For the first 30 years or so, recreational dive time underwater was the no decompression limit of the deepest depth reached.

Today, multilevel diving gaining more no decompression time by ascending to shallower depths where nitrogen absorption is slower-is more the rule than the exception. Thanks to multilevel diving, in most environments you can stay underwater as long as you have air and warmth, this breakthrough springs from modern dive computers, the Wheel version of the Recreational Dive Planner and other advances in decompression theory. Whether you favor coral reefs or inland lakes, most dive sites have opportunities to multilevel dive.

Any place you find a sloping reef, wall or other topography that allows you to start deep and move to shallower depths, you can plan a multilevel dive with your computer or the wheel. Even many wreck dives will allow multilevel dives in which you start on the deeper parts of the wreck, and gradually ascend to the wreck’s upper structure. Multilevel diving is your ticket to more of what you first got into diving for-time under water. No wonder more and more divers consider their computers as essential as their masks and regulators.

Multilevel Diving Theory:

Multilevel diving is a technique for extending your bottom time beyond the no decompression limit (a.k.a ‘no stop’ limit) of the deepest depth you reach. You accomplish this by ascending to shallower levels during the dive, as you ascend; your body absorbs nitrogen more slowly than if you remained at the deepest depth for the entire dive. Because you absorb nitrogen more slowly, you have more time available within the no decompression limits.

Multilevel diving-whether you use a computer, the dive wheel or both- draws directly upon decompression theory, so a rudimentary understanding of this theory helps you understand some of the limitations of multilevel diving and other dive practices. You’ll find that guidelines you use with dive computers and tables come from what we know and don’t know-about decompression.

No Decompression Limits:

Reviewing what you know from your PADI Open Water Diver course, your body absorbs nitrogen from the air that you breathe during a dive. The deeper you dive, the faster you absorb nitrogen and the longer you dive, the more nitrogen you absorb. Your body tolerates a certain amount of excess nitrogen left over from a dive when you surface without developing decompression sickness; dive tables and computers track theoretical nitrogen absorption to keep your bodies nitrogen within tolerable limits.

Interestingly, the ‘tolerable limits’-the no decompression limits for recreational divers – are the starting point for decompression theory. No theory of human physiology or decompression alone can predict the no decompression limits. They’re determined only one way: through the actual results of human dives, preferably test dives.

The Decompression Model:

Since no decompression limits are determined through the results of actual dives, if you limited the results of actual dives, if you limited your diving to one single depth dive in a day, you wouldn’t need a decompression theory. You would just memorize the limits established by successful dive results. But you don’t want to make only one dive, you want to multilevel dive and you want to be ready for emergency decompression, just in case.

Repetitive diving, multilevel diving and emergency decompression theory have too many variables to test all the possible combinations of dives, levels and surface intervals, so physiologists use mathematical decompression models to apply test results to this multitude of diving variables. Virtually all recreational dive computers and dive tables grew from various modifications of a decompression model published by physiologist John Scott Haldane in 1908.

Briefly, a decompression model works by mathematically predicting how much nitrogen the human body absorbs during a specific dive. As dynamic and useful as decompression models are, however, physiologists have learned that decompression theory is imperfect, and that decompression models can predict as ‘safe’ dive profiles that might not be so safe.

For that reason, you can only rely on decompression theory to produce an acceptable risk of decompression sickness as far as it has been successfully tested. Even then, people vary in their physiology and susceptibility to decompression sickness. No dive table or computer can guarantee that decompression sickness will not occur, even when diving within the table or computer limits. This is one reason why it’s important to dive conservatively, well within table or computer no stop limits.

Mounting evidence suggests that repetitive deep dives produce an unacceptable high rate of decompression sickness, despite the predictions of mathematical models. For this reason, plan your repetitive dives deeper no deeper than 30 meters/100ft, regardless of what your dive table or computer might say it permits.

Tables and Multilevel Diving:

Obviously, you can use dive computers for multilevel diving-that’s perhaps their prime function. Computers calculate your exact dive profile and apply a decompression model to write a custom dive table for your dive. On the other hand, you can’t use conventional dive tables for multilevel diving, even by ‘interpolating’ repetitive groups; because doing so can permit dives beyond what human dives show to work.

