There are plenty of incentives for taking part in a scuba diving ‘expedition,’ and some of them at least, may not obvious at first sight.
The best, and a motivation with all the enticement of a chocolate-chip cookie in the eyes of the average six-year old, is to learn something new. Coming a very close second – with the pulling power of a chocolate- dipped coconut macaroon in my world – is having a tightly-held, but wrong-headed notion, kicked into the trash, and replacing it with an idea backed by science, logic, and responsibly collected data. Again, a learning experience.
Earlier this year, Jill Heinerth and I were asked to put together a small expedition to do some work in the Bell Island Iron Ore Mine, in Conception Bay, Newfoundland. We had both been there before, and we were both huge fans of what the place has to offer: essentially, the chance to experience truly unique dives in an environment that screams history and heritage.
There were tonnes of other reasons to sign on for this particular expedition, but diving in what is essentially an underwater museum, was pretty high on the list.
A total surprise – and an unexpected bonus – was having my opinion about heated vests and their potential role in diver safety – specifically decompression stress – turned around about 180 degrees.
The lesson went something like this.
Dr. Neal W. Pollock was part of the Bell Island project, gathering data for his research at Divers Alert Network. Neal is research director there – at DAN – and a senior research associate at the Center for Hyperbaric Medicine and Environmental Physiology at Duke University Medical Centre. And when not pushing an ultrasonic transducer against your rib cage, he’s a handy guy to have around when the chatter turns to many things related to technical diving.
I’d seen Neal’s presentation on Thermal Physiology and Protection at Rebreather Forum 3.0, in Florida a few years before. But, frankly did not really grasp his message. Then, sprawled on the floor of the Bell Island Museum and watching a whole stream of gas bubbles race around in my heart after a dive, helped me – and others on the expedition – listen a little harder to what he was telling us.
A link to his presentation at RB 3 is at the bottom of this page, and if you dive at all, you’d do well to watch it, but the Coles Notes version is this.
If, like me, you figure the safest way to dive is to be warm throughout, you may want to rethink your approach.
Dr. Pollock, suggests that there are three issues to think about when we consider thermal protection. Number one, at depth we must be able to function; secondly, we must take into account the effect of temperature on our decompression stress; and lastly we need to consider comfort.
Based on a “small but significant” study conducted by the Navy Experimental Diving Unit, Dr. Pollock explained: “Divers tend to put the emphasis on the wrong thing… comfort.” And comfort, according to the data, and Dr. Pollock, “should only be a distant third.”
The NEDU study found that the fewest instances of DCS occurred in divers who started cold, and finished warm. Probably the exact opposite of what happens on a significant number of technical dives. Worth noting is that for this study, subjects wore no thermal protection and worked in water at 36 degrees (warm), and 27 degrees (cold).
Significant also was that there were zero cases of DCS in 80 “cold start, warm end” dives, but in warm/cold dives, probably the situation for many long dives, seven out of 32 (22 percent) resulted in DCS. The dive profile incidentally was a seemingly benign 120 fsw with 30 minutes of bottom time and 91 minutes of deco!
When the dive profile was appreciably altered to be more aggressive – 120 fsw for 70 minutes of bottom time but with an unchanged 91 minutes of deco – the results had very similar implications: cold/warm dives resulted in 0.1 percent DCS.
Of interest to those of us who own and use heated drysuit under garments, warm/warm dives following the same profile, returned a 17 percent instance of DCS (four cases in 24 dives).
A slide from Dr. Neal Pollock’s presentation
For technical divers, there are many, many factors with a role to play in decompression stress, but consider this. With a heated vest, several things might happen. The vest is turned on and keeps you warm throughout your dive. The vest is turned on to start the dive, and then is turned off or runs out of power at the end of the dive. The vest is turned off to begin the dive and left off when you start to get chilled as bottom time passes, and only turned on during the latter stages of decompression (probably NOT the most common practice).
Based on the NEDU study, each has possible consequences, and not all of them positive. It would seem that the best option in terms of thermal status, is to start cold and end warm.
As Neal remarked, the NEDU study was across a very small population (73 divers), and one has to take that into consideration when assessing its value and relevance. However, my personal observation of what was happening in my heart after a 90-minute warm/warm dive – thanks to the transthoracic echocardiogram being orchestrated by Dr. Pollock about 20 minutes after I surfaced – is that I won’t do that again.
View Neal’s RB3 presentation here… and in addition to a much more complete interpretation of the NEDU study, he covers several other related and significant issues… watch it.
https://www.youtube.com/watch?v=yixnr07AiTI
Doppler's Tech Diving Blog 