Decompression Planning for TMX Dives between 120-200m Author: Jonas Samuelsson, TBI, Jan 2013 Introduction Brief History Application Table of Guidelines Use of decompression software Decompression Schedules Execution of dives References Introduction This article is going to review decompression planning for dives between 120-200 meters. The objective is not to explain technical terms or the difference between decompression models. We are certain that Dr Buhlmann and Erik Baker explain their theories better than us. What we can do is to share our experiences utilizing their theories in ways that they could not. Buhlmann was not a diver himself and even if Erik Baker is both cave and trimix diver our frequent deep dives can perhaps shed some light which hopefully leads to safer planning. The article is based on a series of trimix dives during 2010-2012 when our team conducted 180 dives to 70-90 meters, 80 dives to 90-120 meters, 30 dives to 120-135 meters and 20 dives to 135-200 meters. The dives took place in the Red Sea, Iceland, Sweden and Mexico. Based on these dives, previous published materials, hours of discussions within our team and others with well documented experience and the most important actual execution of the dives we created a method of decompression planning and execution that been working fairly well for us. We are not saying that these methods are optimal and there is no room for improvement. Diving deeper than 120 meters is still quite rare and with more data from actual dives and more research conducted in the hyperbaric community and other medical fields we adapt and adjust our plans accordingly. Use this as a guide for discussions between your diving buddies and when teaching technical diving courses. In this article we are also going to review to what depth, in our opinion, DIR configuration and dive planning works. At the end of the article there is reference to articles that further explain different aspects of decompression planning. Important: Trimix diving is not a safe activity. Make sure that you have the correct training and experience before any dive. Always bring lots of gas and progress slowly. Follow the guide lines given by your technical instructor and the diving organization you been trained through. Using the information provided in this article might cause serious injury or death. John gave me the ok and we started our descent. Our descent rate was set for 50meters/minute down to 150 meters. At that depth we plan to slow down our descent to 25meters/minute for the last 50 meters. Our objective was to conduct a 200 meter dive and we did not use a line as reference. We both carried doubles 20 liter steels with a 6 liter attached between the twins for inflation of the wing. We each carried 8 aluminium deco tanks. We had one deep support at 75 meters and the other at 20 meters. Both support divers carried extra gas. Our bottom mix was 6/72 which gave us an equivalent narcotic depth of 49 meters with an oxygen partial pressure of just below 1.4. Three minutes later we were down at 150 meters and we started to slow down our descent according to our plan. At 167 meters John signaled that something was wrong. His arm was shaking slightly. We both realized that he got symptoms of HPNS. We decided to abort the descent and begin our 4 hour decompression. It took us another 10 meter of depth to completely stop. Our depth was 177 meters when we started to ascent.
This dive was conducted during 2012 and analyzing dives like this one we learn and adjust. But before we analyze the dive above let's discuss some history and terms in regards to deep open circuit mixed gas diving. History of decompression planning There are plenty of articles already published related to the history of decompression planning and I am sure that you are already familiar with it. The reason why I briefly mention it here is to show the benefit of using both longer shallow stops ala Haldane and to integrate the bubblemodel into your dive plan. Haldanian/Bühlmann models Dr. John Scott Haldane who in 1905 got assign by the British Navy to look into how to make the diving conducted by the British Navy divers safer. Using goats in recompression chambers as test subject he based a strategy for decompression Haldane's perhaps greatest contribution was to realize that he did not need to model every bit of a goat and know its half times just to have representative numbers that were close enough so that their effects overlapped. These are called Compartments. Dr Albert Buhlmann based his work on the Haldanian model and developed the ZH algorithm which is used in many of today's dive computers. Before any animal activitist reacts to the fact that Haldane used pigs in his research we would like you to bear in mind that the modern decompression modelers are using us, the technical diver, as 'pigs'. VPM and RGBM bubble models David Yount designed the Yount's Varying Permeability Model (VPM) - a model that accounts for bubbles formation during the dive. Others like Eric Maiken and Erik Baker based their work on the VPM. Erik Baker also