Air, nitrox, trimix and heliox

The first 3 chapters showed that pressure rises as you go deeper, and that each gas in your breath has its own partial pressure. Your body responds to each of them separately.

The partial pressure of a gas is its share of the total pressure. It is the gas’s fraction of the mix (FO2 for oxygen, FN2 for nitrogen, FHe for helium) times the absolute pressure, which is the pressure of the water plus the air above it. Pressure is measured in ATA (atmospheres absolute, about the same as bar). At the surface it is 1 ATA, and every 10 m of sea water adds 1 ATA. So at 30 m you breathe gas at 4 ATA.

Air is a fixed mix. As you go deeper, the partial pressures of its oxygen and nitrogen rise together, and at some depth one of them causes a problem. Divers change the mix, or add helium, to move that depth. This chapter covers the main breathing gases, how they are named, and how to choose one for a depth.

Why divers change the gas

There are 4 reasons to breathe something other than air:

• Oxygen. A high PO2 (partial pressure of oxygen, also written ppO2) can cause convulsions, called CNS oxygen toxicity. Over long exposures it can also harm the lungs, called pulmonary oxygen toxicity (chapter 6 and chapter 7). A PO2 that is too low causes hypoxia: not enough oxygen to stay conscious. The oxygen fraction must suit the depth.
• Inert gas narcosis. Nitrogen under pressure slows your thinking, rather like alcohol (chapter 8). Replacing some nitrogen with helium reduces it.
• Gas density. Gas at depth is denser, so it is harder to breathe. This extra effort is the work of breathing. It can lead to hypercapnia, a build-up of carbon dioxide in your body (chapter 8). Helium is much lighter than nitrogen.
• Decompression. The body soaks up the inert gases in the mix, nitrogen and helium, which the body does not use. Taking them up is on-gassing. You must release them slowly on the way up, called off-gassing. Less inert gas on the bottom means less to release, and a gas rich in oxygen on the way up releases it faster.

Air and nitrox

Air is cheap, easy to fill and needs no checking. Its limits are narcosis and density at about 30 m. Its no-decompression limit (NDL), how long you can stay and still go straight up without stopping, also gets short at depth.

Nitrox replaces some of the nitrogen with oxygen. You take up less nitrogen, so the NDL is longer. At 30 m, the Bühlmann model (chapter 13), with no extra safety margin, gives 18 minutes on air and 30 minutes on EAN32. You can compare other depths with the no-stop time calculator. The cost is a shallower MOD, so nitrox suits moderate depths.

In the 1970s Morgan Wells of the US National Oceanic and Atmospheric Administration (NOAA) developed nitrox for scientific diving. NOAA published 2 standard mixes in 1979, with 32 % and 36 % oxygen. These are the EAN32 and EAN36 still used today.

Heliox

Heliox is oxygen and helium, with no nitrogen. It causes no narcosis and its density is low, so it allows much deeper diving than air.

In 1939 US Navy divers used heliox during the rescue and salvage of the submarine Squalus, which had sunk in 74 m of water. They stayed clear-headed at that depth. Today heliox is used mainly in commercial and military diving. Most technical divers use trimix, which needs less helium and costs less.

Trimix

Trimix keeps some nitrogen. You choose enough helium to keep narcosis and density within their limits at the planned depth, and as much oxygen as the PO2 limit allows. The rest is nitrogen.

A normoxic trimix, such as 21/35, has about as much oxygen as air and can be breathed at the surface. A hypoxic trimix, such as 10/70, cannot, and needs a travel gas. A deep trimix dive usually uses 3 or more gases: a travel gas, the bottom gas you breathe at depth, and 1 or 2 deco gases.

A closed-circuit rebreather (CCR, chapter 19) holds the PO2 at a set value, the setpoint, for you. The gas it adds to dilute the oxygen, the diluent, is chosen by the same rules for narcosis and density (chapter 20).

How mixes are named

• Nitrox is named by its oxygen percentage. EAN32 is 32 % oxygen and the rest nitrogen. Some divers write NX32.
• Trimix and heliox are named oxygen first, then helium. 21/35 is 21 % oxygen and 35 % helium. The remaining 100 − 21 − 35 = 44 % is nitrogen. Heliox has no nitrogen, so its 2 numbers add up to 100.
• Oxygen means pure oxygen, 100 %.

Before the dive you measure the mix in each cylinder with an analyser and label the cylinder with its mix and its MOD. A mix 1 or 2 % away from the plan changes the MOD, so the plan uses the measured value.

How a mix is chosen

A mix is checked against 3 limits at the planned depth. Each depends on partial pressures, so each moves with depth:

• MOD: the depth at which the PO2 reaches the limit. The limit is usually the working PO2 of 1.4 for a bottom gas and the deco PO2 of 1.6 for a deco gas.
• END (equivalent narcotic depth): the depth at which air would feel as narcotic as the mix. DiveLogic counts oxygen as narcotic, which is the more cautious choice. A common limit is 30 m.
• Gas density: the weight of 1 litre of the gas at depth. Keep it below 5.2 g/L where possible, and never above 6.2 g/L.

The MOD comes from Dalton’s law. It tells you the deepest point where a mix is safe to breathe, so it is written on every cylinder.

The END works the same way in reverse. Take the part of the mix that is not helium, find its pressure at your depth, and ask at what depth air would give the same pressure. Oxygen counts as narcotic here, so only the helium is left out.

Both roundings are on the cautious side.

Gas density rises with depth in the same way, and each gas adds its own weight. Oxygen and nitrogen are heavy. Helium is about 7 times lighter than nitrogen, which is why it keeps a deep mix easy to breathe.

The best mix for a depth follows from these limits. The oxygen is the most the PO2 limit allows at that depth. The helium is the least that keeps both narcosis and density within their limits. The rest is nitrogen. For 60 m with a PO2 limit of 1.4, a 30 m narcosis limit and 5.2 g/L, this gives 20/51. Here the density limit sets the helium.

Try it

Set the depth and the 3 limits. The figure shows the best mix for that depth, its MOD, and its END and density at that depth.

The best mix calculator limits narcosis by the equivalent air depth (EAD): the depth at which air would give the same PN2 as the mix. It counts only nitrogen as narcotic. The END shown also counts oxygen, so it is higher than the EAD.