Dives With/Without Decompression Stops
In the Bühlmann 86 decompression table there are depth/time limit values that allow the diver to climb without having to perform decompression stops . Values above these limits are represented in the yellow zone.
For each depth there is a maximum Bottom Time, within which the tissues do not accumulate nitrogen in sufficient quantity that can cause decompression accidents (nitrogen does not exceed the limits supported by the body). For reasons of prevention and safety it is mandatory to respect the speed of ascent and the safety stop, of which we will talk later.
This does not mean that a certain amount of surplus nitrogen does not remain dissolved in the tissues. This nitrogen will only be completely eliminated some time after diving (between 2 and 24 hours).
IN THE CONTEXT OF RECREATIONAL DIVING, DIVES SHOULD NOT HAVE DECOMPRESSED DECOMPRESSION STOPS
DECOMPRESSION LEVELS (RESIDUAL NITROGEN)
As we have already seen, during the dive the nitrogen slowly accumulates in the diver’s body. However, its release through breathing cannot always be carried out in the same gradual manner, since the diver cannot climb as slowly as would be desirable. The solution found for the return to the surface, in order to release excess nitrogen before it exceeds the limit supported by the body, is as follows:
- Meet a fixed ascent speed.
- Make stops (if necessary) for a certain time and at certain depths (landings).
In table Bühlmann 86, for dives carried out up to 700m above sea level, levels of 3 in 3 meters ( 3, 6, 9 and 12m depth) were defined. For dives between 701 and 2500m altitude, the levels are at 2, 4, 6, 9 and 12m .
WHENEVER THE BOTTOM TIME EXCEEDS THE TIME LIMIT FOR THE MAXIMUM DEPTH REACHED, THE DECOMPRESSION STOPS BECOME MANDATORY, AND IT IS NOT POSSIBLE TO RISE DIRECTLY TO THE SURFACE
The diver must always carry with him a decompression table, either for the planning of the dive or for use during the dive, whenever there is a need to make a quick and easy consultation.
The following table, taken from Table Bühlmann 86, shows the maximum limits of Background Time according to depth, so as not to exceed the nitrogen limit supported by the tissues. In these conditions, to return to the surface, only one safety stop of 1 min to 3m will be required .
|Maximum Depth||Limit Background Time||Maximum Depth||Limit Background Time|
|9 m||335 mins||27 m||20 mins|
|12 m||125 mins||30 m||17 mins|
|15 m||75 mins||33 m||14 mins|
|18 m||51 mins||36 m||12 mins|
|21 m||35 mins||39 m||10 mins|
|24 m||25 mins||42 m||9 mins|
At 9 METERS DEPTH THERE IS A TIME LIMITATION OF 5H 35MIN
RESIDUAL NITROGEN GROUP (GG)
It is the amount of excess nitrogen dissolved in the diver’s tissues after its arrival on the surface. In the decompression table, this amount of nitrogen is represented by a letter.
We have already seen that, after finishing the dive, certain amount of nitrogen remains dissolved in the tissues, which will only be completely eliminated some time later (between 2 and 24 hours), as can be seen in table IS (Surface Interval).
VALUES NOT INDICATED IN TABLES
The actual values of The Bottom Time and Dive Depth may not match the values shown in the table. When this happens, values immediately higher than the actual values of recorded time and depth should always be considered.
- If the Depth is 19 meters and the Background Time is 42 minutes, the choice in the table is: 21m/50 minutes .
- If the Depth is 31m, the choice in the table is: 33m .
- If for the Depth of 24m the Background Time is 26 minutes, the choice in the table will be: 30 minutes .
DIVES WITH EXAGGERATED PHYSICAL EFFORT
In dives with exaggerated physical exertion take time immediately higher than real background time.
DIVING IN VERY COLD WATERS
In dives in very cold waters take the time immediately higher than the actual bottom time.
DECOMPRESSION AFTER DIVING
After returning to the surface, having taken a dive with or without decompression stops, it cannot be considered that the diver has completely eliminated the excess nitrogen dissolved in the tissues. It can only be said that the amount of nitrogen dissolved in the body is no longer so likely to cause accidents.
In case the maximum depth is 16meters and the bottom time of 52 minutes, the Residual Nitrogen Group with which the diver finishes the dive is F (GR=F) .
We have already seen that the diver can only be considered completely free of all residual nitrogen between 2 and 24 hours after the dive. This gives rise to some important restrictions during this period of time: violent physical efforts, flying, mountain climbs and successive dives.
If it is necessary to travel by plane (pressurized or not) or climb a mountain after a dive, the diver will be subject to pressures below the atmospheric pressure at sea level, for which the tables have been calculated. Therefore, it is necessary to wait the time necessary to eliminate the excess nitrogen, at the level of the place where you took the dive.
For safety reasons, even after a dive without decompression stops, you should only fly after 12 hours have passed . Also for safety reasons, mountain trails should be avoided during the first hour after diving. If this happens inadvertently, as soon as the error is discovered, it should be dropped immediately.
Although this situation is difficult to happen in mainland Portugal, in the Algarve, for example, in less than an hour the diver may be climbing the Serra de Monchique (900 meters of altitude) after having been diving in Lagos or the Stª Clara Dam (Ourique). On the other hand, in the Azores and Madeira this problem arises with some frequency and with higher altitudes.
It is also important that the diver does not make violent physical efforts after diving, because increased circulatory rhythm, plus increased CO2 production, can lead to the Decompression Accident.