Extend your limits with Trimix

by Darrell Adams:

So what is this Trimix stuff anyway? No we are not talking about an ED drug (go see your doc for that! And read The P-Valve Struggle for you drysuit divers).  But seriously, novice divers may not know what all this Trimix talk is about. The cliché definition is, “It is a breathing gas, consisting of oxygen, helium, and nitrogen, and is often used in deep commercial diving and during the deep phase of dives carried out using technical diving techniques.”  But in reality, it is a risk management breathing mixture utilized by divers typically seeking to offset the consequences of diving normoxic air (21% oxygen and 79% nitrogen) or nitrox mixtures (22% oxygen or more with a balance of nitrogen) at a planned diving depth by replacing much of the nitrogen and some of the oxygen with more benign inert gases like helium.

As one may have learned in introductory classes, in scuba diving the gases we breathe all effect the body in three different ways: in good ways, negative ways, and sometimes bad ways. In order to understand these effects in better detail, we need to try and recall from the depths of our neurons the stories of the Old Salts of Men like: Robert Boyle (Boyle’s Law), John Dalton (Dalton’s Law), Louis Joseph Gay-Lussac (Gay-Lussac’s Law), Jacques Charles (Charles’s Law), Amedo Avogadro (Avogadro’s Law), and Thomas Graham (Graham’s Law), who came up with or discovered all the gas laws. These discoveries in most cases were derived from the studies of many other “Old Salts” who came before them. The understanding of the relationships between these laws will improve clarity on exactly what is happening when breathing gases are consumed at increased partial pressures.  This foundational knowledge began the day we were born and we all took our first breath. Our bodies, without conscious effort, told us we had too much of this and needed more of that (in regard to gas).  This education continued throughout our lives as we took on additional adventures and discoveries in life, like athletic events, which increase our respiratory rate. More education took place the day we inhaled helium from a balloon and tried to talk. We learned that with certain activities our bodies show a demand for more of these gases or can be effected by the mere presence of certain gases.

So stepping back to our open water class, we learned while diving recreationally at depths of less than 130 feet (40 meters), the effects of oxygen in normoxic air is not of concern. However, the nitrogen exposure can be of concern due to the effects of narcosis and decompression sickness.  We may have been told of the consequences of diving at deeper depths, or with different breathing mixtures like nitrox, in regard to the way these gases will behave. We may have also been taught that we should seek additional training to engage in these types of activities.  Therefore, the reason for diving Trimix mixtures is primarily to circumvent the negative effects, and hopefully prevent the bad effects, of increased oxygen and nitrogen gas exposures. For those divers who have taken the leap into TDI’s Intro to Tech, Nitrox, Advanced Nitrox, Decompression Procedures, and Extended Range courses you already have a grasp of what we have been discussing.

“Twenty years from now you will be more disappointed by the things you didn’t do than by the ones you did. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover.”- Mark Twain


So as we begin to ponder our future journeys, and if our future dive plans may involve helium mixtures, let us take a look at why this may be of interest to us.  First, helium has two advantages over nitrogen in a breathing mixture. One advantage is that helium has very little narcotic effect.  Most divers start to experience the narcotic effects of nitrogen around a nitrogen partial pressure (PPN2) of 3.0 to 4.0. The onset of narcosis is hard to recognize, so most divers do not recognize it at the deeper depths even though it may be present.  With Helium, one can set his or her own “Equivalent Narcotic Depth” (END) by adjusting the Oxygen/Helium/Nitrogen in the mix.  By customizing your narcosis depth you may find you remember more about your dive and are much more “clear-headed” during your dive.  As an example, divers off the coast of North Carolina searching for “megalodon teeth” in waters 80’ to 110’ (24 – 34 meters) using helium mixtures as a bottom gas to lower the narcotic effect, are finding they discover and capture more teeth than when diving normoxic and nitrox mixtures.

The second advantage is that it is less dense, resulting in reduced breathing resistance at depth. Therefore, breathing effort may not be as high as dealing with a more dense gas like nitrogen.  A less dense gas flow delivery rate may be much better through a regulator on much deeper dives as well.  Another advantage of helium is that it is used to control oxygen exposure on deeper divers so a partial pressure of oxygen (PPO2) can be kept at a level desired for the dive. This ability helps to reduce the risk of oxygen toxicity.


There are some disadvantages to helium one must take into account as well, and through effective mitigation these disadvantages can and may be dealt with.  The first concern is that helium has been found to conduct heat approximately six times faster than air. Therefore, heat loss through normal respiration is a factor to consider when compared to denser gases. This factor makes helium less desirable as a drysuit inflation gas when compared to a more dense gas like nitrogen or even argon.  For those long decompression dives, divers must remember to choose appropriate exposure protection and may wish to carry a separate cylinder containing a proper suit inflation gas.

A second disadvantage one most ponder is the cost of helium.  The rising cost of breathing quality helium over the last 15 years has made open circuit trimix diving somewhat more costly.  Many divers may weigh the balance of cost between open circuit helium gas trimix dives and similar rebreather gas fills, and decide to make another educational choice and explore the world of Rebreathers.

The third disadvantage we may find about using helium is that, being a lighter gas, it will dissolve into tissues (on-gassing) faster than denser gases at increased pressures.  During the ascent from a dive, the off-gassing rate is also faster (due to ambient pressure decreasing), which when combined with a decompression gas of choice, careful dive planning may need to include deeper decompression stops and slow ascent rates. These actions can result in mitigating and reducing the risk of DCS that may be associated with breathing a less dense gas.

“The most important reason for going from one place to another is to see what’s in between, and they took great pleasure in doing just that.” ― Norton Juster, The Phantom Tollbooth

So as you begin to wonder where your aquatic journeys may take you, remember the dry portions of this planet only cover a small percentage, and there are many more places to visit under the water. Trimix diving may be vehicle that transports you there. So stop in at your local TDI Facility, speak with an instructor, and discover if this journey is for you.

Another great article to read is “Are you ready for Trimix.”

– Darrell Adams
SDI/TDIERDI Instructor Trainer – Air Hogs Scuba, Garner, NC

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