Shearwater Predator Video November 10, 2009
Posted by deepstop in Equipment, Technical Diving.Tags: Adventure, Dive Computer, Outdoors, SCUBA, Scuba Diving, Scuba Equipment, Shearwater Predator, Shearwater Pursuit, Technical Diving
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A buddy sent me a pointer to an interview at DEMA 2009 by Curt Bowen from Rebreather World and Advanced Diver Magazine with Bruce Partridge from Shearwater Research about the new Predator Dive computer.
Bubble Compression November 9, 2009
Posted by deepstop in Technical Diving.Tags: Bubble Model, DCS, Decompression, Diving, Micronuclei, RGBM, SCUBA, Scuba Diving, Technical Diving, UHMS, VPM
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I saw an interesting slide at the UHMS GLC meeting in October. It compared dive profiles that were virtually identical except for a very deep bounce at the beginning of what was otherwise a “square” profile. The presenter stated that there were less observed bubbles after the dive that began with the bounce than the one that didn’t.
That’s quite astounding. One would think that starting a dive with a bounce would increase the number of bubbles, not decrease it. This result seemed to support some of the claims made by bubble model proponents that neo-Haldanean compression models miss opportunities to exit from deep dives earlier. The effect is explained by the theory of micronuclei, which can be made up of very small bubbles that already exist in the bloodstream, which grow larger in the presence of supersaturated inert gas (i.e. Nitrogen or Helium) and cause problems. The deep bounce crushes these micronuclei to a degree where they do not easily accrete dissolved gases and grow to a size that would cause the diver problems.
Sounds intriguing, but I’m still not convinced that this is safe enough for my personal use. I worry that certain environmental conditions or dive profiles would result in a reversal of the effect and the onset of serious DCS. I’m probably full of “it” on this topic but if one model predicts I’m going to be fine and the other predicts I’m going to be writhing in agony, I’m going to err on the side of staying down a little longer.
Respiratory Minute Volume November 7, 2009
Posted by deepstop in Technical Diving.Tags: Air Consumption, Dive Training, Diving, DSAT, IANTD, PADI, Respiratory Minute Volume, RMV, SAC, SAC Rate, SCUBA, Scuba Diving, Scuba Training, SDI, Surface Air Consumption, TDI, Technical Diving, Training
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It’s interesting that there is no agreement among agencies as to the exact definition of Respiratory Minute Volume (RMV) and Surface Air Consumption (SAC).
My original certification agency, NASDS, defined SAC as the surface equivalent number of PSI per minute on a dive, which is (PSI Consumed/Total Dive Time)/((33+Depth)/33).
IANTD does the same thing, and defines RMV as the SAC/(Working Pressure/Rated Cylinder Volume).
DSAT defines SAC as the same as IANTD and NASDS defines RMV, and defines RMV is the (Tidal Volume minus Respiratory dead air space) times breaths/minute. I added the parenthesis to what I read in the book (p42 Tec Deep Diver Manual) because otherwise it doesn’t make any sense. Oddly the Encyclopedia of Recreational Diving doesn’t seem to mention either term. Wikipedia’s definition of RMV is similar to DSAT but doesn’t factor Respiratory Dead Air Space.
In the SDI solo diving course SAC is defined as the volume per minute consumed at rest on the surface. They recommend breathing from a tank while sitting around watching TV or like activity to measure this. For me, consuming about 325 PSI of a 3000 PSI working pressure tank with a rated volume of 77.4 cubic feet in 30 minutes, this was about .28 cubic feet per minute. They call what IANTD and NASDS call RMV the SRMV, or Surface Respiratory Minute Volume. They then go on to recommend that before each dive the RMV by multiplying by the number of absolute atmospheres of pressure and also by a “Dive Factor”, which takes into account effort and should be at least 1.5 for an easy dive, and perhaps more than 3 for a high effort, cold or stressful dive.
Disconcertingly, their sister agency, TDI, recommends determining SAC using a swim at depth, the very thing that SDI says doesn’t work, and implies that RMV is just another term for SAC.
0.28 seems a bit low. I think having the reg in my mouth called attention to my breathing and slowed it down some. At 33 feet a tank would last over 2 hours at that rate, although my record is about an hour and 20 minutes on a reef dive (and 1h 40 in really shallow water) on a single 80 so maybe it’s not all that far fetched. I know when it’s cold and I’m working a bit hard the rate goes way, way up.
