VII. Knowledge Development Presentation Two A. Equipment II – Fullface masks and multigas dive computers Learning Objectives: By the end of this section, you should be able to answer these questions: 1. What is the primary advantage of using a fullface mask for trimix diving? 2. What are two primary considerations in using a fullface mask for trimix diving? 3. What are two primary advantages of diving with a multigas trimix computer? 4. How do you plan dives with a multigas computer? 5. How do you backup a multigas computer when diving with trimix?
1. Fullface masks a. The primary advantage of using a fullface mask is to reduce drowning risk in the case of a CNS convulsion or other loss of consciousness such as hypoxic blackout. may allow additional comfort in colder water may be fitted with voice communication electronics CNS risk may be higher during trimix dive due to longer decompressions; fullface mask may reduce the risk of drowning should CNS toxicity occur
b. Considerations when using fullface masks with trimix gas switches – you will need to switch gases, at least for air breaks use of valving and switch blocks not favored in tec diving due to configuration complexity and error potential a conventional fullface mask may require entirely removing the mask one or more times during the dive – not typically preferred for tec diving
gas sharing – fullface mask should not interfere with the ability to provide gas to a teammate in an emergency conventional fullface masks are not typically well suited to gas sharing in a conventional tec diving manner using fullface masks compatible with tec diving a fullface mask compatible with tec diving allows use of standard second stages and no significant procedural changes at this writing, at least one version (the Kirby Morgan M-48) exists with M-48, you can breathe from a standard second stage, or second stage with M-48 mouthpod attached with M-48, you use mouthpods for easy gas switches with fullface mask risk reduction divers not using M-48 mask can still breathe from the M-48 mouthpod (it has a mouthpiece) usual tec procedure with M-48 is to use standard second stage (no mouthpod) for bottom gases (long hose, secondary and travel gases) and mouthpods for deco gases (highest CNS risk)
2. Multigas trimix computers a. At this writing, several multigas trimix computers have entered the market You can program these computers with seven or more gas blends: oxygen, enriched air, air or trimix. During the dive, the computer calculates your decompression based on the actual dive depth, time and gases you use (switch the computer as you switch gases per manufacturer instructions) Some interface with desktop deco software. b. Two primary advantages of diving with multigas trimix computers Precision – Your deco schedule is based on your actual dive profile; especially useful in caves, wrecks and other dives that follow varied contours. Simplifies some emergency situations if you switch to a low oxygen gas because of CNS concerns, the computer calculates your decompression based on the lower oxygen gas (as opposed to simply stopping the deco clock) with a deco gas problem (lost, runaway regulator, etc.), computer calculates deco with the gases you have (including back gas if necessary) extra gas compatibility in an emergency – you program computer with additional gases (e.g. Your team is using four gases but your computers allow seven: TMx 21/50, EANx36, EANx50 and oxygen. However, there’s also EANx32 and air available; you set these in two of the “extra” gas choices so that computer can calculate your decompression with them should they be needed in an emergency.)
c. Planning dives with multigas trimix computers You plan dives using desktop deco software. multigas dive computers do not calculate gas supply requirements predive planning modes don’t usually allow easy comparison of differing gases, depths, etc. you usually want backup tables anyway (they’re required with only a single computer)
With experience, you’ll learn which desktop deco software algorithm (if the type you’re using offers a choice) most closely matches your multigas dive computer – generally start conservative.
d. Backing up a multigas trimix computer Simplest method is to have two multigas trimix computers, ideally the same model, or at least two with same decompression model (also most expensive option) Two computers, even of the same model, are seldom exactly identical in their profile (though they shouldn’t differ much); obviously you decompress according to the more conservative one. With only one computer, you carry contingency tables you generate with desktop deco software during predive planning. Many divers carry contingency tables even if they have the two computers, since they’re generating them anyway as part of dive planning.
B. Gas Planning II – Properties of helium, ENDs, choosing bottom and deco gases, hypoxic gas blends Learning Objectives: By the end of this section, you should be able to answer these questions: 1. What four characteristics of helium relate to its use as a dive gas? 2. What are the two primary advantages of helium as a dive gas? 3. What are four disadvantages of helium as a dive gas? 4. What is isobaric counterdiffusion and how do you avoid it? 5. How can putting trimix in your dry suit cause a decompression problem? 6.What is an END (Equivalent Narcotic Depth), and how do you determine it? 7. What four considerations do you apply in selecting a trimix? 8. What is meant by “ideal mix”? For what reasons is the “ideal mix” often not ideal? 9. What is meant by a “hypoxic” trimix? 10. What is a “travel gas”? 11. When do you need a travel gas? 12. What is the recommendation regarding using air or enriched air for a repetitive dive after a trimix dive?
1. The following characteristics of helium relate to its use as a dive gas: a. Helium doesn’t dissolve in lipids as readily as nitrogen or oxygen. Solubility in lipids is associated with a gas’ narcotic potential. b. Helium is less dense than nitrogen or oxygen. c. Helium diffuses more rapidly than nitrogen or oxygen. Graham’s Law of Diffusion states that the relative rates of diffusion of two gases is inversely proportional to the square roots of their densities. helium’s density (grams/litre) approx =.18, square root .42 nitrogen’s density (grams/litre) approx = 1.3, square root 1.14 oxygen’s density (grams/liter) approx = 1.4, square root 1.18 All else being equal, helium diffuses more than 2.5 times as fast as oxygen or nitrogen. d. Helium conducts heat faster than nitrogen/oxygen do.
2. Primary advantages of helium as a dive gas: a. Helium doesn’t cause narcosis (at least within the depths frequented by tec divers), which is one of the primary purposes of trimix. theorized that this is due to poor lipid solubility (Meyer-Overton hypothesis) can cause High Pressure Nervous Syndrome (HPNS – effect on the nervous system characterized by tremors, nausea, dizziness, fatigue, muscle twitching), but typically at depths well below the norm for tec diving (but within commercial diving heliox range – generally below 125 metres/400 feet) b. Helium requires less effort to breathe when diving deep due to its lower density. This reduces fatigue and saves energy.
3. Disadvantages of helium as a dive gas: a. Within the depths/times common to tec diving, helium’s rapid diffusion makes trimix dives longer than comparable air/enriched air dives because more gas goes into solution and must come out of solution during decompression. This is because the pressure gradient driving the helium into solution into body tissues at depth will be greater than the pressure gradient driving the helium out of solution during ascent. Therefore, the high gradient plus rapid diffusion means more helium goes into solution than would nitrogen, all things being equal. During ascent, however, the gradient is less, and not enough to force the helium to dissolve out of solution as fast as it went in despite its diffusion rate. Helium may therefore ingas so much faster than outgas that it takes longer to decompress than with a comparable dive made using a comparable amount of nitrogen. Example: According to one desktop deco software set on a conservative Buhlmann algorithm, a dive to 60 metres/200 feet for 40 minutes, with deco on EANx36, EANx50 and oxygen: 80 minutes deco using air as bottom gas 91 minutes deco using TMx21/35 as bottom gas 123 minutes deco using TMx21/70 as bottom gas Note that all three examples have the same oxygen (21 percent) and therefore the same fraction of inert gas; the deco time rises with more helium and less nitrogen. Note that because of helium’s rapid diffusion, very long dives that approach saturation have shorter decompression times with trimix than with a comparable air/enriched air. However, the bottom times for this are several hours long – beyond what’s common in tec diving. Using the same model and deco gases as above, a dive to 60 metres/200 feet for 720 minutes requires deco of: 48 hours, 50 minutes using air as bottom gas 34 hours, 04 minutes using TMx21/35 as bottom gas 28 hours, 49 minutes using TMx21/70 as bottom gas
b. Helium’s rapid diffusion makes it less forgiving compared to air in case of decompression errors. Because helium diffuses rapidly, it forms bubbles more easily. This means less error tolerance of decompression errors, such ascending past stop depth (even despite returning immediately), because bubbles can form more quickly. Helium mixes are known for producing DCS symptoms while still decompressing. This is relatively rare with oxygen-nitrogen blends (except when large amounts of decompression have been omitted).
