Before the invention of the Self Contained Underwater Breathing Apparatus the only divers were surface supplied, or hard hat divers. These divers preformed tasks at set depths, meaning they spent their entire dive at one depth. All of their decompression status, and any decompression stop execution, were preformed by diver tenders. Once freedom of the surface was achieved, the diver was free to move about at any depth he wanted, for any time he fancied, within reason. This produced the need of the diver to monitor his own bottom time, and depth, to determine his decompression status. In a surface supplied diving, depth, and bottom time were all controlled by the surface support tenders. Now the Scuba diver has to monitor all this. This produced the need for the Scuba divers to monitor, or determine decompression status underwater. Without the unlimited air supply of the surface compressor, the Scuba diver has to return to the surface for fresh tanks, thus repetitive diving, rarely done by hard hat divers.
To keep divers within the safe limits of nitrogen the Dive Tables were developed. These tables take into consideration the amount of nitrogen absorbed by the body at different depths for different times. The United States Navy (USN) had Decompression Tables made to allow for stage decompression and no-decompression diving. The USN Dive Tables are the most used by divers at this time. They are the result of years of study, experimentation and field experience. The U.S. Navy Dive Tables and all others, are based on one depth diving. They do not account for the fact that scuba divers do not stay at the same depth throughout a dive. For this reason, when using dive tables the diver always takes his deepest depth obtained during the dive, and uses it as if the whole dive was done there.
As stated, the Dive Tables are to prevent Decompression Sickness which results from as divers dive deeper, nitrogen is put into the tissues and blood. To remove the nitrogen the process has to reverse, slowly, so bubbles do not form in the blood stream. The classic example of a gas coming out of solution is opening a carbonated drink. While the cap is on, no bubbles are visible, because of the held in pressure. Pop the top, and the pressure is suddenly reduced with bubbles starting to flow. The bends comes on approximately the same way. If a deep diver relives pressure too suddenly, then uncontrolled bubbles will form in the blood.
Divers should understand that although the classic example of the soda bottle is proper it is still a little basic. It should be realized that a diver, with a bottom time of 60 minutes, took that amount of time to put the nitrogen into the blood. By rising to the surface in two, or three minutes after an one hour down time, the pressure is relieved rather fast. This is the reason that many dive educators teach divers to use a three minute safety stop at ten feet at all dives deeper than fifty feet. When doing a safety stop, there is no reason the diver cannot swim around. If diving from shore swimming around at ten feet for a few minutes helps the diver out-gas, the same way he in-gassed.
An useful example of how the tissues hold nitrogen is to observe a cellulose sponge. When it is dry, it is hard and collapsed, this represents the diver before any dives, called a NEW DIVER. Place the sponge in the water, and it starts to soften, pulling water into itself, this represents the diver's BOTTOM TIME. Bottom time is the time the diver is underwater putting nitrogen into his blood and tissues. The longer the sponge is in the water the more it will pull in. In the same way the longer a diver is in the water the more nitrogen will enter his tissues, based on time at depth. When the sponge is taken out of the water some, drips off, but most stays in. This is the post dive diver. After a time the sponge will start to drain off by gravity. This is its SURFACE INTERVAL, or the amount of time a diver is out of the water between dives. Any time the sponge is placed back in the water, before it has thoroughly dried, it will pull more water back in, replacing the water lost while sitting. This is called a REPETITIVE DIVE, and refers to any dive within 12 hours of an earlier dive. Finally take the sponge out of the water and squeeze all the water out that can be. It will be noticed that no matter how hard the pressure applied it will not return to the way it started. The only way that the sponge will return to its dry hard self is time. After a few hours it will be dry and hard again. The body is the same way. The only way to have all the nitrogen removed is time.
Divers must be familiar with these terms concerning diving:
During every dive made on compressed air, nitrogen is absorbed by the body. How much is absorbed is dependent on the depth, and amount of time spent there. If a quantity of nitrogen exceeds certain amounts, and the diver surfaces without following a strict procedure of stops, decompression sickness will result. All diving performed on Scuba should be made within established no-decompression limits. These dives are called no-decompression dives and many divers are under the mistaken belief that no-decompression dives mean no chance of nitrogen problems, this is very wrong. The dives should be called no-decompression stop dives, because no decompression stop is required of the diver, the diver still needs time to decompress or relive all retained nitrogen.
Following any dive, at least 12 hours is required for nitrogen levels in the body to reach normal. If another dive is made before the 12-hour period, the nitrogen remaining in the body must be taken into consideration. The greater the time between dives the less nitrogen there will be in the body.
To keep divers within the safe limits of nitrogen the dive tables provide all information needed by the diver to dive safely. The USN and most other dive tables are made to be used in a loop, or continuous sequence for no-decompression stop diving. The Dive Tables consist of three tables and each leads to the next. To use them each table is given a number instead of using its name. They are numbered as follows:
Dive Tables By Number
Table One is the No-Decompression Limits (NDL) and Repetitive Group Designation Table showing the maximum times a diver can stay at a set depth, without having to perform a decompression stop. It also provides the group designator, a letter of the alphabet that designates the amount of nitrogen accumulated on the dive. The table also shows the decompression stops at ten feet if they become necessary. This part of the table is for emergency use only and not for planning decompression stop dives. .
|Table Two is the Surface Interval Credit Table. This table credits the diver with time out of the water. While the diver is out between dives, the body is gradually out-gassing nitrogen. After 12 hours the body returns to normal levels. This table with Table 3, are only needed if the diver plans to make a repetitive dive, another dive before the 12-hour period has passed. The times are expressed in hours and minutes, and 2:30 stands for, two hours 10 minutes.|
|Table Three is the Residual Nitrogen Time Table, or Repetitive Dive Time Table. This table shows the residual nitrogen time (RNT), for a given depth and group designator. This time is representative of how along a diver has already been at that depth, based on his repetitive group. When this time is subtracted, from the maximum no-decompression time for the desired depth, the diver gets an adjusted no-decompression time (ANDL). After a repetitive dive, the diver takes his actual bottom time (ABT), plus the residual nitrogen time (RNT), to get his total bottom time (TBT). This time is then used on table one to get a group designator. .|
|When using the dive tables general safety
precautions should be followed:
The tables have a rule on them for the diver who missed a required decompression stop. As long as the diver shows no signs or symptoms of decompression sickness, and has been out of the water less than 10 minutes, the diver may descend again and decompress, using the omitted decompression stop procedure. .
When going up in an airplane pressure decreases. This pressure decrease can also lead to decompression sickness. Any flying more than 1000 feet should not be done until you are a new diver. Commercial airliners are not pressurized to one atmosphere but about 9 psi. For this reason any flying to be done by the diver he should wait a minimum of 12 hours, with some experts saying 24 hours. If you have been doing deep dives, or have been pushing the tables, you should wait 48 hours before flying. This change in the altitude also applies to divers who have to cross mountain ranges. If a diver lives above 1000 feet or has to cross a mountain over that height, he may develop the bends.
If diving in mountains or altitudes above 1000 feet then the use of special tables is needed. There are tables that have been used in altitude diving and are available in areas where that form of diving is done. Altitude diving is beyond the scope of this text book, and should not be attempted unless properly trained.
NOTE: Remember NO DEVICE OR TABLE removes the possibility of decompression sickness! And divers should trust no one who says this system or table prevents it.
NEVER DIVE DEEPER THAN THE PREVIOUS DIVE
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