Thursday January 23 2020
September 8, 2013

Go Deep: Trimix Diving

Have you ever thought about diving in extreme depths like 100 meters? Let’s make it 200 meters? The deepest dive approved on the world was made to 318.25 meters (1044 ft) by Nuno Gomes. You can read my article about this dive from here.  But how diving to these depths can be possible? For non-divers, the answer can be pretty quick: “divers have air sources with them, so they can breath and dive to which ever depths they want to.” But in fact, this is not the case. If you know a little bit about diving, you can answer this a little bit different like “this depth can be reached if you have more than one air source (more diving tanks) and physical condition.” These are quite right but there are other serious factors when we talk about really deep dives.

First, we should understand the affects of compressed gas in diver’s tanks. Okay, the gas in diver’s tanks is not pure oxygen. It is air. The very same air when you breath right now, while reading this article. But it is compressed. Air is a mixture of oxygen, nitrogen and the other gases (I’ll omit other gases in this article to have an easier understanding). At the start of a dive, air in divers’ tanks have a pressure of 200 atm/bar (3000 psi). Your car tire almost has an air pressure of 1 atm/bar (14 psi).

Partial pressure (pressure of only one gas in the mixture, at 1 atm/bar -on the sea-level-, for example, partial pressure of oxygen is 0.2 atm/bar, nitrogen has a partial pressure of 0.8 atm/bar) of nitrogen and oxygen increases when the depth increases. So why this should interest us? Let’s go one step further.

When the partial pressure of nitrogen increases, anesthetic effect of nitrogen comes out, which is called nitrogen narcosis. The symptoms are very close to drunkenness. Oxygen is not that innocent either. When the partial pressure of the oxygen increases when the depth increases, oxygen toxicity occurs. It is known that when the partial pressure of oxygen reaches 1.4 atm/bar, which means when a diver dives to 60 meters there is a risk of oxygen toxicity.

To be able to reduce these risks, a third gas enters the equation. Meet with helium! That’s why this is called “Trimix“. They mix nitrogen, oxygen and helium. By doing so, partial pressure of oxygen and nitrogen is decreased. This results in being able to make deeper dives because risk limits of nitrogen narcosis and oxygen toxicity change to a deeper point. Let me give you an example about oxygen toxicity I have already mentioned. When only air is used, in 60 meters, the partial pressure of oxygen is 1.4 atm/bar which can cause oxygen toxicity. This is because 20% of the air is oxygen (on the surface, partial pressure of oxygen is 0.2 atm/bar and in 60 meters, it makes 1.4 atm/bar which is considered as the limit of oxygen toxicity). But in trimix diving, there can be only 10% of oxygen in the mixture. So, partial pressure of oxygen is 0.1 atm/bar on the surface. To have a 1.4 atm/bar of partial pressure, the diver should be diving to 130 meters for the same risk.

This case is also valid for the nitrogen narcosis. When partial pressure of nitrogen is decreased by adding helium in the mixture to form a trimix, risk limit of experiencing nitrogen narcosis is delayed to a deeper point.

Of course there are some disadvantages of using helium in trimix diving. First of all, helium is expensive. Second, it conducts heat much more than air so, it is not suitable for inflating the dry suits during the trimix dive. Trimix divers usually use “argon” in a separate tank to inflate their dry suits. Even air is conducting heat faster than argon and divers get cold underwater. Third, helium dissolves in our circulation system faster than nitrogen causing a bigger risk of decompression sickness if a fast ascend is made.

Sometimes, it is possible not to use nitrogen in trimix diving. Heliox is a mixture of oxygen and helium. But because helium is expensive, it is a good idea to remain some nitrogen in the mix. Also, High Pressure Nervous Syndrome may occur when a diver dives deeper than 150 meters (500 ft) with heliox. Wikipedia describes High Pressure Nervous Syndrome (HPNS) as:

is a neurological and physiological diving disorder that results when a diver descends below about 500 feet (150 m) while breathing a helium–oxygen mixture.

Name of trimix depends on the percentage of the gases. For example, if a trimix tank has 10% oxygen, 65% helium and 25% nitrogen, the name of the mix is “trimix 10-65-25“.

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About Seaman


Hi, my name is Murat Demirağ. I have been diving since 1996 and teaching scuba since 2005. I wanted to share my knowledge through

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