Mix Primer

by Michael Menduno with John Crea

"Safety is the key consideration in diving; it entirely controls depth capability."

Imbert, Ciesielski & Fructus,

Safe Deep Sea Diving Using Hydrogen

We live in a compressed gas environment. The atmospheric air we breathe is a mixture of gases composed of about 78% nitrogen, 20.9% oxygen, 0.9% argon, and a balance of carbon dioxide and rare gases, at a total pressure of 14.7 psi at sea level, conveniently defined as one atmosphere, or atm. Of these, oxygen is the only gas required to sustain human life. The other gases breathed from the atmosphere, or in a diver's gas mix, serve as a carrier and diluent for oxygen in order to maintain it within physiological limits defined by partial pressures (PO2)1.

Humans function optimally at an oxygen partial pressure of about 0.2 atmospheres (atm), and without too much CO2. Above and below this level, oxygen induces a variety of physiological effects depending on the dose (PO2) and exposure time.These can range from hypoxia (oxygen starvation) at partial pressures below about 0.14 atm, to whole body toxicity and central nervous system (CNS) toxicity at elevated partial pressures greater than 0.5 atm and above about 1.2 -1.4 atm respectively.

During the limited exposures encountered in diving, oxygen levels can be extended upward, and in fact it's beneficial to do so. For most surface-based dives scuba operations, elevating oxygen partial pressures to about 1.2-1.4 atm seems to be ideal and is unlikely to induce CNS toxicity, though PO2 are sometimes extended to as high as 1.6 atm and above for limited duration dives and during decompression, when the diver is at rest. Whole body toxicity resulting from very long exposures at PO2s above 0.5 atm is rarely an issue in most technical dives, but can become critical during the lengthy exposures encountered in saturation diving. In this case, oxygen levels (PO2) are usually maintained near or below 0.5 atm.

Carbon dioxide (CO2), which serves to regulate respiration, also plays an important role in diving physiology. CO2 can be toxic at elevated pressures and is believed to "interact" with both oxygen and nitrogen increasing the diver's susceptibility to oxygen toxicity and narcosis.

Even though oxygen is the most vital gas physiologically, inert gas is important to any discussion about diving as it is the source of decompression and other physiological problems. With the important exception of helium and also neon, other inert gases such as nitrogen and sometimes hydrogen, used to dilute oxygen levels in diving mixtures, act as a narcotic at elevated partial pressures. Though this narcotic property is generally highly undesirable from a diving perspective, it can sometimes be used to advantage in dealing with another problem encountered by deep sea divers; the effects of rapid increases of hydrostatic pressure to about 600 - 800 fsw and beyond, referred to as High Pressure Nervous Syndrome (HPNS) and usually characterized by tremors.

Diving gases have thermal consequences as well and can also effect vocal communications. All of these effects must be taken into account when considering the use of diving gases. A summary of the diluent gases that are used in diving is shown in Table 1. (pg. 71) along with their physiological properties.

The Problem With Air

As a diving gas, air has several real limitations from a safety and performance perspective. For shallow water dives in the 30-130 fsw range, the high nitrogen levels found in air result in excessive decompressions (and short no-stop times); the oxygen levels being far below optimal.

Conversely, for deep dives beyond about 190- 220 fsw, the partial pressure of oxygen becomes excessive, increasing the likelihood of a CNS toxicity hit. However, the most immediate problem is usually narcosis. Nitrogen, the primary diluent gas in air, becomes increasingly narcotic beyond 100 fsw (a partial pressure of nitrogen of about 3 atm and greater), impairing the divers ability to perform and respond. Density is also an issue on deep dives; air is difficult to breathe at depth and can effect equipment performance. In the midrange, from about 130 to190 fsw, oxygen levels found in compressed air are close to optimal, and the narcosis is manageable with experience. In practice this is a workable range for air, though it is sometimes extended deeper for short duration dives.

