Thailand – Australia – United Kingdom

Posts tagged “extended range

Deco and Nitrox diving in Thailand

Technical Diver underwater during deco stop

Koh Tao, Thailand

Big Blue Tech Thailand celebrates the graduation of Phil Clegg and Paw Mac-Mullit from their TDI Advanced Nitrox and TDI Decompression Procedures course conducted over 4-5 days on Koh Tao Island by Technical Instructor Ash Dunn.

Koh tao has been struck recently by severe flooding, this flooding receded but the success of the course was in jeopardy as the seas were too rough to go diving in.

The conditions on the island improved, power was restored and the sun came out. This meant the course could being and the team headed out to sea for some deep technical diving.

The diving conditions proved hard and challenging putting the fortitude of each novice technical diver to test. The surface current was strong, visibility was limited and the skills had to be performed perfectly to the instructors demands.

In the end the students walked away from the course successfully having been challenged under the worst conditions Thailand has to offer. Thankfully the students remained to continue on to their TDI Trimix Course starting tomorrow which won’t be as severe conditions wise but just as challenging skill wise.

The focus of these combined courses is to train students to be successful in technical diving. These classes are taught as if they are a small piece to a much larger picture, not as entry-level technical diving. These courses build the foundation for sound technical diving. What you will learn will be utilized in higher level trimix courses. This is not a course to learn or re-learn fundamental diving skills. Students will be held to a higher level of performance not found in many technical diving courses.
Students are taught and evaluated, not only on skill proficiency, but control, leadership, situational awareness, teamwork, and judgment. Successful students will have a finesse that few divers have. You will finish this class with the confidence, competence, and comfort to be able to complete dives at this level of training prior to receiving a c-card.

This course requires a minimum of 5 days with 10 or more dives and involves a minimum 40 hours of instruction. Expect to dive every day with lectures in the afternoon and evening for each day. The majority of dives will be conducted in shallow water for critical skill evaluation. However, each day will get deeper as the class progresses. The final day is reserved for experience dives that will be at depth with a real decompression obligation.

Course content will include, but not limited to: enriched air Nitrox usage, decompression mixtures, diving physics & physiology, dive tables, advanced decompression theory, oxygen exposure/management, team diving procedures, and contingency planning.


BSAC Extended Range Instructor Workshop in Thailand

A workshop to deliver technical diving instructor qualities and skills conducted in thailand.

Koh Tao, Thailand

Big Blue Tech celebrates the graduation of James Foleher from his BSAC (British Sub Aqua Club) Extended Range Instructor SDC Workshop conducted by BSAC Extended Range Instructor Trainer Ash Dunn over 2 days on Koh Tao Island off the coast of Thailand.

The BSAC ERD Instructor course allows qualified technical divers and BSAC Instructors the ability to teach the BSAC ERD SDC. The course covers academic presentations, skill assessment, open water skill delivery and a stress test.

The course was accompanied by BSAC Dive Leader intern Chris Shaw who acted as a student for James to practice his skills.

As a BSAC ERD Instructor James is authorized to teach the BSAC Advanced Nitrox and BSAC Extended Range course through any BSAC Centre worldwide!

Valentines Tech Expedition: Similans Technical Liveaboard

Technical divers return to shore after 4 days at sea

Khao Lak, Thailand – The Valentines Tech Expedition contingent of Big Blue Tech return to Khao Lak today after coming ashore from the Mv Pawara after the completion of a 4 night technical diving liveaboard on the Similan and Surin islands.

The liveaboard was the base of our diving while completing a TDI Extended Range course which trained the divers to conduct accelerated decompression dives to depths of 55 meters / 180 ft using 3 mixes of gas with air, nitrox and oxygen over 4 cylinders worn simultaneously.

This course was delivered by James Thornton-Allan and Andy Cavell for students Yvonne Fries, Helen Artal, Duncan Tyler and Thomas Hallstrom.

This would be the second technical diving trip in the similan islands for Big Blue Tech this season and again it proved to be a holiday setting with challenging and interesting dives. While the diving conditions are described below it was the extra touches of relaxing watching movies, sun tanning on the roof and trips in the dinghy to the beaches, which really made this trip relaxing and enjoying as a holiday.

