Monday, July 21, 2008
Here is the article Nuno wrote:
Diving history has been made in Lago Maggiore, Northern Italy. On 10th May 2008 three divers, Pim van der Horst, Mario Marconi and Alessandro Scuotto did “The Deepest Wreck Dive”. They dived the wreck of the “Milano” located at a depth of –236 meters of fresh water, the divers used the “Ouroboros” closed circuit rebreather. The logistic preparations for this dive were both comprehensive and impressive. The support by the Italian diving community was total and it included both sport divers as well emergency diving personnel. The dive was planned by Marco Braga, Andrea Cortesi and Fabio Manganelli with the assistance of many other volunteers. An ROV, positioned on the wreck of the “Milano” at –236 mfw, provided a visual beacon for the divers (with its lights), it also monitored the safe arrival of the divers at depth and established visual proof that they had been there. The most critical infrastructure that was available for this dive was a diving bell (fitted with a hot water and surface gas supply as well as visual and audio communication). The diving bell allowed the divers to decompress in comfort for the last four hours of the seven-hour epic dive. The dive went off without incident and as planned until Pim arrived back at –120 meters at which point his dry suit flooded (this was a problem because the water temperature is around 4 to 6 degrees centigrade), he managed to survive and was assisted into the bell at –21 meters by his support diver Remko van de Peppel, at that stage he was very weak and unable to do so on his own. At –100 meters, on the way up Alex started to feel dizzy and had to bail out to open circuit due to vomiting, with the assistance of his support divers and Mario he too was able to reach the safety of the diving bell. Mario’s dive went on without incident and he communicated with the surface from the diving bell, on the condition of his dive buddies. He also assisted them when possible because they were both vomiting frequently while decompressing in the safety of the bell. Upon surfacing Alex was evacuated for further treatment in the Recompression Chamber while Pim whose condition had improved remained under observation. Mario, after a medical examination, required no further attention except for a good night’s sleep. The whole team can be congratulated on doing a very difficult record dive and on recovering from a number of potentially fatal emergencies. I had the pleasure of being a witness to a well-executed dive instead of being one of the divers doing the record, for a change(236m).
Wednesday, July 16, 2008
Decompression sickness or DCS is the body’s reaction to bubbles. These bubbles can lodge anywhere in your system (brain, spine, lungs, heart, muscles) and tend to be made of either nitrogen or helium (this obviously depends on what you are breathing when you are diving, nitrogen and or helium). The gases that you breathe are transported from your lungs to all areas of your body (tissue) in the blood along with the oxygen you need to survive. The deeper you go, the more ‘thirsty’ your tissues are for these gases, so the faster they absorb them (a process called in-gassing). This absorption process occurs until the amount of gas outside the tissue is exactly the same as inside (as measured by the partial pressure of the gas). When you change depth and ascend the tissue suddenly has more gas than blood and so releases the gas, a process called out gassing. This is the simplest picture I can paint…and so has its own set of inaccuracies, but you get the idea.
Not all tissues have the same reaction to an increase in the amount of gas available. Some are thirstier than others. When a tissue is greedy and absorbs gas fast it is called a fast tissues and when it takes its time to take on board the new gas, it is called a slow tissue. The speed at which tissues absorb gas is used to classify every tissue in the body. To make it easier tissues are further grouped together into compartments that behave in more or less the same way, absorbing and releasing gases at the same rate (Buehlmann created 16 compartments). The compartments are important for decompression programs as the maths then treats all tissues in that compartment as having the same absorption features.
When discussing tissues the phrase tension is often used (tissue tension) which is simply another way of saying the amount of gas (nitrogen or helium) that is absorbed by that tissue (or put yet another way, the gas pressure). The tables ensure that the tension of the gas in the tissue does not drastically exceed the outside tension. When it does.. bubbles result.
The amount of time it takes for a tissue to release (or outgas) its nitrogen or helium is referred to as that tissues half time. This is a standardised unit that measures the amount of time it takes for a tissue to halve its gas tension (halve the amount of gas it has absorbed).