c. Helium’s rapid diffusion rate makes isobaric counterdiffusion (a.k.a. inert gas counterdiffusion) a theoretical concern. An issue if switching from a slow diffusing gas to a fast diffusing gas during decompression, such as making an air/enriched air dive and using trimix during decompression. Helium diffuses into the tissues faster than nitrogen diffuses out, making the total dissolved inert gas high, possibly leading to bubble formation and DCS. Not generally a significant issue with the standard practice of switching from a fast diffusing gas (helium in TMx) to a slow diffusing gas (nitrogen in EANx) during decompression. (Note: Inner ear DCS commonly results after switching to air or EANx from heliox. The theory is that because heliox trapped in the ear is in gas form, it doesn’t diffuse out rapidly, but nitrogen diffuses in, raising the inner ear’s inert gas pressure.) This is one reason why gas compatibility between teammates is important – in a gas sharing emergency, it creates a risk for a diver using EANx/air as bottom mix to breathe the long hose of TMx diver.
Isobaric counterdiffusion can also be an issue by causing DCS through counter diffusion through your skin if you’re surrounded by a rapidly diffusing gas (helium/TMx) while breathing a slow diffusing gas (air/EANx). This is primarily an issue if you put helium in a dry suit (which you wouldn’t do anyway). Not generally an issue with argon (very slow diffusion) or wet suits.
d. As you already learned, helium’s heat absorbing characteristics (as well as isobaric counterdiffusion) makes it inappropriate for a dry suit inflation gas. As you know, trimix requires a separate inflation system, with argon the most common because it has better insulating capabilities than air/EANx. Some TMx divers put an inflator hose on an EANx deco cylinder as a backup inflation source.
4. ENDs (Equivalent Narcotic Depths) a. For a given depth using TMx, an END (Equivalent Narcotic Depth) is the depth at which you would expect the same narcosis if diving with air. b. END can be calculated assuming that oxygen is, or is not, narcotic. Because oxygen appears to be as or slightly more narcotic than nitrogen, the prevailing method is to treat oxygen as narcotic. Desktop deco software usually determines ENDs. You may need to select whether it treats oxygen as narcotic.
c. END is calculated by finding the absolute partial pressure (in msw or fsw) of the nitrogen/oxygen portion of trimix at a given depth, and then determining the depth at which that partial pressure is the total pressure.
Metric END = [(1 – fraction of He) X (D + 10)] – 10 Example: What is the END at 50 metres when diving TMx 18/33? END = [(1 – .33) X (50 + 10)] – 10 END = [(.67) X (60)] – 10 END = 40.2 – 10 END = 30.2 metres For simplicity, use the Equivalent Narcotic Depth table (metric or imperial) in the Tec Trimix Diver Manual .
Imperial END = [(1 – fraction of He) X (D + 33)] – 33 Example: What is the END at 165 feet when diving TMx 18/33? END = [(1 – .33) X (165 + 33)] – 33 END = [(.67) X (198)] – 33 END = 132.7 – 33 END = 99.7 feet For simplicity, use the Equivalent Narcotic Depth table (metric or imperial) in the Tec Trimix Diver Manual .
5. Several factors go into selecting the gases you will use on a given dive, as well as at what depth you will make gas switches. The gases you choose will be a judgment that balances these four considerations:
a. Narcosis What is the planned depth? What is the maximum tolerable narcosis for the dive and mission? (Recommended general maximum END is 40 metres/130 feet; the highest accepted maximums are 50 metres/165 feet for open water in good conditions, and 40 metres/130 feet for complex dives and overhead environments. In many areas it is common to use a maximum of 30 metres/100 feet because it doesn’t require much more helium.) b. Oxygen exposure What is the bottom oxygen partial pressure? What are the total CNS “clock” and OTU exposures for the entire dive?
c. Decompression issues What will the decompression duration be? Is that a thermal consideration? Is there ample gas for the decompression and reasonably possible contingencies? d. Logistics What gases are available? Do some gas blends have substantial advantages (available premixed to save time/cost, tables already prepared for specific blends, etc.)? What is best for the team in choosing compatible mixes?
6. The “ideal” trimix is defined as the trimix that has an oxygen percentage that yields a PO2 of 1.4 and an END of 40 metres/130 feet at the planned depth. For a given set of deco gases it is the trimix with the theoretically shortest decompression for that depth based on using the highest possible oxygen content and the lowest possible helium content.
a. You can find the ideal by using the Maximum Depth Table and the Equivalent Narcotic Depth table (metric or imperial). On the Maximum Depth Table, follow the 1.4 column to the desired depth, or next greater depth if the exact depth isn’t listed; follow the row left to the oxygen percentage in the right column. This tells you the oxygen the blend should have. On the Equivalent Narcotic Depth Table, follow the depth column for the planned depth to 40 metres/130 feet or the next shallower depth if 40 metres/130 feet isn’t listed, then follow the row left to the helium percentage in the right column. Example: What is the ideal trimix for 64 metres/210 feet? In the Maximum Depth Table, in the 1.4 column find 64 metres/210 feet. Go left to find 19 percent. Find the 66 metre (rounding up from 64)/210 foot depth column on the Equivalent Narcotic Depth Table, and follow it down to 40 metres/130 feet. Go left to find 34 percent (metric)/33 percent (imperial) helium (slight difference due to rounding in metric) The ideal trimix would therefore be TMx19/34 metric/TMx19/33 imperial You can also use a calculator to find the ideal mix.
Example: What is the ideal trimix for 64 metres/210 feet? In the Maximum Depth Table, in the 1.4 column find 64 metres/210 feet. Go left to find 19 percent. Find the 66 metre (rounding up from 64)/210 foot depth column on the Equivalent Narcotic Depth Table, and follow it down to 40 metres/130 feet. Go left to find 34 percent (metric)/33 percent (imperial) helium (slight difference due to rounding in metric)
The ideal trimix would therefore be TMx19/34 metric/TMx19/33 imperial You can also use a calculator to find the ideal mix.
b. The ideal trimix is often not ideal. Doesn’t consider oxygen exposure – may be useful to have less oxygen to minimize CNS “clock” and OTUs, especially when making repetitive dives or multiday dives. Better to deco longer than to push your oxygen limits. Mission may call for more conservative narcotic limit of 30 metres/100 feet END or shallower. Ideal trimix is seldom a blend that divers commonly use.
7. Hypoxic trimix a. “Hypoxic” means “low oxygen.” b. “Hypoxic trimix” means a trimix with less than 21 percent oxygen, with 18 percent the recommended minimum for use at the surface. As low as 16 percent can be breathed at the surface. Trimix with as little as 10.5 percent oxygen is common for deeper exploration, esp. in the 90 metre/300 foot range.
