As you might also expect, trimix training is quite involved and quite expensive. Trimix courses typically require certification as an advanced open-water diver, an advanced nitrox diver and a decompression procedures diver as prerequisites. The course itself will range from $900 to $1,200 in the United States, depending on geographic location, and the cost is exclusive of gas fills, charter fees, etc. When all the expenses are totaled, divers starting a trimix class should anticipate spending between $1,500 and $2,000, assuming they already have all their own gear.
Calculating a Narcosis Depth
One of the most important criteria for selecting the right trimix blend is calculating a narcosis depth air equivalency–a comparison of the expected narcotic effect of a tri-mix blend at a given depth to a more easily understood air depth.
To accomplish this, we must first select a narcosis depth limit and determine the partial pressure of nitrogen (PPN2) while breathing air at that depth. For example, a dive to 99 feet, or four atmospheres (ATA), yields a PPN2 of 3.16 on an air dive. However, if we reduce the nitrogen content to 35 percent by replacing some of the nitrogen with helium, we can dive the resulting gas mixture to 300 feet while experiencing a PPN2 of approximately 3.16. That means our narcosis level at 297 feet (10 ATA) on that blend of trimix would be roughly equivalent to the level experienced at 99 feet on air. Similar calculations must be made for oxygen exposures at depth and during decompression.