Your body absorbs nitrogen during a dive: after the dive, your body can tolerate a certain level excess nitrogen without developing decompression sickness. The question is, how do you know what that level is, and then stay within it?

To answer this question, physiologist and another scientists created mathematical decompression models that track the theoretical nitrogen you have in you have in your body before , during and after diving. For practical field use, these models are expressed by dive tables and in dive computers, which as you read earlier, you use primarily to determine your maximum allowable time at given depths.

The fact that you derive your dive time limits your tabled m a model explains why you need to dive conservatively and avoid the maximum limits Your table or computer provides. Theoretical models can't account for variations front one individual to the next, so it's prudent to stay well within the limits a table or computer predicts. This is especially true if any of the factors that contribute to decompression sickness (vigorous exercise, cold, age, etc.) apply to you or the dive situation. You want to stay well within limits, AM take extra precautions to avoid the secondary contributed is, you can't change your age, but you can keep yourself' from becoming dehydrated.

So, because people differ in their susceptibility to decompression sickness, no dive table or computer can guarantee that decompression sickness will never occur, even though you dive within the table or computer limits. It is always wisest to plan dives well within table and computer limit, especially if any contributing factors apply.

As a recreational diver, you'll be learning no decompression diving. No decompression diving means that you ll plan your dives and dive so that you can always ascend directly to the surface without stopping, yet without significant risk of decompression sickness. This is also called (somewhat more accurately) no stop diving ,because you don't have to make ( to make a stop (though you usually will - more about that in Section Five). As a recreational diver, you always plan your dives as no decompression dives.

Decompression diving means that the divers absorb so much nitrogen (or other gas) during a dive that it's not possible to ascend directly surface without a substantial risk of DCS. Instead, diver makes a series of stops, each progressively longer, to allow sufficient time for the body to release dissolved nitrogen. 

Decompression. diving usually calls for using special synthetic breathing gases, requires a good deal of surface support. and even when done proper. compared to recreational diving the diver has more risk from DCS and other hazards. Obviously, this type of during is beyond the scope of the course an : recreational diving, though you'll learn - procedures for making emergency decompression stops in the unlikely event you accidentally exceed a no decompression limit.

Dive tables have been around since 1907, and until the late 1980 where primary method for planning dive. Although you'll probably end up using a dive computer for most of 'your diving tables still have their place for two primary reasons. For one, understanding how to plan dives with table gives a "feel" for what your computer does - it gives you a sense (a precise time) of how long a computer will let you stay at a given depth. 

Second, being electronic, dive computers can malfunction on you due to battery failure, impact. baking in sunlight, you name it Dive tables, being primarily printed plastic, are much less likely to file unless your dog chews them up or something. So you'll want to have them along as back up - if your computer crashes on a dive trip. (I tables may mean the difference between being in the water, or sitting at the dock in a very foul mood.

Until 1988, the dive tables recreational divers used were really hand-me-downs from commercial and military diving. Although they adequate for planning recreational dives, they were tables for decompression diving and had to accommodate large amounts of theoretical nitrogen, and consequently "penalized" recreational divers, who by making no decompression dives, had far less theoretical nitrogen. Further more, these tables were tested on predominantly young, male military divers, which didn't fully represent the population spectrum you find in recreational diving.

Commercial/military tables worked. but they weren't ideal. In 1988, DSAT (Diving Science & Technology) introduced the Recreational Dive Planner (RDP) which were the first dive table, designed for planning and making no decompression recreational dives. They were the first (and at this writing. still the only) such tables validated by test dives by volunteer recreational divers - men, women, younger. older, etc. 

This remains one of the largest and most extensive decompression tests in recreational diving. Distributed by PADI, the RDP quickly became (and remains) the world's most popular dive tables: quite a few popular dive computers even employ 1ZDI' teat data in their electronic decompression models.

It's available in a Table(conventional) format, and in The Wheel (circular slide rule) format, in both metric and imperial versions. For divers accustomed to conventional tables, DSAT developed The Wheel. To simplify use and to make multilevel diving possible without a dive computer 'more about multilevel diving in a moment. MAT developed The Wheel. You'll he learnig to use one or the other as part of this course - you should know which already.

Dive computers do the same job as dive tables, and they do it in the same way - by using a model to determine how much nitrogen you theoretical have in your body. They're neither more nor less valid than dive tables, ,so don't let the electronics, lights, beeps and digital displays impress you. 

The difference between a dive table and a dive computer is this: to be workable on a piece of plastic, a dive table uses a series of enough approximations for possible dives into which You fit Your dive. whereas a dive computer uses a depth gauge, timer and software to write a custom dive table for your exact dive. Throughout the dive. your computer updates this "custom table" as you change depth. constantly showing you how much no stop dive time you have left.