As a result of these limitations, air is generally considered to be inefficient as a diving gas for most applications, except from an availability and cost standpoint and was abandoned some time ago as the gas of choice in most commercial and government operations.

Special Mix Technology

First conceived in the early 1900's and eventually applied to the problems of underwater breathing, the use of custom or special mix diving gases -- mix technology -- was developed to overcome the inherent problems and limitations associated with diving compressed air.

The fundamental idea behind mix technology is to improve safety and performance by optimizing a diver's breathing gas during various phases of the dive. This is best done by treating the dive in two distinct phases; the descent and working phase and decompression. Each has it's own set of issues and objectives.

The important issue during the working phase of the dive is to insure that the diver maintains a safe, sustainable level of oxygen based on the exposure, and that undesirable inert gas effects, such as nitrogen narcosis and excessive decompression, are minimized. Reducing breathing gas density is also important on deep dives as it improves the ability to ventilate the lungs, facilitating CO2 removal and increasing work capacity. These objectives are accomplished by setting reliable oxygen levels based on the planned working depth, and selecting an appropriate inert gas to serve as a diluent, for example, substituting helium for all or a portion of the nitrogen found in air on deep dives to reduce or eliminate the effects of nitrogen narcosis, and to reduce gas density (see Table 1.).

During decompression, which generally represents the longest phase of many technical dives, the objective is to maximize oxygen levels subject to reliable limits; oxygen management is the key to reliable decompression. In addition, inert gas switches are also used to facilitate off-gassing and minimize additional gas loading during decompression. In open-circuit scuba2, this is accomplished by switching mixes (usually by switching cylinders ) during decompression according to a predetermined plan, normally included as a part of the decompression schedule. The types of gas used and number of switches depend on the specific exposure and operation.

In the case of a simple dive, for example a open water no-stop dive to 70 fsw, a single enriched air mix such as an EAN 36 (36% oxygen, balance nitrogen) might be used as the diving gas for both the working and decompression phases of the dive (in this case a controlled ascent and safety stop). Longer decompression dives might involve a single bottom mix such as air or enriched air supplemented with oxygen for decompression. For deep operations, multiple mixes are used, including a helium-based bottom mix and intermediate or decompression mixes (typically nitrox), before switching to pure oxygen at 20 fsw. This necessitates carrying and/or staging multiple cylinders, called stage bottles, in addition to the diver's set, containing the appropriate mixes. Staging is a matter of operations and depends on the specific environment and exposure.

Decompression Management and Other Tools

Though air tables and dive computers can sometimes be used directly for special mix diving, for example; breathing enriched air while following an air-based DC or table as a safety hedge, most mix dives require special application tables. However, in the case of enriched air, an equivalent air depth (EAD) can be calculated making it possible to use air tables (see technically speaking).

Because most shallow water (30 -100 fsw) dives tend to be multilevel, the prospect of being limited to using EAD tables when diving enriched air can represent some disadvantage. However, a number of EAN-compatible dive computers (DCs) are currently in development and will be available soon.

In addition, because of the number of cylinders that must sometimes be carried by the diver using open circuit scuba, diver propulsion vehicles (DPVs) are becoming increasingly important for transport in long and deep diving applications and offer a tremendous advantage in range and reduce diver workload.

Anatomy Of A Special Mix Dive

Figures 1. and 2. span the range of today's special mix applications representing the simplest and one of the more involved type of mix diving.

Enriched Air

Figure 1. illustrates a series of no-stop dive profiles to 70 fsw utilizing two different enriched air mixes compared to a similar dive on air. These profiles were calculated using the DCIEM tables and an EAD calculation method. A five minute safety stop at 15 fsw is included on all profiles.