Technical diving is saturated with bravado and peer pressure which have lead to serious diving accidents around the globe, we’re more about having fun on the surface and focusing as a team underwater which allows each diver to relax and progress at their own pace. In addition to our relaxed atmosphere we also encourage a alcohol free environment and no smoking during the diving day which has contributed to our perfect record of no diving related injuries.

The Similan Islands is located off the west coast of the west side of Thailand just north of Phuket from a town called Khao Lak. There are several ways to reach the similans by speed boat, long tail or liveaboard and is listed as one of the best diving destinations in the world.  The Similan and Surin Islands are protected marine parks managed by the Thai government to prevent fishing and destruction to help the marine environment sustain for generations to come.  Divers must pay a park fee to enter the marine park which is enforced by roaming park police boats. It seems the greatest enforcement in the area is getting the money from the dive tour operators rather than protecting the environment from fishing or negative effects like litter or pollution but it’s a better system than nothing at all.

The dive sites we visited on our trip was East Of Eden, Boulder City, West of Eden, Elephant Head Pinnacle, Christmas Tree Point, Koh Bon, Koh Tachai, Richelieu Rock, Richelieu Rock(sunset), Koh Bon Pinnacle, Boonson Wreck.

Over these sites we found ourselves at the mercy of very strong currents pushing us in all directions with changing temperature. On a dive a Elephant Head Pinnacle at 55m a freezing cold wall of cloudy water washed over us making the visibility very limited and giving all of us instant brain freeze that took your breath away. While we were struggling to adjust to the temperature we were being pushed all over the place at a very fast rate, so strong that you couldn’t kick against it to keep in place, our only option was to hide behind rocks and do strategic zig zag movements through the dive site back up to recreational diving depths where is was warm and clear again but it was an experience that taught everyone how to handle vicious currents and how to stay together as a team.

On a dive to Koh Bon we finally saw Manta Rays, thankfully our instructor dropped his mask off the back of the boat, as he went do to get it just below the surface we noticed two large manta rays circling us about 10m below us. This would be a first for some of the divers who have had plenty of chances but never actually seen one. The Giant Manta Ray or “Manta Birostris” is mostly black with a white underbelly, long triangular wings and a tail without stinger. It also has a pair of movable flaps just in front of its mouth. They can grow up to 3-4 meters wide and are recorded as up to 22ft or 670 cm in diameter or “disc” size making these very exciting animals to be witness to. These gentle giants are also one of the few rays that don’t sting so you can get quite close without worry of harm. We spent in total about half an hour with these majestic animals, while other divers were restricted by their single cylinder and no decompression limits we spent over an hour at depth without any concern for air or decompression since the dive was planned well in advance.

As the final night rolled around many started falling asleep after dinner showing clear signs of fatigue from the days diving. It was decided as a group that we would skip the last 2 dives and sleep in, we would come back with the speed boat to visit Koh Bon Pinnacle at a later date. It was also the 11th of February which is Andy Cavell’s 27th birthday. We all knew that coming back from the trip and it being Andy’s birthday that we would be well into a few drinks so it’s good to rest up for such vigorous Olympic style consumption.

Returning to shore we unloaded the boat into our taxi and headed off to our hotel for a nap, shower and relaxed for the next few days until the 13th when we would head off to the big shipwreck off the coast called the Sea Chart 1 which is 85m long in 40m of water.

Special thanks to the staff of Big Blue Khao Lak and Mv Pawara for taking such good care of us and bending over backwards to accommodate our trip.

Big Blue Tech – April in Review

April proved to be our busiest month with our biggest cavern course to date with 10 people on a 5 day exploration of the caves and cavers, an enrollment of 3 technical diving students at different stages progressing on to Trimix diving on the west coast and videography internships later. The arrival of new staff, Cory Lewis, to keep up with our nitrox distribution to the local diving community. All during this we had the arrival of “Golden Week” where 50 Japanese customers arrive for 3 weeks of diving on our boats, the overhall of all our compressors (7), Thai New Year, an electricity crisis and the renovation of the tech equipment room.

