A dive separated from the previous one by more than 8 hours is called a "single dive" and takes place normally.
Here is the diagram of a dive; the main parameters of this dive are the following:
* Dive time (A-B) is the interval between the dive and the moment the diver ascends to 15 m./min. .
* The maximum depth is the greatest depth reached during the dive.
* The total ascent time (B-D) is the distance B-C divided by 15 m/min, plus the time for the stops (the stairs on the diagram), plus 30 seconds between each stop and between the last one and the surface.
To calculate the stop time to be made using the ND90 tables, we look up the maximum depth of the dive in the table, associate the dive time (A-B) with it, and this gives us the depth and the stop time to be made.
A succesive dive is a second dive that occurs at least 15 minutes and no more than 12 hours after the first dive. This second dive must not be deeper than the first.
Tissue nitrogen desaturation can last for about 12 hours after a dive. So if a diver re-swims during this interval, he leaves with excess nitrogen in his tissues. For this reason, he must increase the decompression stops on his second dive.
This increase is calculated using the following parameters:
* The Dive Group of the 1st dive
* The time interval between the 2 dives
* The depth of the 2nd dive
A diver gets out of the water after a first dive at 10:00 am and his GPS is H. He dives again at 12:00 pm to a depth of 30 meters. After 20 minutes, he decides to go back up.
To calculate the stops for this 2nd dive, you must:
* Associate the GPS of the 1st dive with the interval between the exit of the 1st dive and the launch of the 2nd dive, in table 2: we obtain 0.98.
* Associate this last value with the depth of the 2nd dive, in table 3: we obtain an increase of 14 minutes.
* Repeat Table 1 and calculate the decompression time for the second dive by adding this increase to the dive time: 20' + 14' at 30 m. gives 17 minutes of decompression time at 3 meters.
A successive dive must be prepared in advance.
Depending on the add-on and the depth of the dive, you can set a dive time that leaves you enough air for the ascent.
Let's take the previous example: if the diver does not plan his dive and stays 30 minutes at 30 meters.
At the moment of ascending, he takes his table, adds his increment to the dive time and obtains 1 minute of stop at 6 meters and 31 minutes at 3 meters! He then says to himself that he was right to install a safety tank on the hanger...
In the case of a consecutive dive (e.g. when a diver re-enters the water to unhook the anchor), the stops will be calculated from the maximum depth of the 2 dives and the addition of the 2 dive times.
This curve defines the dives for which, depending on their depth and dive time, it is not necessary to make a decompression stop.
Dives below the curve do not require a decompression stop.
Examples: an 18 minute dive at 24 meters, or 1 hour at 15 meters...
Dives above the curve require one or more decompression stops.
Examples: 57 minutes at 20 meters, 7 minutes at 40 meters.
Some values of this curve are to be known by heart in the federal examinations:
10 meters : 5h30min
12 meters : 2h15min
15 meters : 1h15min
20 meters : 40 minutes
25 meters : 20 minutes
30 meters : 10 minutes
35 meters : 10 minutes
40 meters : 5 minutes