Traditional dive planning, the kind you learn as a beginning diver, has you following tables rather than using a dive computer. Even if you use a computer now, it’s a good idea to pre-plan your dives using tables to give yourself plenty of buffer and avoid decompression sickness.

That’s understandable, but the discrepancy between a square profile from a table and a dive computer can be quite large, costing you precious bottom time. One solution to give yourself more bottom time in the pre-planning stage is to use multilevel dive planning.

The problem

When dive planning with a table, you give it your depth, and it tells you how long you can spend at that depth. The problem with this is that it conservatively assumes you spent the entire dive at that depth, when in fact that’s never how it goes. You end up with a theoretical profile that looks like this:

Your dive times are limited based on this square profile.

A dive computer, on the other hand, constantly samples your depth to give you a very accurate profile:

This increased accuracy pays off in the form of more bottom time, while still staying within conservative limits. The table profile assumes you are at one depth for the entire dive, which is rarely the case. Multilevel dives strike a balance, although they err more towards the square profile side.

Multilevel dives

Multilevel dives assume more than one depth for the duration of a dive. However, it doesn’t give as accurate a profile as a dive computer. Instead, we get something in between:

For example, imagine we are diving to 70 feet (22 meters). Using my standard table, this gives me a maximum of 40 minutes of bottom time.

The problem is, I know I won’t spend all 40 minutes at 70 feet, and because my air will last longer, I’ll have to cut my dive short because of my limiting dive table. What I will actually do is begin my dive at the bottom of the reef, then gradually come shallower. In fact, I’ll plan on spending the first 20 minutes at 70 feet, then the rest of my dive at 35 feet (7 meters).

With this in mind, my total bottom time goes from a meager 40 minutes to practically unlimited. Once I move up to 35 feet, I can spend over 200 minutes at that depth! That’s more than enough for my air supply.

As another example, let’s say I want to dive to 90 feet (30 m). My maximum bottom time at that depth is 25 minutes. Let’s say I spend 21 minutes there and want to move up to 60 feet. Instead of ending my dive, I can spend 22 minutes at 60 feet, doubling my total bottom time.

The multi in multilevel

It gets even better. Nothing says you have to limit yourself to a two-level dive. You could even do a three level dive. While theoretically there’s nothing stopping you from a 4, 5, or 6 level dive, it can get a bit tedious at that point. Not in the planning stage, but when actually executing your dive.

The catch

The one catch is you need to actually follow your planned profile. This means keeping an eye on your dive timer and depth gauge, moving to the next level at the appropriate time. This becomes especially important when diving multiple times in a single day.

Should you miss one of your levels, take note of how long you were at each depth and modify your profile back on the surface. This way, you have an accurate measure for planning more dives.

Equipment

Obviously, you’ll need a watch (timer) of some sort and a depth gauge, so you know when to change depth. You’ll also need a planning table appropriate for multilevel dives.

Ordinary tables are not designed for multilevel dives. Using them as such can push you over known limits for preventing decompression sickness.

PADI used to use The Wheel, but have discontinued it. They now use the eRDPML (electronic Recreational Dive Planner, Multi Level). Other organizations have similar devices. Contact your local dive shop for more info on what you need.

Unfortunately, I can’t describe the process without knowing what you are planning with. Nonetheless, planning is simple with just about all devices, especially with instructions right in front of you.

Dive computers

A lot of this information may seem largely irrelevant with a dive computer, and that’s partially true. With a dive computer, there is no need to painstakenly track your dive profile—the computer does it for you.

However, dive computers usually have a very limited dive planning. This is often restricted to allowing you set one depth and it tells you what your no decompression limit is. You may want more planning power.

While you can’t start using multilevel dive planning in between dives, you can use it at the beginning of the day to give you an idea of what your day will look like, including about how long your surface intervals should be. This can be useful for many things, including coordinating non-diving activities, meals, and more.

Specialty courses

Most agencies would be more than glad to offer you a multilevel dive planning specialty course. Let me say, though, it’s really not necessary. Purchase the required dive planning equipment and read the instructions. You should get a decent feel for how it works. If you are really not sure, or are scared of messing up, talk to a professional, and take a class if that’s what they recommend. Diving safe is the number one priority.

Disclaimer: Do not attempt to dive with enriched air unless you have completed the appropriate speciality course. Doing so without complete training can be very dangerous.

