The "120 Rule" in scuba diving is a simplified method to estimate the No Decompression Limit (NDL) at a specific depth, especially around 70-80 feet. It states that NDL + Depth = 120.
Explanation
This rule of thumb was used as a quick mental calculation during dives using Imperial units (feet). It's based on the observation that at certain depths, the sum of the depth in feet and the approximate No Decompression Limit in minutes would be around 120.
How it Works:
- Depth: The current depth of the dive in feet.
- NDL: Estimated No Decompression Limit in minutes.
To estimate the NDL, you subtract the depth from 120. For example:
- At 80 feet: 120 - 80 = 40 minutes (approximate NDL)
- At 70 feet: 120 - 70 = 50 minutes (approximate NDL)
Important Considerations:
- Approximation: The 120 Rule is a very rough approximation and should NOT be used as a replacement for dive computers or dive tables.
- Conservative Diving: Always err on the side of caution and dive conservatively.
- Dive Tables/Computers: Modern dive computers and dive tables provide far more accurate NDL calculations, taking into account various factors like ascent rate, previous dives, and altitude.
- Modern Diving Practice: The 120 rule is outdated and rarely used due to the availability of more accurate methods. Reliance on this rule alone is not recommended for safe diving practices.
Example
Let's say you are diving at a depth of 75 feet. Using the 120 rule:
120 - 75 = 45 minutes.
This suggests an approximate NDL of 45 minutes. However, you should always consult your dive computer or dive tables for the accurate NDL based on your specific dive profile.
Conclusion
The 120 rule offers a simple historical insight into how divers used to estimate NDLs. However, it's crucial to prioritize using modern, accurate tools and techniques for dive planning and execution to ensure safety.
