Planning Gas Mixtures
Written by Robert Palmer, European Training Director, Technical Diving International

INTERMEDIATE MIXES

Deep trimix dives require the use of multiple gas mixtures, and generally use a nitrox travel gas containing between 21 and 36 percent oxygen for use during the period from leaving surface to changing to bottom mix. There are two reasons for this. The first, and physiologically most important, is that many "bottom mixes" (as the trimix is often called) do not contain sufficient oxygen to be capable of supporting consciousness at the surface. Any mixture containing less than 16% oxygen falls into this category. Additionally, oxygen lean mixtures do not provide the best decompression schedules, and one or more oxygen rich mixtures are usually employed to ensure an acceptable decompression regime.

Travel Mixes
Travel mixes usually contain between 21 and 40% oxygen, with a nitrogen balance. Richer nitrox mixtures do not allow a convenient depth range, and weaker mixes offer no advantage. The depth at which the switch to trimix is undertaken depends on the PPO2 of the travel gas. A diver on EAN40 must switch by 30m (95'), at which point the PPO2 is 1.6 bars, whilst a diver on air can switch at depths of up to 60m (200'), at which point the PPO2 is at its maximum recommended limit, and the narcotic effect of the gas is becoming cause for concern. The richer travel mixtures allow a more effective decompression, and limit ongassing of nitrogen to a small degree during the initial descent. The weaker mixtures allow some room for safer bailout from bottom mix at greater depths, though offer a less effective decompression schedule than their richer counterparts. In most cases, a compromise is reached, and EAN32 or 36 are used for travel gases, allowing some room for deep bailout, and a slightly improved decompression schedule.

Decompression Gas
The gases used for the final stages of decompression usually contain oxygen percentages in excess of 50%. Pure oxygen can be used, but only for the final stops above six meters, unless a full face mask is used or special supervisory practices are employed. We do not recommend that pure oxygen is used below 6 meters (20') for in water decompression unless in an emergency. Mixtures containing 60 to 80% oxygen are most commonly employed, ensuring that the inert gas gradient is appropriately steep without creating the danger of oxygen toxicity in the final stages of the dive, when CNS percentage loading is at its greatest. The richer the mixture, then the better is the offgassing at the shallowest stops. However, weaker mixtures such as EAN50 or 60 allow the offgassing process to begin more effectively at slightly deeper stops, and allow a deeper changeover to the decompression mixture if the travel mixture is unavailable for any reason (e.g. equipment failure). This may be even more appropriate where very weak oxygen mixtures are used for the bottom gas (e.g. less than 12% oxygen). The choice of both travel and decompression gases should take into account possible bailout scenarios and the final CNS percentage dose, as well as the most appropriate decompression schedules.

SELECTING A TRIMIX

Before choosing your gas mixtures for a mixed gas dive, you need to know You should not exceed a PPO2 of 1.4 bars.

Each gas mixture should maintain a PPO2 of 1.4 bars or less for the working portion of the dive. EAN mixtures reaching 1.6 bars PPO2 can be used during the decompression phase as long as the total oxygen CNS percentage does not exceed 100% at the end of the dive. Your required Equivalent Narcotic Depth (END). Once you have your required END, the gas mixture to be used can be selected in association with the 1.4 bar oxygen limit. The depth at which you will operate, and the time you wish to spend there. This will govern both the final selection of bottom mix and the amount of gas that must be carried to maintain a credible reserve.

Finding Your Equivalent Narcotic Depth.
Your END is the comparative depth on air at which you are entirely in control of narcosis, even under stress. You find the partial pressure of nitrogen at this depth by using the Dalton's Law formula : PPN2 = FN2 x P. For example, an END of 35 meters would give a PPN2 of : 0.79 x 4.5 = 3.55 bar.

THE "IDEAL MIX" CONCEPT

Custom mixes can be blended to generate the best trimix for a particular depth/duration. This is a mix which optimizes both oxygen and nitrogen levels to control the effects of both gases. Most trimix divers use a PPO2 of 1.4 bars and an END of 40 -50 meters.

How to Find the "Ideal Mix"
Use Dalton's Law formulae to first find the appropriate oxygen and nitrogen percentages, knowing your depth and the partial pressure of nitrogen of your required Equivalent Narcotic Depth. The balance is helium.

BLENDING TRIMIX

Firstly, do be aware that the actual blending of gases should only be done by divers or technicians who have successfully completed a recognized training course in that discipline. Not all EAN facilities or filling stations will encounter a need to blend trimix. Trimix, as its name suggests, is a blend of three gases, in this case oxygen, nitrogen and helium. Trimix is used to dive to depths greater than those encountered in recreational sport diving, most usually in the 40-100m range (130-330 feet). The use of helium reduces both the effects of oxygen toxicity and nitrogen narcosis, but requires considerable additional training and experience on the part of the diver, well in excess of that required for the safe use on EAN mixtures.

There are several ways in which trimix may be produced. A diver may prefer to use one of the standard, or may select to use what is referred to as an "ideal mix", where the partial pressures (and thus percentages) of oxygen and nitrogen are preselected according to the precise depth of the dive. Alternatively, a third option of "heliair" exists, where the diver may accept a slightly higher than normal equivalent narcotic depth to enable the simplest form of production of trimix by adding air to helium.