The strategy here is to reduce nitrogen uptake which is the source of decompression problems, by substituting oxygen which is used up metabolically by the body. As shown, no-stop times can be greatly increased over air by using EAN 36 (36% oxygen, balance nitrogen), sometimes refered to as NOAA Nitrox II, yielding 75 minutes versus 40 minutes. However the maximum PO2 on the dive is only 1.1 atm at it's maximum depth of 70 fsw which is below optimal levels (PO2 = ambient pressure (3.12 atm) x the oxygen fraction, FO2, of.36 = 1.1 atm) . By increasing the maximum PO2 to 1.4 atm, which represents a reliable and sustainable oxygen level for most sport dives, in this case by using an EAN 45 , no-stop times can be increased to 175 minutes, a dramatic improvement.

The example also highlights two of the operational issues involved in enriched air diving; operating range and gas supply. All breathing mixes, including enriched air (and air as well!) have a maximum "reliable" operating depth, or MOD from an oxygen toxicity standpoint, which must be respected by the diver(see technically speaking for calculations). In the case of enriched air, these MODs can be quite shallow; for example 95 fsw for EAN 36, and 70 fsw for EAN 45 at a PO2 of 1.4 atm (see technically speaking for calculations).

Also, as seen above, in the case of shallow water dives, the diver is far less limited by no-stop (decompression) times. Consequently, gas management becomes all the more important. From a mix perspective, decompression dives using enriched air, with or without oxygen, are only slightly more complicated.

Trimix

Figure 2. profiles a typical trimix dive (an oxygen-helium-nitrogen mixture) to 250 fsw for 30 minutes showing the planned diving gases used during the working and decompression phases of the dive. The decompression procedures and calculations are based on the Haldane-Workman-Schreiner model using Matrix 11F6 (Hamilton Research DCAP computational program).The diving gases used are discussed below.

travel mix is sometimes breathed during descent on specific operations to provide decompression advantages (minimize inert gas loading during descent) and/or to avoid problems of hypoxia (too little oxygen; less than about 0.14 atm) near or at the surface when using a bottom mix with a low oxygen fraction (FO2). Travel mix is not used in the profile shown above .

bottom mix, in this case trimix, is breathed during the working and usually deepest portion of the dive. Both the oxygen and helium content (fractions ) are predetermined based on the planned working depth and environmental specifics of the dive eg. physical overhead, open water etc.. For the profile shown in Fig.2, a trimix 17/50 (17% oxygen, 50% He, balance N2) is used. The oxygen fraction is set at 17% to insure an oxygen partial pressure of 1.45 atm at 250 fsw, the deepest portion of the dive. The mix was set with 50% helium to create an equivalent narcosis depth (END) of 85 fsw (see technically speaking for calculations-ed.). ENDs are commonly set at depths ranging from 75 feet to as high as 200 or more, depending on the dive and what needs to be accomplished.

For very deep dives, typically beyond about 800 fsw, High Pressure Nervous Syndrome, HPNS, associated with oxygen-helium (heliox) mixtures, becomes a critical limiting factor, though with the rapid ascent rates used in technical (self-contained) diving, this phenomenon may ocurr at depths as shallow as 400 to 500 fsw. One way to reduce HPNS is to greatly slow descent rates with depth, providing the diver with time for pressure adaption. However this method is impractical for most deep dives. The method most commonly used today is to counterbalance the excitatory effects of pressure with gas narcosis. In this approach, nitrogen and sometimes hydrogen which cause narcosis at elevated pressures, are added to the basic heliox mix used in deep diving in suitable concentrations to make rapid descents (compression rates) possible.

intermediate or decompression mixes, usually one or more increasingly enriched nitrox mixtures, and sometimes air, are breathed during the decompression phase of the dive, prior to switching to oxygen at 20 fsw. The strategy here is to boost oxygen levels back up to an optimal PO2 of normally about 1.4 - 1.6 atm during the decompression3, and to get off of the helium-based bottom mix as soon as operationally feasible by substituting nitrogen -- a slower diffusing gas -- for helium, to facilitate helium off-gassing and slow additional gas loading. For the profile shown in Figure 2., a single enriched air mix, EAN 36, is breathed from 110 to 30 fsw before switching to oxygen at 20 fsw. For deeper and longer exposures requiring deeper stops, a series of mixes might be used, for example; air (from 220 to 150 fsw) to EAN 30 (from 150 to 80 fsw) to EAN 50 ( from 70 fsw) to oxygen at 20 fsw.