There are a number of different factors that affect whether or not a tissues is fast or slow, these include the degree of blood flow to that tissue (perfusion), the amount of blood vessels within the tissue and so the distribution of the blood to a tissue and the solubility of the tissue (i.e. how easily it absorbs nitrogen and helium.) This means that areas that have a lot of blood vessels (good perfusion) tend to be faster tissues (lungs and abdominal organs). Slower tissues are normally fat and joints. Fat also holds onto nitrogen better (incidentally this is believed to be one of the reasons we are susceptible to narcosis, our neurons are sheathed in fat and fat likes nitrogen).
One common misconception is that as you ascend all your tissues will be outgassing. But not all the tissues find themselves in a situation where the pressure outside is less than the pressure inside… which means that they are in fact still ongassing (the tension of the tissue is less than the ambient pressure rather than greater than). This is more often the case on deeper dives and has been used as an argument AGAINST deep stops ( personally believe the value of the deep stop is to outgas your fast tissues effectively and stop these bubbling and causing problems at shallower levels. My philosophy is to let your slower tissues absorb, I will deal with them when I get to them J , i.e shallower)
A tissue is deemed to be saturated when the tension outside and inside is the same. The tissue is in a state of equilibrium, it neither takes up gas nor does it let it go. Supersaturation is when the tissues have more gas than outside (ambient pressure outside is less than the pressure of the tissue). Bubbles form when based on how high this supersaturation is (i.e. how great the pressure difference) and how long this state remains true. This is termed critical supersaturation.
Another element often referred to in tables is M-values. These are the maximum nitrogen tensions for a tissue after which bubbles form (or the supersaturation point). Fast tissues tolerate higher supersaturation rates than slower tissues and so have higher M-values.
Tables and decompression programs put all this together and use maths to calculate the times you need to stop to allow the tension in your tissues to subside (i.e. gas to be released into your blood, move to your lungs and then be breathed out). Most tables are parallel models in that they assume that all tissues are exposed to the effects of the gas at any one time…as opposed to serial models where each compartment reacts one after the other (which is obviously not true).
Simple right ? J
Tuesday, July 8, 2008
You may already be a fan of deep stops or you may be like a lot of divers, you have heard of them but have been diving successfully for years without them so don’t see why you need to change. Here is some food for thought (my sources for this blog are the NAUI Journal of Underwater Education, Vol 20, Issue 2 and the published journal on the “Effect of Varying Deep Stop Times and Shalo Stop Time on Precordial Bubbles” by Bennett, Marroni, Cronje, Cali-corleo, Leonardi Bonucelli, Balestra, Undersea and Hyperbaric Medicine 2007; 34(6).)
NAUI (as an example which is not meant to imply that other agencies do not already recommend this) had a recommended deep stop of 1 minute at a depth that is half that of the deepest depth reached on the dive. They have now changed that stop from 1 minute to 2 or 3 minutes ( the ideal being 2.5 minutes). This is based on the research by Bennett et al (for us more technically minded divers this ‘deep’ stop should be half the absolute depth and not half the actual depth…and yes, they really mean at least 2.5 minutes…read on)
What does the research say ? Well, it has already been proved that a deep 5 minute stop at 15 m (on a 25 meter dive) in addition to the typical 3 to 5 min shallow stop (6 m) significantly reduces the number of bubbles and fast tissue compartment gas tensions (this research also indicated that the optimal ascent rate from 25 m was 10 m/ minute). This led to a number of agencies introducing the 1 minute ‘deep’ stop. BUT, the new research indicates that 1 minute is too short and actually increases the bubbling rather then decreasing it. Dives with 2.5 minute deeps stops showed the highest bubble reduction. Interestingly the research also found that if you have a deep stop, you could drastically reduce the time you spent shallow without changing the bubbling.
Some underlying theory - most decompression computer algorithms and dive tables are based on the original ideas of Haldane or Hill. Haldane modelled gas uptake and elimination on 5 tissue compartments that were either fast or slow to uptake/ eliminate gas (the compartments are virtual entities into which the various parts of the body are assigned a group, for example, brain tissue and nerves are fast tissues, fat is a slow tissue…I will blog on the basics of decompression next week).
