by Jeffrey Bozanic, Ph.D.
Scuba diving involves a degree of inherent risk. As we dive deeper or enter caves or wrecks or utilize advanced equipment, the level of risk increases. This is why some experts consider technical diving to be too dangerous for recreational divers. However, risk can be reduced by many factors, including education, experience, proper equipment, and practice.

One of the most important factors in reducing risk is proper education. Many certification agencies have been established that offer training in technical diving activities. Yet, the endeavor of technical diving is still young and still evolving. One of the ways it changes is through the evaluation of incidents that occur to divers while diving. As a community, we are always learning lessons from new incidents and modifying training and practices as a result of those lessons.

In an effort to broaden dissemination of this information, Advanced Diver is establishing a column that will examine one incident every issue. Our goal is that you, the readers, will continue to learn from the mistakes of others, helping to make technical diving safer for all of us. While the cases are based on actual incidents, minor editorial changes have been made to protect the identity of the divers involved.

Case history: What’s my mix?

Steve and Jim were diving off Catalina Island, California. They were looking for a wreck that was reported to lay off Ship Rock. Their first dive was to 170 fsw (52 msw) using trimix with 21% oxygen and 45% helium (TX 21/45). They mixed their bottom gas using partial pressure blending and confirmed the mix with their oxygen analyzer.

They utilized Deco Planner® to plan their dive using nitrox with 50% oxygen (EAN50) for their decompression stops, which began at 70 fsw (21 msw). Their bottom time was 25 minutes, resulting in a decompression time of 49 minutes. They completed their decompression without incident, but were unsuccessful in finding the wreck.
Steve finished his dive with 1,800 psi (125 bar) of bottom mix and Jim with 1,500 psi (102 bar). They decided to conduct another dive to attempt to locate the site. Motoring into Two Harbors, they left their doubles to be topped off with air to 3,000 psi (204 bar). While this was being done, Steve calculated that their resulting mix would be TX 21/30. As they did not have their computer with Deco Planner® on the boat, they used air decompression tables to calculate their decompression for the repetitive dive. Upon receiving their cylinders, they analyzed them for oxygen content and prepared to dive again.

Two hours after surfacing from the first dive, they began their second dive. They quickly swam back to the same depth (170 fsw/52 msw) for 20 minutes, followed by 39 minutes of decompression. They finished their dive and exited the water about 5:00 pm.

A short time after surfacing from the dive, Jim began complaining of a dull ache in his shoulders. Ten minutes later he began to notice a slight loss of strength in his left arm. He breathed oxygen on the surface for 10 to 15 minutes with no relief and decided that the symptoms were probably from overexertion and being cold during the dive. To warm up, he took a hot shower, which made him feel better, but did not improve the pain he was feeling. Since the pain seemed to come and go in waves ranging from intense to a dull ache, Jim tried to ignore it.

Finally, at 9:30 pm, he contacted a recompression chamber, and was told to come in. Upon evaluation, he was found to have other neurological symptoms, including minor loss of fine motor skills and leg weakness. Following a twelve-hour treatment, all of the pain had resolved, as well as most of the weakness he was experiencing. A second treatment two days later resolved the remaining problems.

Steve and Jim made several errors during their dives. The first was in mixing their bottom gas. When they blended their initial mix, they started with oxygen, placed helium on top of that, and then topped off with air. When they analyzed their cylinders, they only tested for a single gas (oxygen). This did not provide them with adequate information about their breathing gas. In fact, it left them almost in the dark about the two most important gases from a decompression status, the fractions of helium and nitrogen.

It is impossible to plan decompression unless you know the inert gas fractions of the mix. Early in the days of mix diving, helium analyzers were expensive (the one I used to use was about $25,000!) and temperamental. They were used infrequently, so there was always some uncertainty about the mix in use. To account for this, tables were generated that allowed for a wide variation in the inert gas fractions. While this allowed an imprecise mix to be used, it resulted in much longer decompression profiles than those that would be incurred if an exact mix could be programmed.