A second concern is that attempting to calculate multilevel dives with conventional tables is at best tedious and at worst complex and error-prone. Not a good place to have an ‘oops’ The Wheel differs, though, because it was designed and tested with multilevel diving in mind. It keeps your dive plan within accepted limits, and its designed simplifies planning.

Ascent Procedures:

There are two potentially hazardous conditions related to a diver’s ascent that you’re already familiar with from your Open Water diver course; lung overexpansion injuries and decompression sickness. Ascent recommendations help you avoid these.

So far as decompression theory is concerned, an ascent procedure consists of three parts;

1) no decompression limit,

2) rate of ascent and

3) safety stop.

No Decompression Limit:

From a theoretical point of view, the no decompression limit dictates when you’ll start to ascend. This component of ascent has no bearing on lung over expansion injuries.

Rate:

The ascent rate for The Wheel has been established at a maximum of 18 meters/60 feet per minute based on human tests, though some computers specify slower rates. Ascend no faster than 18 meters/60 feet or per minute or at a rate prescribed by the table or computer you are using whichever is slower. Using The Wheel (or table) RDP, you may ascend slower than 18 meters/60 feet per minute.

Safety Stop:

A safety stop is a 3-minute pause at the 5-meter/15 foot level.  Safety stops have been tested to a limited degree, and show significant benefit in reducing the probability of decompression sickness. When analyzed mathematically with a decompression model, the safety stop theoretically also produces a significant reduction in absorbed nitrogen. A safety stop at 5 meters/15 feet also allows you a moment to double-check your depth and time information.

In addition, the stop gives you a moment to readjust your buoyancy, so it may help prevent runaway ascents through the last few meters/several feet of water, and thereby minimize the possibility of lung over expansion injuries. It’s with these reasons in mind that you want to make safety stops on virtually all dives.

Emergency Decompression:

Within the scope of recreational diving, decompression diving is exclusively an emergency procedure. However there’s an important aspect to note about combining a dive that requires emergency decompression with repetitive dives: it’s a bad idea.

A 1986 U.S. Navy test revealed an unacceptable rate of decompression sickness resulting from repetitive decompression dives, despite table predictions.  Furthermore, anecdotal reports from recompression chamber facilities indicate many cases of decompression sickness result from combining decompression diving with repetitive diving. Apparently, even if only one dive in the series is a decompression dive, the probability of decompression sickness increases.

It seems that in many cases, mathematical decompression models cant’ adequately predict the combination of a repetitive dive with a decompression dive. For this reason, avoid combining a dive that requires emergency decompression with a repetitive dive. If you make a dive and accidentally end up at an emergency stop, after the stop make that the last dive of the day, even if your computer permits more dives.  (The Wheel requires at least a six our surface interval).

Using your computer:

It’s your dive computer that really makes multilevel diving practical.  While The Wheel makes it possible without a computer, the primary advantage of using a computer for multilevel diving is that it computes your exact dive profile for maximum allowable no stop time. The Wheel helps you understand how a computer calculates, and it’s your best option for resuming diving if your computer crashes (it happens), but 99 percent of the time, you’ll probably want to use your computer for the convenience and the precision it offers.

However, as wondrous as your computer is, it’s imperative that you make computer-assisted, not computer controlled dives. Computers appear almost magical to some people; but they’re just highly sophisticated calculators that read the depth and time and then apply the same type decompression model your dive tables, and the same guidelines apply to computer diving as to table diving.

Eight rules for computer diving (some of these apply to tables, too) help you stay within the limits of what has been proven to produce an acceptably minimal probability of decompression sickness.

1. Don’t dive to the no decompression limits and avoid mandatory emergency decompression.

Stay well within the computers limits.  You should have ample time before reaching a no stop limit at all times during your dive.

2. Topography permitting make multilevel dives that start deep and work shallower.

Avoid ‘sawtooth’ dive profiles with repeated significant shallow and deep depth changes, such as starting a dive at 30 meters/100 feet, then ascending to 18 meters/60 feet and after a while descending to 28 meters/90 feet. While its unclear what added risk there may be for this kind of diving, if any within the realm of no stop diving the vast majority of test data is based on ‘forward’ profiles that start deep and work progressively upward. To stay within the envelope of proved test data, start at the deepest point and progress shallower (minor variations aren’t a problem). Your computer may permit sawtooth profiles by calculating them, not to encourage this kind of profile, but so you have information if you accidentally do one.