contributed with the gradient factor inclusion in decompression planning. VPM-B is a more conservative version of the original VPM. (which is a good thing because the previous VPM gave many divers decompression sickness when released). Another bubble model that surfaced was Dr Weinke's Reduced Gradient Bubble Model (RGBM). RGBM is used in SUUNTO computers among others. Our team use a bubble model (VPM-B/E with gradient factors) for our dive planning with extra time added for our stops starting at 9 meters for extra conservatism. This method has been proven to work well for us. By combining the two models we believe that we can make our decompression more efficient and hopefully safer. For the deeper dives we consult Steve Burton, 'CDM-18 analyzer' to find any irregularies which need to be adjusted before the execution of the dive. Essential terms of dive planning Let's now review some of the essential terms of technical deep diving planning and how we apply or done apply them during our dives. Team Approach We always plan our dives as a team. For dives between 120-150 meters we use two support divers on at 75 meters and one at 20 meters. For dives between 150-200 meters we use 3 support divers. The deep support diver meets us at around 85-90 meters. Second support diver at 40 meters and the shallow support diver at 12 meters. We always have surface support and we always contact the local recompression chamber and inform DAN, Divers Alert Network or the Russian Insurance Company T.I.T. well before any dives deeper than 100 meters to make sure that they are aware and approve of the dive respectively. DAN is using a special form for approval of deeper dives. Contact your local DAN office for more information. Equipment configuration We are strong believers of a standardization of equipment and we are thanking the DIR community for promoting the Hogarthian, named after William Hogarth, equipment configuration so successfully. For dives to 120 meters we are using a strict DIR configuration and find it is the most efficient way of carrying your equipment for all our diving regardless of environment. However for dives deeper than 120 meters once again the DIR way
has to be adjusted some. When carrying for example 8 decompression cylinders we find the most efficient way is to carry our two deepest gas mixes on our left side, our shallow gas mixes like 70% and 100% on our right side and all gases between on a leach on our left side. Reason for carrying the rich gases more of an old school way on the right side is that they are heavy due to the oxygen and have the tendency to hang low during the dive. The tanks containing helium is lighter and can be comfortably be carried during a leach. Then we 'roll' the tanks the during the dive, except for the rich oxygen tanks, on our left side so the tank that we always breath is on top left side. (William Hogarth Deompression Rates (Ascent) We set our ascent rate of 15meters/minute to 150 meters, then we slow down to 10meters/minute until we reach our first decompression stop where we continue our ascent with 3meters/minute. (US Navy, NOAA) Compression Rates (Descent) For a single dive to 120 meters we use a descent rate of 25 meters per minute and for single dive between 120-200 meters we use a descent rate of 50 meters per minute until reaching 25 meters from the planned maximum depth when we slow down the descent to 25 meters/minute. Slowing down for the last 25 meters got more to do with the fact that it takes some time to actually slow down your descent so to avoid going deeper than the planned depth. This method of descending might contradict with the methods used by commercial divers successfully over many years where they slow down their descent rate significantly during deeper dives. A commercial diver can take a full day to descent to 200 meters and it's been proven to be an effective way to avoid High Pressure Neurological Syndrome. However in our case without the surface support that commercial divers have we don t have that priviliege. To have any reduced effect of HPNS you need to slow down your descent to 5meter/minute or less, Bennett 1982, which in our case would result in saturated tissues and unrealistic decompression times. We have to battle the HPNS in other ways as described further down. (Bennett 1984) Gas Management & Surface Air Consumption (SAC) Rates Looking into many cases of injuries and fatalities within the technical diving community we have seen a pattern resulting in accidents. First and most common is to dive deeper than your training and experience allows and secondly having lack of gas supply. We battle this by carrying all our gases with us to complete our decompression independently including reserve and by having deep and shallow support divers carrying extra gas from the first decompression stop and for the rest of the dive beyond that point. We adapt the 1/3 rule for all our dives. (Sheck Exley, Wes Skiles) M-Values, Gradient Factor and Conservatism M-Values are as you know the maximum pressure each tissue compartment (TC) can tolerate before you are in