Dive Training: Double Your Bottom Time | Scuba Diving

Dive Training: Double Your Bottom Time

| |Photography by David Fleetham|

Editor's note: All dive profiles listed here are for demonstration purposes only. They should not be used to plan or execute a dive.

How would you like to dive to 60 feet for 90 minutes — with no decompression? Or make back-to-back dives at 60 feet for 60 minutes with a surface interval just long enough to change tanks? It's possible with advanced nitrox blends containing 40 percent or more oxygen. Because these so-called "hot mixes" contain less nitrogen, they have traditionally been used by tech divers as decompression gases at the end of deep air dives. But with proper training and equipment, you can use hot mixes as a primary breathing gas and radically extend no-decompression limits at traditional sport diving depths.

Stretch the No-Deco Limits ...

Take the example of those back-to-back 60-foot dives for 60 minutes each. How is this possible? Simply apply the equivalent air depth (EAD) principle you learned in the basic nitrox course. On 50 percent nitrox, a 60-foot dive has the equivalent nitrogen exposure and no-decompression limits as a 30-foot dive on air.

According to the U.S. Navy dive tables, after 60 minutes at 30 feet, you'll exit the water as a D-group diver. With virtually no surface interval, you'll have a residual nitrogen time of only 37 minutes and a new no-deco limit of 163 minutes (based on a second dive to 40 feet, because the Navy Repetitive Dive Table starts at that depth). You can almost immediately re-enter the water for another hour on the bottom — effectively doubling your no-deco time compared to the same dive using an air profile.

For another table-busting application of advanced nitrox, consider the possibilities of an extended drift dive in 80 feet of water. Using 43 percent nitrox, you have an EAD of only 48 feet with a no-deco limit of 100 minutes according to the Navy tables, a full 60 minutes more than an air-breathing diver would have.

... Or Don't

Of course, the more traditional use of hyperoxic nitrox blends is as a decompression gas. For example, a diver making a 40-minute air dive to a depth of 150 feet might carry a small bottle of 80 percent nitrox. After ascending to a depth of 30 feet, the diver would then switch to the hyperoxic gas and use this mixture to complete all of his decompression stops, an added safety stop and his slow ascent to the surface.

Thanks to the simple principle of gas diffusion (gas always moves from higher concentrations to lower concentrations, and the larger the difference in concentrations the more rapid the diffusion process), this decompression gas of 80 percent oxygen and 20 percent nitrogen accelerates offgassing of nitrogen.

Technical divers apply hyperoxic deco gases in two ways. The first is to cut decompression time in hazardous conditions. A more conservative approach is to complete decompression on an air profile while using the hyperoxic gases for added physiological safety.

Managing the Risks

As with most beneficial technologies, there are downsides to using hyperoxic gases--specifically the risk of oxygen toxicity and the need for oxygen-compatible dive gear. As basic nitrox divers know, prolonged exposure to oxygen-rich breathing gases in a high-pressure environment can be toxic to the central nervous system. Oxygen toxicity has nasty and life-threatening side effects like convulsions and seizures.

Divers breathing hyperoxic mixes must closely monitor their oxygen exposure or PPO2. Depending on their training and tolerance for risk, divers typically choose an exposure limit of 1.4 to 1.6 PPO2 and consult tables or dive planning software for the appropriate time limits at a given depth and oxygen percentage. A diver breathing 80 percent nitrox at 33 feet, for example, experiences the same oxygen exposure as a diver breathing 36 percent nitrox at 115 feet — 1.6 PPO2. Both divers have a single dive exposure limit of 45 minutes and a 24-hour exposure limit of 150 minutes.

The Fire-Breathing Regulator

As a general rule, gas mixtures containing less than 40 percent oxygen require no special equipment. However, once we exceed the 40 percent oxygen point, regulators must be constructed of oxygen-compatible materials and all of the components must be specially cleaned for exposure to pure oxygen to limit the risk of fires. In an oxygen-rich environment, the heat generated by the compression of gas into small orifices and the sparks created by the movement of microscopic particles in the gas stream can ignite. Although rare, these fires can be serious, and the risk increases with any increase in the percentage of oxygen in the breathing gas.

Thanks to the demands of tech divers, it's easy to find regulators that are oxygen-compatible and oxygen-clean. Oxygen-compatible means that all of the component parts (base metals, platings, O-rings, hoses, seats, etc.) are made of materials that are safe for exposure to higher percentages of oxygen. To be oxygen-clean, all petroleum-based lubricants must be removed from all surfaces through a very thorough cleaning process.

THE BOTTOM LINE: Divers using hyperoxic mixes should make doubly sure their regulator is prepped to handle hot mixes by consulting the manufacturer and a qualified service technician.

You Do the Math

What's Your PPO2?

To determine your PPO2:

>Convert the depth of the dive to atmospheres absolute of pressure (ATA) by dividing the depth in feet by 33 and adding one. (Example: 132 feet/33 + 1 =5 ATA)

>Multiply the percentage of oxygen in your breathing gas by the ATA. (Example for 32 percent oxygen: 5 ATA x .32 FO2= 1.6 PPO2)

What's Your Equivalent Air Depth (EAD)?

Equivalent air depth is calculated based on the fraction of nitrogen in a gas mix. To obtain an equivalent air depth, consult dive tables or:

>Multiply the fraction of nitrogen in your breathing gas (FN2) by the combined total of the depth in feet, plus 33. (Example for a dive on 50 percent nitrox to 80 feet: .50 FN2 x (80+33) = 56.5)

>Divide the result by .79, the percentage of nitrogen in air. (56.5/.79= 71.52)

>Subtract 33 from the result to get your equivalent air depth. If you're using tables, round up to the nearest depth. (71.52-33 = 38.5 EAD, rounded up to 40 feet for dive planning)