Buelmann increased the number of tissue compartments to 16 / 18 creating an algorithm that is perhaps more accurate and safer as a result. Most models are based on the common premise that as long as none of these compartments become supersaturated beyond a certain critical threshold, decompression sickness (DCS) is avoided. Haldane also (and critically) introduced the concept that it is as safe to come from 6ATA to 3ATA as it is to come from 4ATA to 2ATA, or a 2:1 ratio of absolute depth. This ratio of one half the absolute depth has been modified over the past decades and now ranges from 4 to 1 for fast tissues to less than 2 to 1 for slow tissues. Interestingly though, most tables adopted Hills approach of a linear ascent of 10m/ min with a safety stop at 3 to 5 meters. This meant that the concept of staged decompression at one half the absolute depth was eliminated… and so were the positive effects there-of. In essence this is what Bennett et al are doing, re-introducing a stop at one half your absolute depth. Their research indicates that such a deep stop (for their 25 meter dive) often completely eliminated type 3 and 4 bubbles (bubbles were graded as follows 0 (none), 0.5 (sporadic), 1 (15 bubbles over 1 minute with bubble showers), 2 (30 bubbles), 2.5 (>30 bubbles with showers), 3 (virtually continuous bubbling), 3.5 (continuous bubbling) and 4 (continuous bubbling with continuous showers).
What did they find across their various profiles ?
- After a 2.5 min deep stop, decreasing the shallow stop time from 5 min to 1 min had no significant difference to bubbling. They are do not come out and quite say it, but the evidence seems to indicate that the safety stop at 5 meters can be seriously reduced to as little as 1 minute although they still recommend it for recreational diving to prevent pulmonary barotrauma.
- Without a deep stop increasing the shallow stop time did not give you the same effect as a profile with a deep stop. So basically, without the deep stop you had significant bubbling and nothing you do changes that. This was supported when they increased the shallow stop to 10 minutes (from 5 ) with only a slight reduction in bubbling being reported
- If a deep stop is used, any times less than 2.5 minutes ACTUALLY INCREASES YOUR BUBBLING as compared to not having the deep stop at all. So basically either do not do the deep stop or do it properly spending at least 2.5 minutes, otherwise you are making it worse.
I find this research quite interesting as deep stops have always been part of my profile. In fact, I remember quite clearly asking Dr Cronje what advice he had to prevent decompression and him stating “prevent bubbles from forming” (well his actual answer was do not do the dive J ), which is what this research seems to indicate.
If you want to stop bubbling you have to stop it occurring, which is normally deep in your ascent. Staying shallow is there to remove bubbles that have already formed.
Dives that produce bubbles tend to produce DCS!
Wednesday, July 2, 2008
Here in South Africa the diving dry suit is more the exception than the norm. Well, for most divers. For those of us diving caves (or Cape Town or long decompression) a dry suit is a must. With the slow adoption of technical diving more and more divers are starting to move towards dry suits. The question is which one ?
For those of you who are reading who are not quite sure what a dry suit is… simply put it is sealed suit that prevents nasty cold water from ever reaching/ touching your skin (well ok, not your hands and your head, although you can get dry gloves and hoods but that is a bit of overkill in Africa). It does this through the use of seals that prevent water from entering the suit at your neck and wrists.
There are many brands of dry suit available to a diver, but only these tend to fall within two main types, a shell or membrane suit or a neoprene suit. Each 'type' has its own set of pro’s and cons, not the least of which is price (indeed price is often the most important factor when selecting a suit).
MEMBRANE DRY SUITS
Shell suits are so called because they are made of a thin, strong and durable material that creates a shell around the diver. This resembles a hard wearing membrane and is made up of layers. For our use the only way to go is a tri-laminate membrane suit as the bi-laminate is just too thin and prone to breaking (a membrane dry suit is worthless if it leaks). As the suit is a thin'ish membrane there is little insularion (unlike a wet suit). The intention is too keep the water out so with a membrane suit inners must be worne to provide warmth. Without the inner (think portable sleeping bag) diving in cold water is akin to standing in a gale with a dry mac on…i.e. cold!
There are a number of advantages to a membrane suit :
- They are cheaper than neoprene (especially crushed neoprene)
- They do not change their thermal insulation and buoyancy properties with depth (neoprene compresses and so becomes less effective)
- They are versatile as you can change your inners (and so your warmth factor), moving from a thin inner (or even just a track suit) to something hectic and more appropriate for arctic conditions.
