Dive Planning Terms and Concepts
Before starting to look at the dive tables themselves, we need to review some terms and concepts associated with dive planning, residual nitrogen, and repetitive dives.
The next part of the tutorial will address the dive planning worksheet and dive tables in detail. The dive planning worksheet is used to pre-plan the dive profile, which is simply a statement of the depth and duration of a dive. For the sake of being cautious, planning is based upon the most extreme cases — depth is taken as the maximum depth reached during the dive and duration is the total time that the diver is in the water. This "square profile," then, assumes that the diver spends all of their time at the maximum depth of the dive.
A dive profile is expressed as depth/time, so that a 60 ft. dive for 45 minutes would be written 60/45. Divers need to take care to note what unit of measurement is being used to express depth; a 30/27 dive has very different ramifications depending upon whether the 30 is measured in feet or meters!
As we will discuss later, the dive tables are all about estimating the amount of residual nitrogen in the body. To do this, divers are assigned a letter group based upon the depth and duration of the dive; the higher the letter group, the more residual nitrogen in the body. Time on the surface and the dive profile of subsequent dives all affect the letter group. This will be seen clearly in the next part of the tutorial.
Other important terms for dive planning include:
Actual Bottom Time (ABT) or Actual Dive Time (ADT): The duration of the dive; this is the amount of elapsed time from getting into the water until getting out of the water. (This definition is conservative; some sources suggest that you do not need to count the time spent at the Safety Stop. The diver should follow the practice of their certifying agency.)
Maximum Bottom Time (MBT) or Maximum Dive Time (MDT): Also called the decompression limit or Maximum Allowable Dive/Bottom Time, this is a measure of how much time the diver can spend at a given depth without having to make decompression stops.
Residual Nitrogen Time (RNT): A measure of how much nitrogen remains in the body; the diver needs to account for the RNT in the dive plan as if they had already spent this amount of time at a given depth of a repetitive dive. For the first dive, RNT = 0.
Surface Interval Time (SIT): The time spent on the surface between dives. During the SIT, the body eliminates nitrogen. Note: If the SIT is less than 10 minutes, the second dive needs to be considered as just a continuation of the first dive. The SIT should be at least an hour.
Adjusted Maximum Bottom Time (AMBT) or Adjusted Maximum Dive Time (AMDT): For a repetitive dive, this is the actual amount of time that the diver can safely stay in the water at a given depth. The AMBT/AMDT is calculated as the maximum time that would be allowed at this depth if this was a first dive (i.e., MBT or MDT), corrected for the level of residual nitrogen remaining in the body (i.e., RNT). In arithmetic terms:
Adjusted Maximum Bottom/Dive Time = Maximum Bottom/Dive Time - RNT Total Bottom Time (TBT) or Total Nitrogen Time (TNT): For a repetitive dive, this is the sum of the residual nitrogen left over from previous dives prior to starting this one (i.e., RNT) and a measure of the amount of residual nitrogen accumulated on this dive (i.e., ABT or ADT). In arithmetic terms:
Total Bottom/Nitrogen Time = RNT + Actual Bottom/Dive Time On a final note, although each individual dive should be planned within the no-decompression dive limits, sport divers should usually plan multiple dives so that the deepest one is first and each subsequent dive is more shallow than the previous one. In this way, the body can be ridding itself of excess nitrogen even while underwater.