Unfortunately a few of our tech students hit a bit of trouble, Christos is still out of the water with some awful spider bite infection he got for passing out in the bushes in Khao Sok and John who picked up an ear infection for diving 5 times a day. However, this leaves room for some time off, with current courses on hold and no more planned until a CCR Megaladon Course in the end of May we’ve all decided to do some fun diving, relaxing, working on our tan and enjoying the island we all came here for.

Elsewhere on the island things have been busy. Mv Trident have been in and out of the harbour all month on technical wreck diving trips, PADI gave a discussion about how DSAT are changing their technical courses and another successful group of new instructors graduated from several different schools on the island.

We’ve got some pictures of Cory’s Tech course, he’s about 1/3 the way though and is only certified to dive in tech gear without anything too fancy. He’ll be joining the rest of the lads in mid May.

Surviving a Technical Dive

Though technical training standards have been well established by the American Nitrox Divers, Inc. (ANDI), International Association of Nitrox & Technical Divers (IANTD), Professional Scuba Association (PSA), Technical Diving International (TDI), and Professional Association of Diving Instructors (PADI) and are continuing to evolve. There is currently no set of agreed upon operational guidelines, i.e. guidelines used to direct diving operations, similar to those developed by the professional and commercial diving communities.

Originally presented in aquaCORPS Journal N6/Computing (93JUN), the guidelines which were compiled by technical diving pioneer Capt. Billy Deans/Key West Diver and dive technologist Michael Menduno with the help of many individuals throughout the community in response to the number of technical-level accidents that were occurring during that period. They are based on what are perceived as the best practices from the technical diving community drawing heavily on accident analysis techniques developed by the cave diving community, and were offered as a starting point for the development of “community consensus” guidelines for technical divers.

Now several years later, in light of the tremendous growth in technical diving, and with the majority of the recreational training agencies accepting enriched air Nitrox we felt it was necessary to update and republish these critical guidelines. Many people’s comments and suggestions for the development of these guidelines have been incorporated in this document.

About twenty years ago in response to the then growing number of fatalities, the cave diving community developed a set of safety principles based on the then-new tool of accident analysis. Later refined by pioneer Sheck Exley and elucidated in his book Basic Cave Diving: A Blueprint for Survival (Exley, 1979, 1986), accident analysis is a means to rigorously dissect an accident into its constituent parts with the goal of determining what went wrong. Applying this tool to cave diving, it was discovered that most diving accidents could usually be attributed to a primary causal factor and typically one or more contributing factors. What’s more is that these factors could be boiled down into five basic cave diving safety principles: be trained, use a continuous guideline to the surface, manage your gas according to a thirds rule or better, don’t dive deep (on air), and carry at least three lights. A sixth principle, known as Eternal Vigilance, states: Anyone Can Die At Any Time On Any Cave Dive. Accident analysis of these resulting safety procedures have become the cornerstone of cave diving safety ever since.

Numerous other analysis of sport diving accidents have been conducted following the early cave diving work. In 1989, Mano and Shibayama published a study titled Aspects of Recent Scuba Diving Accidents (Mano and Shibayama, 1989), which analyzed 264 fatalities and 319 incidents of decompression illness and arterial gas embolism. According to the authors, over 45% of sport diving fatalities that occurred were due to reckless diving or lack of technique. Most appear to have been preventable. In another study, Chowdhury, in affiliation with the National Underwater Data Center (Chowdhury, 1989) conducted an analysis of wreck diving accidents. His conclusions were that 73% of the accidents involving wreck penetration were due to the lack of a continuous guideline, while 42% of the fatalities that occurred external to a wreck were due to out of gas emergencies.

In 1990, the late Sheck Exley revisited his earlier work in a paper published in Underwater Speleology. Based on the recent trends in accidents, Exley concluded that perhaps too much emphasis was being placed on the basic cave diving principles in light of more recent tools and techniques being employed by cave divers (e.g., mix technology), and that an expanded list of safety recommendations should be developed.