3. Control your rate of ascent to 18 meters/60 feet per minute or slower.

Virtually all computers have rate indicators that alert you if you start to go too fast.

4. Take a safety stop at the end of all dives at 5 meters/15 feet for at least 3 minutes.

5. Allow a surface interval of at least 60 minutes with a computer; even if it permits the dive time you want in less time.

6. Limit repetitive dives to 30 meters/100 feet or less.  Make your deepest dive first, with subsequent dives progressively shallower.

7. Don’t get so caught up in your extended bottom time limit that you neglect your air supply.

8. Be aware that no computer or table can account for physiological variations caused by factors such as age, dehydration, alcohol consumption, strenuous exercise, excessive fat tissue, injury or other factors that predispose you toward decompression sickness.

The more of these factors that apply, the more conservatively you should use your computer or table. Surprisingly, being conservative doesn’t always mean you have to cut your dive short- usually, you can simply move shallower sooner so you always have lots of time before reaching a no decompression limit.

Multilevel dive equipment:

Equipment for all multilevel dives: Besides the equipment you normally wear in your local dive environment, you want to have three pieces of equipment along for multilevel dives.

The Wheel:

As the table you can plan multilevel dives with, The Wheel is recommended for two reasons.

First, to plan a multilevel dive with or without your computer.

The Wheel will give you a rough idea of what your computer will allow as you ascend during the course of the dive.

Second, if your computer goes south on you, use The Wheel as a back up that permits multilevel diving – something you’ll want especially if all your buddies are still humming along fine. If you’re on an exotic dive holiday and your computer quits, you’ll kick yourself up and down the dock for leaving your wheel at home.

Depth gauge and timer:

These two pieces of equipment are necessary to back up a dive computer. You’ll also need these to multilevel dive with The Wheel without your computer.

Equipment for computer assisted Multilevel Dives:

When making computer-assisted multilevel dives, you and your buddy should each have your own computers. Multiple divers shouldn’t share one computer.  Once a diver begins diving with a computer, that diver uses that computer for the entire diving day or longer if specified by the manufacturer.

The reason for this is that dive computers track your dive profiles so closely that even minor variations between you and your buddy affect your allowable dive time.  Likewise, the computer tracks nitrogen release between dives, so it’s inappropriate to let someone borrow your computer between dives.

Multilevel dive hazards:

There are a few potential hazards and mistakes in multilevel diving that you’ll need to avoid.

No worries – none of these are particularly mysterious or difficult to evade. Just things to watch out for.

Potential Hazards:

Three main potential hazards of multilevel diving are not unique to multilevel diving. You can encounter these in almost any diving situation, but we give them special attention here because multilevel dives means extended bottom times, which means more potential to run into them.

Hypothermia:

Extended bottom times increase your exposure to cool water. Your exposure suit that’s adequate for a short dive may not be enough for a longer one. Be sure to wear adequate thermal protection.  If you begin shivering, end the dive immediately.

Running out of air:

We touched on this earlier.  Your available no stop time on a multilevel dive can easily exceed your air supply. Pay close attention to your air supply.  Pay close attention to your air supply and allow plenty for a safe return to boat or shore. A good habitat is that when you check your computer, you check your SPG too.

Disorientation:

A long period underwater means you can go farther, which means more opportunity to get turned around. Use a compass and your other navigational skills so you know where you are and where to exit the water at all times. Take your time and go slower.  Just because you can go farther doesn’t mean you have to. See the Underwater Navigation section for more information about staying oriented.

Common Mistakes:

There are two common mistakes to avoid, both of which we’ve touched on already.

Sharing a computer:

Don’t.  Computers follow dive profiles too closely to make sharing feasible. The diver not wearing the computer can’t be confident in the dive profile, and it would be especially cumbersome if that diver wants to dive with someone else after a surface interval. If you have two divers and one computer, have the computer-less diver calculate a multilevel profile with The Wheel. This should provide ample no stop time for both divers to fully enjoy the dive without necessary risk or complications.

