on a Rebreather!
Below you will find an article detailing one of the longest cave dives ever completed, certainly the longest DEEP cave dive... and it was done on a Rebreather!
On the afternoon of April 25th, 1997 at 1:00pm divers began to enter the water. This was not to be just another dive, or even just another cave dive. By early the next morning, one of the longest cave dives ever attempted was added to the long list of accomplishments of the organization known as the WKPP.
The WKPP is the Woodville Karst Plains Project. It consists of a group of dedicated volunteers from many backgrounds with overlapping talents and interests. Included in this group are extreme cave divers, engineers, scientists, environmentalists and students. All with the common goal of the exploration and mapping of what is probably the worlds largest underwater cave system. This cave system also happens to be the water supply for northern Florida. For this reason, one of the primary goals of this group is to gather information which helps to protect Florida's aquifer.
The dive was conducted in Northern Florida in Wakulla Springs. A location where the WKPP has been involved in exploration for over 10 years. The statistics of the dive include:
How did they do it?
They did it with years of practice and experience. There were 3 lead divers who made the final push and added line out to the maximum distance of 11,000 feet. They were supported by other WKPP members both in and out of the water. Dives of this magnitude require careful planning and implementation to remain safe, not to mention the coordinated effort of dedicated support personnel. The lead divers are only the tip of a well oiled machine which requires all of it's parts to function efficiently.
To extend the time available to explore this cave system, the three lead divers were using the Halcyon, semi-closed circuit rebreather. This rebreather adds gas to the breathing loop as a function of the divers breathing rate. The Halcyon's discharges a fraction of each breath into the surrounding water while simultaneously removing any excess CO2 with a chemical scrubber. At the surface this fraction is approximately 20%, with the fraction becoming smaller as the diver goes deeper.
WKPP members have found the the rebreather provides between 6 and 10x's the usable volume in any given gas cylinder. In other words, a single 80ft3 cylinder yields between 480ft3 and 800ft3 of usable volume. The available volume depends on the nature of the dive profile. The flatter profiles limit the need to refill the countering thus extending the range of a given cylinder. The entire dive covered 22,000' of cave (11k each way) and consisted of 3.5 hours of bottom time at depths below 300', yet the team consumed less than 150ft3 of bottom mix. The rebreather also proved highly beneficial during the decompression, allowing the team to significantly reduce the volume of decompression gas consumed. The divers used air down to approximately 190 ft where they switched to the the bottom mix. (This is no longer the case, for safety. WKPP divers use air only above 130ft. TriMix is now used below 130ft.) The Halcyon has quick connect fittings which allow the divers to connect different gas mixes while under water.
Extra rebreather supply bottles and OC bailout bottles as well as spare scooters for each of the lead divers were placed well within the cave system by other members of the WKPP team. The dives were carefully planned and conducted so that at any point during the dive, there were back up systems in place that would allow each of the divers to exit the cave, even with a complete failure of their primary system. Although they always plan for the worst contingency, the dive was conducted with no problems and their rebreathers worked flawlessly. After about 5 hours on the rebreathers, the divers switched to open circuit to complete their decompression.
How did they feel?
According to the team members, the extreme time of the dives takes it's toll in many ways. With the rebreather it is necessary to keep the mouthpiece firmly in place to avoid excess water leakage into the breathing loop. After five hours, jaw fatigue can become a real problem for some of the team members.
Another problem experienced by the divers is difficulty concentrating at the end of the dive. Try to imagine intense concentration for many hours, eventually your brain is just plain tired of concentrating. Only a shear act of will and considerable experience allows these divers to maintain the alertness required to complete this type of dive.
Are they tired? Yes, but just like a marathon runner they keep on going. Of course, they do have a little more incentive to complete the race!
If you would like more information on the WKPP and it's efforts to protect and explore Florida's cave systems. You can go to their web site. For information on the Halcyon rebreather, contact the manufacturer (see the manufacturer's section of this web site).
Special thanks George Irvine and to Jarrod Jablonski for much of the information contained in this article. (along with Brent Scarabin, they were the team members who accomplished the final push on this dive.) For more information on Jarrod Jablonski and his cave training courses, you can *link broken*visit his web page.
All pictures by Barrie Miller (Rat). Underwater pictures were taken using a Sony VX-1000 3-ccd digital camera. Barry can be contacted by e-mail.
Picture Key: (from the top down...)
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Revised: September 07, 2005.