Planning, preparing for and conducting a deep dive requires attention to detail and the desire to meet the demands that deep diving presents.
Dive planning
Like all dive plans, the first step is to determine if the site is favorable for diving, both on the surface and under water. The weather should be good and the seas should be calm. Locations that provide some of the best deep diving are typically subject to more severe variable weather conditions.
Deep dives will usually be conducted from a boat, as many popular locations are farther from shore; however, for shore dives, the surface swim to the descent point must be taken into account. Shore dives present additional concerns, such as the increased time for rescue personnel to reach the descent area, and an increased risk of DCS due to fatigue following the dive. Regardless of the dive type, shore or boat, there should always be a qualified individual, whose sole purpose is to provide surface support to the dive team. As previously discussed, the law in many countries requires that a qualified surface support individual is present during the course of boat diving operations.
Dives deeper than 24 meters / 80 feet will have shorter bottom times, and longer ascents. As such, it is important to determine a proper dive profile. While the majority of divers use their dive computer to establish their dive profile, it is always recommend to establish a square profile based on the no-decompression tables, and cross reference it with that of your computer. The profiles will not be exact, but when cross referencing a deep dive they should be close. It is possible that your computer profile may be more conservative than your table. Because tables are based on a square profile, you will not be able to take advantage of the computer’s ability to recalculate bottom times based on multilevel dive depths. In fact, if you were to cross check your dive’s max depth and total bottom time against a table, you likely would discover that the table shows that you entered a decompression profile. This is a good demonstration of one of the many benefits of using a dive computer. With that said, it is always a good idea to dive conservatively when using a computer, and avoid pushing their limits of their calculations.
Take the necessary time to familiarize yourself with your dive buddy’s equipment, especially his computer. Determine which computer amongst you is the more conservative and agree to follow its time parameters for the dive. To be clear, you are both diving according to the recommendations of your own individual computers. You are simply agreeing to follow the more conservative time. Remember, one diver, one computer. Ultimately, factors such as gas availability and nitrogen saturation must be accounted for in the dive plan, and will often determine the available bottom time. Both divers should calculate and compare their gas consumption, and monitor it regularly throughout the dive.
The pre-dive check
Conduct the pre-dive check with your buddy carefully and meticulously. It is more difficult to deal with a problem at deeper depths than in shallower ones; consequently, you should spend additional time ensuring that everything is in order. Pay particular attention to the first and second stage regulators and the cylinder valve. Make sure the valve is completely open, and that the SPG needle does not bounce erratically after breathing from the second-stage. Verify that the cylinder is full. Sometimes leaks go unchecked, and a cylinder that was full when you set up your equipment may no longer be full. Double-check your computer. Make sure it is on and programmed for the dive.
Mentally prepare yourself for the dive during the last few minutes before entering the water. Close your eyes and take a few deep, cleansing breaths. Visualize your team descending, swimming along the bottom and ascending to the safety stop, all without error. If you have made this dive before, you will have a basis for your visualization; adhere to it until your dive team has successfully exited. Visualization is a very useful technique that has been proven to lead to improved performance and ultimately a more successful activity.
Finally, as with all dives, do not enter the water until you are ready. Your confidence, comfort and well being while under water are your responsibility. Do not let anybody, including your buddy, the dive master, other divers, or the boat captain, rush you to get into the water. Conversely, you should never rush any other person. If and when to dive is your decision, no one can make it for you, as you should not make it for anyone else.
The descent
It is recommended to use a descent/ascent line for deep dive. As previously discussed, the anchor line may be used, but it is not the ideal choice. Anchor lines are typically sloped, which creates more work for the divers, and can hasten fatigue during a deep dive, especially if there is a current.
Deep dive descent rates should be controlled and slower than normal to help prevent the onset of nitrogen narcosis. As such, it is recommended to descend no faster than 18 meters (60 feet) per minute.
The descent line is there for you to use any time you need it. It will help you control your descent and provide a point of orientation in an otherwise deep blue sea where the bottom is not always visible. While this can be disorienting, especially for divers new to this diving, there should be no rush to reach the bottom, or planned depth.
The descent line is there for you to stop as needed, to equalize, make equipment adjustments, or simply relax for a moment. Use it when you need to. The descent line also helps to keep you and your buddy together while descending.
When the water visibility is not ideal, slow your descent as you approach the bottom to avoid stirring up the sediment and decreasing the visibility.
A free descent, without a line, is only recommended for those with a significant amount of deep diving experience and a high-level working relationship with their buddy.
