What is Trimix IANTD ,Warning:You must be Qualified To use this Gas Mixture
Normoxic and Trimix
Trimix Diving: The History, Purpose and Percentages of Each Gas
Nick Barr
• Posted: January 25, 2024
Last Updated: April 3, 2026
Trimix is a gas blend consisting of helium, oxygen and nitrogen. It allows divers to explore depths that were once considered unattainable. The specific ratio of these three gases depends on the depth of the dive. In this overview we will delve into the aspects of Trimix diving so that you can make an informed decision, about its suitability, for your technical diving endeavours!
Trimix refers to a gas mixture that combines proportions of oxygen, nitrogen and helium used to explore depths. Trimix allows trained technical divers to safely conduct dives beyond recreational depth limits, when properly planned and executed. However, before attempting diving, it is very important to obtain Trimix diving certification and use equipment that is compatible with Trimix diving, such as a dive computer. While most recreational diving equipment can handle Air Nitrox mixes, they may not be suited to handling Trimix.
You will need to adjust the oxygen and nitrogen levels while increasing the proportion of helium. These adjustments help prevent nitrogen narcosis and minimise the risk of oxygen toxicity. The exact calculations required in determining the percentage are quite complex.
There is no “standard” composition for Trimix. Each blend is designed according to the target depth, the acceptable partial pressure of oxygen (PO₂) limits, and the desired Equivalent Narcotic Depth (END). The deeper the dive, the higher the helium fraction is typically increased in order to reduce nitrogen narcosis and lower gas density, while the oxygen content is adjusted to remain within safe limits.
How do you determine the percentages of each gas in Trimix?
The percentages of oxygen and helium are included in the naming convention for Trimix. Nitrogen percentage may be included in certain cases. For example, 21% oxygen, 45% helium and 34% nitrogen are contained in Trimix 21/45.
Trimix is generally divided into two main categories: Normoxic and Hypoxic. Normoxic Trimix contains 18% oxygen or more, allowing it to be safely breathed at the surface. These blends are commonly used for dives in the 50–70 metre range, depending on the planned partial pressure of oxygen (PO₂) and Equivalent Narcotic Depth (END).
Hypoxic Trimix contains less than 18% oxygen and cannot be safely breathed at the surface. These mixtures are used for deeper technical dives and require a separate travel gas for descent and ascent.
What is the purpose of using helium for breathing?
When it comes to breathing gases, oxygen and nitrogen are pretty standard, and you can ask why helium was chosen as a third element. First, helium has a lower density that makes breathing at extreme depths more comfortable.
Trimix is primarily reserved for professional and technical divers, as opposed to other mixtures of breathing gases. In general, recreational divers would not have reached the depth required for these special measures. We’ll talk about why this mixture is for special purposes in a moment.
The Military Origins of Trimix Diving
The development of mixed-gas diving has its roots in military and commercial diving research. In the mid-20th century, the U.S. Navy and the Royal Navy began experimenting with helium-based breathing gases to enable deeper and longer dives. Early deep diving primarily used Heliox, a mixture of helium and oxygen, particularly in saturation diving operations.
However, divers discovered that breathing pure helium-oxygen mixtures at extreme depths could contribute to High Pressure Nervous Syndrome (HPNS). To mitigate this effect, nitrogen was reintroduced in controlled amounts, leading to the development of Trimix.
In the 1970s and 1980s, pioneering technical and cave divers began adapting these mixed-gas techniques for exploration beyond recreational limits. By the 1990s, formalised training standards were established, making Trimix more widely accessible within the technical diving community.
Today, Trimix is widely used in deep technical diving to reduce nitrogen narcosis and oxygen toxicity at extended depths.
Are there any drawbacks and risks in considering diving with Trimix?
Interestingly, not all tanks are filled with this material. However, there are several reasons why it is not commonly used and not everyone is a trimix diver.
One reason is that helium costs extra. This is largely due to the fact that producing helium is not easy and requires special training. Decompression with trimix is a little more complicated than with traditional mixtures, so divers should be trained on how to avoid decompression sickness.
Another disadvantage of helium is that it decreases the body temperature and increases the risk condition called HPNS – High Pressure Nervous System, when it is used in Trimix Diving. This condition, which can occur when diving deeper than 500 feet (150 metres) in the ocean, manifests as fatigue, nausea, tremors or cognitive impairment, and may in some cases be caused by a helium tremor. Although it’s very rare, the risks of exposure to helium are constantly being investigated by scientists.
Trimix represents one of the most important advancements in modern technical diving. By carefully balancing oxygen, nitrogen, and helium, divers can manage nitrogen narcosis, control oxygen exposure, and reduce gas density at extended depths.
However, Trimix is not simply a “deeper diving gas”. Each blend must be precisely calculated according to depth, partial pressure limits, and Equivalent Narcotic Depth planning. Proper training, specialised equipment, and disciplined dive planning are essential to ensure safety.
For divers looking to explore beyond recreational limits, Trimix opens the door to deeper wrecks, remote cave systems, and advanced technical expeditions, but only when approached with the knowledge, preparation, and respect that mixed-gas diving demands.
Contact one our IANTD Technical Diving Facilities and ask about our Recreational Trimix, Normoxic and Trimix Courses.