Exley’s conclusions provided motivation for the original paper. Our approach was to attempt to identify and address the factors that could potentially result in diver injury or death, building on the cave diving safety principles and practices from the community. The resulting guidelines are organized into seven categories: Requirements, Training, Gas Supply, Gas Mix, Decompression, Equipment, and Operations.

The generalized requirements for conducting technical dives were aptly summarized in the form of the acronym AKTEE.

These are:

Why are you doing this? A proper attitude is essential to conducting technical dives safely. There is no room for recklessness or machismo.

Without the proper knowledge, there are no options when problems occur.

Every dive requires an appropriate set of tools.

Skills must become second natural part of muscle memory.

Experience is exposure and environmental specific and takes time to build. Extensive wreck diving experience does not qualify a diver for cave diving and visa versa.

Note that the more challenging the dive and the further the dive goes beyond mainstream sport diving limits, the more risk the diver must accept. No amount of training or equipment will completely mitigate this risk.

Technical training is an ongoing process similar to training for an athletic season or fight training. Continual practice is the key. Completing a formal course is a good first step, but is only a starting point. It does not in itself prepare you to make the dive. Technical diving is a discipline and a “mind-set” it’s not just a card.

1. Always be prepared and trained for the dive you plan to conduct. Perform a risk assessment. Ask yourself if you, and your partner, meet the AKTEE criteria and if the dive is worth the risks involved. If the answer is no to either question, call the dive.

2. Review and practice emergency procedures frequently so that they become second nature.

Gas Supply
Ensuring adequate gas supply is the major constraint factor in self-contained diving and represents the single largest risk factor. In particular, planning and carrying adequate gas reserves is critical.

3. Always dive an appropriately redundant breathing system (minimally first and second stage redundancy) in an overhead environment, or when diving in open water beyond 130 f/ 40 m. Second stage redundancy and a dive partner is an acceptable redundant system in open water no-stop diving (recreational diving) to 130 f/40 m in good conditions though an independent redundant system is recommended for dives deeper than 60 f/18 m and/or in less than ideal conditions.

4. Pre-plan and calculate the gas required to conduct the dive (Gas requirements = planned consumption plus required reserves) and dive your plan. Gas calculations should be based on the most conservative breathing rates of you and your partner. Always dive your bottom gas using at least the Rule of Thirds [Turn the dive when one third of your gas is exhausted, leaving two thirds for the exit and reserve] in an overhead environment, or a suitable equivalent in open water, depending on the operation. There should be sufficient reserves for the dive team to exist safely in the event one diver suffers a catastrophic gas loss. For extended open water dives, the consensus seems to be to reach your first decompression stop with one third of your bottom gas remaining.

5. Plan at least a 33% reserve (1.5 x planned usage) for your decompression gas. Depending on the operation, decompression cylinders should be equipped with redundant regulators.

6. When possible, carry all the gas you will need for the dive unless it can be reliably staged, depending on the operation and environment. Note that the ability to reliably stage gas is one of the major differences between cave and wreck diving. In open water diving the goal is to be self-sufficient, to the maximum extent possible. Based on an analysis of gas logistics, the self-sufficiency breakeven point for extended open water dives appears to be about 250-300 f/77-92 m for a two-person team, depending on the duration of the dive. Open water dives beyond this require an extensive support team and effective communications.

Gas Mix
Mix technology is a tool designed to improve underwater safety and performance when properly applied. The most critical factor in special mix diving is oxygen management due to the risk of a CNS toxicity convulsion.

7. Always dive the safest possible mix(es) for the dive you plan to conduct.

8. Always analyze and label your gas and regulators before making the dive. Make sure that you know what you are breathing. Use a contents tag that specifies the type of gas and maximum operating depth. Any cylinder or regulator carrying a potentially hyperoxic gas (PO2=1.6+ at any depth during the dive) should also ideally have touch ID capability for zero visibility conditions (see below).