A dive computer offers both advantages and disadvantages (which is yet another reason for havin, both a computer and the RDP) :

Advantages:
. They're easier to use than tables because the track time and depth automatically, avoiding some human error. But you can't turn your brain off just because you turn your dive computer on.

. They give more no stop time on multilevel profile,. As you ascend. Yon take Up nitrogen more slowly and dive computers credit you for this by increasing your no decompression time. Tables must assume you spend the whole dive at the deepest depth you reach, so your allowable dive time is much shorter. The increased no stop time from a multilevel profile is substantial and the primary benefit of diving with computers. (Note: The Wheel let, you plan multilevel profiles with more dive time, making it the best table backup for your dive computer; it offers more no decompression time than any other table.)

. They track your more no-stop tine by eliminating the rounding and coarse fit that You have with a dive table.

. They track your the oretical nitrogen throughout the entire dive day (or longer). With tables, you have to calculate different allowable no stop times for each dive, which vary depending on the first dive time and depth, how long you're out of the water, and how deep the next dive will be. This isn't hard, but dive computers it automatically

Disadvantages
. They can fail. This can happen before a dive, during a dive or after a dive.
. They will "let you do them that, aren't recommended while happily crunching and displaying the theoretical numbers. (This is why you can't turn your brain off.) While they eliminate some human errors, they create the opportunity for other.
. You gain dive time by eliminating hounding. but you give up the extra conservatism that the rounding provides.

Don't let these disadvantages dissuade you from using dive computers - their advantages are overwhelming, and today ifs more unusual to a see diver with motif one than with one. Rather he aware of these problem; in Section Five, you'll learn how to avoid them.

The illustration gives you an idea of this works. Before your first your body has its normal nitrogen. Upon surfacing, your nitrogen level is higher, even though , you're within the safe limits estab you're by your computer. After some time at the surface, your body some of the residual nitrogen, but not all of you can also see that you're still closer to the maximum limit that you were before your dive, so a repetitive dive will have a sorter no decompression limit. After the repetitive dive, you're still within accepted limits, but your nitrogen level has -en and includes the extra nitrogen absorbed during this dive, Am the residual nitrogen left from your first dive. The RDP d/or your dive computer helps you determine acceptable time and depth limits for your first and repetitive dives. Accounting for theoretical changes in body nitrogen.

How long you have to wait before a dive isn't a repetitive dive depends on the computer or table. A computer tracks theoretical nitrogen for varying intervals, but it isn't really important to how long because the computer accounts for it automatic

Using the RDP, if you don't plan to dive for at least six hours, the residual nitrogen has little consequence. On the other hand, if you do plan to dive within six hours, you must account for the residual nitrogen when you plan your dive - and that's part of at you're about to learn to do with the Recreational DivePlanner.

During ascent, your body needs time to adjust to changing pressure, and you need time to regulate your buoyancy, keep track of your buddy and watch for obstructions overhead. It's important to ascend slowly - no faster than 18 metres/60 feet per minute, which is slower than you may realize.

As a new diver, you may find it a little difficult to judge your ascent rate at first. No worries. Star your ascent with plenty of air so you can make a slow, leisurely trip to the surface. Preferably, ascend along a line or follow the bottom contours to give you a visual reference and help you gauge your speed. Use your depth gauge as you ascend to help you know how fast you're going up, particularly when ascending without a visual reference. It should take you at least 10 seconds to ascend 3 metres/10 feet - but don't worry about being exact, as long as you're not exceeding this rate. In fact, it's a good idea to come up slower - most computers and gauges warn you if you exceed 10 metres/30 feet per minute.

Whenever possible, stop your ascent when you reach 5 metres/15 feet and wait three minutes - more is fine - before continuing your ascent, particularly after deep dives or dives close to the no stop time limit. This is called a safety stop (you'll learn more about safety stops in Section Five), which gives you an extra margin of safety.

Think of the 18 metre/60 foot per minute rate of ascent as a speed limit. It's fine to go slower, but don't go faster. Be a S.A.F.E. diver: Slowly Ascend From Every dive.

Dive tables and dive computers use mathematical models to estimate the theoretical nitrogen in your body before. during and after a dive. People vary in their susceptibility to DCS, so no computer or table can guarantee you'll never get DCS, even Within its limits. So, dive well within table/computer limits. Dive computer has some use advantages and disadvantages compared to tables, but it is neither more nor less valid. Recreational divers only make no decompression (no stop) dives.