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When recreational diving, it is always a good idea to plan your dive and include the maximum depth and the allowable dive time. The maximum depth may be the easiest part to determine. Consult a dive site chart or map. Note the depths indicated, or ask the local guide/dive master for details about the recommended depths.

Next, determine the maximum dive time. You have the following factors to consider that will limit your dive time.

  • Exposure to the environment
  • Open circuit gas supply limit
  • NDL for the depth (maximum allowable dive time)
  • Depth limit for your level of training

We will consider each of these one at a time, although the answer will be the most conservative time for any of the above.

Here are a few terms you should know.

  • Maximum operating depth (MOD): This is usually based on the maximum depth at which the nitrox can be used. For air diving, this is not relevant at Open Water 20 depths.
  • Target operating depth (TOD): This is the depth the Open Water 20 program is planning to dive to.
  • Surface air consumption (SAC): This is calculated using the amount of gas consumed over a set time at a set depth.
  • No decompression limit (NDL)
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