“Sawtooth diving”:

Because computers and tables are mathematical devices, they calculate any variety of dives, even deep dives following shallow dives or on multilevel dives, deep levels following shallow levels. These kinds of “sawtooth” profiles, however take you outside the body of known test data for reliable diving. It seems that decompression models aren’t as reliable in this kind of diving, and in any case, there’s been virtually no testing of “sawtooth” diving.

As stated earlier, avoid this.  Make your deepest dive first, with repetitive dives progressively shallower. Multilevel dives should begin at the deepest point and work to progressively shallower levels. Once you’ve ascended to a shallower depth, don’t drop back down.  Computers or tables let you calculate it not because it’s recommended, but so you’ve got information if it happens accidentally.

Multilevel dive planning:

You’re going to find that planning a multilevel dive differs little from planning a single depth dive, making it easy to benefit from extended no stop time at shallower depths.

Considerations:

There are three considerations to take into account when planning multilevel dives.

Topography:

You need to estimate ascending depth levels when planning your dive. A sloping reef, wall or other moderate rise in the bottom is nearly ideal for multilevel diving because it offers almost any depth level you want. Some “single-depth” sites, such as wrecks or flat reefs have few practical opportunities for making a multilevel dive.

At an unfamiliar dive site where you have you have no information about depth levels it may be impossible to plan your first visit to the site as a multilevel dive.  Of course your computer will extend your no stop time if you do, in fact, find appropriate topography and follow it.  Just be sure to include this possibility in your planning.

Air supply:

As mentioned in the section on hazards, your no decompression time can be longer than your air supply. Be sure you watch your air supply when you dive your plan.

Contingency plans:

Multilevel dives require contingency planning.  If your computer fails, you need to know what to do. If intermediate depth levels are deeper than you planned, you need to be ready to skip up a level or revert to a single depth dive plan, especially if you’re using The Wheel without a dive computer.

Multilevel Planning with Computers:

If you’re diving with a computer, there are a few steps to take for planning. You will get your initial and repetitive dive no decompression limits from your computer and you can use The Wheel to estimate the time your computer will allow on a multilevel profile.  You also need to know what to do if your computer crashes. Obtaining no decompression limits.  For first and repetitive dives, most dive computers display their no decompression limits for various depths when you activate the scroll mode. Different models activate their scroll models in different ways, so consult the computers instruction manual or your instructor.

Using The Wheel to plan computer dives.  The Wheel helps estimate your computers no stop time. For your first dive, simply calculate the multilevel profile you intend to follow; The Wheel will approximate the time you can expect at each level.

For repetitive dives, it’s necessary to find a pressure group for use on The Wheel.

First, set your computer in scroll mode and find your o decompression limit for a repetitive dive to 12 meters/40 feet.

Next set The Wheel’s yellow p.g. Index for the scrolled time and align it with the NDL mark on the 12-meter/40 foot curve.

Now look at where the white p.g. Index crosses the 12-meter/40 foot curve to determine your pressure group.

(This is similar to setting your Wheel for a minimum surface interval problem).

Use that pressure group on The Wheel to approximate the time your computer will permit on a repetitive multilevel dive.

In using The Wheel to assist in planning computer dives, remember that dive computers follow your exact profile, and may have different no decompression limits than The Wheel.  When diving with a computer, stay with the computers no stop limits. The computer is your source of no decompression time; The Wheel helps you plan a dive.

If your computer fails:

If your computer fails between dives, you will have to wait at least 24 hours (longer if recommended by the manufacturer) before resuming diving with another computer or The Wheel.  This is because there’s no accurate way to account for the nitrogen in your body. You must wait until the excess nitrogen has left your body for all practical purposes. An exception is if your maximum depths and times fall with the single depth limits of the RDP. In this case you can calculate the days dive profile and determine a pressure group for planning another dive. But this is often impossible because multilevel dive times frequently exceed single depth limits, especially if your first level was deeper than 18 meters/60 feet.

If your computer fails, while you’re diving, immediately stop the dive and ascend according to the manufacturers’ guidelines. If there are none, ascend no faster than 18 meters/60 feet per minute, or the computers ascent rate, whichever is slower, to 5 meters/15 feet. Make a long stop – perhaps as long as your air supply permits.  Do not dive again for 24 hours, or as stipulated by the manufacturers instructions.