Although the blend may have insufficient oxygen PO2 to breathe at the surface, it will at depth due to increased partial pressure of oxygen. c. Hypoxic trimix is used to reduce oxygen exposure on deeper dives.
d. You may need a travel gas , which is simply a higher oxygen mix that you use until reaching a depth deep enough to use the trimix. Failure to use a travel gas can cause you to lose consciousness without warning due to hypoxia. You use a travel gas at least until you reach a depth at which you have a PO2 of .16 ata or greater with bottom gas. (Use the Trimix Oxygen Management Table to determine how deep you have to descend to reach a PO2 of .16 ata or greater). Example: Using TMx 14/33, how deep do you have to descend for a PO2 of .16 ata or greater? On the 14 percent Trimix Oxygen Management Table, follow the oxygen partial pressure column down until you find the first PO2 of .16 or higher, in this case .18. Follow the row left to find 3 metres/10 feet. You would use travel gas until reaching 3 metres/10 feet.
Community practice is to use some of the lowest oxygen deco gas (consumption included in gas planning), especially when descent is direct and short. In previous example, use EANx36 or other deco gas until you pass 3 metres/10 feet. Travel gas may be an intermediate depth trimix or enriched air nitrox. Common when you have a long descent or a long swim at an intermediate depth before descending to the bottom depth. Use of intermediate travel gas can reduce deco time by allowing higher oxygen content for long intermediate level. Example: Dive planned on a wreck to 69 metres/230 feet for 25 minutes bottom time using TMx14/40. However, you plan 10 minutes of the bottom time swimming along the upper hull at 30 metres/100 feet before descending to the bottom depth, so you use EANx32 from the surface until you descend from 30 metres/100 feet (not an isobaric counterdiffusion issue because you continue descent). Travel gas may be used as a decompression gas, especially for some of the deep stops (more about deep stops shortly) Example: Dive planned to 75 metres/245 feet using TMx11/50 for 25 minutes. You have to use travel gas to 5 metres/15 feet and you need the extra gas volume, so you take TMx21/15 and use it to 48 metres/160 feet. On ascent, you use it beginning with your second deep stop at 42 metres/140 feet until switching to EANx36 at 33 metres/110 feet. In the above example, note that the use of “thin” TMx as a travel gas allows you to stay on the higher O2 blend deeper than if using air.
[!] Remember that if you need a travel gas on the way down, you will need it on the way backup. You will also need it for any breathing at the surface (waiting for the boat, etc.) Your back gas becomes unavailable on ascent at the depth you needed your travel gas to reach on the way down. Back gas may be breathable for much of your decompression, but you need to be aware of when you’re too shallow to use it in an emergency. Even 10 percent oxygen gas is breathable as shallow as 6 metres/20 feet, so this is primarily an issue for the 5 metre/15 foot and shallower stops with less than 10 percent oxygen (blends this low in oxygen not likely needed for new trimix divers). Handle travel gas like any other stage/deco gas with respect to NO TOX switches. You cannot go to back gas between switches when above your back gas minimum depth, so you go to travel gas between deco cylinders. If you expect a high exertion level, or you’re diving at altitude, 16 to 18 percent oxygen may be too low to use at the surface. In that case, you might use a travel gas with 21 percent or more oxygen at the surface.
e. As you recall from the Tec Deep Diver course, if you experience any CNS oxygen toxicity symptoms, your first option is to switch to a low oxygen gas to lower your PO2. Typically this is your back gas but use caution with a hypoxic trimix . Assuming proper gas switches and staying on back gas at your deep stops (more about these next), CNS oxygen toxicity is most likely at shallow depths near the end of the dive with your high oxygen deco gases. A hypoxic trimix back gas may not have enough oxygen if you’re too shallow, such as at 5 metres/15 feet or shallower using TMx10.5/50. You would need to switch to your travel gas or lowest oxygen deco gas. With 14 percent or more oxygen in your back gas, you can use back gas shallow as 3 metres/10 feet.
f. As a beginning trimix diver staying within the depth limits of this course (75metres/245 feet), you should seldom if ever need less than 16 percent oxygen in your back gas.
8. Repetitive dive gas. a. Some environments do make it feasible to make a repetitive dive. b. You should make repetitive dives to the same or a shallower depth, so your oxygen content can usually be the same or higher (if shallower).\\ c. Helium content should be appropriate for the depth.
d. Many decompression experts recommend that you do not follow a trimix dive with a repetitive dive that uses air or enriched air as the bottom gas. Although the concerns are theoretical and not universally agreed upon, this is the conservative approach.
C. Decompression II – Deep Decompression Stops, Air Breaks, Deco Gases and Ascents Learning Objectives: By the end of this section, you should be able to answer these questions: 1.What is a deep stop? 2. What information supports the use of deep stops in trimix diving? 3. What are three ways that you determine deep stops using desktop deco software? 4. What are two ways to determine deep stops using a multigas trimix dive computer that does not calculate deep stops? 5. As a general rule, what gas should you use at your first deep stop and why? 6. What are air breaks? Why and how should you do them? 7. What are the most common decompression gases for trimix diving? 8. Why is there some theoretical benefit to having a small percentage of helium in some of your deco gases? 9. What is the optimum way to ascend when making a trimix decompression dive?
1. As you learned in the Tec Deep Diver course and as mentioned earlier, deep stops have become common practice in tec decompression diving. a. Deep stops are stops for one to five minutes (one or two min most common) between the bottom and the first deco stop predicted by most computers/software running a neo-Haldanean model (Buhlmann most common). b. As noted previously, some desktop deco software can be set to put in deep stops.
2. Deep stops have not been tested formally to any great extent, but field experience and theory support using them, and they’re becoming more common. a. Divers using deep stops report feeling less fatigued after a long deco dive, which is attributed to less decompression stress. b. Deep stops are used widely (depending on location), and there have been no indications that they cause a problem. c. Bubble dynamics models all predict a benefit to making decompression stops deeper. The deep stops procedure is consistent with these models.
3. With your desktop deco software, you can calculate deep stops three ways. a. Automatic deep stops – set the software to generate them; the deep stops are generally indicated differently. b. Use a bubble dynamics model if part of your desktop deco software. These are technically not “deep stops” but a model that begins decompression deeper than modified neo-Haldanean models like the Buhlmann model. Note that bubble dynamics models have less field experience and predict shorter deco schedules than the neo-Haldanean models. Most conservative approach is to use deep stops with a neo-Haldanean model. c. Determine the deep stops and insert them as waypoints if your desktop deco software does not generate them automatically. The first deep stop is the approximate halfway point between the bottom and the first required stop. In trimix diving, it’s common to use multiple deep stops because you’re frequently ascending from a deeper depth and have a longer decompression compared to tec diving with air/enriched air. Each successive deep stop is the next halfway point between the current deep stop and the first required stop, until reaching a stop that’s approximately 3 metres/10 feet deeper than the first required stop. [!] Deep stops should be entered into your deco software (to cover gas requirements and oxygen exposure as well as deco considerations). Deep stops are typically one to five minutes, with one-two minutes most common. Be sure to enter the planned duration into your desktop deco software and decompress accordingly because deep stops may affect shallower stops. To find the deep stop depth (halfway point), subtract the first required stop depth from the bottom or previous stop depth. Divide this by two and subtract it from the bottom or previous stop depth. If the answer isn’t a standard stop depth, it’s typical, but not essential, to round to the next deeper stop in 3 metre/10 foot increments: e.g. round 68 metres to 69, or 155 feet to 160 You can also use the Deep Stop Calculation Table – find the bottom or previous stop depth on the left and the first required stop depth at the top. Round to next greater depth if exact depth not shown. Where the two intersect find the halfway point. If not on a typical 3 metres/10 foot stop increment, you may (but it’s not essential) round to the next deeper 3 metre/10 foot increment stop depth. Example (based on table and rounding to next deeper 3 metre/10 foot increment stop depth): You’re ascending from 60 metres/200 feet. Your first required stop is at 9 metres/30 feet. What are your deep stops? First deep stop is approximately 35 metres/115 feet (midpoint between 60 metres/200 feet and 9 metres/30 feet) . Second deep stop is 23 metres/75 feet (midpoint from 35 metres/115 feet, rounded to 36 metres/120 feet on table, and 9 metres/30 feet.) Third deep stop is 17 metres/55 feet (rounded to 18 metres/60 feet). Fourth deep stop is 14 metres/45 feet (rounded to 15 metres/50 feet).
