The Depth of the Matter

A primer on deep diving

Few things hold more mystique or interest for human beings than the depths of the ocean. The deeper we go, the more intriguing the site and the more fear-inducing the prospect of going there. Deep diving can be hazardous, but the risks can be mitigated through training and the proper use of procedures, planning and equipment. The first question in the planning of a deep dive is how deep is "deep"? What constitutes a deep dive varies by region and circumstance, but dives between 66 feet and 130 feet are generally considered deep recreational dives. To go beyond those limits requires additional specialized training, so for the purpose of this article, we'll stick to recreational dive considerations.
Planning and Preparation
The first step in a safe, deep dive is proper planning. Deeper depths have a physiological effect; inert gases accumulate, and breathing gas supplies deplete much more rapidly. Both of these factors equate to shorter bottom time and an elevated risk of decompression illness (DCI). Hedge your bets against DCI by being fit, well rested and well hydrated; this applies to every dive, but especially a deep dive. Limited bottom time is dealt with through careful planning. Shallower dives may not require a definitive plan to accomplish objectives, but deep dives, with bottom times often shorter than 10 minutes, do. A deep dive plan starts well in advance of the actual dive, and it begins with equipment selection.

Deep dives are certainly done in inland waters, but the vast majority are offshore, and the deeper the water, the more variable the conditions are prone to be. Well beyond the sight of land, strong currents and rougher sea conditions are likely to be encountered, so safety dictates additional gear requirements. Deeper dives put more pressure on exposure protection, especially wetsuits. The added pressure at depth compresses the suits, decreasing their ability to keep a diver warm. If diving deep, you may want to consider a thicker suit, especially if you get cold easily. Surface signaling devices such as a safety sausage, a surface marker bag, a whistle, a sonic-alert device or the new Nautilus Lifeline are a must. Also recommended is a good-quality dive light with several hours of available burn time. The light pulls double duty for deep divers: It allows illumination of colors underwater (everything appears blue and gray in deep water), and it serves as a signaling device after dark.

Assembling the appropriate equipment is only part of the challenge; next it must be properly configured. At 100 feet, the pressure is four times greater than on the surface, and just about everything requires more effort. It holds true with any dive, but gear must be streamlined on a safe and efficient deep dive. Stow safety sausages in BCD pockets or clip them to a D-ring; do the same with dive lights, though make sure nothing clipped dangles down. Either way, make sure equipment is easily accessible at all times. Once gear is configured, go diving — in a pool. Go to the pool or another controlled site, and practice swimming with your gear configured exactly as it will be on the deep dive. The added pieces of equipment may have an impact on trim and buoyancy control; more important, a controlled environment provides a chance to practice accessing and deploying items like a safety sausage or marker bag before heading for big water.

Another key equipment decision for deeper dives is cylinder size. Once comfortable with your gear configuration, verify or calculate your gas consumption rate. When your consumption rate is determined, select the cylinder size that provides adequate air to complete dive objectives. If the size required is different from your accustomed cylinder, head back to the pool for another practice session.

Once comfortable in the water, it's time to plan the dive. Gas management is a critical component of any plan; the goal is to accomplish dive objectives while leaving an emergency reserve. On a deep dive, safety dictates that divers use no more than two-thirds of available gas supply on the descent and bottom, always leaving at least one-third for ascent, a safety stop and emergencies. Many deep divers choose to carry added insurance in the form of additional tanks or pony bottles, but backup gas supplies are for emergencies only and should never be factored into planning as available gas.
Doing the Dive

A scuba diver explores one of the deeper wrecks off Key Largo, the
USCGC Duane. Resting perfectly upright in 125 feet of water, she
offers an opportunity to practice both deep and wreck diving skills.
Most deep divers will encounter currents, especially during ascents and descents, but dive boats utilize a series of current lines to help divers reach the descent line. Enter the water with all gear checked and ready to dive, descend as soon as possible to conserve gas, and go down the line hand over hand; maintain contact with the line to ensure a current doesn't take you away from the target site. Descend at a rate no faster than about 66 feet per minute. The descent rate should certainly be included in the planning; on a 100-foot dive, it will take nearly two minutes to reach the bottom. Upon reaching the bottom, check your gas supply, your time and your buddy, and take a moment to get neutral before moving toward the dive objective.

As a general rule, use no more than one-half of the time available for your no-decompression limit (NDL) on the trip out from the ascent line; this leaves at least half of your NDL for the return to the ascent line at the end of the dive. Some divers utilize the more conservative rule of thirds: Use one-third of available NDL time swimming away from the line, one-third in the return to the line, leaving the last third in reserve to explore the area immediately around the ascent point before the end of the dive.

Time is only one thing that needs to be monitored closely; gas must be even more closely tracked. Make a habit of glancing at the gauge every one to two minutes at the beginning of the dive; check it more frequently as the gas supply begins to dwindle. If new to deep diving, keep your submersible pressure gauge (SPG) in your hand throughout the dive. The rule of thirds applies to gas as well: Use no more than one-third of available gas supply swimming away from the line, one-third on the return and the last third on the ascent and safety stop. When calculating the thirds, remember to deduct whatever gas is to be left in the tank from the available supply.

Ascent rate is critically important. Many divers have been trained to ascend no faster than 60 feet per minute, but current protocols recommend a rate of 30 feet per minute. Ascending at the slower rate can feel like a crawl and actually requires practice. It's important to stay in contact with the ascent line, as the slower pace makes it easy to be swept away by currents. Once on the surface, try to minimize physical activity, use the available lines and currents to drift to the platform, and relax on the tag line while waiting to board. Your body will still be offgassing, and excessive physical exertion can increase the risk of decompression sickness.

If involved with a gas-sharing emergency on a deep dive, it is generally best to ascend immediately rather than to try and reach the ascent line; this helps ensure enough gas for a safe ascent. In most cases, it is better to drift than to risk drowning or a rapid ascent. If a free ascent is necessary, control the ascent rate by monitoring gauges and controlling buoyancy. Do not attempt to fight the current. Weigh the risk factors of completing a safety stop and continuing to drift against the risk factors of making an immediate ascent. Remember, you can always inflate your surface marker from the safety stop to assist the boat in spotting you. Regardless, as soon as the surface is reached, utilize any and all surface signal devices to alert the boat, and remain as calm as possible.

Given the added risk factors, why dive deep? Deep diving is not for everyone, but there are a variety of reasons for deeper dives, all of which boil down to a bigger playground. For many divers, the added risks are worth the benefits of diving deeper, especially since the risks can be managed. No matter the dive objective, the keys to safer deep diving are preparation and good planning.
Calculating Deep Stops
Some divers incorporate a deep stop into their dive plan. If a deep stop is a part of your decompression algorithm, it should be completed at a depth one-half the distance between the bottom and the safety stop. For example, if the maximum dive depth is 95 feet and the safety stop is to be completed at 15 feet, divide the difference in half and add the safety stop depth for the deep-stop point (in this case, 55 feet). Most profiles recommend a one- to two-minute stop time, but this needs to be balanced with remaining gas supply. If gas supply forces a choice between a deep or safety stop, the shallower stop has priority.

© Alert Diver — Winter 2011