Enriched air can mean any gas blend other than the standard 21% oxygen / 79% nitrogen that comprises what we call “air.” For our purposes in this article, enriched air will refer to a blend of oxygen and nitrogen in which the oxygen content has been raised, more commonly called “Nitrox” (or EANx, Enriched Air Nitrox). We won’t be concerned with Tri-mixes and the like—usually the domain of tech diving.

Nitrox blends increase the oxygen content of a cylinder, which simultaneously lowers the nitrogen content. This is usually accomplished by partially filling a cylinder with air (21% oxygen / 79% nitrogen), then “topping it off” with pure, 100% oxygen. Some basic math tells us the final oxygen content.

For example, imagine we fill a cylinder to 2500 PSI with air, then fill to 3000 PSI with pure oxygen. What is the final oxygen content? 500 PSI (3000 PSI – 2500 PSI) is 100% oxygen, while 21% of 2500 PSI is also pure oxygen. 2500 PSI * 0.21 = 525 PSI. Add the two together to get 500 PSI + 525 PSI = 1025 PSI oxygen. Divide by the total contents of the cylinder to get the percentage of pure oxygen, 1025 / 3000 = 0.3416 = 34.16% oxygen. That means the remaining 65.84% must be nitrogen (ignoring trace amounts of other gases in the air). We call our blend is EANx35, Enriched Air Nitrox 35 (you round up with enriched air blends).

Why enriched air?

What advantages does Nitrox provide divers? Nitrox reduces exposure to nitrogen compared to diving with air. Nitrogen is known to instigate Decompression Sickness (DCS), so less exposure is always good.

Divers take advantage of this benefit in two ways. Bottom times provided by dive tables and computers are controlled by estimates of nitrogen exposure. Less exposure means more bottom time. So the maximum bottom time on a Nitrox dive to a given depth is always higher than the maximum bottom time of an air dive to the same depth. My dive table says that an air dive to 60 ft (18 m) has a maximum bottom time of 47 minutes. The same dive on EANx32 has a maximum bottom time of 77 minutes, half an hour longer!

Not everyone wishes to dive for an hour and a half, however. The other advantage, then, is that with two identical dive profiles, one with air and one with Nitrox, the Nitrox dive will have a higher buffer of nitrogen exposure, significantly decreasing any risk of DCS. This isn’t a big deal for any one dive, but for many consecutive multi-day dives, where DCS causes are less understood, this buffer becomes important. The benefit of Nitrox also decreases required surface intervals, allowing a diver to easily fit four dives in a day.

What’s the catch?

Diving with Nitrox introduces a few more variables to track. For one, you must always check your cylinder for what percentage Nitrox it contains. The blender uses the above calculations, but any errors can affect your diving in a bad way, so for safety it is not only recommended, but required that the diver personally check their enriched air before every dive. Shops will have the appropriate equipment for doing so. I would refuse to dive with an enriched air mix I have not personally checked.

Fortunately, most dive computers come equipped for enriched air, making it straight-forward to dive with Nitrox. However, Nitrox requires modified dive tables, due to different exposures. Shops sell tables for common mixes such as EANx32 and EANx36, and your Nitrox specialty course may even come with them. The most versatile method is using something called Equivalent Air Depth (EAD).

Equivalent Air Depth calculations tell you, given a specific Nitrox blend, at what depth you would have to dive with air to get the same nitrogen exposure. With this depth, you can use normal dive tables to plan dives. Since Nitrox contains less nitrogen, this depth is always less than the actual dive depth. We can compute EAD as

EAD = (1 – O%) * (D + 10) / 0.79 – 10,

where O% is the percentage of oxygen, and D is the depth of the dive, in meters. Suppose we are planning a dive to 60 ft (18 m) with EANx34 and don’t have an appropriate table. We compute our EAD as (1 – 0.34) * (18 + 10) / 0.79 – 10 = 13.39 m, or 45 ft. We then use our ordinary dive tables, entering 45 ft (or 13.39 m) as the dive depth. This gives me a 63 minute bottom time.

Oxygen exposure

Diving with air within recreational limits, oxygen exposure never reaches critical limits—our body is always able to metabolize the available oxygen. However, with Nitrox oxygen content is increased to amounts that could be dangerous.