SELECTING A TRIMIX

A diver planning a trimix dive will generally select a trimix that gives an oxygen partial pressure of 1.4 bar (ata) and a nitrogen partial pressure of 4 to 5 bar (ata) at the deepest point of the dive. This allows a maximum oxygen exposure of up to 150 minutes, and an equivalent narcotic depth (END) of 40 meters (130') to 50 meters (160'), which means that the diver would suffer no more nitrogen narcosis during the dive than on air at those depths. Where a specific rather than a generic mix is used, this would be termed the Ideal Mix for that dive.To produce the appropriate trimix, all the blender needs to know is the depth of the planned dive and the partial pressures of oxygen and nitrogen required. The depth of the dive will give the absolute pressure in bar or atmospheres, and the proportions of the constituent gases can be worked out and related to the pressure of the cylinders to be used.

REAL AND IDEAL GAS LAWS.

The actual compressibility of gas is related to several things. The main ones are :
The molecular density of the gas.
The temperature of the gas.
The pressure to which the gas is being compressed.

Most mixing calculations work with what are termed Ideal Gas Laws. These assume that all gases have the same molecular density and react in the same way to temperature and pressure. Thus if temperature is kept constant, mixing oxygen, nitrogen and helium would be a simple and predictable process.

In reality, though oxygen and nitrogen have similar molecular densities, helium is considerably less dense than either. Real Gas Laws take such variations in compressibility and molecular density into account. Helium, being the least dense of the three gases, is the most compressible at higher pressures. The higher the pressure, the greater the variation in actual gas percentage content of the mixture. For example, if 100 bar of oxygen and 100 bar of helium were compressed together at the same temperature, the resulting mix would contain about 54% oxygen and 46% helium. We tend therefore, when dealing with trimix percentages, to round oxygen down to the nearest whole percent and helium up.

In actual fact, minor variations in compressibility, temperature, gauge accuracy and analyzer accuracy during the actual blending process all tend to even the process out. To compute for Real Gas Laws that take into account the variations in compressibility due to molecular density and the temperature generated by an actual on site mixing system requires equations that can only reliably be done by computer, and a very slow blending process to keep temperatures constant. While such computer programs exist, and are available to the Gas Blender, the final variation in analyzed trimix blended using Ideal rather than Real Gas Laws is, in real life, virtually indistinguishable from blends made from Real Gas Laws. The difference is far more in the quality and skill of the actual blender.

BLENDING TRIMIX IN THE CYLINDER

There is a standard procedure for blending trimix, taking into account the different properties (and costs) of the component gases. It is usual to place helium (the most expensive gas) in first. Oxygen (if required) is then added, both gases being decanted slowly, with intervals in between to allow temperature to be as constant and stable as possible. Once both helium and oxygen are in, it is possible to analyze the resulting mix to ensure that the correct percentage of oxygen (as a proportion of the 2 gases) is present.

The table at the end of this chapter is used for blending trimix into empty cylinders. It assumes that 1 bar of existing gas is already in the cylinder. The gas makeup of the 1 bar of gas in the cylinder (as long as it is air or trimix) will not materially affect the final balance of the new mixture. (Remember, in a 210 bar / 3000 psi cylinder, 1 bar of gas = 0.5% and 1 psi = 0.03% of the gas in the cylinder.) A the maximum depth given for the above mixtures, the PPO2 is approx. 1.4 bar and the Equivalent Narcotic Depth is approximately 40m / 130'.

Once this has been done, and the mixture is analyzed as acceptable, air should be added vigorously to encourage mixing. The tank should be laid on its side during this process, as the less dense helium will tend to layer out unless agitated. Often the tank is rolled from side to side while being filled, or immediately following filling, to ensure mixing is adequate. The cylinder should be allowed to stand till cool, at which point it should be analyzed for oxygen percentage. If the mix is too rich, more air can be added. Too low, and oxygen can be added with a booster pump if required.

Once mixed, the gases will stay mixed due to the constant movement of the gas molecules ("Brownian Motion"). It is still good practice to re-analyze trimix immediately before the dive to ensure the original analysis was correct.

If the oxygen percentage is within 1 percent, it is assumed the nitrogen and helium percentages are also accurate. In practice, a percentage variation of 3-4 percent in these inert gases will not compromise decompression requirements or nitrogen toxicity, and it is unlikely that, if the oxygen is within 1% of calculated value, that the inert gas percentages will be much adrift.

HELIUM SAFETY

Pure helium should never be breathed during the blending process, or at any other time. Irreversible asphyxiation may occur as a result of the rapid diffusion of the gas into the lung tissues, essentially blocking the passage of oxygen once the helium source is removed. Diving grade helium generally contains about 2% oxygen (which also allows it to be breathed from directly at saturation depths in emergency) and this helps maintain a conduit for oxygen. Be especially aware of children using trimix or helium cylinders to fill balloons unsupervised, or using it to make funny voices. Balloon grade helium contains between 20-30% oxygen to prevent asphyxiation, and this makes it unsuitable for diving purposes. Do not allow diving grade, or any other grade, of helium to be used for balloon filling or for play.

TRIMIX CYLINDERS

Trimix cylinders need not be oxygen service unless pure oxygen is used in the blending process. Where the cylinder is simply used for heliair (helium/air mixtures) or where helium is blended with the appropriate EAN mix produced from a continuous blending system, an ordinary air cylinder will suffice. If oxygen is used in the blending process, the cylinder should be in oxygen service. In either case, the cylinder should be labeled TRIMIX in large letters, and have a label on it indicating the relative percentages of the mixtures in the cylinder and its maximum depth. Several training agencies market Trimix stickers for cylinders, usually in a red color.