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.
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 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 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.
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.
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.
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.