serious risk of getting DCS. With other words going beyond the M-Values would be insanity if you asked the man who coined the phrase Dr Buhlmann. We follow Erik Bakers strategy by ascending to a point where our Gradient Factor (GF) is not above 0.3, 70% buffer to the M-Value, which means as you know that the Tissue Compartment (TC) pressure is 30% of the way between ambient pressure and Buhlmann s M-value. Then we do our stop there until the TCs dropped enough pressure so that you can ascent to your next stop. Our end goal surfacing is to never reach a GF of more than 0.85 (15%buffer to the M-Value). We use +2 Conservatism in our configuration of VPM-B/E which add an extra 12% conservatism based on the baseline set by the program designers. (Understanding M-Values, Erik Baker and http://www.hhssoftware.com/v-planner/faqhelp.html). CNS Oxygen Toxicity and Limits (Paul Bert effect) For dives between 0-150 meters we use a standard max 1.4 ppo2 for our bottom mix and 1.6 ppo2 for any decompression gases. For dives deeper than 150 meters we use 1.6 ppo2 during the complete phase of the dive. This once again contradict everything we learn from our technical diving manuals so what is our rational for using such a high ppo2 on dives deeper than 120 meters? High PPO2 at bottom depth enables us to keep a higher partial pressure to the first gas switch. The benefit of a higher PPO2 at depth and to first deco exchange result in shorter overall decompression and also have an positive effect on the Oxygen Window. By making sure our PCo2
low by using low levels of energy and making sure that our body temperature is steady by proper exposure we believe the risk for a Whole Body Oxygen Toxicity is very small. So pro's of shorter decompression times versus con's of adding risk of Whole Body Toxicity our conclusion for dives deeper than 120 meters the 1.6PPo2 has the advantage. (Albert R. Behnke, R.W. Hamilton) Whole Body Oxygen Toxicity and Limits (Pulmonary Oxygen Toxicity/Lorrain Smith effect) We use a maximum of 850 OTUs per day and don t dive 24 hours before dive deeper than 120 meters. This way we reduce the amount of exposure we have prior to the dive. 850 OTUs is following the guidelines by R.W. Hamilton and are within the limits of the Repex Method. (Tolerating Exposure to High Oxygen Levels: Repex and other Methods. R.W.Hamilton, Hamilton Research) Oxygen Window We choose to lengthen the decompression stops where ppo2 is high and to push gradient at the shallower depths of the decompression curve, thus creating an S-shaped curve. We add 5 minutes for each gas switch conducted at first decompression stop and shallower depth. If we also have a required decompression stop, 5 min or more, at that gas switch then we don t add time. We avoid adding additional time during our deepest gas switches due to the concern that absorption of gases in our tissues during the extended stop takes out the possible benefit of the Oxygen Window effect. (Albert R. Behnke, 1967) Air breaks Air breaks for 5 min every 20 minutes have been proven to be effective in reducing or postponing pulmonary oxygen toxicity due to long exposure of breathing oxygen. Because we don t breath air during a dive to 120 meter we either take an 'air break' using shallow support divers gas or we use the decompression gas with the lowest, above PPO20.18, oxygen content, (Lambertsen 1988; Butler and Thalmann 1984; Henricks 1977) High Pressure Neurological Syndrome (HPNS) As you know HPNS is a condition that can occur at depth deeper than 120 meters breathing a mix of oxygen and helium. Symptoms like tremors, vertigo, nausea are widely known and published. The believed reason for HPNS is that the nerves, consisting of nerve cells as conductors and lipids as insulation, are exposed to an increasing pressure. This results in compression of the lipids and the insulation, causing "short-cuts" of the CNS. High lipid solubility means a large swelling of the lipid tissues with increasing cell volume, which counteracts the effects of compression. There are three possible ways to suppress HPNS effects: adding narcotic gas to the breathing gas; reducing the compression rate; and taking longer stops during compression for the pressure adaptation. Compression rates of 5m/min have been used successfully during descent on dives to 180 meters. Out of the three options there is only one feasible for the technical diver and that is adding narcotic gas to the breathing gas. Compression rates of 5m/min or slower, utilized by NAVY and commercial divers, would result in unrealistic decompression times. (Bennett 1984) Equivalent Narcotic Depth (END) This is one of the most discussed points in the technical diving community today. Our opinion regarding ENDs is that for any dives conducted in an overhead environment regardless of depth and conditions we use an END of 30 meters. For environments without hard overhead (caves, wrecks) we use an END of 30-40 meters during dives to 120 meters. The lower 30 meter END for cold water diving and in strengious conditions and the higher 40 END when conditions are favorable