The first deep stop is the approximate halfway point between the bottom and the first required stop. In trimix diving, it’s common to use multiple deep stops because you’re frequently ascending from a deeper depth and have a longer decompression compared to tec diving with air/enriched air. Each successive deep stop is the next halfway point between the current deep stop and the first required stop, until reaching a stop that’s approximately 3 metres/10 feet deeper than the first required stop. [!] Deep stops should be entered into your deco software (to cover gas requirements and oxygen exposure as well as deco considerations). Deep stops are typically one to five minutes, with one-two minutes most common. Be sure to enter the planned duration into your desktop deco software and decompress accordingly because deep stops may affect shallower stops.
To find the deep stop depth (halfway point), subtract the first required stop depth from the bottom or previous stop depth. Divide this by two and subtract it from the bottom or previous stop depth. If the answer isn’t a standard stop depth, it’s typical, but not essential, to round to the next deeper stop in 3 metre/10 foot increments: e.g. round 68 metres to 69, or 155 feet to 160 You can also use the Deep Stop Calculation Table – find the bottom or previous stop depth on the left and the first required stop depth at the top. Round to next greater depth if exact depth not shown. Where the two intersect find the halfway point. If not on a typical 3 metres/10 foot stop increment, you may (but it’s not essential) round to the next deeper 3 metre/10 foot increment stop depth.
Example (based on table and rounding to next deeper 3 metre/10 foot increment stop depth): You’re ascending from 60 metres/200 feet. Your first required stop is at 9 metres/30 feet. What are your deep stops? First deep stop is approximately 35 metres/115 feet (midpoint between 60 metres/200 feet and 9 metres/30 feet) . Second deep stop is 23 metres/75 feet (midpoint from 35 metres/115 feet, rounded to 36 metres/120 feet on table, and 9 metres/30 feet.) Third deep stop is 17 metres/55 feet (rounded to 18 metres/60 feet). Fourth deep stop is 14 metres/45 feet (rounded to 15 metres/50 feet).
Example (based on table and rounding to next deeper 3 metre/10 foot increment stop depth): You’re ascending from 60 metres/200 feet. Your first required stop is at 9 metres/30 feet. What are your deep stops? First deep stop is approximately 35 metres/115 feet (midpoint between 60 metres/200 feet and 9 metres/30 feet) . Second deep stop is 23 metres/75 feet (midpoint from 35 metres/115 feet, rounded to 36 metres/120 feet on table, and 9 metres/30 feet.) Third deep stop is 17 metres/55 feet (rounded to 18 metres/60 feet). Fourth deep stop is 14 metres/45 feet (rounded to 15 metres/50 feet).
4. If using a multigas trimix computer that doesn’t provide deep stops, despite your preplanning with desktop deco software, you may have to determine deep stops on the fly. This is because your computer follows your actual profile, and you may be ascending from an actual bottom depth slightly deeper (contingency situation) or shallower (multilevel profile) than planned. a. The first option is to calculate the stops in your head as described before by subtracting your present depth (bottom or current deep stop) from the first required stop shown by your dive computer, dividing by two and subtracting that from your current depth. This works, but is obviously not recommended. b. A simpler procedure is to determine the stops with the Deep Stops Calculation Table slate (as previously described) based on your current depth (bottom or deep stop), and the first required stop shown by your computer. c. Either way, you may round to the next deeper 3 metre/10 foot increment stop depth on the table, though this is optional. d. The dive computer will automatically calculate your deep stops into your decompression requirements.
5. Generally, you make your deep stops using back gas (your lowest oxygen gas). a. This helps oxygen exposure by reducing your PO2, which is generally near 1.4 ata on the bottom. b. Your first stop after a gas switch often puts your PO2 up to 1.6 ata, so it’s wise to keep your oxygen exposure lower as long as possible – the big depth change will be creating a decompression benefit without raising oxygen. (Note that you can reduce your oxygen exposure by planning gas switches for the depth at which the gas has a 1.4 ata PO2.)
c. On some dives, your deep stops may be deep enough that you want to stay on trimix to manage narcosis. d. Even if a trimix travel gas or breathable deco gas (PO2 below 1.6 ata) is available, it is typically recommended to stay on lower oxygen back gas until reaching a deep stop shallow enough for the travel gas PO2 to be less than 1.4 ata or lower. This keeps oxygen exposure low immediately following the deep portion of the dive. e. May be possible to plan a deep stop as an extended second level of a multilevel profile for more time on task.
6. Air breaks a. As you learned in the Tec Deep Diver course, an air break is a break from high oxygen deco gases to air (or lowest oxygen gas available) for five minutes each 20 to 25 minutes maximum. reduces risk of CNS toxicity does not count as deco time on an accelerated decompression schedule switch multigas computers to the break gas some desktop deco software will automatically add air breaks air breaks more frequently are fine – some theory suggests breaking more frequently makes deco a bit more effective by reducing oxidative stress on the lungs to maintain optimum gas exchange; many divers opt to break for two to five minutes each 10 to 12, which is fine. 20 to 25 minutes is the maximum without a break, but more frequent may be better b. Total oxygen exposure is typically higher on a trimix dive due to decompression time, so air breaks are even more important. To manage oxygen exposure, it’s best to keep your PO2 on the low side during the deep portion of the dive so you have adequate exposure margin during decompression. c. When diving air/enriched air, the break gas is always the lowest oxygen gas (back gas), but this is not necessarily true with trimix . switching to trimix with a high helium content late in the dive poses some theoretical risk of isobaric counterdiffusion, though in practice this has not been a major issue the lowest oxygen deco gas without helium or with very low helium may therefore be the appropriate choice there are varying opinions on this – be conservative d. Based on bubble dynamics models, there’s a growing practice of having some helium in the break gas. Thus, it may be appropriate to break with a travel or deco gas trimix with five to 20 percent helium (and least oxygen available). More about helium in deco gases shortly. Use of helium in the break gas/deco gas varies somewhat internationally. The practice is most common with the more serious trimix dives with longer decompression requirements. e. Air breaks do not count as deco time when following a dive table unless you entered them, with the appropriate gas, into your deco software. f. Air breaks count as deco time with multigas trimix computers – switch on the fly to the break gas during the air break and the computer adjusts your deco schedule accordingly.