Steve and Jim used Deco Planner® to compute their initial decompression. This calculates decompression based on exact gas fractions. I have seen mixes in which the fractions of inert gas were off by 10% from those expected from partial pressure blending, with the oxygen fraction only varying by 1% from expected (within the accuracy range of most oxygen analyzers used by divers). The only way to identify this problem is to analyze a second gas constituent. Helium analyzers now only cost about $1,000 and are readily available. If you are going to use a decompression planning software package like Deco Planner®, which incorporates exact fractions to minimize decompression, then you must use such an analyzer.

They compounded this problem by topping off their bottom mix cylinders for the second dive. This second generation blend further increases uncertainty, by adding another level of potential errors (I have seen errors exceeding 20% when this has been done). In fact, they miscalculated their resulting mixes for the second dive. Had they worked the math properly and done so for both sets of cylinders, they would have found that Steve’s final mix would have been TX 21/27 and Jim’s TX 21/23, not the TX 21/30 they had figured. This error could have easily been detected had they properly analyzed their gases.

The next error they made was switching decompression tools during a series of repetitive dives. Besides using the wrong tool for the first dive, they then switched to another wrong tool for the repetitive dive (air tables for a trimix dive). Jim’s comment was that, “since we were using air tables, and they are always more conservative than trimix, the mix really didn’t make a difference.” In fact, the tables you use do make a difference.

All tables and decompression software programs are based on an underlying model or algorithm. Different tables, decompression software packages, and dive computers often use different models. Since these involve different assumptions and on- and off-gassing parameters, you cannot switch indiscriminately between them during a series of repetitive dives. This is especially true when breathing helium-based breathing gases.

Finally, Jim demonstrated one last error in judgment. Upon surfacing from the second deep dive, he began experiencing symptoms consistent with decompression sickness. He attempted to diagnose that by breathing oxygen on the surface, and when that did not resolve the problem, he attributed the pain and other symptoms he was experiencing to another cause.

It can be extremely difficult to diagnose any type of decompression illness on the surface. Even experienced hyperbaric physicians often must conduct a test of pressure by recompressing the affected individual in a chamber to arrive at a differential diagnosis. In fact, Jim’s symptoms did not begin to subside until he had been in the chamber for almost an hour. His symptoms gradually resolved over the next eleven hours in the chamber. Had he not delayed seeking treatment, it is probable that resolution would have occurred much sooner and possibly without the residual symptoms that ultimately required a second treatment.
Steve and Jim were lucky. Steve experienced no problems at all. Jim’s problems could have been much worse. As it is, he is diving again, and enjoying the underwater realm he loves so much. And hopefully, they have both learned from this experience….

Lessons learned:
1. Never breathe a gas without knowing what you are breathing. When using trimix, always analyze at least two of the mix constituents (oxygen and helium).
2. When planning decompression, always use the appropriate tools for the job.
3. Do not mix decompression planning tools when planning a repetitive diving sequence.
4. If you suspect decompression sickness, do not delay… get on oxygen and seek a qualified medical evaluation.

Jeffrey Bozanic

Jeff was certified as a NAUI Instructor in 1978, is certified to teach diving for IANTD, TDI, NAUI, and the NSS-CDS. He is active in teaching rebreather, nitrox, technical nitrox, and trimix diving courses. Together with his wife, Rebekah, he has maintained the combined accident files for the cave diving community (a joint project of the NSS-CDS, NACD, and IUCRR). He has published extensively on diving education topics, with heavy emphasis on cave diving safety techniques. He has edited/reviewed many diving textbooks and is the author of Mastering Rebreathers. He has served on several Boards of Directors in the diving community, including as Chairman of the NSS-CDS and as Vice Chairman of NAUI, and as Treasurer on the AAUS Board. He has received the NAUI Outstanding and Continuing Service Awards; the Silver Wakulla, Abe Davis, and Henry Nicholson Awards for safe cave diving; the SSI Platinum Pro 5000 Award, and is a NAUI Hall of Honor inductee.