For 60 years, the tanker SS Cities Service Empire lay undisturbed under 250 feet of ocean after being sunk by a German U-boat off the Florida coast during World War II. Then, in mid-2002, a group of five technical divers using new breathing techniques that allow deeper dives, reached the well-preserved wreck. More than 800 Merchant Marine freighters and oil tankers were sunk, many of them right off the U.S. coast during the early months of the war, according to military historians.
    News items such as the above (WSJ July 1,2002) piqued the interest of SGR editors, suggesting that a look into the niche market of mixed gases for SCUBA (Self Contained Underwater Breathing Apparatus) diving would be of interest to our readers.
    The term "Technical Diving" can be applied to all diving methods that exceed the limits imposed on depth and/or immersion time for recreational scuba diving. The key to all deeper diving is the use of something other than compressed air (O2=21 percent, N2=79 percent) as a breathing mix.
    Divers have used compressed air as their breathing medium since the advent of diving in the 1950s. The principal advantage of air is that it is readily available and relatively inexpensive to compress into cylinders. Air, however, is not the "ideal" breathing mixture for diving. With a concentration of approximately 79 percent nitrogen, compressed air poses two potential problems for all divers—susceptibility to nitrogen narcosis (a condition resembling alcoholic intoxication) at deeper depths and decompression sickness. Either can be fatal. To reduce the dilatory effects of nitrogen on divers a special mixture, called Nitrox, was developed.
    Mixed-gas diving and other advances have made it possible for a new breed of divers to get to previously unreachable depths and explore sunken ships, perform salvage work, and do some construction. It is a technique that has been known for decades, but has been used almost exclusively by the Navy and commercial exploration companies. Special computer software is now available that makes it possible for new technical divers to calculate when to switch from, say, a helium/ oxygen/nitrogen mixture to just oxygen and nitrogen.
    "Non-air" gas mixtures help avoid nitrogen narcosis. Mixed-gas diving also alleviates decompression problems and helps avoid oxygen toxicity. Mixed-gas diving operations require detailed planning, sophisticated equipment and, at times, extensive support personnel and facilities. The fact that such dives are often conducted at great depths and for extended periods increases the risks associated with them. It is extremely important for the breathing mixture to be properly identified, because breathing the wrong gas can lead to a fatal accident. One of the pioneers of the sport, Sheck Exley, died in 1994 trying to be the first to dive 1,000 feet to an underwater cave.
    More than one breathing gas is used under different conditions, with the diver switching gases for different depths. The type of gas mixture used is determined either by the maximum depth planned for the dive, or by the length of time that the diver intends to spend underwater. While the recommended maximum depth for conventional scuba diving is 130 ft, technical divers typically work in the range of 170 ft to 350 ft, sometimes even deeper.
    One or more mandatory decompression "stops" are usually required during ascent in technical diving. During this process, the diver may change breathing gas mixes more than once. Decompression stops are necessary so that gases that have accumulated in the diver’s tissues (primarily nitrogen) can be released in a slow, controlled manner. If an individual exceeds the limits of time and/or depth for recreational diving, and/or ascends too quickly, large bubbles can form in tissues, joints, and bloodstream. The formation of these bubbles leads to an extremely painful condition known as Decompression Sickness (DCS)—more commonly known as the "bends," which can cause paralysis and even death.
 