9. Maintain your PO2s at or below 1.45 atm during the working phase of the dive and anytime more than light work is being done, boosting oxygen levels to a maximum of 1.6 atm with care, during resting decompression. The community standard today is to run travel and bottom mix at about 1.2-1.45 atm, depending on conditions and the operation and PO2s of 1.4 -1.6 atm are generally treated as a caution zone. Take regular air breaks, as a safety hedge every 20-25 minutes when breathing oxygen. If air is not available, the lowest FO2 travel gas should be used. Some trainers take breaks during the decompression phase of the dive whenever the CNS index exceeds 80%. Note that the CNS indices being used today are just guidelines and are not necessarily supported by hard data. As succinctly summarized by Terry Billingsley (Hamilton, 1985): CNS toxicity is like sand beside the road. If you stay on the road, you won’t get into trouble.

10. Just Say No to nitrox mixes ( like air) beyond about 180-200 f/55-61 m or less, depending on the operation and environment. In particular, keep equivalent narcotic depths (END) as shallow as operationally and economically feasible, preferably 150 f/46 m or less. Note that ANDI limit s nitrox (air and EAN) exposures to 165 f/ 50 m. PSA allow s very short non-working exposures to 240 f/ 72 m and deeper under the supervision of two instructors per student.

Decompression illness is not an accident. It happens and will continue to happen as a predictable part of diving.

11. Always use appropriate and reliable decompression methods and tools for the dive you’re planning to conduct and be conservative. Carry bailout tables for gas loss scenarios.

12. Utilize a hyperoxic mix for decompression (e.g., oxygen and/or suitable EAN mixes) whenever possible when conducting a staged decompression exposure. Oxygen at 10 and 20 f/3 and 6 m stop is preferred in some circles [Air, and to a lesser extent EAN mixes, have been regarded as inefficient at reducing decompression risk (Vann, 1992)], though EAN 80 (80% O2, 20% N2) has grown in popularity as it is thought to reduce CNS toxicity risk and can be used at 30 f/ 9 m. Note just as it is recommended that divers make a safety stop on no-stop dives, some individuals prefer to treat the first (deepest) stop on a mix dive in the same manner and make a couple minute safety stop at least 10 f/3 m deeper than required.

13. Limit oxygen decompression to 20 f/6 m or less (max. PO2=1.6 atm) and use care. The diver breathing a decompression mix or oxygen should avoid anything that would increase the likelihood of CNS oxygen toxicity, or specifically, anything that might raise the diver’s C02 level. Use an oxygen regulator guard to prevent the accidental use of pure oxygen at depth. Color coding and labeling are insufficient safeguards.

14. Plan for and always be prepared to deal with decompression illness (DCI). In particular, have plenty of oxygen immediately available for treatment after any diving operation and know how to use it. Many people believe that low-cost portable on-site chambers will eventually become the order of the day.

Your equipment is your life support system which allows you to survive in a physiologically hostile environment. Second only to breathing equipment in importance, safety lines and a decompression line system are critical to diver safety.

15. Always use the best possible equipment that is well-maintained and appropriate for the dive you plan to conduct and the environment. Redundancy on all essential subsystems is key. In particular, always carry appropriate emergency equipment and know how to use it, for example: three lights (overhead environment), a decompression reel and lift bags (open water) surface signaling device (open water) and a bail-out bottle when diving as a team of one.

16. Always use a continuous guideline when diving in an overhead environment, and/or a decompression line system when conducting extended and/or deep open water dives. Note that conducting multi-level extended range open water hangs without a safety line home can be problematic and difficult. They require skills and practice to perform without compromising effective decompression, particularly when using hyperoxic decompression mixes where depth control is critical.

Note that the original set of guidelines (1993) included the following point; If possible, wear breathing equipment that allows you to survive an underwater convulsion/loss of consciousness, such as a full face mask system or retaining strap. The use of full face masks is growing and will likely become a standard for many technical diving applications due to their many advantages.

In practice this point has not stood up in the field. Technical divers have not embraced full face mask technology, nor have FFMs become a standard. This may change when rebreathers finally hit the market and/or when an effective mask is developed for technical diving applications. In the meantime practice the effective and conservative use of oxygen management in order to avoid a CNS hit.

Technical dives are operations: a project or venture involving planning, preparation, organizational structure, the use of proper equipment, teamwork, competent execution, and the capacity to respond to emergencies effectively and immediately. Diver safety is always the first priority. In terms of support requirements, technical dives fall somewhere in between recreational dives and commercial operations. Note that all dives are operations. In the case of traditional *recreational diving, the requirements are minimal.