It would be nice to have a consensus between agencies on this topic. Maybe ISO will define it some day.
The Avro Arrow Models October 28, 2009
Posted by deepstop in Diving Books and Films, Shipwrecks, Technical Diving.Tags: Adventure, Avro, Avro Arrow, Diving, Lake Ontario, Outdoors, rocket, SCUBA, Scuba Diving, Sea Hunters, Shipwreck, Sport, Supergun, Training, Trimix, Wreck Diving
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I just watched, not for the first time, a Sea Hunters episode from 2005 on a search for several models of the famous Canadian fighter aircraft, the Avro Arrow, in Lake Ontario. In this episode, the crew took their boat from Port Dover in Lake Erie to Point Petre in Prince Edward County in Lake Ontario, where during the mid-to late 50s Avro Aircraft Limited shot models of their aircraft into the lake at supersonic speeds, on the nose of a rocket.
They didn’t find it. They found an unidentified sailboat from the mid 19th century in great condition at a depth of 200′. They also found a rocket which they believed to be a Canadair rocket booster used to test the Velvet Glove Missile, which was being designed as armament for the Arrow. This booster was designed in part by Gerald Bull, a Canadian engineer who was assassinated (reputedly by the Mossad) while working on Project Babylon, a supergun for the Iraqi government.
The show then rambled on into a rather pointless expedition off the Virginia coast where the visibility was so back they could only feel the object they were trying to investigate.
Despite the squirrely storyline, the Sea Hunters is my favourite underwater TV show. It’s short on the wonders of the ocean environment and long on hard core diving and exploration (not that I’m against the environment, sharks or pretty fish, it just gets repetitive after a while). In Lake Ontario they were diving surface supplied trimix with a hardhat and ship to diver voice communications, running sidescan sonar, and a ROV. What could be more fun than that?
A Sad State of Affairs October 25, 2009
Posted by deepstop in Technical Diving.Tags: AGE, DCI, DCS, Decompression, GLC, HBOT, Recompression, UHMS
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Yesterday (October 24th, 2009) I attended the 30th annual meeting of the Great Lakes Chapter of the Undersea and Hyperbaric Medical Society. For reasons I’ll explain at another time, this is probably the last meeting of the chapter, due to reorganization of the parent society.
There were many interesting talks, but one thing really stuck in my mind about the state of hyperbaric medicine in Canada – and that is that it isn’t taken seriously enough. Hyperbaric medicine is used in the treatment of several ailments, including
- Air or gas embolism
- Carbon monoxide poisoining
- Clostridial myositis and myonecrosis (gas gangrene)
- Crush injury, compartment syndome and other acute ischemia
- Decompression sickness
- Enhancement of healing in selected problem wounds
- Exceptional anemia
- Intracranial abscess
- Necrotizing soft tissue infections (like necrotizing fasciitis a.k.a. flesh-eating disease)
- Ostemyelitis
- Delayed radiation injury
- Grafts and skin flaps
- Thermal burns
One particular aspect of the above was in the treatment of diabetic foot ulcers, which often would otherwise result in amputation. With the population (like me) aging, diabetes is becoming more common and the incidence of this problem will grow proportionately.
However, when we visited the Hyperbaric Facility at Toronto General Hospital and asked why it wasn’t in use at that time, the reply was that there was no funding to pay for technicians. Meanwhile it seems that private hyperbaric facilities are popping up all over the place for treatments that have no proven benefit, putting, as we heard, the situation as on where a clinic “can charge patients for hyperbaric oxygen therapy for conditions that is doesn’t work for, but cannot charge for condition for which it can”.
Per capita Canada has about 10% of the number of chambers as the US available for physician-prescribed HBOT. Part of this I’m sure is the stringent controls on private facilities that can avail themselves of government funding, and of public facilities that can access private funds (i.e extra-billing which was outlawed decades ago by the government of Pierre Trudeau). But it also seems there is a general ignorance and/or scepticism with regard the to usefulness of HBOT, and the leaders of the chapter are campaigning for greater awareness and education amongst the medical community.
Let’s hope they succeed. Access to recompression chambers for emergency use is essential to divers, but actually provide greater benefits to the general public. That’s a double win.