b. Total oxygen exposure is typically higher on a trimix dive due to decompression time, so air breaks are even more important. To manage oxygen exposure, it’s best to keep your PO2 on the low side during the deep portion of the dive so you have adequate exposure margin during decompression. c. When diving air/enriched air, the break gas is always the lowest oxygen gas (back gas), but this is not necessarily true with trimix . switching to trimix with a high helium content late in the dive poses some theoretical risk of isobaric counterdiffusion, though in practice this has not been a major issue the lowest oxygen deco gas without helium or with very low helium may therefore be the appropriate choice there are varying opinions on this – be conservative d. Based on bubble dynamics models, there’s a growing practice of having some helium in the break gas. Thus, it may be appropriate to break with a travel or deco gas trimix with five to 20 percent helium (and least oxygen available). More about helium in deco gases shortly. Use of helium in the break gas/deco gas varies somewhat internationally. The practice is most common with the more serious trimix dives with longer decompression requirements. e. Air breaks do not count as deco time when following a dive table unless you entered them, with the appropriate gas, into your deco software. f. Air breaks count as deco time with multigas trimix computers – switch on the fly to the break gas during the air break and the computer adjusts your deco schedule accordingly.
7. Deco gases a. As you know, practically speaking, trimix diving requires accelerated decompression. b. The practice is to NO TOX switch to higher oxygen, low helium gases as soon as possible to speed elimination of helium and nitrogen. As a reminder, your PO2 should not exceed 1.6 ata while decompressing. In planning your oxygen exposure, you can often greatly reduce your OTUs and CNS clock by keeping the PO2 even lower as much as possible while not significantly increasing your decompression. To manage oxygen exposure, the first deep stop and commonly the first two or three, are made with back gas to keep the oxygen exposure low following the higher oxygen exposure at the bottom depth. Example: Suppose you dive to 90 metres/300 feet using TMx14/50. Your bottom PO2 is the maximum allowable 1.4 ata (not recommended that you push limits; but to make a clear example). Assuming your first deep stop is 70 metres/230 feet, staying on TMx14/50 your PO2 falls to 1.1 ata., providing an oxygen exposure break. If you switched to a travel/deco gas of TMx20/45, you’d spike your PO2 to 1.6 ata., increasing your CNS toxicity risk. c. The most common deco gases are: 100 percent oxygen from 6 metres/20 feet to the surface (recall that it’s typical to make 6 metres/20 feet the last stop, or 5 metres/15 feet if necessary to control oxygen exposure), EANx50 from 21 metres/70 feet and EANx36 from 33 metres/110 feet EANx32 and air sometimes used. In some areas, EANx80 and EANx40 are the common choices in place of oxygen and EANx50, respectively. Other variations of EANx may be used if necessary, of course, due to availability, logistics, etc. Remember that oxygen content is the primary factor in a blend’s deco efficiency. Just because three or four deco gases are available doesn’t mean that’s the best dive plan – the simplicity of two deco cylinders may be better.
d. Based on bubble dynamics theory, there’s a growing trend of decompressing with five to as high as 20 percent helium in the deco gases (except for the final stop on 100 percent oxygen). Because helium diffuses rapidly, the theory is that a small amount of helium reduces helium’s off gassing gradient in the lungs. This slows the speed at which the pressure of the helium dissolved in the blood drops. This theoretically creates a lesser gradient between the tissues and the blood, thereby reducing potential bubble formation during offgassing. Because of helium’s rapid diffusion, it still leaves the body quickly compared to nitrogen. The fewer bubbles, the more efficient and effective the decompression is as well (theoretically). Helium in the deco gases may be important on extreme dives where deco begins deep, to offset narcosis and ease breathing resistance. Isobaric counterdiffusion is not a substantial issue with this little helium in the deco gas (the potential rises with the fraction of helium, but doesn’t appear to be an issue with the low percentages being used for this purpose). The typical deco gases correspond to the traditional EANx blends with respect to oxygen content, such as TMx36/10, TMx50/10, etc. The benefits are largely theoretical, but it is becoming a common practice with widespread anecodotal success, with no risks or negative issues reported at this writing. In programming a multigas trimix computer or writing tables with desktop deco software, you enter the appropriate trimix as the deco gas so the deco algorithm accounts for the helium in the deco schedule. Addition of helium slightly increases the deco requirements with most models. Because helium in deco gases can significantly increase dive cost (helium costs vary widely), in some areas helium in the deco gases isn’t common on dives with relatively short decompression obligations. You can also keep helium in your decompression by using deco for stops shallower than you otherwise might. For instance, you may plan to deco with EANx32 and EANx80; you can switch to the EANx32 at 40 metres/130 feet, but instead, you plan your decompression so you don’t switch until you reach 21 metres/70 feet. This allows you to deco with helium longer, usually with minimal extra hang time required. e. In all cases, it’s important to balance the advantages and disadvantages of different deco gas choices. To avoid high PO2s and high oxygen exposure (especially multiday and repetitive diving), it may be wise to start each deco gas use from slightly shallower (e.g., use EANx50 starting at 18 metres/60 feet instead of at 21 metres/70 feet) On shorter/shallower dives with only one or two deco gases, you may want lower oxygen deco gas or gases so you can begin efficient decompression deeper. (E.g. Instead of oxygen and EANx50, you might want oxygen and EANx36 so you switch off the bottom gas deeper for more efficient deco.) Desktop deco software allows you to easily compare the pros and cons of differing gas combinations.
8. Ascents a. You learned in the Tec Deep Diver course that ascents should be gradual, controlled and within the prescribed ascent rate of the deco model you’re using (generally not faster than 10 mpm/30 fpm). b. Use your gauges to assist with your ascent rate – watch your time and track how fast you pass each 3 metres/10 feet. If you get ahead, stop to allow the time to catch up. c. Many divers stop or slow at each 3 metres/10 feet to keep their rate in control. d. Make deep stops a habitual part of ascents, even if not prescribed by your desktop deco software or dive computer. Evidence increasingly supports their benefit (enter them as waypoints in your deco software).
e. It’s better to not skip stop depths even if a multigas computer or a desktop deco table allows it – take a minute or so at each 3 metres/10 feet (write them in as waypoints on deco software). E.g. After a six minute stop at 18 metres/60 feet on EANx50, the deco software says you can ascend directly to 6 metres/20 feet. Instead, enter and make a one minute stop for each 3 metre/10 foot stop to 6 metres/20 feet. f. Don’t neglect the final part of your ascent. The last part creates the biggest gas volume changes. Go up slowly – take an entire minute or longer to ascend the final 5 metres/15 feet. If you’ve decompressed properly this shouldn’t be essential (so you can hurry in an emergency, for instance), but it should reduce your risk by minimizing bubble formation. Follow this with your surface deco stop. g. With many stops in the profile, team work and communication are essential, especially when ascending with a lift bag (teammates monitor depth so reel diver can concentrate on depth control and the reel).
D. Techniques and Procedures II – Travel gas handling Learning Objectives: By the end of this section, you should be able to answer these questions: 1. Where do you wear travel gas when using it? 2. What is the procedure for beginning a dive on travel gas?
