Scuba diving is one of the most exciting things you will ever do. Becoming a scuba diver is great fun.
When we scuba dive we make decisions, and those decision must be based on accurate information. That is why we need to learn the classroom material - - but no one ever became a scuba diver while sitting in a classroom nor while training on the internet. Your diver comfort and skill will be determined by the quality of your open water training.
This tutorial does not replace your open water diver training manual nor does it replace your scuba instructor. It is intended to make this one subject easier to learn and to understand. Then get ready to have fun.
Step One, The Basics
While the diver is underwater excess nitrogen gas is absorbed through the lungs into the blood and other body tissues. The deeper and longer the dive, the greater the amount. If too much nitrogen becomes absorbed, roughly twice as much as normal, the diver must not ascend to the surface without first pausing in shallow water. The pause, called a decompression stop, allows some nitrogen to be safely removed from the body tissues and eliminated through breathing, the same way it entered. Sport divers limit their dives so that they do not have to make these pauses, and then they often pause in shallow water anyway for added safety. To disregard this procedure would put one at risk of having the nitrogen form gas bubbles within body tissues; a serious condition called Decompression Sickness or The Bends.
It is easy to stay within safety limits, because every depth has a matching maximum time called its No-decompression Limit. A dive to 50 feet for example, has a maximum time limit of 70 minutes (using Doppler ultrasound modifications of the Navy tables). If the dive was for only 45 minutes, would one be safe diving to 50 feet a 2nd time, staying for the full 70 minute maximum? Absolutely not! Being safe to ascend to the surface does not mean the amount of nitrogen within body tissues has been reduced to surface normal. That takes up to 12 hours. Until then one must allow for the extra nitrogen, called residual nitrogen, when planning a repetitive dive.
There are several ways to safely make a dive within 12 hours of a previous dive. One very simple way is to limit the depth of the repetitive dive to shallower than 25 feet. This works because depths shallower than 25 feet have very large no-decompression limits. Another method is to consider the repetitive dive as a continuation of the previous dive. Using the 50ft/45min example above; A 70 minute limit minus the 45 minutes of the dive would allow a repetitive dive to a maximum depth of 50 feet for a maximum time of 25 minutes. Although this method is very easy to use, it is excessively conservative. While on the surface between dives some extra nitrogen is being eliminated, and we have not given “credit" for this reduction.
Other methods give credit for the nitrogen eliminated during the surface interval or time between dives. One of these is the use of an electronic instrument, the dive computer - highly recommended and discussed as a separate subject.
Another method is the use of the Navy tables.
Step Two, The Tables
Tables are available in different forms. Some require starting at the top of the table, some on the left, or right, and some are circular, having you start on the outer ring. In all forms, the concept is the same and, in many, even the numbers are the same.
We advise using the U.S. Navy Dive Tables that have been updated with improved, conservative limits based on recent Doppler ultrasound studies. This new version is published on a plastic, waterproof logbook page by Scuba Schools International.
Follow along with your copy of the SSI/Navy tables as we explain how it is used. If we were to plan actual rather than practice dives we would include safety stops at 15 feet for from one to three minutes at the end of each no-decompression dive.
We start by knowing the depth and bottom time of the first dive. Depth is the deepest point of the dive, and bottom time is all the time underwater except for the final ascent.
Knowing depth and bottom time, we determine the amount of the diver's excess nitrogen. This amount is given a letter from A to Z, called a repetitive group designation letter.
Using your tables, look up a 55ft./22min. dive. You will find that this dive produces a repetitive group letter of “E". First locate the “Depth" column in the upper left corner. Looking down the column you will not find a 55 ft. row, so continue looking down to the next greater depth. Use the 60 ft. row. Moving across the row to the right you will see the number 50. You are being reminded that 50 minutes is the no-decompression limit for this dive. Continue looking across. You will not find 22 minutes, so continue to the next greater time. At 25 minutes look down to the column bottom. The repetitive group letter “E" represents the amount of excess nitrogen absorbed during the dive.