17. Pre-plan all aspects of the dive you intend to conduct and dive your plan. Design your operation with the goal of being able to provide effective and immediate assistance to a diver in distress at any point in the dive. In particularly be prepared for the worst, and always have plenty of oxygen on hand and know how to use it. Above all, if you’re not prepared to do it right, don’t do it.

18. Always dive as a team, using surface support personnel, and when appropriate, in-water support divers, whenever possible. In particular, designate an operations manager, who is responsible for overseeing diver safety and record keeping. Note that the buddy system is not reliable enough for technical diving. A team approach based on individual self-sufficiency and competency is required, though an team of one is acceptable in some circumstances, depending on the operation and environment. Above all, always honor rule number one of team diving: anyone can “call” the dive at any time for any reason (anyone can die just as easily).

19. Utilize an effective communications system between the dive and support team whenever possible. In the future, wireless communications systems will likely become commonplace.

20. Stay within your “comfort zone” during all phases of the dive.

21. Remember: YOU, and YOU ALONE, are responsible for your own safety. Never permit overconfidence or peer pressure to allow you to rationalize compromising safety procedures. It could ruin your whole day.#

Technical diving is a discipline that uses special methods and equipment to improve diver safety and performance enabling the user to conduct dives in environments and perform tasks beyond the scope of traditional recreational diving i.e. no-stop dives in an open-water environment to 130 f/40, Europe limited decompression dives to 50 m/165 f.

For more information about technical diving, contact us at

Technical Extended Range course – (en francais )

Today Big Blue Tech started a Technical Extended Range course for Jean-Paul Canto, a veteran french diver with over 9000 logged dives from the past 40 years.

Jean-Paul had been searching high and low for a technical diving school with multiple languages and found Big Blue Tech with an in house French Instructor Guillaume Fargues.

Today we covered the basics of technical diving from gas planning and equipment setup. The goal for the course as requested by Jean-Paul was not to go deep but learn about increased safety and because of his age be able to use increased amounts of oxygen to ensure he can dive well into retirement. Naturally the course will expose him to challenges and in excess of 40m.

For the rest of the tech team we’ve been rushing to stock the boat with red wine and cheese and have place special hangers for beret’s in the dive shop.

We’ll endeavor to post about this course daily with pictures and developments as it continues.

Full Day Trip – Nitrox

Big Blue Tech conducted this seasons first Sail Rock full day trip taking us over 2 hours away from Koh Tao wil food, sun, drinks and 3 dives.

Today was, however more about nitrox diving then anything else as we trained up the interns on the use and methods of enriched air nitrox.

Many would later continue on to their deep course and the upcoming wrck trip, but for today it was a great chance to relax and feel like students again.

Returning to Shore we were met by other members of the technical team who decided to have a day off and enjoy the sunset soaking in the sea, having a beer and floating in their BCD’s. Otherwise called a “Beer Float”

By the way Emily, you are actually topless in that picture.. just to clarify that.

PADI Enriched Air and Deep Diver Specialty Course

Big Blue Tech has been busy again with the completion of a PADI Enriched Air and Deep Diver Specialty Course. Although this course is not specifically technical diving it is a great introduction to it and is a prerequisite for further more challenging diving.

Because of the amount it was also a pleasure to be able to take a boat to ourselves and park at South West Pinnacle for the duration of the morning for 2 back to back dives on nitrox. Later diving was completed again Chumphon pinnacle.

With another great group completed some progressed on to technical diving while others went back to their divemaster internship or fun diving on nitrox.

Below are some pictures from the event.

The Search for the Perfect Gas

Take a deep breath. You just filled your lungs with air, the common name for a gas mix of approximately 21 percent oxygen and 79 percent nitrogen (give or take less than 1 percent for trace gases like argon and helium). Without it, you’d keel over and die. But when it comes to diving, “Nobody ever said air was the best gas to breathe,” says technical diving pioneer Dick Rutkowski.