Take this job and . . . (Surface supplied diving (SSD) is an alternative to SCUBA diving. It consists of lowering divers into the water on a support platform and supplying them with breathing gas (air or another gas mixture) through a flexible hose attached to a diving helmet, which is connected to an "umbilical" that supplies breathing gas, two-way communications, a depth measurement tube, and (optionally) hot water to warm the dive suit. The editors thought you might share a bit of humor attached to one incident.)     Next time you have a bad day at work think of Rob, a commercial saturation diver for Global Divers in Louisiana. He performs underwater repairs on offshore drilling rigs. Below is an e-mail he sent to his sister. She then sent it to radio station 103.2 on your FM dial in Ft. Wayne Indiana, who was sponsoring a Worst Job Experience contest. Needless to say she won.     Hi Sue, Just another note from your bottom-dwelling brother . . . Last week I had a bad day at the office. I know you’ve been feeling down lately at work, so I thought I would share my dilemma with you to make you realize it’s not so bad after all.     Before I can tell you what happened to me, I first must bore you with a few technicalities of my job. As you know, my office lies at the bottom of the sea. I wear a suit to the office. It’s a wetsuit. This time of the year the water is quite cool. So what we do to keep warm is this: We have a diesel powered industrial water heater. This $20,000 piece of equipment sucks the water out of the sea. It heats it to a delightful temperature, then pumps it down to the diver through a garden hose, which is taped to the air hose. Now this sounds like a darn good plan, and I’ve used it several times with no complaints. What I do, when I get to the bottom and start working is take the hose, and stuff it down the back of my wetsuit. This floods my whole suit with warm water. It’s like working in a Jacuzzi.     Everything was going well until all of a sudden, my butt started to itch. So of course I scratched it. This only made things worse. Within a few seconds my butt started to burn. I pulled the hose out from my back, but the damage was done. In agony I realized what had happened. The hot water machine had sucked up a jellyfish, and pumped it into my suit. Now since I don’t have any hair on my back, the jellyfish couldn’t stick to it. However the crack of my butt was not as fortunate. When I scratched what I thought was an itch. I was actually grinding the jellyfish into the crack of my butt.     I informed the dive supervisor of my dilemma over the communicator. His instructions were unclear due to the fact that he, along with five other divers were all laughing hysterically. Needless to say I aborted the dive. I was instructed to make three agonizing in-water decompressions stops totaling thirty-five minutes before I could reach the surface to begin my chamber dry decompression.     When I arrived at the surface I was wearing nothing, but my brass helmet. As I climbed out of the water. The medic with tears of laughter running down his face handed me a tube of cream, and told me to rub it on my butt as soon as I got in the chamber. The cream put the fire out, but I couldn’t poop for two days because my butt was swollen shut.     So next time you’re having a bad day at work think about how much worse it would be if you had a jellyfish shoved up your butt. Now repeat to yourself, " I love my job, I love my job, I love my job . . . " Love, Rob

 
Nature of Nitrox
   Nitrox is a gas mixture of oxygen and nitrogen, but with a higher oxygen percentage than found in ordinary air. As a result of its higher oxygen concentration, the percentage of nitrogen in nitrox is always lower than in air. Two standard mixtures of Nitrox are recognized by NOAANitrox I and Nitrox II.
    Nitrox I has 32 percent oxygen and 68 percent nitrogen. Nitrox II has 36 percent oxygen and 64 percent nitrogen. While an increase of 12 to 16 percent oxygen by volume may not seem significant, nitrox makes it possible for divers to extend their bottom time dramatically, and at the same time, decrease their risk of developing DCS.
    While nitrox offers definite benefits, it also presents associated risks. The major hazard is oxygen toxicity, which results when oxygen is inhaled in high concentrations for an extended period. Toxicity occurs most often when a diver exceeds the recreational limits for depth. Under these circumstances, a diver can experience an epileptic-like seizure, which may lead to drowning. Due to this potentially fatal hazard, divers who use nitrox are trained to adhere to special dive tables, which list the maximum safe length of bottom time.
    Nitrox is usually prepared by mixing pure oxygen from one source (e.g., from a tank of 100 percent oxygen) with air, until the desired oxygen concentration is reached. Adding oxygen to air always lowers the percentage of nitrogen in the final nitrox mixture, because the sum of gas percentages cannot add up to more than 100 percent. The process requires quality control to ensure that the desired oxygen concentration is reached, and that the two gases are thoroughly mixed in whatever container holds the nitrox.
    When nitrox is discussed in relation to a specific dive profile, it must always be qualified with the exact percentage of oxygen used. Nitrox I and Nitrox II present different risks.
 