in terms of visibility, temperature and currents. So what happen for dives deeper than 120 meters. We discussed earlier about out descent rate and the fact that we did not have the resources which commercial divers have to reduce the effects of HPNS by slow compression. So what can we do to reduce the effects? The only way in our opinion except for the obvious of keeping fit and taking your vitamins and being prepared for the dive both mentally and physically is to increase the END. This is where DIR goes out of the window in our opinion during dives deeper than 120 meters. Based on that study and our many dives to these depths and also from discussions with some of the most experienced deep divers in the world we now apply an END of 50 meters for dives between 120-200 meters. We don t conduct dives deeper than 200 meters at this point but if we were we would apply an END of 60 meters between 200-250 meters and an END of 70 meters
between 250-300 meters. We know this might be controversial but it's based on real studies and actual application by many dives to these depths. (Steve Burton, 'CDM-18 analyzer', R.W. Hamilton). Standard mixes If you use the same gas, same equipment, same plan over and over again and it's been proven to work you reduce the risk of accidents and injuries. We use standardized mixes as well for the majority of our dives. However as you will see when we look into our dive planning for dives between 120-200 meters there are no 'standard mixes' so we have to rely on our own custom mixes. Today's 'custom mixes' for dives deeper than 90 meters is perhaps tomorrows 'standard mixes'. (GUE, NOAA) Deco ratio We believe that a basic understanding of deco ratio increases the safety of the dive. However deco ratio does only apply to standard mixes so for many of our dives deco ratio does not apply. We plan our minimum one day ahead using decompression software and discuss the dives so all the team approve. By planning the dive early you get some time to mentally review it before the execution. We use a little different configuration than traditional DIR. We use bottom timers mainly for our dives in accordance with DIR. The reason that we prefer to use bottom timers is that they are very reliable. Another reason is that diving computers does not at always follow the decompression schedule of the decompression software. There are exceptions but they are few. But the main reason is that computers fail. So we prefer that all in the team are using bottom timers both as primary and secondary gauges. So the configuration we use during the dive is to wear the wrist slate on our right hand. Yeah that s correct. The reason for this is that we carry our primary plan on the slate regardless of depth and because we don t use the wrist slate to write on. As a result we don t need to have it on our left arm (most people are right handed so carrying the slate on the left arm makes writing easier). Another benefit to carry the wrist slate on the right hand is that it makes deflation while looking at the plan easier. So bottom timer and wrist slate with a primary plan on the right arm and a secondary bottom timer and compass on the left arm. The wetnotes are carried in our right pocket and is used for carrying backup plans and to write messages with. (GUE, UTD) Isobaric Counter Diffusion (ICD) We take Isobaric Counter Diffusion into consideration when planning our dives. ICD is proven to effect divers when breathing mixes of helium and nitrogen. We have seen a few cases the last few years on dives in the range 150-200 meters where the diver got inner ear bends due to ICD. It's not easy to decompress for hours while not knowing what is up or down and puking at the same time. Dehydration and not being able to stay level during decompression and more could lead to serious injury or death. So we like to take ICD into consideration. So how do we apply it for our deeper dives. You heard of the 1/5 rule which state that as long as you allow an increase of nitrogen one point for each 5 points of helium you reduce during any gas switch you should avoid it. We made it even more simple, based on research by Steve Burton, 'CDM-18 analyzer', for any gas exchange we lower the helium equal to the increase of oxygen and keep the nitrogen level same or lower. (Steve Burton, 'CDM-18 analyzer') Pyle Stops (Deep Stops) Your first deep stop being half way between your max depth and your first deco stop does not always apply for deeper diving in our opinion. However we do our first gas exchange close to the on-off gassing point and as a result we many times get a "bonus" deep stop without planning it, due to the fact that the first deep stop occurs normally close to on/offgas. Decompression software, like VPM-B, accounts for deep stops so in our opinion it's not necessary to add extra 'Pyle' Stops to your decompression schedule. (Richard Pyle, Erik Baker) Nitrogen
