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Demystifying the Deep

Deep diving sometimes gets a bad rap because diving’s perceived risks become more pronounced the deeper you go. It is true that the nitrogen in your air builds up in your blood at an increased rate as you go deeper, which shortens your bottom time. Your gas consumption increases exponentially, making accurate time and air management essential skills. And mental focus becomes imperative because that same nitrogen can affect your judgment and cause feelings of euphoria and somewhat lowered inhibitions–like the ones you feel after downing a couple of rum and cokes at the post-dive beach bar.

But statistically speaking, the most hazardous place to dive is in the first 33 feet, and learning how to deep dive will open up a whole new range of diving opportunities: Walls that drop into the open ocean play host to marine life not seen in the shallows. Blue water is a magnet for large pelagics. Then there are the wrecks, most of which lie at the deep end of recreational diving depths, usually defined as between 60 and 130 feet, because the ones in shallow water are either destroyed so they don’t pose hazards to large ships, or they don’t survive the ravages of Mother Nature’s storms. And with the right equipment, knowledge and a little extra pre-dive planning, deep diving can be both safe and fun.

Air Consumption Calculator

The first thing most divers notice about diving deep is that their air supply is used up much more quickly. At 99 feet, or 4 atmospheres, a diver uses air four times faster than at the surface and twice as fast as at 33 feet. So, to accurately plan your dive times, calculate your air consumption and apply it to your available air supply on each dive. Here’s what you’ll need:

  • Precise pressure gauge–digital is best.
  • Precise depth gauge.
  • Small slate and pencil.
  • Reasonably calm diving area where you can maintain a consistent depth.
  • Bottom timer or waterproof watch with a stopwatch function.

Step 1–Wear all of your equipment exactly as you will when you deep dive, including the same exposure suit and tank configuration.

Step 2–Drop to a set depth, say 33 feet to keep the math simple, and get neutrally buoyant and comfortable in the water.

Step 3–Check your SPG, write down the air pressure at the start of your swim, check your timer or push the button on your stopwatch, and swim at a consistent, relaxed pace for 5 minutes, maintaining a constant depth.

Step 4–At the end of your swim, check your SPG and write down your ending air pressure. Tuck these away for use at the surface and enjoy the rest of your dive.

Step 5–Calculate your Surface Consumption Rate (SCR) with these formulas:

  • Depth/33 + 1 (the atmosphere around the earth) = Total atmospheres of pressure (atm). In the example, 33 feet/33 + 1 = 2 atm.
  • Divide the psi used during your timed swim by the length of the swim to find average psi usage per minute. If you used 200 psi in 5 minutes, 200/5 = 40 psi per minute.
  • Finally, divide this number by the atm to get your surface consumption rate: 40 psi per minute / 2 atm = 20 SCR.

Step 6–Find your consumption rate at any given depth by converting the depth to pressure to see how much air you’ll use per minute. A depth of 99 feet is 4 atmospheres, 99/33 + 1 = 4. Using the example, multiply 20 SCR by 4 to find the consumption rate at depth, 20 x 4 = 80 psi/minute at 99 feet.

Step 7— As long as you always use the same volume and pressure-rated tanks that you used when calculating your SCR–aluminum 80s at 3,000 psi, for example–you can find out how long your air will last by dividing the total psi you’ll use on your dive by the consumption rate at depth. Since you should always arrive back to your starting point with one third of your tank left, you should plan to use 2,000 psi on the dive. 2,000 psi/80 psi per minute = 25 minutes at 90 feet.

Using nitrox on deep dives

Gas consumption is, of course, only part of the picture. You also have to plan for your no-decompression limits (NDL), which get significantly shorter as you go deeper, especially when using normal air. For example, a dive to 60 feet can last between 50 and 60 minutes, depending on the table used, but a dive to 100 feet will be limited to between 20 and 25 minutes. You can maximize your bottom time by using nitrox on deep dives. There’s a common misconception that nitrox helps divers go deeper, but it doesn’t. In fact, because of its increased oxygen content, nitrox can be more dangerous on deep dives if it’s not used properly. Check the table below for the maximum depths of some common nitrox mixes and the maximum bottom times they offer compared with normal air (from the U.S. Navy Doppler Air Table and the NOAA equivalent air depth table).

Narcosis Management

Nitrogen is the culprit behind problems like decompression sickness, and on deep dives another effect of this gas is nitrogen narcosis (unrelated to decompression sickness). This condition caused by high partial pressures of nitrogen is a reality that divers need to take into consideration. Back in the old days, hard-drinking divers called the relation between diving depth and the effects of narcosis Martini’s Law. The law said that for every 50 feet of depth, narcosis was roughly equivalent to drinking a martini, or about 2 ounces of 80 proof alcohol.