Heliox
    Another type of mixed gas diving incorporates the use of heliox, a mixture of 79 percent helium and 21 percent oxygen. This mixture is used for very deep diving — usually depths greater than 200 feet. Unlike nitrogen, helium does not have an intoxicating effect at any depth. It has a lower density than nitrogen, which makes breathing easier, and in cases of extended submersion, it improves decompression.
    Heliox does have its drawbacks, however. It is expensive, supply is limited, and its thermal conductivity is six times greater than that of nitrogen. A diver breathing heliox will lose body heat six times faster than someone breathing compressed air or nitrox, a condition that leads to hypothermia. To avoid hypothermia, divers often wear special suits that are filled with hot water, which is pumped down from the surface. Heating the heliox before the diver inhales it is another strategy that combats hypothermia. Both of these procedures require specialized equipment and highly trained personnel.
 
Trimix and Computers
   Trimix is a mixture of oxygen, helium, and nitrogen. Nitrogen, usually in a small percentage (e.g., 15 percent), is added to heliox to create trimix, in order to lessen the risk of the high pressure nervous syndrome seen with helium breathing. Nitrogen slows down nerve conduction. Trimix is used in very deep dives in place of air to reduce the partial pressure of oxygen (to avoid oxygen toxicity) and nitrogen (to avoid nitrogen narcosis). The percentages of gas components vary depending on the dive. The deeper the dive, the lower the percentage of oxygen and nitrogen, and a higher percentage of helium. Trimix mixes are labeled for example as "Trimix 10/50" or "Trimix 10/50" (10 represents the percentage of oxygen in the mix, and 50 is the percentage of helium).
    The requirements of technical diving make it necessary for extensive training and a deep understanding of customized dive tables. New technology has made available Mixed Gas Decompression Computers to handle the complexities of multiple gas switching—trimix and nitrox breathing mixes, as well as closed circuit rebreathers. Companies like Delta-P Technology Ltd., Abysmal Diving, and Dive Rite offer computers for this purpose. It is targeted primarily at the trimix open circuit market and the rebreather market. A number of open circuit decompression computers are already available for open circuit nitrox divers, but these do not permit a switch between open and closed circuit modes.
    Trimix divers cut individual decompression tables on PC-based software for each dive. A dive computer sidesteps the need to cut tables for each dive and is also a multilevel decompression computer. While a decompression computer for technical diving permits diving to be carried out with a great degree of flexibility, planning gas management is still essential to ensure that sufficient gas is available to carry out the decompression penalties incurred.
 

One diver's experience I have worked in the commercial diving industry here in Los Angeles since 1986. I have made hundreds of mixed gas (HeO2) dives and have more than 600 Saturation days logged, with over 40 days at 750 fsw. Most of our commercial gas uses include He and O2 mixes blended on-site to meet the safe PPO2 levels at our working depths. Generally that means at 200 fsw you breathe an 80/20 or 86/ 14 blend of HEO2, and by the time you reach deeper depths like 400- 500 fsw, your O2 is down to 9-10 percent. Our use of Nitrox is limited to surface diving decompression. We will breathe a 50/50 mix only while on in-water Deco stops above 60 ft. We do not get into the tri-mixes and nitrox diving that has become the craze with the sport scuba divers. Consensus is that although the tables for these mixes are safe, not everyone using them observes the rules of safety, and believe me (from experience) if you get bent, you had better have a decompression chamber nearby—and most sport divers don’t. — Dave Gilbert

 
Where No Man (or Woman) has Gone Before
Technical divers are the new breed of explorers on the planet. Already divers around the world are exploring deep wrecks, caves, and reefs in the 500-ft to 1000-plus-ft depth ranges. New gas mixes have plowed the way for deep exploration by individuals. No longer is it the realm of commercial divers working for oil companies and navies of the world......   
 

Dave McClure is an avid recreational diver. He is President and Director of US Internet Industry Association, and an active member of the SGR Editorial Advisory Board.
Many thanks also, for editorial review, suggestions, and editing to: Dr William E Adams Jr, TRIMIX diver instructor; TRIMIX gas blender instructor; Nitrox gas blender Instructor; Nitrox rebreather Instructor; Life support equipment techncian instructror; Mixed gas closed circuit rebreather diver.