Next we determine the reduction in nitrogen during the surface interval. For example, a diver who ends the first dive in repetitive group “E" and waits two hours (2:00) before making a second dive, will be making the repetitive dive in group “C". To determine this for yourself; locate the “E" group on the diagonal line in the center of the table and look to the left. The first box shows “0:10 over 0:54" and is for surface intervals between 10 minutes and fifty four minutes. The third box “1:58 over 3:22" is the one we need for a 2 hour surface interval. Now look down the column. Repetitive group letter “C" represents the amount of excess nitrogen still present after two hours.
Finally we convert the repetitive group letter into something useful; minutes of residual nitrogen time or RNT. RNT is the time you “pretend" to have already spent on the next dive before you start, to account for the extra nitrogen. (Reread that last sentence. It is important to understand it.)
Using your tables, confirm that a group “C" diver, planning a 50 feet dive will have a residual nitrogen time of 21 min. Turn over to Table 3 (Residual Nitrogen Time). Look down the “C" column to the 50 feet row. That is where you will find the numbers “21" over “49" The diver will have to make an allowance for 21 minutes worth of excess nitrogen already absorbed during previous dives, and his no-decompression limit time remaining is 49 min. (21 minutes less than the no decompression limit)
Example; The 50 ft dive (above) has a maximum no-decompression limit of 70 minutes. However, it is a repetitive dive, and you start the dive with an RNT of 21 minutes. You can only dive for a maximum of 49 minutes. The reason is that you started the dive with 21 minutes of excess nitrogen. 49 minutes later you have reached the 70 minute limit. Look at Table #3 and notice that the number 49 is already on the table, just under the 21.
Example; Your 2nd dive is to 40 ft for 35 min, and you started the dive with an RNT of 12 min. You must consider this as though it were a 47 min dive. You had started the dive with 12 min of excess nitrogen. 35 min later you ascend with 12+35, for a total of 47 min of excess nitrogen. If you plan to make a 3rd dive you must calculate your repetitive group based on 47 minutes of bottom time even though you actually spent only 35 minutes diving.
Step Three, Perfect Practice Makes Perfect
1. After a 90ft./15min. dive and a 2:00 surface interval, what is the maximum bottom time allowed on a repetitive 80 ft. dive? Solution: A 90ft./15min. dive results in group “E" which, after 2:00 surface interval, becomes group “C", which has an RNT at 80ft. of 13min. Since the no-decompression limit at 80 feet is 30min., the maximum time allowed is 30-13 or 17min.
2. After a 65ft./15min. dive and a 3:30 surface interval, what is the maximum bottom time allowed on a repetitive 82 ft. dive? Answer: 18min.
3. After a 75ft./18min. dive and a 2:33 surface interval, what is the maximum bottom time allowed for a repetitive 50 ft. dive? Answer: 49min
4. After a 80ft./20min. dive and a 4:20 surface interval, a diver makes a repetitive dive to 60ft. for 15 min. What is the repetitive group at the end of the 2nd dive? Solution: A 80ft/20min dive results in group “E" which, after 4:20 surface interval, becomes group “B", which has an RNT at 60feet of 11 minutes. After 15 minutes of bottom time the diver ascends with 11+15, or 26 minutes worth of nitrogen. To determine the new repetitive group, treat the 2nd dive the same as the first dive of the day, but for 26 min., not 15 minutes. (why?) A 60ft/26min dive results in group “F"
5. After a 70ft./20min. dive and a 2:50 surface interval, a diver make a repetitive dive to 60feet for 15 minutes. What is the repetitive group at the end of the 2nd dive? Answer: Group “G"
6. After a 60ft./30min. dive and a 4:20 surface interval, a diver make a repetitive dive to 60feet for 15 minutes. What is the repetitive group at the end of the 2nd dive? Answer: Group “F”
7. After a 90ft./10min. dive and a 4:20 surface interval, a diver makes a repetitive dive to 55ft. for 15 min. What is the repetitive group at the end of the 2nd dive? Answer: Group “F"
8. After a 120ft/9min dive and a 2:45 surface interval, a diver makes a dive to 90ft/15min followed by another 2:45 surface interval. What is the maximum time for a 3rd dive to 60 feet? Answer: 26min
9. After a 99ft/10min dive and a 5:00 surface interval, followed by a 70ft/10min repetitive dive and a 3:30 surface interval, what is the maximum bottom time for a 3rd dive to 60 feet? Answer: 39min
10. After a 55ft./35min. dive and a 3:00 surface interval, a diver made a 2nd dive to 55 feet for the maximum time allowed. Then, following a 2:00 surface interval, she made a 3rd dive to 55 feet for 10 minutes. What was her repetitive group letter at the end of the 3rd dive? Answer: group “G"
11. After a 70ft./30min. dive, a diver plans to dive 60ft./20min. What is the shortest surface interval needed to keep the 2nd dive within the Doppler No-decompression Limits? Answer: 46min
12. How long must a diver wait after making a 60ft./50min. dive before she can duplicate (repeat) the same dive a 2nd time? Answer: 12:01
Stop at 15 ft for 1 to 3 minutes at the end of each no-decompression dive.