1. When using travel gas, you need to follow procedures a. to avoid using back gas during descent and blacking out from hypoxia. b. to avoid failing to switch to back gas at depth and risking CNS oxygen toxicity. 2. Whether you wear stage deco cylinders all left or right/left, travel gas is normally upper (if wearing two on that side) left cylinder, the lowest oxygen gas. a. may be separate travel gas or lowest oxygen deco gas
3. You and your teammates begin dive by confirming for each other that you’re breathing correct travel gas. When diving with a hypoxic bottom mix, it’s a good idea to clip your primary second stage as soon as you’re geared up so you can’t instinctively stick it in your mouth at the start of the dive. b. If conditions are rough, you may enter water with travel gas already secured so you can breathe from it at the surface. 4. After teammates confirm that everyone’s breathing travel gas, dive begins.
5. At depth, team switches to back gas together at predetermined depth (part of dive plan). a. commonly combined with descent check b. if staging deco cylinders, switch to back gas commonly made when you stage cylinder c. entire team switches together and confirms that all have gone off travel gas and on to back gas 6. If travel gas will be used for decompression, during ascent it is treated as any other deco gas (handling, NO TOX switches, etc.) 7. If isobaric counterdiffusion or ascending above a hypoxic trimix’s usable depth are possible issues, you go to your travel gas instead of your back gas between cylinders when NO TOX switching.
E. Emergency Procedures II – Lost deco gases and computer failure Learning Objectives: By the end of this section, you should be able to answer these questions: 1.What are your options if you lose your decompression gases on a trimix dive? 2. How do you handle a failure of your multigas trimix computer during a dive?
1. Loss of decompression gases a. As you learned in the Tec Deep Diver course, loss of deco gases greatly limits your options and the emphasis must be on prevention. In many air/enriched air tec diving scenarios, you may have sufficient back gas to decompress even without deco gases. This is very unlikely in trimix diving. Never stage your deco cylinders if you have any real doubts about being able to return to them, find them or anything else that would prevent retrieval. Don’t rely exclusively on deco gases supplied by a dive boat; that may be your preferred option, but you should have the gases you need with you. The advantage of boat-supplied gas should be that you don’t have to refill your carried bottles after each dive. b. In the event you lose your deco gases on a trimix dive: Your best option is to have support divers bring down the gases you need. If you have no support divers, or they don’t have the gases, you may be able to deco with your teammate’s reserves as they finish with each cylinder. This assumes they didn’t lose theirs, too. You’ll finish deco later, since you’ll have to wait for them to finish decoing first. If there’s no decompression gas, deco as long as you can as best you can using your back gas. Trimix is very inefficient, so it’s likely you’ll not decompress sufficiently. However, the more you decompress, the less severe DCS is likely to be (but there are no guarantees). c. Again, the emphasis is on prevention. There’s nothing wrong with wearing your deco cylinders for the entire dive.
2. Multigas trimix computer failure a. The simplest option is to complete the dive using your backup multigas trimix computer. b. If you don’t have a multigas trimix computer, decompress following your backup tables with a depth gauge and bottom timer. c. Use the following steps to enter your backup tables after a computer failure during decompression. Find the contingency table for the time and maximum depth of your dive. Find the decompression stop depth at which your computer failed. Complete the time indicated by the table. Finish your decompression following the table’s stop times. Example: You’ve made a dive to 73 metres/240 feet for 18 minutes bottom time, but your actual profile is a gradual ascent over a rising bottom so you started your direct ascent from about 60 metres/200 feet. You decompress with your multigas trimix computer, which requires considerably less time than your 73 metres/240 feet for 18 minute backup table. At the 12 metre/40 foot stop, your computer fails. You find the 73 metres/240 feet for 18 minutes schedule and complete the time indicated by your tables for 12 metres/40 feet. Complete decompression according to the table’s remaining stops.
F. Team Diving II – Safety/support divers Learning Objectives: By the end of this section, you should be able to answer these questions: 1.What are the roles of a support diver on a trimix dive? 2.Why are support divers especially beneficial on a trimix dive? 3.What characteristics does an effective support diver have?
1. In the Tec Deep Diver course, you learned the duties of a support diver. Some of these take on added advantages when trimix diving. a. Checking on divers and assuring they have ample gas, etc. This is especially beneficial on a trimix dive because lost or inadequate deco gas likely means insufficient gas to decompress adequately. The ability to bring down deco gas can literally be life saving. b. Shuttling gear by taking up exhausted cylinders, unneeded gear, etc. Because trimix dives commonly employ more cylinders than air/enriched air tec dives, this can be especially helpful . c. Watching for and locating divers separated from their teams. Notifying teams that missing divers are located.
d. “Baby-sitting” – hovering near decoing divers to be ready to assist. With longer deco times typical, this is especially reassuring. Confirming gas switches, tables and runtimes allows you to anticipate needs for extra gas, etc. e. Sitting standby on the boat or shore, fully geared up or ready to gear up, ready to go in to assist in an emergency. If this support requires going to deep depth, support divers need to be qualified and equipped for the depth. Standing by in the water (baby-sitting) is more common, but surface standby may be advantageous when exposure is an issue (temperature, etc.) and the support diver should be fresh and ready to go. f. Shuttling communications between the divers and surface support.
2. The characteristics needed of a support diver depend upon the dive requirements, but generally include: a. Qualified for the support dive and depths a skilled recreational diver can handle tasks typically needed during the upper decompression levels a Tec Trimix Diver may be needed for deeper deco levels especially complex missions require greater qualification, such as trimix and cave certification for support of long distance deep cave dives b. Good communicator c. Creative problem solver – able to take care of what a diver needs
d. Patient – needs to be able to hang out and watch divers deco without getting bored. 3. Support divers stay out of decompression, except on highly complex dives where it’s planned for – in which case, the decoing support divers may need support divers. a. some groups distinguish between set up divers , whose mission supports another mission (such as staging in cylinders for exploratory divers), from support divers, who aid returning divers during decompression
G. Thinking Like a Trimix Diver II Learning Objectives: By the end of this section, you should be able to answer these questions: 1.Why does becoming a trimix diver put you in a position of distinction in the dive community? 2. What four responsibilities do you have in that role?
1. Whether you want it or not, becoming a Tec Trimix Diver sets you apart in the dive community. a. As a new trimix diver, you will be “at the bottom of the top,” one of the less-than-one-percent of divers qualified to dive in the sport’s deepest ranges. b. You will be among those who accept the sport’s greatest potential risks. c. Your level of expertise with respect to dive skill and knowledge will be expected to be almost professional.
2. This distinction comes with responsibilities. These include: a. Being a role model for other tec divers and recreational divers by respecting and following all the rules and procedures for whatever type of diving you’re doing. By now you know that these exist for safety. Disregarding them or lip service is the mark of an amateur, not a Tec Trimix Diver. b. Showing humility. Being a trimix diver is an accomplishment, but keep it in perspective. It’s diving deep, but it’s still just diving. Diving is a diverse, broad and evolving discipline. No one knows everything about it and no one can; it reflects poorly on those who think and act like they do. Don’t present your opinion or the opinion of someone else as fact. In tec diving as in most things, there’s usually more than one right way to do something. Be open minded so you can continue to learn. Remember and understand where we’ve come from as a community – it helps us see where we’re going.
c. Bringing up less experienced tec divers. It took other divers giving you a chance as a teammate and student to get where you are today. Do the same for those coming up the ranks behind you. Pass your experiences on to the less experienced. Chances are, you’ve avoided mistakes because others who didn’t told you about them. d. Staying current. Much of what you know today will be obsolete tomorrow. Keep informed about the latest advances in dive technology, safety, medicine, decompression and procedures. Dive frequently to keep your skills proficient. If you’ve not been diving in awhile, get in the water and refresh your skills before a “big” dive. Develop a reference library and information sources you can draw on.