Any diver worthy of his C-card understands why–all that inert nitrogen does funny things in your body under pressure. Much of what you learned in open-water training is designed to mitigate the accumulation of nitrogen in your tissues to prevent decompression illness (DCI). And on dives below 100 feet, nitrogen starts to produce a narcotic haze that can become quite debilitating as you go deeper. Somewhere between 150 and 180 feet, most divers will be so narced that they are incapacitated.

Go beyond 180, and oxygen starts to be a problem. Somewhere between 190 and 220 feet, oxygen becomes toxic, resulting in sensory distortions and seizures that can be fatal under water.

Clearly, air has its limitations as a diving gas, particularly for divers who want to stay longer or go deeper than the traditional recreational diving limits. Which is why tech divers have long been experimenting with alternative blends in the search for a better diving gas.


The logic behind nitrox is simple: Replace some of the nitrogen with more oxygen. Less nitrogen in the tank means less nitrogen in the diver and fewer problems with DCS and narcosis.

There’s just one catch: When diving with enriched air, you have to monitor your oxygen exposure to avoid toxicity. There are two crucial factors to consider. The first is the relative percentage of oxygen (or PPO2) in your tank. The second factor is the time of exposure. The combination of the two tells us the total oxygen dose. An oxygen dose of 1.6 PPO2 for 45 minutes is recognized as the maximum safe limit for divers who aren’t working strenuously.

Fortunately for recreational divers, this dose limitation gives us a wide latitude to dive in the 60- to 130-foot range using gas mixtures from 32 to 55 percent oxygen. With proper planning, you can significantly increase the duration of your dives without any increased risk of decompression illness or narcosis and with an extremely low risk of oxygen toxicity.


To go beyond traditional recreational depths, technical divers employ trimix, the general term for gas blends that replace much of the nitrogen and some of the oxygen with more benign inert gases, such as argon and helium.

Based on current research and practical experience, helium is the inert gas of choice. Its narcotic properties are negligible in comparison to nitrogen and it’s a thinner, more compressible gas that helps regulators work more efficiently at extreme depths. Helium-based mixtures allow properly trained and equipped divers to routinely go to 400 feet (and beyond) with a remarkable safety record.

Trimix divers custom blend their breathing gas to suit each dive, allowing them to more precisely control oxygen limits and more dramatically reduce narcosis. For example, let’s look at a 240-foot dive using 17/50 trimix. That’s 17 percent oxygen, 50 percent helium and 33 percent nitrogen. (When expressing percentages in a trimix gas, the oxygen is always stated first, followed by the helium. The balance of the gas is assumed to be nitrogen unless otherwise stated.) This dive yields a conservative PPO2 of 1.4 with an equivalent narcosis depth of only 57 feet. In other words, the diver would experience the same level of narcosis on this trimix dive as he would on an air dive to 57 feet.

The Cost of Deeper Diving

Of course, these benefits do not come without cost. The deeper the dive, the more complicated the dive plan and the gear configuration become. The first thing most recreational divers notice is all that redundant gear, particularly the twin back-mounted cylinders coupled together with an isolation manifold. The manifold allows the gases from the two tanks to “communicate” so that the diver can use the regulator attached to either tank while consuming the gas supply equally from both tanks. On deeper trimix dives, the oxygen content in the main cylinders may be too low to safely breathe at shallow depths. In these cases, the diver must also carry separate gas mixtures in additional tanks–one cylinder of travel gas (typically a nitrox blend of 32 to 40 percent oxygen) to breathe while passing through traditional recreational diving depths and another cylinder of decompression gas (either 80 percent or 100 percent oxygen) for shallow stops.

Decompression and Helium

While helium is extremely useful in combating the ill effects of nitrogen and oxygen at extreme depths, it’s not without its problems. Because it is a lighter, faster gas, divers load and unload helium more quickly than nitrogen. For some profiles, this requires slower ascents or deeper decompression stops to keep the helium from off-gassing too rapidly and causing DCI.

Finally, the major drawback to helium-based gases is cost. Helium supplies are tightly regulated, making the gas very expensive. The cost of filling a set of doubles with a trimix will typically range anywhere from $30 to $120, depending on the amount of helium used.