During the dive
Once you have reached the pre established depth, it may take a moment to orient yourself. Using a compass during a deep dive is recommended, because you cannot return to the surface to get your bearings like you can on shallow dives. However, natural navigation is usually sufficient as the limited bottom times associated with deeper depths prevent the dive team from venturing too far away from the descent line. If the visibility is poor, you may want to consider using a buddy line, as described in the limited visibility diving section of the night diving chapter of this manual.
Check your instruments more often than you would on a shallow dive. It is easy to get distracted on a deep dive, partly because of the excitement of the different environment, and partly because of the increased partial pressure of nitrogen. Also, the deeper you dive, the more gas you will consume than you do at shallower depths. Strong self-discipline is very important. You need to remember to check your instruments often, and remind your dive buddy to do the same, especially if you are diving on a wall where remaining at or shallower than your planned depth.
As we have discussed throughout this chapter, gas inhaled at deeper depths has a higher density and there is additional mechanical breathing resistance, which combined increases the chances of feeling short of breath. Should this occur, slow your kicking and if necessary, stop and take a break, calm down and focus on your breathing rate. As you may already be aware, many professional and technical divers dive deeper for longer periods of time. For these divers to be successful they must breath custom gas mixtures that have lower overall gas density. Many of these gas mixtures contain lower percentages of oxygen, which helps to decrease the potential problems associated with breathing higher partial pressures of oxygen. The mixtures have a lower percentage of nitrogen, to help decrease the affects of nitrogen narcosis, and they have the addition of helium, which is a very low density gas, it does not have the same narcotic effects as nitrogen and is, for the most part, inert. This type of diving is beyond that of recreational diving and this course.
Managing your gas while deep diving is as important as monitoring your depth and time. In fact, it may be your gas supply that dictates the end of the dive before your allowed bottom time has been exhausted. When deep diving it is absolutely necessary for you to remember that when it is time to return, you must have a sufficient supply of gas to reach the ascent point, ascend and conduct your safety stop. This is why following the rule of thirds, as discussed in the underwater navigation portion of this course, is more important than ever. The dive begins and continues until a team member has used a third of his gas supply. At which time, the team turns around and returns to the ascent point. The remaining third is used for the ascent and any other needs that may arise. You should arrive at the surface with no less than 50 bar / 750 psi in your cylinder. As always, the dive plan should be based on the gas consumption of the member of the team that consumes his gas the fastest. To help offset this, the diver with the higher gas consumption rate should use a bigger cylinder. Remember, during a deep dive, when the time to end the dive has come, the dive must be ended. There is no reserve to rely on.
Surfacing and Exiting
When the dive plan is followed, there should be an ample amount of gas to perform the ascent and safety stop and arrive on the surface with gas to spare.
Do not ascend faster than the recommended ascent rate of 9 meters / 30 feet) per minute (3 meters or 10 feet every 20 seconds). If you need to, hold onto the descent/ascent line to maintain a controlled ascent while monitoring your instruments. As you get closer to the surface, you may notice a greater increase in your positive buoyancy, especially if you are using an aluminum cylinder and, or a thick wet suit. Aluminum cylinders that are low on gas will be positively buoyant and macro-cellular wet suit cells will expand, also increasing buoyancy. Be more vigilant and maintain your buoyancy control as you near your safety stop depth.
Stop for three to five minutes upon reaching your predetermined safety stop depth, usually between 5 to 6 meters / 15 to 20 feet for a deep dive. It is recommended to stay longer at your safety stop if you have enough gas, but make sure to stay for a minimum of three to five minutes.
If you or your buddy are nearing or are lower than the recommended gas pressure of 50atm / 750psi, use the spare cylinder, or, spare second stage regulator (that is attached to long hoses leading to the boat) that was left at the safety stop depth. As a reminder, If a cylinder has been placed at the stop location, the valve should be closed to prevent accidental discharge and the second-stage regulator hoses should be pressurized to help prevent water from entering, and potentially damaging the internal parts.
Some dive boats are equipped with an underwater safety bar that allows the divers to perform the safety stop more comfortably than by hanging onto a line. In this case, follow the instructions provided by the dive master or boat captain regarding its use.
After the Dive
The dive is not over once you have reached the surface and stepped onto the boat deck or beach. Your last decompression stop is on the surface, after exiting the water.
The fact is that the dive is not truly over until your tissues have reached equilibrium with the surface conditions and all of the residual nitrogen has left your body. This process takes at least 12 hours following your last dive. Consequently, it is important to keep your activity levels moderate, avoid physical stress and keep your body well hydrated during this period of time.
Finally, you should avoid flying for at least 24 hours after a deep dive.