VIII. Practical Application Two Practical Application Two has four primary purposes: 1) to formalize student practice in analyzing trimix, 2) to further team building among the student divers, 3) to continue developing student fluency with desktop deco software, and 4) to build upon mission planning skills learned in the Tec Deep Diver course. To successfully complete this Practical Application, the student will be able to: 1. Demonstrate the use of oxygen and (if available) helium analyzers to determine the contents of trimix blends. 2. Working as a team and using desktop deco software, plan two trimix dives, both with deep stops, one requiring a travel gas and three deco gases and one requiring three or four deco gases but no travel gas. The plans must include gas consumption, oxygen exposure, decompression schedules, turn pressures and contingency decompression schedules for each team member. A. Trimix Analysis 1. Provide the class with two or more cylinders of trimix. a. Working in teams, students each take turns using an oxygen analyzer to analyze the contents of all cylinders. Each cylinder should be marked with the target blend. b. Students should be able to tell you the oxygen content of each cylinder, tell you whether the content is close enough to the target blend to use as that blend, and what to do if not. c. If available, have student divers also analyze the blends using a helium analyzer. A point of discussion: the oxygen may be dead on, but the helium may be off anyway. Does this lead to a serious dive planning concern? Use desktop deco to compare deco profiles and ENDs. (Conclusion -- if the oxygen content is accurate, the variation in helium should not be sufficient to make a meaningful difference in dive planning.) Give students a trimix cylinder without the target blend indicated. Ask them to determine the trimix with only the oxygen analyzer. They can’t -- requires a helium analyzer. Ask them if they can even determine if it contains helium at all. (Yes, but not with an oxygen analyzer -- take a few breaths of it and then talk; the helium speech distortion confirms helium.) Remind them, “When in doubt, throw it out!” If you can’t tell the helium and nitrogen content, you can’t plan your decompression or narcotic depth. B. Dive Planning 1. Divide student divers into teams . Ideally, these should be the teams that will dive together in Training Dive Two. 2. Assign students two dives to plan. Provide them with the simulated depth and time to plan based on the following. a. The first dive should require a travel gas and deco gases. It is recommended that you assign a travel gas and two deco gases. This may be the plan for the simulated trimix dive in Training Dive Two. You may allow student divers to determine what the gases would be, and the travel gas may be one of the deco gases (with the volume used on descent considered in planning. See Training Dive Two for more details. The actual dive will be an air/enriched air decompression dive no deeper than 50 metres/165 feet, with actual decompression gases but the bottom gas air/EANx simulated as trimix; the class will decompress following the trimix schedule.) Depending upon the planned actual dive depth and bottom time, you may assign a deeper simulated depth and bottom time or use the actual depth and bottom time. (See Training Dive Two for more details.) b. The second dive planned should require a travel gas and three deco gases . It is recommended that you assign three deco gases. The travel gas may be one of the deco gases with the volume planned accordingly. c. Both dive plans must include deep stops . If the software doesn’t generate them, then students enter them manually as waypoints. gas supply requirements for each diver based on personal SAC rates decompression schedule with runtime turn pressures contingency (bail out) decompression schedules oxygen exposure (OTUs and “CNS clock”) 3. After teams complete the desktop deco software planning and successfully fulfill the required criteria, assign them a mission to plan in Training Dive Two within the planned limits. a. The mission is within your discretion, and may be based on the environment and other local variables. b. Choose something complex enough that it requires forethought and teamwork, but simple enough to accomplish within the context of learning new trimix skills and procedures. This could include: lay down a four equal-sized box grid using line measure something (reef section, wreck feature, etc.) attach a lifting device to something with three different knots required (but the actual lift is not made) c. If the mission duration would be longer than available bottom time, check that team revises plan to accommodate actual dive time. d. Remind student divers that completing the mission is not the priority; completing the dive safely as planned is the priority. 4. Training Dive Two is optional for Tec Deep Diver certified students who were certified in the previous six months or who have made at least one technical decompression dive requiring a decompression cylinder to 40 metres/130 feet or deeper in the previous six months, but they must complete the planning practical application whether they will actually make the dive or not.
X. Training Dive Two To successfully complete this training dive, the student will be able to: 1. Working within the student’s assigned team, rig gear, including stage/deco cylinders, and plan the dive following the A Good Diver’s Main Objective Is To Live procedure, and perform predive checks following the Being Wary Reduces All Failures procedures. 2. Working in a team, plan and execute a simulated trimix accelerated decompression dive using a table and/or multigas computer using air/enriched air as the bottom gas, and air, enriched air and/or oxygen for decompression gases. The dive will be an actual decompression dive, but will follow a trimix decompression schedule. 3. Deploy a lift bag from the bottom as a team and ascend along the lift bag line following the decompression schedule. 4. Working as part of a team, respond appropriately to spontaneous simulated emergencies presented by the instructor throughout the dive. 5. Monitor and record depth, time and gas supply information during decompression to determine the deco SAC rate. 6. Working as a team, attempt to complete a dive mission assigned by the instructor. 7. Demonstrate time, depth and gas supply awareness by recording depth, SPG pressure and time at intervals set before the dive by the instructor. Note that Training Dive Two is optional for students certified as DSAT Tec Deep Divers who were certified in the previous six months or who have logged at least one technical decompression dive requiring a decompression cylinder to 40 metres/130 feet or deeper in the previous six months. It is required for student divers entering the Tec Trimix Diver course with a qualifying prerequisite certification from another training organization. You may require Tec Deep Diver certified students to complete this dive in any case at your discretion, such as to improve mastery, allow additional learning time, or simply for added experience with slightly more depth. Training Dive Two is recommended for any students who are not showing the polished technique expected of trimix level divers, especially maintaining a tightly controlled decompression stop depth. Use this dive to emphasize streamlining, buoyancy control and body position. Assign additional skills that require deploying and properly restowing gear, with an emphasis on a clean rig before and after the drill. A. Training Dive Two Standards 1. Training Dive Two is conducted in open water as a decompression dive. The minimum depth is 27 metres/90 feet and the maximum depth is 50 metres/165 feet. 2. Ratios – 2 students to 1 instructor, with 1 more student permitted with a certified assistant to a maximum of 3. (See Section Two for specific requirements necessary to qualify as a certified assistant in this course.) These are maximums – reduce ratios as necessary to accommodate student characteristics and environmental/logistical considerations. 3. Students and instructor must be equipped as described in the Tec Deep Diver course, with accommodation for environmental needs. This includes but is not limited to: a. Manifolded double cylinders with dual, independent regulator posts. b. Technical diving BCD, redundant buoyancy device (double bladder BCD, or dry suit if appropriate for weight of gear worn) and harness as described in the equipment requirement section, and following the rigging philosophies described in the Tec Deep Diver course. c. Two stage/decompression cylinders configured as described in the equipment requirement section, and following the rigging philosophies described in the Tec Deep Diver course. 4. Gas requirements. Student divers and staff may use air or enriched air, any suitable blend. These may be blends such as air, EANx 32, 36, 50 and oxygen . It is recommended that you have students simulate starting the dive using one of the deco gases as a travel gas, figuring the gas consumption into their dive plans. Back gas should be a simulated trimix that cannot be breathed shallower than 3 metres/10 feet, but should actually be air or enriched air and marked accordingly. The decompression gases should be the actual gases called for in the decompression schedule. 