They do however have disadvantages:
- They are not as flexible as their neoprene cousins, so if you are dressed up for warmth reaching fins can be a problem
- They have little to no thermal insulation properties so if your suit floods, you stand the chance of freezing unless you have some hectic inners or a heating system (they do exist)
NEOPRENE DRY SUITS
The neoprene counterpart comes in two varieties, normal neoprene and of course the deluxe and ultimate... crushed neoprene. The big difference with a neoprene dry suit is that you tend to use them without inners (unless you are using crushed neoprene of which the DUI is the only choice).
Advantages of neoprene:
- They have in built thermal insulation like a wet suit so if you have a loose fitting one (not normally the case, they normally fit tight like a wet suit) you can add warm inners and so create the ultimate suit for long, cold stays
- Because they are neoprene, if you flood your suit you stand a better chance of heating the water up and surviving
- They are more flexible than a membrane with more give
- Unfortunately unless you are diving crushed neoprene, you lose a good part of you insulation properties at depth thanks to compression and you end up with buoyancy issues
- Price! Crushed neoprene suits will make you weep and even normal neoprene is more expensive than a membrane
So which to choose ? I did my world record using a Scubapro membrane suit. Nuno Gomes prefers neoprene because of the thermal insulating properties. I have now moved to a DUI crushed neoprene but the jury is still out. There was nothing wrong with my membrane and the only reason I moved to neoprene was because my next dive will be over 6 hours and I need extra warmth (read on for the paragraph on pee valves). Most of the divers we talk to make their choice based on price, which means membrane (especially if they need a custom fit).
Once you have chosen between membrane or neoprene you still have to choose from selections like;
- telescopic torso (a must as it gives you manoeuvrability, flexibility and comfort),
- where the dump valve goes shoulder or cuff (shoulder is better as it is out of the way and high up on the suit, on a cuff dump you have to spend your ascent with your hand above your ear….kinda inconvenient)
- and whether or not you want extra thigh pockets (yes, you do, they are damn handy)
Suits also come with back entry or shoulder/ front entry. To be honest, I still need help on the self donning shoulder entry suit but only with that last centimetre. Having said that, the self donning is still way easier to get into and out of than the standard and more common back entry which requires either a second person or some interesting antics as you try and ‘catch’ the zip tag and then pull it closed (or open) using a convenient protrusion (think cat scratching its back)
When it comes to a brand you have a wide choice, especially with the internet. Our recommendations are Otter or Scubapro (naturally) as these fit within our target price range (you do not want to know how much a DUI is these days). We find that we sell more Otter's simply because you can get a custom fit at a really reasonable price (and to my eternal regret, Scubapro does not make custom fit suits). Why is custom fit so important ? Well most divers actually battle to fit into the standard off the shelf cuts, especially if you are female and buying a male cut (which is often the case). Custom fit also means you do not have to spend your life with a size 8 or 10 boot when you feet are a size 6 (based on the off the shelf suit you had to choose to get your length and girth to fit).
Another question that is hotly debated is when to purchase a dry suit ? Normally divers leave the dry suit to last, well after they have bought their Liquivisions or VR3's. Which I find odd. You can always (and normally do) plan your dive on paper so a helium computer is a luxury BUT you can not easily unfreeze yourself doing those 2 to 4 hour dives in cave cold water. From a pure risk management view point maintaining your body temperature should be one of your primary and critical factors that you are managing. Being cold is just way more of a risk than having a snazzy helium computer that means you can cut corners and not plan on paper before a dive. My advice is always to get the suit before the computer (they cost more or less the same).
The final topic that does need to be discussed (and one that I am not going to blog about, I will leave that to Gerhard) is that of pee valves. This is a conversation that I really do not want to know more about and is exclusively an option for the male dry suit diver and yes, I am jealous. When I do a deep dive I have to wear incontinent nappies which last like twenty minutes after which I am wet and cold . The guys fit their condom catheters and spend a blissful 4 hours warm and more importantly dry.
Would I ever go back to diving wet ? No way! I have only ever been too warm in a dry suit once and compared to being almost permanently cold and unable to warm up after repetitive dives I think the warmth of kitting up in mid summer is a small price to pay. Admitedly sea dives take some getting used to these days as it is harder to get back into the boat because I have a power inflator on my chest that catches me as I try and get in but then again, sea dives are not really on my agenda much so I guess I can live with the inconvenience. If you are a technical diver my only question to you is, why are you not diving dry yet ?