Though Deep Stops are regarded as a major development in diving, the first experiments were more trial-and-error than scientific in nature. Just like so many other important developments in the real world. Underlying science with mechanistics would follow in the late 80s and 90s, albeit with considerable flack from the "experts" of the time. . .
. . .Individuals, particularly in the cave diving community, toyed with decompression regimens in hopes of minimizing their decompression time. The cave exploration Woodville Karst Plain Project (WKPP), mapping subsurface topographies in Florida, pioneered deep stop technology, establishing many rule-of-thumb protocols to be imposed on conventional tables. Irvine and Jablonski stand at the forefront here, successfully conducting 6 hour dives at 280 ft in the Wakulla cave complex with deep stop decompression times of 12 hours versus traditional Haldane hang times of 20 hours. Also, the horizontal penetrations of 19,000 ft are world records (Guinness). . . Spectacular is a gross understatement. Certainly such contributions to diving science and spin-off model validation parallel Haldane a hundred years ago.
WKPP initially found that common decompression assumptions subjected divers to extremely long decompression obligations, and ones that, regardless of their length, were inefficient. Divers also felt badly upon surfacing from extended deco dives. Operationally (many dives over many years), WKPP divers found that the insertion of deep stops permitted shortening of shallower stops with an overall reduction in total decompression time. The decompression schedule was more effective, with effectiveness represented by subjective diver health and sense of well being.
But even before these deep stop protocols emerged, utilitarian diving practices among diving fisherman and pearl gatherers suggested traditional staging was in need of rethinking. And early deco models, such as the so-called thermodynamic model of Hills, suggested why and how. Deep stops likely evolved from cognizance of both by tech divers. . .
Pearling fleets, operating in the deep tidal waters off northern Australia, employed Okinawan divers who regularly journeyed to depths of 300 ft for as long as one hour, two times a day, six days per week, and ten months out of the year. Driven by economics, and not science, these divers developed optimized decompression schedules empirically. As reported by Le Messurier and Hills, deeper decompression stops, but shorter decompression times than required by Haldane theory, were characteristics of their profiles (Note: The current NEDU Study Conclusion implies significant DCS incidence for these profiles). Such protocols are entirely consistent with minimizing bubble growth and the excitation of nuclei through the application of increased pressure, as are shallow safety stops and slow ascent rates. With higher incidence of surface decompression sickness, as expected, the Australians devised a simple, but very effective, in-water recompression procedure. The stricken diver is taken back down to 30 ft on oxygen for roughly 30 minutes in mild cases, or 60 minutes in severe cases. Increased pressures help to constrict bubbles, while breathing pure oxygen maximizes inert gas washout (elimination). Recompression time scales are consistent with bubble dissolution experiments.
Similar schedules and procedures have evolved in Hawaii, among diving fishermen, according to Farm and Hayashi. Harvesting the oceans for food and profit, Hawaiian divers make between 8 and 12 dives a day to depths beyond 350 ft. Profit incentives induce divers to take risks relative to bottom time in conventional tables. Repetitive dives are usually necessary to net a school of fish. Deep stops and shorter decompression times are characteristics of their profiles (-Again the NEDU Conclusion shows high DCS incidence). In step with bubble and nucleation theory, these divers make their deep dive first, followed by shallower excursions. A typical series might start with a dive to 220 ft, followed by 2 dives to 120 ft, and culminate in 3 or 4 more excursions to less than 60 ft. Often, little or no surface intervals are clocked between dives. Such types of profiles literally clobber conventional tables, but, with proper reckoning of bubble and phase mechanics, acquire some credibility. With ascending profiles and suitable application of pressure, gas seed excitation and bubble growth are likely constrained within the body's capacity to eliminate free and dissolved gas phases. In a broad sense, the final shallow dives have been tagged as prolonged safety stops, and the effectiveness of these procedures has been substantiated "in vivo" (dogs) by Kunkle and Beckman. In-water recompression procedures, similar to the Australian regimens, complement Hawaiian diving practices for all the same reasons. So deep stops work and are established. . .
The earliest prescriptions for deep stops were imbedded in conventional tables. Something like this was employed, trial and error, and this one is attributed to Pyle, an underwater fish collector in Hawaii:
-calculate your decompression schedule from tables, meters, or software;
-half the distance to the first deco stop and stay there a minute or two;
-recompute your decompression schedule with time at the deep stop included as way time (software), or bottom time (tables);
-repeat procedure until within some 10 -30 ft of the first deco stop;
-and then go for it.
Within conventional tables, such procedure was somewhat arbitrary, and usually always ended up with a lot of hang time in the shallow zone. Such is to be expected within dissolved gas deco frameworks. So, deep stop pioneers started shaving shallow deco time off their schedules. And jumped back into the water, picking up the trial and error testing where it left off.
Seasoned tech divers all had their own recipes for this process. And sure, what works works in the diving world. What doesn't is usually trashed.
Concurrently, full up dual phase models, spawned by the inadequacies and shortcomings of conventional tables, emerged on the diving scene. Not only did deep stops evolve self consistently in these models, but dive and personal computers put deco scheduling with these new models in the hands of real divers. And real on the scene analysis and feedback tuned arbitrary, trial and error, and theoretical schedules to each other.
One thing about these bubble models, as they are collectively referenced, that is common to all of them is deeper stops, shorter decompression times in the shallow zone, and shorter overall deco times. And they all couple dissolved gases to bubbles, not focusing just on bubbles or dissolved gas. . .
So let's consider some recent tests, and see how they relate to deep stops. Analysis of more than 16,000 actual dives by Diver's Alert Network (DAN), prompted Bennett to suggest that decompression injuries are likely due to ascending too quickly. He found that the introduction of deep stops, without changing the ascent rate, reduced high bubble grades to near zero, from 30.5% without deep stops. He concluded that a deep stop at half the dive depth should reduce the critical fast gas tensions and lower the DCI incidence rate.
Marroni concluded studies with DAN's European sample with much the same thought. Although he found that ascent speed itself did not reduce bubble formation, he suggested that a slowing down in the deeper phases of the dive (deep stops) should reduce bubble formation. He will be conducting further tests along those lines.
Brubakk and Wienke found that longer decompression times are not always better when it comes to bubble formation in pigs. They found more bubbling in chamber tests when pigs were exposed to longer but shallower decompression profiles, where staged shallow decompression stops produced more bubbles than slower (deeper) linear ascents. Model correlations and calculations using the reduced gradient bubble model suggest the same.
Cope studied 12 volunteer divers performing conventional (Haldane tables) dives with and without deep stops. His results are not available yet but should be very interesting.
The Bottom Line
To most of us in the technical and recreational diving worlds, the bottom line is simple. Deep stop technology has developed successfully over the past 15 years or so. Tried and tested in the field, now some in the laboratory, deep stops are backed up by diver success, confidence, theoretical and experimental model underpinnings, and general acceptance by seasoned professionals.
Amen.
And dive on.
http://www.marinventure.ca/down/30065.pdf