As you might also expect, trimix training is quite involved and quite expensive. Trimix courses typically require certification as an advanced open-water diver, an advanced nitrox diver and a decompression procedures diver as prerequisites. The course itself will range from $900 to $1,200 in the United States, depending on geographic location, and the cost is exclusive of gas fills, charter fees, etc. When all the expenses are totaled, divers starting a trimix class should anticipate spending between $1,500 and $2,000, assuming they already have all their own gear.

Calculating a Narcosis Depth

One of the most important criteria for selecting the right trimix blend is calculating a narcosis depth air equivalency–a comparison of the expected narcotic effect of a tri-mix blend at a given depth to a more easily understood air depth.

To accomplish this, we must first select a narcosis depth limit and determine the partial pressure of nitrogen (PPN2) while breathing air at that depth. For example, a dive to 99 feet, or four atmospheres (ATA), yields a PPN2 of 3.16 on an air dive. However, if we reduce the nitrogen content to 35 percent by replacing some of the nitrogen with helium, we can dive the resulting gas mixture to 300 feet while experiencing a PPN2 of approximately 3.16. That means our narcosis level at 297 feet (10 ATA) on that blend of trimix would be roughly equivalent to the level experienced at 99 feet on air. Similar calculations must be made for oxygen exposures at depth and during decompression.

Agencies offering trimix certifications:

> American Nitrox Divers International (ANDI)

> International Association of Nitrox and Technical Divers (IANTD)

> National Association of Underwater Instructors (NAUI)

> Technical Divers International (TDI)

Eiffel Tower Offers Free Scuba Lessons

The weather was nippy and overcast and the water just chest-high, but a new scuba-diving pool in Paris has something Bali, Belize and other diving hotspots don’t: a terrific view of the Eiffel Tower.

To promote the sport, scuba instructors began offering free lessons Friday — with wet-suits, scuba gear and even a biodegradable towel — at the foot of the French capital’s famed landmark.

“Through the water you can see the monument. It’s magnificent,” said New Zealand tourist Adrian Carter, one of the first to try it.

He and a group of friends had planned to go up the 1,063-foot high Eiffel Tower, but opted for a dip instead.

“This is better than the Eiffel Tower,” said Carter, a 28-year-old computer programmer, his hair dripping from the 30-minute dive — his first ever.

The lessons include a safety lecture and a how-to demonstration in which instructors share tips. One first-time diver did a double-take when his guide told him to spit into his goggles to help keep them from fogging up.

The above-ground pool is under the Tower, between its four legs. It’s small, at 50 feet by 50 feet, about half the size of a basketball court. Just 4-feet deep, it’s safe for beginners and children aged 8 and older, said the event’s organizers, an umbrella group of scuba associations. To add a touch of realism, the bottom of the pool is studded with waterproof photos of fluorescent fish.

Though heated, the water temperature hovered Friday around a cool 71 degrees.

That, combined with icy winds that whistled down the Champs de Mars, a grassy promenade leading to the Tower, dissuaded many would-be divers. More people milled around the pool’s perimeter — watching the instructors as they floated on their backs staring up at the tower’s steel girders — than actually queuing up for a lesson.

This was not the first time the Eiffel Tower has become a sporting venue. Three winters ago, an ice-skating rink was installed on the lower of its three observation decks to draw Parisians to the monument that mostly attracts tourists.

Organizers of the 10-day diving event said they were angling for tourists and Parisians alike.

“We want to give as many people as possible a taste of scuba-diving,” said Gerard Puig, the pool’s manager and head of a diving company on France’s Mediterranean coast.

He said organizers also hope the diving experience will focus attention on the environmental dangers threatening the ocean.

“Once you’re underwater and face-to-face with all sorts of creatures, you can’t remain insensitive to the destruction of the sea,” he said.

Organizers expect up to 3,000 people to take the plunge before the lessons end June 10 — so long as the dismal weather improves.

Another first-time diver who took the plunge, English tourist Jonathan Doneley, said the experience was “awesome” despite the cold.

“I’m still shivering,” he said through chattering teeth. “I’m going again tomorrow.”