As a safety measure, it is recommended when using dive tables to plan for depths beyond your planned depth. You should write these calculations down on a slate and have them available on the dive. If the planned depth is accidentally exceeded, you can refer to the slate. You can find out what the adjusted allowable dive time is for the new depth. Note: Do not plan to dive deeper. This step is designed to help you in case you accidentally exceed your planned depth .
If using a PDC, you do not need to worry about the calculation of the NDL because this is completed by the PDC. The PDC calculates the allowable dive time every 2 or 3 seconds while underwater. Although the PDC does calculations underwater, it is still recommended to plan the dive on your PDC beforehand. You need to determine what the limiting factor will be and which limit may be the first to consider: Will it be the NDL or the gas supply?
The PDC has a few limitations.
- The PDC will not be able to account for the gas supply unless the PDC has a high-pressure sensor link.
- The PDC will not be able to account for previously completed dives that the PDC has not logged.
- PDCs cannot account for the tissue loading of another diver. It must be used by only the same person within 24 hours.
- It is recommended not to use any PDC that has not cleared from the no-fly time and/or has a dive log history within 24 hours of your use.
- PDCs (and dive tables) do not consider your individual physical diving fitness and hydration levels.
The most common reason a PDC fails is a spent battery. Make sure a battery check is part of your pre-dive procedure. If your PDC does fail and the screen goes blank, call off the dive and surface normally with your dive buddy. Since a failed battery clears the data on your PDC from previous dives, the best option is to stay out of the water for at least 12 hours.
Note: You may dive many times a day, provided you dive within the limitations of all the factors and stay within the NDL limits.