The Future of Diving October 21, 2009
Posted by deepstop in Technical Diving.Tags: AGE, DCI, DCS, Decompression, Dive Computer, Dive Medicine, Haldane, Technical Diving, The Bends
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I picked up this pointer on the Ontario Diving web site. It’s the proceedings of the Baromedical and Environmental Physiology Group of the Norwegian University of Science and Technology, 18–19 December 2008, in Trondheim, Norway and looks pretty interesting, although it’s still downloading as I write this. It has an appendix the entire seminal document on decompression sickness (“The Prevention of Compressed Air Illness”) by John Scott Haldane, which makes up about a third of its almost 300 pages.
I was pleased to read the following sentence in one of the papers: “Nevertheless, there is no clear evidence, to date, that diving, in the absence of serious acute damage, leads to long-term neurological dysfunction.” That’s good news, I think.
Blending Trimix October 19, 2009
Posted by deepstop in Technical Diving.Tags: Diving, DSAT, Enriched Air, Nitrox, SCUBA, Scuba Diving, Scuba Training, Technical Diving, Trimix
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As described in a prior post, I recently took the DSAT Gas Blender course which includes Trimix blending. No formulas were provided for trimix blending but you get a handy little program for calculating the blend for a partial pressure mix or a top-up for trimix, which is a whole lot simpler than doing it by hand. However, it’s not really difficult to calculate trimix blends using the method I came with during the class.
The most difficult part of blending Nitrox is that the Oxygen in air comes premixed with 78% Nitrogen. If we were making Nitrox from pure Oxygen and pure Nitrogen anyone proficient in mental arithmetic could figure out the blends in his or her head. For instance, a 3000 PSI tank of EAN36 would need 1,080 (3,000 x .36) PSI of Oxygen and 1,920 PSI of Nitrogen, but as we generally top up with air, only 570 PSI of Oxygen is needed because the air already contains some.
As Helium is not a significant component of air, we can use this simple method to calculate the required partial pressure of Helium in any mix. So a 16/40 mix (40% Helium) has 40% x 3,000 or 1,200 PSI of Helium and 1,800 PSI of some combination of Oxygen and air. Similarly, a 4/80 mix has 2,400 PSI of Helium in a 3,000 PSI tank, with 600 PSI of Oxygen and Nitrogen.
So what about the rest? This is a little more complicated, but there’s a fairly easy shortcut. Let’s look at an 18/50 mix, since this is an easy example. This mix is 50% Helium, which would be 1,500 PSI in our AL80. The other 1,500 PSI is Nitrox, and our goal is to get a final blend of 18% Oxgyen. To do this, we need to top up the tank with 36% Nitrox. Why? Because ½ a tank of 36% Oxygen has the same O2 content as a whole tank of 18% Oxgyen. From there, it’s a simple matter of looking up what pressures are needed to blend 36% Nitrox at 1,500 PSI. Nitrox blending tables should show this, and most people who have done Nitrox filling will know that 36% into a 3,000 PSI requires 570 PSI of Oxygen, so a 1,500 PSI fill would need ½ that which is 285 PSI.
So our 18/50 mix will need 1,500 PSI of He, 285 PSI of O2, and be topped off to 3,000 with 1,115 PSI of air. I contrived this example so that the numbers would work out conveniently for me, especially with exactly ½ the tank containing Helium. What about other blends?
If we were looking for a 21/40 mix, clearly we’d need 1,200 PSI of Helium. The other 1,800 PSI would contain some Nitrox blend. To figure out the Oxygen percentage, we divide the desired percentage of 21% by one minus the Helium fraction, so in this case it’s (1-.4) or 60%. This turns out to be EAN35. So you can look up a 35% mix in a 1,800 PSI tank for the amount of Oxygen and air that needs to be added. You can also use the Nitrox blending formula of (W-.21)/.79 x PSI (where W is the percentage of Oxygen wanted) to calculate it. This works out to 319 PSI of Oxgyen with the remaining 881 PSI topped off with air.