We all know what can happen if you don t decompress your tissues after breathing inert gases like nitrogen at pressure. We try not to have more nitrogen than 30% in any gas mix during the complete dive. This is based on a recommendation given by commercial diving company COMEX. This could be in conflict with high END during a dive and something you need to balance when you planning your dives in regards to both decompression sickness and HPNS. Regarding nitrogen narcosis our belief is that you can't build up a physiological tolerance for it, however saying that, by diving frequently we seen a better function in dealing with tasks underwater even with relatively high ENDs. The reason for this in our opinion is simply that you become more comfortable during the dive as you build up more experience and due to the experience level you are breathing less gas resulting in a less carbon dioxide buildup. Carbon dioxide is 20 times more narcotic than nitrogen so by keeping calm and by breathing slow, deep and regular you reduce the effects of narcosis. (COMEX Commercial Diving, France) TBI Guidelines TMX Diving Depth Exceeding 120m Topic Guidelines Rational Reference Compression (Descent) 50m/min to 25m away from bottom where you slow down to 25m/min. Fast descent rate equal less inert gas absorbtions equals less deco time. To reduce effect of HPNS a slower descent rate of 5m/min has be applied which in our case is unrealistic. P.B. Bennett and D.H. Elliot. Decompression (Ascent) Ascent of 15m/min to 150m. Then 10m/min to first deco stop where you slow down to 3m/min 15m/min during ongassing and 10m/min for off gassing integrated with a slow descent from the first deco stop has shown to be optimal balance between decompression and absorbtion of inert gases. The U.S. Navy and the National Oceanic and Atmospheric Administration (NOAA) CNS Oxygen Toxicity * Partial Pressure Oxygen (PPO2) PPO2 1.6 during the complete phase of the dive. High PPO2 at bottom and gas exchange increase decompression efficiency and keep the 02 higher during the complete ascent. R.W. Hamilton, Hamilton Research. Equivalent Narcotic Depth (END) 120-200m: END 50 * 200-250m: END 60 * 250-300m: END 70 High END (increase of Narcotic Gases in the mix) reduce the risk of HPNS. Steve Burton, J.C. rostain, C. lavoute, J.J. risso, N. Vallée, M. weiss Gas Management Rule of thirds. All divers carry all gases to complete the dive independently. Rule of thirds is a good balance between gas reserve and work load of carrying many tanks. Enough gas so you can complete the total deco safely even without support divers. Sheck Exley, Wes Skiles Decompression Models (Bubble versus Buhlmann) Bubble model (VPM-B with GF) with rule of thirds applied to all deco beyond 9 m (to benefit of longer Buhlmann stops) Controlling the micro bubbles during descent by ascending slow from first deco stop and earlier deco stops enable the tissus to offgas earlier. Extra time at shallower stops for extra precaution. Haldane, Buhlmann, Erik Baker Gradient Factors Gradient factors of 0.3 and 0.85 0.3 control microbubbles expansion during ascent and 0.85 at the end of the dive is a good compromise between amble deco time and dcs precausion. Erik Baker Isobaric Counter Diffusion (ICD) Disregard the 1/5 rule and instead lower the amount of helium equal to the increase of oxygen during each gas switch during ascent. Keep END lower during the complete ascent. By lowering both He and N2 during the complete ascent and during each gas switch you can avoid ICD simply because for each switch both He and N2 decrease. Steve Burton
Team Approach Dive in teams of two. Min one surface support. 2 surface support for dives to max 150m. 3 support divers for dives to 200m. 4 or more for dives beyond that. Diving in teams means more gas and a helping hand during HPNS and other physiological problems at depth. Support divers provide more gas and assists during an ICD. They also provide food and dehydrations. GUE, UTD Deep stops We don t add any deep stops beyond the ones that already are included in the bubble model. VPM-B with GF 85% provides ample deep stops already in its algorihtm. Pyle Stops. Richard Pyle Oxygen Window Slow down the ascent rate to 3m/min, and add 5 minutes to each gas exchange, from the first deco stop. Adding additional time for any gas switch depth only increasing the overall decompression quality. Behnke. (1967) Standard Mixes Standard mixes is not available for dives deeper than 90m Today's 'custom mixes' might be tomorrows 'standard mixes' for dives deeper than 120m. NOAA, GUE, UTD Deco Ratio Deco ratio calculations does not apply for dives deeper than 120 m. Deco ratio only applies to dives to 90m Mark Powell, Deco for Divers Whole Body O2 Exposure * Partial Pressure Oxygen (PPO2) Maximum 800 Oxygen Tolerance Units (no exposure 24 hours prior to the dive) Addcording to R.W.Hamilton and the Repex Method staying within 800 OTUs for a single dive has shown being suffucient. R.W. Hamilton Hamilton Research. 'Repex and Other Methods" Airbreaks' 5 min every 20 min of oxygen breathing. Proven to reduce or postponing CNS O2 Poisoning. Support diver bring 'air tanks' to be used or divers cylinder with lowest o2 content (min PPO2 0.18) Lambertsen 1988 * Butler Thailmann 1984 * Henricks 1977