5. Decompression planning. Trimix schedules call for more decompression than air/enriched air dives for the same depth and time, so decompressing according to a trimix schedule is acceptable. The dive must be planned according to a simulated trimix schedule that requires at least four deco stops and two deco gases. The actual required decompression schedule may require fewer stops and/or deco gases. Student divers should have the actual schedule available for ending the dive more quickly in a contingency situation. The simulated decompression schedule should include deep stops, either generated by desktop deco software automatically or determined manually and inserted as waypoints. Deep stops that would not be possible because the simulated depth is deeper than the actual depth may be omitted. B. Predive Planning, Briefing and Preparation – suggested sequence 1. Predive briefing a. Group in teams, students set up their rigs, analyze gases, but do not yet don exposure suits. Encourage teamwork. Inspect each rig for correct setup, ample gas supply, etc. Pay particular attention to proper cylinder markings written and placed so team can read them while worn. b. Dive site overview Depth, temperature, entry/exit points, noteworthy features. Facilities – parking, lockers, boat dry and wet areas, where to find emergency equipment, etc. c. Dive overview Depth/time limits It’s recommended that you have student divers write down the dive overview and notes on a slate for reference during the dive, and to have them do this for each dive. Skill overview – describe each skill, the performance requirement and how you’ll conduct it, including signals, etc. appropriate entry, don stage/deco cylinders (at surface or before entering water) students use travel or appropriate deco gas at surface bubble check descent NO TOX switch to back gas at planned depth ; descent check dive mission lift bag deployment follow trimix deco schedule with NO TOX gas switches Spontaneous emergencies (surprise drills). The exact emergency and timing is left to the instructor’s discretion, but there should be at least two unannounced emergencies per team during this dive. Suggested emergencies include lift bag failure, exceeded depth or bottom time, freeflowing regulator, etc. Since this is an actual decompression dive, do not assign emergencies that could compromise buoyancy control or an effective decompression schedule, e.g., failed BCD, unresponsive diver, decompressing with no reference, etc. Midwater gas shutdowns (freeflowing regulator) should follow all required decompression. Inform students that the simulated emergency ends when you signal it, and that they may discontinue the emergency drill if it affects their ability to properly decompress or control the dive. Tells students to signal when they’ve completed required decompression, but not ascend to the surface until you signal them to (except in a real emergency, of course) so that you can assign further emergency exercises midwater after completing decompression. Time, depth and gas supply awareness – assigned by instructor. May be depths, times, pressures, turn pressures, etc. Goal is to get divers to constantly monitor time, depth and gases. Instructor will not remind divers to do this. Teammates are encouraged to assist each other with this. Students must do this no matter what else is going on, short of a real emergency. Review hand signals, emergency protocols, descent and ascent procedures, final details. Assign each team to complete individual dive plans. Remind divers they are carrying gases that they cannot safely breathe at the bottom depth. d. Teams plan dives and gear up Teams go through A Good Diver’s Main Objective Is To Live and plan dive. Have them plan based on a decompression schedule they generate from a depth, trimix and at least two deco gases. The trimix will be simulated, but the deco gases should be the actual gases they will use. They should generate an entire plan with deco schedule, contingency schedule, gas volume requirements, turn pressures and oxygen exposure for each diver. You may have them do this by hand or use desktop deco software, or you may have them use the schedule and plans from Practical Application Two as appropriate. Allow ample time for proper planning, which may take an hour or more. The final plans should include gas volume requirements for all divers and may be presented on the TecRec Dive Planning Slate or on a computer printout. It’s also recommended that you have students laminate copies of their tables to carry on the dive. (Note: Remind divers that losing their tables is not only potentially unsafe, but environmentally unfriendly.) Teams gear up and finish their checks with Being Wary Reduces All Failures. Students use TecRec Equipment Checklist to confirm each other’s equipment setup. Touch drill while geared up and seated, students reach back and touch (grasp) regulator and isolator valves as if to close/open them. Teammates adjust equipment/assist each other as necessary. C. Training Dive Two – suggested sequence 1. Entry – appropriate for environment, deep water entry recommended. a. Divers check their weight if necessary due to environment or gear change. b. Divers don stage/deco cylinders at surface with minimal assistance, or prior to entering water (as appropriate for environment and logistics). c. Divers bubble check teammates. It’s a good idea to have spare o-rings at hand. d. Divers use appropriate deco gas as simulated travel gas at surface. Teammates confirm correct gas use. 2. Descent a. Descend and stop at planned depth for switch to back gas. b. Student divers NO TOX switch to back gas. c. Descent check . d. Stage cylinders at deco levels (if appropriate to environment). e. Continue descent to insensitive bottom. f. Position class for skills 3. Dive skills – for each, instructor demos (if necessary), then has students perform a. Dive mission The assigned mission should be simple and reasonably within the students’ ability to carry out. Completing the mission is not required because the primary dive goal is the dive. The purpose is to give student divers a chance to practice planning and carrying out missions while managing tec diving equipment and procedures. b. Lift bag deployment and ascent and decompression with NO TOX gas switches Each team deploys lift bag. It is recommended that they ascend along the lift bag line, following the planned trimix decompression schedule as a neutrally buoyant hang with a fixed line or reference available. Watch for proper NO TOX gas switches. If necessary for logistics in a current, teams may also maintain contact with an anchor line or other stationary object to prevent drifting. Student divers should not vary more than 1 metre/3 feet from the stop depth during gas switches nor more than .6 metres/2 feet after completing gas switch. Encourage teammates to help each other maintain depth during switches and decompression. c. Spontaneous emergencies At least two emergencies per team, either on the bottom or during decompression. Avoid assigning emergencies that could affect buoyancy control and an accurate decompression schedule. Assign midwater emergencies that could effect stop depth after completing all required decompression. Remind students that the drill ends on your signal, or if the emergency procedure affects their ability to decompress effectively or control the dive in any way. It is acceptable if emergency drills compromise the mission. (The lesson is that the mission is always the lowest priority in a dive – coming back safely is the priority.) Midwater emergencies (after completing all required decompression) should focus on realistic situations and emphasize stop depth control while handling the problem. These can include having divers face away from the line and maintain stop depth (loss of line) or writing the procedures they would follow for an omitted deco problem while maintaining stop depth without physical contact. 4. You may have students stage cylinders as appropriate for local dive environment or for additional cylinder handling practice. 5. Exit water (as appropriate for environment). D. Post Dive 1. Performance review. After giving divers some time to rest, get a drink, etc., but while memories remain fresh, have teams identify what happened, what they learned, what worked and what didn’t, etc. Comment and fill in missing information as necessary, but have students critique themselves constructively while you guide the process. 2. Have divers show you their slates with the recorded times/depths/SPG readings assigned prior to the dive. 3. Have divers calculate and show you their swimming and decompression SAC rates based on the information gathered during the dive. 4. Divers disassemble and stow their gear as appropriate. 5. Students log dive for your signature.
DSAT Tec Trimix Diver Course Knowledge Development Presentation Two