Note that there are some blends that can’t be made The 4/80 mix, for instance, can’t be made exactly. I chose this in my example because it is the mix that was used by David Shaw when he died cave diving in South Africa at a depth of 270 metres. The reason it can’t be made exactly from Oxygen and air is the remaining 20% of the mix that isn’t Helium is only 20% Oxygen, and this Nitrox mix can’t be blended with a combination of Oxygen and air. I’m sure though, that in this case his diluent was just air, and the mix was 4.2% Oxygen, rather than exactly 4%.
So what he was diving was actually “Heliair”, the simplest Trimix blend around, made from combining Helium and air with no added Oxgyen. The disadvantage of this mix is that at the optimum MOD of Oxygen, usually considered to be a partial pressure of 1.4 (although many rebreather divers use 1.3, and some open circuit divers use 1.5 or 1.6 under ideal diving conditions or in deco), has the same END (Equivalent Narcotic Depth) as air at the same O2 partial pressure. This is true at any depth as the ratio of Nitrogen to Oxygen is the same as air for any mix of Heliair.
Deon’s END was 46 metres or about 152 feet, which is greater than many divers would care to venture, although less than many others including me have regularly dived, and his PPO2 1.12, which is well within acceptable boundaries. You can see from this though that he was getting quite exposed to narcosis while being well within the acceptable limits for Oxygen Exposure (at least from a CNS perspective, it was scheduled to be a long dive and Pulmonary exposure would undoubtedly be a concern). This points out the problem with Heliair – long before you’re at your CNS limits you will be well-narked. This extreme example has other considerations, so don’t take this as saying that for this dive his mix was incorrect or that narcosis contributed to his accident.
Equivalent Narcotic Depth October 13, 2009
Posted by deepstop in Technical Diving.Tags: Adventure, Decompression, Diving, Diving Accident, Diving fatality, Enriched Air, Nitrox, SCUBA, Scuba Diving, Technical Diving, Trimix
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I’m glad to see that the Wikipedia definition of Equivalent Narcotic Depth uses (1-FHe) instead of FN2 as the basis of the calculation. The difference between these two is that the older way of using the partial pressure of Nitrogen leaves out the current theory that Oxygen is more narcotic than Nitrogen, and the current practice of assuming that their narcotic potential is equivalent, as it is believed that in practice Oxygen is less narcotic than the theory would suggest due to partial metabolism by the body.
So how does this change the END? So let’s use 20/40 trimix like the Wikipedia example. With this mix, 60 metres should feel like 32 metres. Using the old formula, where Oxygen is treated as equivalent to Helium rather than Nitrogen, the END would be .40/.79*70-10, or just over 25 metres. That’s quite a difference.
I also found a web site that took into account the theoretical narcotic properties of Helium in one method of calculation, which is considerably less than Nitrogen. I’m not convinced that is valid as at some of the depths people are diving at now (over 200 metres), the Narcosis would be intolerable.
For instance, in the Deon Dreyer accident at Bushman’s Hole, he was diving at 270 metres with 4/80 trimix. If Helium was counted at 23% of the Narcotic effect of Nitrogen, his END would have been 97.5 metres! That’s almost double the value if Helium isn’t counted.
Brockville 2009 & my Shearwater Pursuit October 1, 2009
Posted by deepstop in Dive Log, Shipwrecks, Technical Diving.Tags: St. Lawrence River, Decompression, Nitrox, Shipwreck, SCUBA, Diving, Scuba Diving, Dive Computer, Brockville, Enriched Air, Wreck Diving, Scuba Equipment, Technical Diving, Shearwater Pursuit, Adventure, Sport
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The deep drift dive from the Lillie Parsons to the James B. King was a good test of my recently acquired Shearwater Pursuit SPOT. I have to say it performed admirably, calculating and tracking my deco schedule as it should. The first stop it asked for was at 80′, which is about what I’d expect, but it cleared before I got there and so my first was at 70′. I had a bit of trouble finding a good handhold in the current there, and made an unplanned excursion to 68′ for a few moments, but as these are deep stops with very low gradient factors it was of no concern. The US Navy tables show 30′ as the first stop even on a square dive profile to 5′ deeper than my maximum depth. You can see in the graph from my logging program the jump in inspired Oxygen that occurs part way through the 40′ stop, when I switched my computer after switching gases.
As the stops got shallower and longer, the gradient factors get higher and maintaining stop depth becomes critical. I’d planned to switch to my deco gas (curiously 71% 02 because someone topped off my 80% tank by mistake) at 30′, but changed my mind and used it on the 40′ stop as well, as the deco MOD of the mix was within limits.
