Understanding the Recreational Dive Planner (RDP) and eRDPml
The Recreational Dive Planner (RDP) is a critical tool used by scuba divers to calculate no-decompression limits (NDLs) and plan their dives safely. As an instructor candidate preparing for the PADI IDC and IE, understanding how the RDP/eRDPml works, and how it relates to dive computers, nitrogen absorption, and decompression theory, is essential. This blog covers everything you need to know for the IDC Dive Theory exam, focusing on nitrogen absorption, tissue compartments, M-values, dive computers, and important decompression limits.
1. Compartments and Nitrogen Absorption
Nitrogen absorption in the body occurs during a dive as pressure increases. The human body can be modeled as consisting of various theoretical compartments, each absorbing and releasing nitrogen at different rates. These compartments are typically categorized by their half-times, which refer to how long it takes for a compartment to absorb or eliminate half of the nitrogen needed to reach equilibrium with the surrounding pressure.
- Fast compartments (e.g., brain, blood) absorb and release nitrogen quickly.
- Slow compartments (e.g., bones, fat) take much longer.
- PADI Tables based on 14 tissue compartments.
Understanding these differences helps divers and dive planners predict nitrogen absorption and potential risks of decompression sickness (DCS).
2. Tissue Halftime
Tissue halftime is a key concept when working with dive tables and dive computers. It’s the time it takes for a specific tissue (or theoretical compartment) to absorb or eliminate half of the nitrogen in response to pressure changes.
- Short halftime compartments might have halftimes as low as 5 minutes.
- Long halftime compartments could take up to 120 minutes.
These values play a role in determining how long you can safely dive at a given depth and how much surface time is required between dives.
3. M-Value
The M-value represents the maximum amount of nitrogen that a compartment can tolerate at a specific depth before the risk of decompression sickness increases. It’s the limit beyond which excess nitrogen can cause bubbles to form, leading to DCS.
The RDP, eRDPml, and dive computers all use these limits to calculate how long a diver can stay underwater without needing decompression stops. As an instructor, you must be able to explain this to students and help them understand how exceeding M-values can increase their risk of injury.
4. US Navy Tables vs. PADI Recreational Dive Tables
The PADI RDP was adapted from the US Navy Dive Tables but adjusted for recreational diving limits. The US Navy tables are designed for more aggressive diving, often used by military and technical divers who can tolerate higher risks.
Key differences include:
- Depth and time limits are more conservative on the PADI tables to prioritize safety for recreational divers.
- The PADI RDP has simplified rules for no-decompression dives, making it easier for recreational divers to follow.
- PADI Tables based of 14 tissue compartments, US Navy Tables has just 6 compartments.
- PADI Tables credit 60 minute surface intervals, US Navy tables are 120 minute surface intervals.
- PADI Tables designed for multiple dives in the day with shorter surface intervals.
- Navy Tables designed limited dives in a day with longer surface intervals (military use).
Understanding these differences is critical when teaching dive theory to students, as PADI’s focus is always on safety and recreational limits.
5. Dive Computers and Their Relation to Dive Tables
Modern dive computers have largely replaced manual dive tables like the RDP, providing real-time data and adjusting calculations for multiple dives, depth changes, and ascents. However, the underlying principles are still based on the same decompression models and nitrogen absorption rates.
- Spencer Limits and Buhlmann Limits are two common decompression models used by computers.
- Gradient factors are also applied in computers to let divers add conservatism to decompression models. These factors adjust the M-values, in percentages (ie 80%) to account for different tissue compartments and individual diver safety.
- GF low (first number) The percentage of the M-value that establishes the first stop during ascent.
- GF high (second number) The percentage of the M-value that should not be exceeded at any point during surfacing.
As an instructor, you must explain the relationship between dive tables and computers, emphasizing that while computers are useful, understanding the dive tables is still essential for safe diving.
6. Spencer Limits
The Spencer Limits are a set of no-decompression limits used in dive tables and computers, based on studies that set conservative parameters for nitrogen absorption. These limits are particularly important in multi-level diving.
7. Buhlmann Limits
The Bühlmann Limits are used in certain dive computers and are based on the work of Swiss scientist Albert Bühlmann. These limits account for both fast and slow tissues, offering a more comprehensive decompression model for modern diving.
8. EE Washout and 60-Minute Washout
EE Washout (Extended Exponential Washout) and the 60-Minute Washout relate to how nitrogen is eliminated from the body. These models help divers and dive computers estimate the amount of nitrogen still present after a dive and the surface intervals needed before a second dive. The 60-minute washout implies that after one hour on the surface, much of the nitrogen absorbed during a dive is eliminated.
9. General Rules for Using the PADI RDP
When teaching dive theory, it’s essential to drill in the general rules for using the RDP or eRDPml. These rules ensure safety during dive planning:
- Always stay within the no-decompression limits provided by the table.
- Plan dives from the deepest to the shallowest when conducting multiple dives in a day.
- Ensure appropriate surface intervals between dives to allow for nitrogen elimination.
- Use dive computers as a supplementary tool but maintain a strong understanding of manual dive planning techniques.
- Never exceed the maximum depth limits set by recreational standards.
- Presure groups from the PADI RDP are not interchangeable with other organisation’s tables.
- For cold water or strenuous diving plan the dive as if it was 4m deeper than actual depths.
- If planning multiple dives and the ending pressure group is W or X surface intervals need to be 60 minutes or longer.
- If the ending pressure group is Y or Z then surface intervals need to be 3 hours or longer.
- A safety stop for 3 minutes at 5m is recommended for all dives. The safety stop time is not added to dive time when planing multiple dives.
By teaching these rules thoroughly, your students will have a clear understanding of how to plan and execute dives safely, whether they are using traditional dive tables or modern dive computers.
Conclusion
Mastering the use of the RDP and understanding nitrogen absorption, tissue compartments, M-values, and decompression limits are fundamental for any instructor candidate. This knowledge helps you not only pass your IDC and IE but also ensures your future students are well-prepared to dive safely. Whether using the RDP or dive computers, understanding these concepts will enhance your ability to plan dives effectively and reduce the risk of decompression sickness.