For instance, it is known that exposure to oxygen partial pressure beyond 1.6 can be fatal. Oxygen partial pressure is the portion of pressure due to oxygen at a given depth. Air at 1 atm has an oxygen partial pressure of 0.21 * 1 = 0.21. At 2 atm it’s 0.21 * 2 = 0.42. With this knowledge, we can calculate a maximum allowable depth on any gas blend. For air, this maximum is 1.6 / 0.21 = 7.619 atm, which corresponds to a depth of about 66 m (215 ft). In practice, it is recommended (and wise) to use 1.4 as the maximum oxygen partial pressure, to build in a safety buffer. For air then, we wouldn’t want to exceed a depth of 1.4 / 0.21 = 6.6 atm, or 56 m (185 ft). I’m guessing that won’t be a problem for most of you.

We can repeat these calculations for our Nitrox blends. Given EANx36, the maximum depth is 1.4 / 0.36 = 3.8 atm, or 28 m (93 ft), which is above the recreational limits! Keep this in mind for high oxygen content blends. What is the maximum allowable depth for EANx40 (the highest oxygen content permitted with most certifications)?¹

This is a maximum partial pressure in a single dive, but over multiple dives, your body accumulates excess oxygen that it can’t burn. Given enough time this oxygen can become toxic. Therefore, you must also track accumulation of oxygen over a running 24 hour period. You will learn how to do this and be given the appropriate tables in your Enriched Air Diver specialty course.

If these advantages sound appealing to you, I highly recommend completing your Enriched Air specialty. Most regular divers will do it at some point. Don’t let the calculations and variables deter you; they become easier over time. Again, let me reiterate that you should not dive with Nitrox unless you have completed such a course. It can be done in an evening, with no dives required (at least with PADI).

1. 82.5 ft (25 m)

Following basic certification, many divers immediately move on to an advanced certification level. Especially since some activities (like deep dives) require an advanced card.

Advanced certification has been criticized for being somewhat of a misnomer. In all certification agencies, the advanced level is attained with a relatively small number of dives. For this reason, it has been suggested to rename the advanced open water level to something like, “Open Water II” (as it was previously called by NAUI) or “Comprehensive Open Water”, to better indicate the nature of the course. Regardless of what it is called, those that dive regularly will want to complete the course at some point.

Advanced certification is built around specialities. The idea is that exposure to many different varieties of diving increases your experience level and encourages further development. How much exposure is required differs between agencies. I’ll cover the four largest.

PADI

PADI requires five “adventure” (i.e., specialty) dives to qualify for Advanced Open Water (AOW) certification. One of these dives must be an Underwater Navigation specialty and another the Deep Diving specialty. The other three are the diver’s choice. (Note: Until 2006, night diving was required.)

Unlike other organizations, PADI does not require a minimum number of dives for certification. This means that a diver could be Advanced certified with only 9 dives (4 open water dives + 5 adventure dives), all of which are instructor certified. This makes PADI’s AOW certification the quickest, and the easiest to criticize.

NAUI

NAUI calls their second certification level, “Advanced Scuba Diver”, with requirements similar to PADI’s. However, NAUI requires six open water dives to qualify for certification. One must be navigation, another must be deep diving, and a third is required to be night or low visibility diving. The other three are chosen at the diver’s discretion.

Also like PADI, there is no minimum number of dives required. With only one additional dive required, NAUI Advanced certification is almost as quick as PADI.

SDI

SDI prides itself on its Advanced Diver Development Program having higher requirements and demanding more experience before certification. As opposed to just a certain number of speciality dives, SDI requires candidates to complete four specialty courses, with no requirements on the type of specialties. Each specialty requires about two dives, so eight dives total.

Another distinction is that SDI requires a minimum of 25 logged dives to complete certification. Once reached, the scuba diver brings in their dive log and four specialty cards to be awarded Advanced Scuba Diver certification.

SSI

SSI’s Advanced Open Water Diver certification is more similar to SDI than any other agency. They also require completion of four specialties, with no requirements on the specialty subjects.

24 logged dives are required to complete certification.

This should give you an idea of what’s required to complete Advanced certification. The student must decide what specialties they want to explore. In my opinion, the most valuable non-required dive is a buoyancy related specialty. Proper weighting and buoyancy control throughout a dive are valuable skills to master early. Other than that, it should be straight-forward to follow your interests in deciding.

What specialities would you recommend? What do you think about the whole “Advanced Diver” misnomer?