As I mentioned before from now on if I’m only carrying one deco gas, it will be EAN50, as the penalty is low and it is a good backup at greater depths than EAN80. My next tank acquisition will likely be something smaller like an AL30 for pure O2, if I feel the need to carry something else for longer dives.
Deco stops being based on exponential models often work out close to a Fibonacci sequence, where the length of the next stop is approximately the sum of the current stop and the one before. So if your 30′ stop is 5 minutes, and your 20′ stop is 8 minutes, then the 10′ stop is likely to be around 13 minutes. This was certainly true with this dive, after adjusting for the gas switch, which shortens everything. I use this principle to memorize a deco schedule that closely approximates my plan, so if I have nothing else (which should never happen, of course) I can have something close to a reasonable decompression. All it requires is for me to remember the first two stops.
Finally, I’ve mentioned the importance of a slow ascent from the last stop, where the gradient factor is highest, and you can see from the graph that I spent a full minute ascending the final 10 feet.
Brockville 2009, Lillie Parsons to John B. King September 30, 2009
Posted by deepstop in Dive Log, Shipwrecks, Technical Diving.Tags: St. Lawrence River, Decompression, Shipwreck, Drift Diving, SCUBA, Diving, Scuba Diving, Brockville, Wreck Diving, Technical Diving, Adventure, Outdoors, Sport
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As usual, the 4 technical divers on the boat sat out the dive on the Daryaw. September 7th 2009, Labour Day Monday, was a fine sunny day, and buoy leading down to the Daryaw was crowded with dive boats. Captain Lawrence deftly swung his craft around between two of these to back himself on the buoy much to the delight of all who were watching. We checked our equipment and waited for the divers the return. One pair returned rather quickly due to a leaky mouthpiece which was quickly fixed so they could salvage some of their dive.
Once everyone was back on board we took our time getting down to the Lillie Parsons, and once in position the four of us, Brad, his Tec Deep Student Chris, Rich and I strode off the stern in a near simultaneous negative entry like the day before, quickly heading down to the shelf at 45′ or so for a fast bubble check. Before continuing, I spotted Rich’s deco reg hanging loose and went over to help him put it back under its bungee, but he waved me off and just clipped the second stage to something near the neck of the deco tank, and on we went.
The descent was fairly rapid, taking 2 1/2 minutes to go from 50′ to 175′. Once my eyes were adjusted to the light, the visibility was pretty good, and I made a point of looking far and wide to take in the full landscape, rather than letting the narcosis narrow my field of vision. I felt great (naturally) and really enjoyed looking around for things on the bottom, although I didn’t find much.
The four of us stayed together, with Brad leading, Chris very close to him, Rich behind him and me behind Rich. We had been concerned with Chris’ air consumption as his SAC check the day before showed an alarming rate of consumption, but we thought that might have been due to the current. So we planned the dive for 20 minutes bottom time, which we thought wouldn’t be enough time to make it all the way to the John B. King. Sure enough, when 20 minutes were almost up, Brad signalled Rich and I to ask if we wanted to continue to 25 minutes, and as I had 2000 psi remaining I was happy to comply.
Along the way, there’s a large anchor leaning up against the wall, which I always hold on to for a second or to, then letting go. I had been told about a second anchor which I’d not seen in the 3 previous dives, but not long afterward there it was. It was about 1/2 the size of the first one and away from the wall out in the river a bit. I then picked up some large china object which I hoped to examine a bit later. It was large enough that I had to a fair bit of air to my BC. I was also holding on to a metal ring about 3″ in diameter that I’d found, but it was so encrusted I couldn’t identify it.
Right on the 25 minute mark, we reached the King. By that time, we’d lost sight of Chris and Brad. I’d been looking for him, and caught a flash from his light briefly, but that was it. At least I knew we weren’t far behind. The bottom had become shallower by that time – around 155 to 160′, requiring me to dump a little air. Rich and I made our way up to 130 feet at which point there was a bit a traverse to do while exposed to the current. I could see Rich was having trouble holding on, which was probably because of the extra drag from his dry suit, and had to pull him onto a good handhold at one point. By this time I’d dropped the China object and the metal ring to concentrate on the dive.
To be continued…