13. After a 90ft./15min. dive and a 3:00 surface interval, what is the maximum bottom time allowed for a dive to 70ft?
14. After a 70ft./25min. dive and a 3:00 surface interval, what is the maximum bottom time allowed for a dive to 70ft?
15. After a 65ft./11min. dive and a 2:00 surface interval, what is the maximum bottom time allowed for a dive to 60ft?
16. After a 50ft./35min. dive and a 3:00 surface interval, what is the maximum bottom time allowed for a dive to 40ft?
17. After a 70ft/20min dive and a 3:50 surface interval, a diver makes a repetitive dive to 60ft for 18 min. What is the repetitive group at the end of the 2nd dive?
18. After a 80ft/10min dive and a 2:50 surface interval, a diver makes a repetitive dive to 70ft for 10 min. What is the repetitive group at the end of the 2nd dive?
19. After a 60ft/25min dive and a 4:20 surface interval, a diver makes a repetitive dive to 50ft for 15 min. What is the repetitive group at the end of the 2nd dive?
20. After a 60ft/10min dive and a 3:15 surface interval, a diver makes a repetitive dive to 60ft. for 15 min What is the repetitive group at the end of the 2nd dive?
21. After a 65ft/22min dive and a 3:00 surface interval, a diver made a 2nd dive to 55 feet for the maximum time allowed. Then, following a 2:00 surface interval, she made a 3rd dive to 55 feet for 10 minutes. What was her repetitive group letter at the end of the 3rd dive?
22. After a 55ft./35min. dive and a 3:00 surface interval, a diver made a 2nd dive to 40 feet for the maximum time allowed. Then, following a 2:00 surface interval, she made a 3rd dive to 40 feet for 10 minutes. What was her repetitive group letter at the end of the 3rd dive?
23. After a 90ft/15min dive and a 4:00 surface interval, a diver made a 2nd dive to 65 feet for the maximum time allowed. Then, following a 3:00 surface interval, she made a 3rd dive to 55 feet for 7 minutes. What was her repetitive group letter at the end of the 3rd dive?
24. After a 80ft./20min. dive and a 3:00 surface interval, a diver made a 2nd dive to 60 feet for the maximum time allowed. Then, following a 2:00 surface interval, he made a 3rd dive to 55 feet for 10 minutes. What was his repetitive group letter at the end of the 3rd dive?
25. After a 40ft./45min. dive and a 2:00 surface interval, a diver made a 2nd dive to 40 feet for the maximum time allowed. Then, following another 2:00 surface interval, he made a 3rd dive to 40 feet for 17 minutes. Then, following another 2:00 surface interval, he made a 4th dive to 40 feet for 15 minutes. What was his repetitive group letter at the end of the 4th dive?
* Stop at 15 ft for 1 to 3 minutes at the end of each no-decompression dive. *
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