oxygen is normally breathed beginning at 20 fsw (PO2 = 1.6 atm) to complete the decompression prior to surfacing, though it is sometimes used as deep as 30 fsw under special circumstances (Technical divers are advised to limit the use of in-water oxygen to 20 fsw-ed.). Often both the 20 and 10 fsw stops are pulled at twenty to avoid wave surge in the case of open ocean diving, and to maximize the inert gas off-loading gradient and benefit from what is called the "oxygen window" effect. Generally in-water oxygen is recommended whenever a decompression exceeds about 20-30 minutes. Five minute "air breaks" are usually taken every 25 minutes or so while breathing oxygen to decrease the divers sensitivity to elevated oxygen pressures and avoid CNS toxicity problems. Note that these breaks generally do not count towards the diver's decompression obligation unless calculated into the tables.

Oxygen is sometimes breathed on the surface as well, about two to three hours after long deep (beyond 250-300 fsw) trimix dives. Doppler bubble scores for this type of dive seem to increase several hours after surfacing indicating potential decompression stress. Surface oxygen can also be used to shorten surface intervals and reduce time-to fly limitations, and has long been recognized as a requirement in treating dive injuries.

Mix Operations

It's important to understand that the improved safety and performance offered by mix technology comes at the price of increased planning, logistics and expense. Even though making a simple enriched air dive is as easy as air, and involves minimum additional training, requirements on a big dive including; planning and set-up, water skills, gas and gear management and insuring adequate back-ups, can be significant and demand an experienced team, particularly for long and deep dives.

Conducting a deep mix dive safely requires caution, special training, knowledge, understanding, the proper equipment, and practice -- lots of it; the ability to manage multiple mixes, tanks and regulators requires it. With practice these aspects of the dive become " second nature" and the individual is able to focus on his or her specific mission and objectives.

Expense is also an issue; special mix diving costs more. However, the primary expense is usually in equipment investment, eg. tanks, regulators, exposure suits etc, which would be partly needed in any case. Gas is generally a relatively small part of operational costs compared to hardware, though it can be significant in open circuit technical diving operations (see The Case For Heliox , pg.10). However this extra cost can usually be more than justified by the safety and performance it adds. Presently, enriched air fills in the U.S. are running about $8-14 per hundred cubic feet, while a trimix dive can cost $100 and up. If you think about it, it's a small price to pay for increased safety and the ability to do much more4.

As special mix technology becomes more available, we can expect to see improved and simplified methods, better informational materials, standarized training courses, and more readily accessible supplies. Until that time it's wise to approach this technology with care, and work with experienced individuals and facilities who understand what they are doing, and have invested the time and money to do it right.

Footnotes:

1. Note that from a physiological perspective, it is the partial pressure of oxygen that's important, not it's fraction (percentage) in the breathing mix. Human oxygen limits are defined by partial pressures.

2.In closed circuit systems, both the oxygen and diluent are mixed dynamically "on the fly" to maintain a constant partial pressure of oxygen called the set point. The inert gas used as a diluent, for example usually helium, can be switched during decompression through a special valve manifold on some systems or by using stage bottles.

3. The 1.4 - 1.6 atm seems to be reliable for decompression on surface-based scuba dives. Oxygen limits are generally reduced as the length of the dive increases. For example during long saturation dives the partial pressure of oxygen is usually set at around 0.4-0.6 atm during decompression.

4. Calculated on a $/ diving hour basis, the added cost is insignificant in many cases.