This rule is simplistic. Like alcohol, nitrogen affects all of us differently and at different levels. However, in testing, nearly all divers show some degree of slowed reflexes and impaired problem-solving abilities by about 100 feet. At 130, the effects can be quite pronounced, with some divers experiencing a decrease in their reaction times by 30 percent or more. The effects of feeling “narced” can be unnatural feelings of euphoria or anxiety that can lead to panic and potential death or injury. To manage narcosis effectively, divers must do five things:

  • Learn basic dive skills, like buoyancy control and sharing air, until they are almost reflex actions. If these skills are second nature to you, you will have fewer problems when using them in an emergency
  • Plan the dive and dive the plan. If you dive a well-thought-out dive plan you are much less likely to run into unexpected issues.
  • Never change your gear configuration before a deep dive. You need to know where everything is and how to use it, so make any modifications to your gear when you have time to use the gear on several shallow dives (even pool dives) before going deep.
  • Accept narcosis as reality. Many divers think they are immune to narcosis, but nitrogen is like a drug and it affects everyone. Accept that you will be impaired on the dive and dive conservatively to compensate for your slowed thought processes.
  • Stay in tune with your body. If you think you are too impaired to dive, you probably are–ascend until you feel better. Be aware that the impairment will go away if you move to shallower depths, but it might not happen immediately. And the narcosis will probably come back if you go deep again.


Before making the long descent into deep water, make sure your standard dive gear is up to the task and that you have the necessary deep diving-specific equipment to make the dive properly and safely.

Thicker is better on deep dives. Not only does the water get colder the deeper you go, but the increased pressure compresses the neoprene. In warm water, consider a thicker suit or one made from high-density neoprene (less stretch = more warmth). In temperate conditions use multiple layers like an overvest or a farmer john-style wetsuit or, even better, a dry suit.

Make sure your regulators are well maintained and properly serviced no matter what kind of dives you do. On deep dives this is even more critical because, in the case of a major malfunction, the surface isn’t just a short swim away. See for Scuba Lab’s review of 17 high-performance regulators and a breakdown of their features and performances on the ANSTI breathing machine.

Diving analog gauges is like using a manual typewriter for word processing. Low-end computers cost about the same as gauge consoles, and most computers on the market will provide emergency deco information, along with accurate depth, time and no-decompression limit calculations. If you must use older gauge designs, be sure your depth gauge has a max-depth indicator and reset it before each dive. You’ll also need a timer or dive watch that allows precise calculation of your bottom time.

Because they consume gas more quickly at depth, many deep divers choose to use a completely redundant air supply, called a pony bottle, as backup on deep dives. The pony bottle is a small cylinder, usually 13 to 20 cubic feet, with its own regulator that is generally strapped to the side of the primary cylinder. Divers choosing to use a pony bottle should consider their gas consumption rate to select the cylinder size that will allow them to ascend from their deepest planned dives to the surface and safely complete a safety stop on the way up. Smaller, handheld redundant air supplies can be useful and sufficient for shallower dives, but most do not provide enough air for a safe ascent on deeper dives.

On deep dives, it’s not uncommon to descend and ascend in areas where you can’t see the bottom from the surface, and vice versa, which can lead to vertigo without a visual reference. Add currents, surge and recommended safety stops into the equation, and it’s a good idea to have a physical reference to follow on your descents and ascents. If you’re diving from a boat, use the anchor or mooring line for this purpose. A weighted line attached to a float is another possibility. In some locations, you can also start your dive in shallow water and follow the wall or bottom contour to your planned depth.

Surface signaling devices, both visible and audible, are standard equipment for all dives. Deep-water environments tend to be in less sheltered or protected areas subject to rougher seas and more pronounced currents, as well as rapidly changing conditions. These factors add to the time it takes to ascend from depth, including a required safety stop, increasing the chances of divers surfacing down current from their exit points, so these safety devices are even more critical for deep diving. To maximize your chances of being heard over open ocean, carry a tank-air-powered sonic alert device (like Dive Alert) with a whistle as a backup. It’s also a good idea to attach your brightly colored surface marker bag or safety sausage to a line and reel, so if necessary, you can deploy the float underwater and use the line as an impromptu ascent line.

RMV and Calculating Consumption for Various Tanks

When using tanks of different sizes and pressure ratings, you need to convert your Surface Consumption Rate (SCR) into a Respiratory Minute Volume (RMV), which is the volume of gas in cubic feet you’ll use in 1 minute of dive time. First, find the conversion factor (CF) for the tank involved in your consumption rate calculation. The standard aluminum 80 is rated at 3,000 psi of pressure and contains slightly less than 80 cubic feet of air. To get its CF we take 80/3,000 = about 0.027 cubic feet, which means 1 psi equals 0.027 cubic feet of air. Then multiply the SCR by the CF. From the example, a diver with 20 SCR will have a RMV of 0.54 cubic feet, 20 x 0.027 = 0.54, meaning the diver uses 0.54 cubic feet of air per minute on the surface.

Now, when using a different tank, multiply the RMV by the pressure in atm at depth. In the previous example at 99 feet this would be 0.54 x 4 = 2.16, meaning the diver uses 2.16 cubic feet of air for every minute at depth. Using a 98-cubic-foot tank, you’ll plan to consume two-thirds of the air, 65 cubic feet. Divide the air volume by the RMV at depth, 65/2.16 = 30.25 minutes at 99 feet.

Note: Another way to find your air consumption rate is to use one of several types of dive computers that have integrated SPGs, and track your consumption rate automatically.

Nitrox mix table*

130 32 percent 5 min. 15 min. w/ EAN32
120 34 percent 10 min. 25 min. w/ EAN34
100 38 percent 20 min. 30 min. w/ EAN38
80 40 percent 30 min. 60 min. w/ EAN40
* This chart is for comparison only and should not be used to plan a dive.

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