Mixed Gas Frequently Asked Questions

What is mixed gas?

Mixed gas for the diver is any combination of oxygen, nitrogen, and helium. These three gases mixed together are called trimix. Different combination percentages of these gases allow you to dive to various depths while reducing nitrogen narcosis and reducing decompression requirements.

What are the benefits of diving mixed gas?

Safety, reduced nitrogen narcosis, and reduced decompression requirements are the primary benefits of diving mixed gas.

How deep can you go on mixed gas?

Generally, divers are seeking to dive mixed gas for depths of 200 to 300 feet (60 to 91 m). Most of the deep wrecks, ledges, and caves are within 300 feet, so these are the typical training depths offered by training agencies. Divers have gone much deeper than 300 feet using a variety of mixed gases for the descent, bottom time, ascent, and decompression. Divers have descended between 500 and 1000 feet (150 and 300 m) using mixed gas, but diving to these depths is dangerous and requires extreme training and experience.

What is the difference between trimix, normoxic trimix, and heliox?

Trimix is any combination of helium, nitrogen, and oxygen. Trimix is typically used for dives deeper than 200 feet (60 m) and reduces the percentage of oxygen to below 21%.

Normoxic trimix is also the combination of helium, nitrogen, and oxygen, but the oxygen percentage typically remains at 21%. Normoxic trimix is typically used for dives between 130 and 200 feet (40 and 60 m).

Heliox is any combination of oxygen and helium. Heliox is not used as much as trimix is today, but is a viable option for deep diving. By eliminating nitrogen, the effects of nitrogen narcosis completely go away.

How are mixes labeled on tanks?

Properly labeling your tanks is essential for safety. The two main components in labeling a tank is the MOD (maximum operating depth) and the gas mixture percentages.

The MOD is the maximum safe operating depth for the gas mixture in the tank. MOD should be labeled on both sides of the tank in three-inch numbers. The numbers should be oriented so that a diver can read the MOD number from either side of the tank when it is clipped to your harness.

The gas mixture percentages should also be labeled on the tank. Using a piece of duct tape or the pre-made labels some training organizations provide work great. Place the tape or label at the top, tapering area of the tank. Write the gas mixture percentages for trimix in the following way: TXxx/xx (e.g. TX16/40, where the 16 represents the oxygen percentage and the 40 represents the helium percentage).

Where do you get mixed gas?

Some technical dive shops have the ability to mix trimix provided you have a trimix certification. Some experienced technical divers premix their own helium and oxygen in their tanks and then top off with air from their local dive shop. Knowing how to properly mix gasses is essential if you plan on mixing your own dive gasses. Getting trimix when traveling over seas can be very difficult and expensive. If you plan on needing helium in a foreign country you need to prearrange it before you arrive, otherwise you might be out of luck. If you’re mixing your own gasses, pre-pure helium and aviator grade oxygen are usually available at your local gas company. Contact them for details of getting T-cylinders of helium and oxygen.

Do you need a special certification to dive mixed gas?

Yes. Proper training is important in understanding the mechanics, physics, and physiology of breathing and mixing different gas mixtures to be used at depth. Oxygen, nitrogen, and helium react differently in your body to the pressures at depth. Knowing how and why to use certain mixes at certain depths and understanding the principles behind it are important. Knowing how to obtain the desired mixes requires precision and some math skills.

Do you need special equipment to dive mixed gas?

You do not need special regulators, BCs, etc. to dive mixed gas. However, if you’re a dry suit diver, you’ll want to invest in a pony bottle system to use when diving a helium based backgas mix. Pumping helium into your dry suit negates the thermal/warmth abilities of your underwear and suit, which means you’ll be very cold. Pony bottle systems include, a pony bottle, first stage regulator, low-pressure inflator hose, an over pressure relief valve (OPV), and a mounting system so that you can mount the pony tank to your back mounted tank.

If you’re mixing your own gasses, you’ll need the proper fill-whip set up. Some divers buy the parts and make their own, others by pre-assembled fill-whips. During your blender training course you should be exposed to the different kinds of filling whips and systems.

What is the difference between partial pressure filling and premix filling?

Partial pressure filling is the process of separately adding oxygen and helium to a cylinder and then topping off the tank with standard air from a compressor. The oxygen and helium are added to the tank to predetermined pressures, so that when you add air and fill the tank to 3000 psi, the right mixture is obtained. Basically, you create the mix by adding part oxygen, part helium, and part air.

Premix filling is the process of adding oxygen, helium, and air to a large storage cylinder, which is then used to fill other tanks. Tanks are filled directly from the storage cylinder meaning that the exact mix goes directly into the tank and thus there is no “partial” filling of separate gasses.

Most dive shops partial pressure fill trimix. It can be expensive to have and store premixed gasses and knowing that everyone probably wants different mixtures makes it almost impossible and costly to store premix trimix.