Trimix is by far the most complex and staggeringly expensive breathing gas available from a dive centre or store, but it’s generally only reserved for the technical elite.
Trimix is our third and final breathing gas, consisting of helium, oxygen and nitrogen. Trimix is not to be confused with Heliox, which contains only oxygen and helium with 0% nitrogen.
UNDERSTANDING GAS NARCOSIS
To aid your understanding as to why we use helium, all certified PADI divers should have a basic comprehension of gas narcosis, but for those who don’t, lets have a quick recap.
Gas narcosis is the name given to the anaesthetic phenomenon when a diver breathing underwater suffers from an assortment of adverse effects related to the depth they are at. The exact causation for this is not yet fully understood, but it is widely believed that the gas a diver breathes whilst under pressure can have an adverse effect on signal transmission in their central nervous system, thus causing them intoxicating effects.
Whilst breathing air, divers experience the onset of gas narcosis at approximately 30m, and cold water and limited visibility can often predispose them further to exposure.
Initially, the effects of gas narcosis include slowed thinking, a mild feeling of euphoria, and increased problem-solving times, but as a diver goes deeper, the intoxication becomes more profound. Reaction times grow significantly, physical movements and motor skills are reduced, with simple skills becoming significantly harder to achieve. The diver will not be able to concentrate on the tasks at hand and can lose control of the dive through lack of awareness or poor judgment. A diver can also experience visual and auditory disturbances, vertigo, and adverse psychological reactions at extreme depths, including fear, panic, depression, and paranoia.
REDUCING THE NARCOTIC EFFECTS WITH HELIUM
Helium is usually added as a substitute for some of the nitrogen to reduce the narcotic effect of breathing gas at depth. But what some may not know is that helium is also used to reduce the oxygen component of a blend, and thus the inherent risk of a diver experiencing oxygen toxicity.
When combining the above three gasses, it is possible to create the most suitable gas for any given depth by adjusting each respective components accordingly. Helium is a much faster gas to both saturate and desaturate tissue compartments and because helium has a much lower molecular weight than nitrogen, a divers tissues never become as heavily loaded as it does with other denser gasses.
THE ADVANTAGES OF RETAINING SOME NITROGEN
Firstly, nitrogen is a lot less expensive than helium. Retaining nitrogen can also contribute to the prevention of high pressure nervous syndrome (HPNS) or ‘helium tremors’ which can occur when using heliox at depths of around 120m and deeper.
NORMOXIC & HYPOXIC TRIMIX
In open circuit scuba diving, there are two classes of trimix which are commonly used. The first is known as normoxic trimix which has a a minimum PPO2 at the surface of 0.18, and the second, hypoxic trimix which has a PPO2 of less than 0.18 at the surface. Again, we are only really concerned with the oxygen content at this stage.
A normoxic mix such as a 19/30 (19% O2, 30% Helium, 51% Nitrogen) is used in the 30 to 60m depth range, and a hypoxic mix such as 10/50 (10% O2, 50% Helium, 40% Nitrogen) is used for deeper diving, as a bottom gas only. A mix like a 10/50 cannot safely be breathed at shallow depths where the PPO2 is less than 0.18 due to the risk of a diver loosing consciousness (hypoxia).
ASSOCIATED RISKS OF USING HELIUM
As briefly touched upon earlier, HPNS or helium tremors is a neurological condition that affects divers using concentrations of helium often at depths of approximately 100 to 120m, although this depth is not considered exact.
HPNS is characterised by neurological, psychological, and electroencephalographic (EEG) abnormalities that are reported during significantly deep dives that have involved breathing helium-oxygen gas mixtures. Signs and symptoms of HPNS depend on the speed of compression and the hydrostatic pressure attained, but generally speaking the faster the rate of compression rate and the higher the pressure, the more severe the clinical presentation of HPNS will be.
Another concern for divers using gas blends containing helium is a phenomenon known as ICD or ‘isobaric counter diffusion’.
Isobaric counter diffusion is the diffusion of one inert gas into body tissues while another inert gas is diffusing out. During decompression, this can result in the formation or growth of bubbles without direct changes in environmental pressure. If the gas that is diffusing into a tissue does so at a rate which exceeds the speed of the other leaving, it can raise the combined gas concentration in the tissue to a supersaturation sufficient to adversely impact bubble development within a divers tissues.
A GLOBAL SHORTAGE
Although helium is the second most abundant element in the universe, most of it within the earths atmosphere bleeds off directly into space.
Most people understandably associate helium with party balloons and squeaky voices but why is it so expensive? Because helium is inert and has extreme melting and boiling points (both near absolute zero) scientists use it in cryogenics, high energy accelerators, arc welding and silicon wafer manufacturing. It is also a critical ingredient in hospital MRI (magnetic resonance imaging) machines.
Due to reserves allegedly drying up and a decreased supply from major producers, helium has unfortunately, in recent years, rocketed in price. With Russia now out of the market subject to the war in Ukraine and the USA solely producing 75% of global stocks, there is most certainly a valid concern that running out of helium could be the next great global health care crisis.
