Introduction to Dive Tables
Dive tables are tools used by scuba divers to plan dives and avoid decompression sickness (DCS), also known as "the bends". They provide a visual representation of how nitrogen is absorbed and released by the body at different depths and over varying periods. Before the advent of dive computers, dive tables were the primary method for determining safe dive profiles.
The fundamental principle behind dive tables is based on the understanding that nitrogen, an inert gas, is absorbed into the body's tissues under pressure. The deeper you go and the longer you stay, the more nitrogen your body absorbs. Ascending too quickly doesn't allow the nitrogen to be released slowly enough, potentially leading to the formation of bubbles in the bloodstream and tissues, causing DCS.
Dive tables are typically based on a mathematical model that divides the body into theoretical "compartments", each with a different rate of nitrogen absorption and release. These compartments are used to estimate the amount of nitrogen in your body at any given time during a dive.
It's crucial to understand that dive tables are based on averages and assumptions. Individual physiology, fitness level, hydration, and other factors can influence nitrogen absorption and release. Therefore, dive tables should be used conservatively, and divers should always prioritise safety.
Calculating No-Decompression Limits
The core function of a dive table is to determine the no-decompression limit (NDL) for a given depth. The NDL is the maximum amount of time you can spend at a particular depth without requiring mandatory decompression stops during your ascent. Exceeding the NDL significantly increases the risk of DCS.
Here's a simplified example of how a dive table might be used (note: specific values vary between different table versions):
- Determine your maximum depth: Let's say you plan to dive to 18 metres.
- Consult the dive table: Find the row corresponding to 18 metres.
- Find the NDL: The table will show an NDL for 18 metres, perhaps 56 minutes. This means you can stay at 18 metres for up to 56 minutes without needing decompression stops.
- Plan your dive: Ensure your bottom time (the time spent underwater) does not exceed the NDL. If it does, you'll need to plan for decompression stops.
Dive tables also provide information on surface intervals – the time spent on the surface between dives. Surface intervals allow your body to off-gas nitrogen, reducing the risk of DCS on subsequent dives. The longer the surface interval, the more nitrogen is released.
Understanding Group Designators
After a dive, you'll be assigned a pressure group, which represents the amount of residual nitrogen in your body. This group designator is crucial for planning repetitive dives. The dive table will have a section to determine your pressure group after a dive based on your depth and bottom time. This is then used to calculate the adjusted NDL for your next dive.
Understanding Residual Nitrogen
Residual nitrogen is the amount of nitrogen remaining in your body's tissues after a dive. It's a critical factor in planning repetitive dives because it affects how much additional nitrogen your body can absorb on the next dive.
Dive tables account for residual nitrogen by using the pressure group system. After each dive, you consult the table to determine your pressure group. This group then corresponds to a specific amount of residual nitrogen. When planning your next dive, you use this information to calculate a reduced NDL, effectively subtracting the nitrogen already in your system from the amount you can safely absorb.
For example, if you are in pressure group 'E' after your first dive, you would consult the table to find the adjusted NDL for your planned depth on the second dive, taking into account the residual nitrogen associated with group 'E'. This ensures that you stay within safe limits and minimise the risk of DCS.
Divemaster recommends careful planning and conservative diving practices, especially when using dive tables for repetitive dives. Always double-check your calculations and consider the individual factors that can influence nitrogen absorption.
Using Dive Computers for Real-Time Monitoring
Dive computers are electronic devices that continuously monitor depth, time, and other dive parameters to calculate nitrogen absorption and remaining no-decompression time. Unlike dive tables, which rely on pre-calculated values, dive computers provide real-time information based on your actual dive profile.
Here are some key features and benefits of using dive computers:
Real-time calculations: Dive computers constantly update your NDL based on your current depth and time, providing a more accurate assessment of your nitrogen loading than dive tables.
Ascent rate monitoring: Dive computers monitor your ascent rate and provide warnings if you ascend too quickly, helping you avoid DCS.
Safety stops: Dive computers automatically calculate and display required safety stops, which are short pauses at specific depths during ascent to allow for further nitrogen off-gassing.
Dive logging: Most dive computers store dive data, including depth profiles, bottom time, water temperature, and other information, allowing you to review your dives and track your nitrogen loading over time.
Multi-gas support: Many dive computers support multiple gas mixes, such as nitrox, allowing for extended bottom times and reduced nitrogen narcosis.
Dive computers use sophisticated algorithms to model nitrogen absorption and release in different tissues of the body. These algorithms are based on the same principles as dive tables but are more complex and can account for variations in depth and time more accurately.
While dive computers offer significant advantages over dive tables, it's essential to understand their limitations. Dive computers are not foolproof and can fail. Divers should always be familiar with their computer's operation and have a backup plan in case of malfunction. It's also crucial to dive conservatively and avoid pushing the limits of your computer.
Comparing Dive Table and Computer Approaches
Both dive tables and dive computers have their advantages and disadvantages. Dive tables are simple, reliable (as long as you have the table itself), and don't rely on batteries or electronics. They are also relatively inexpensive. However, they are less flexible than dive computers and require careful pre-dive planning. Dive tables assume a square dive profile (descending directly to the maximum depth and staying there), which is rarely the case in real-world diving. This can lead to more conservative dive plans and shorter bottom times.
Dive computers offer greater flexibility and accuracy by continuously monitoring your dive profile and adjusting your NDL accordingly. They can also provide valuable information, such as ascent rate and safety stop recommendations. However, dive computers are more expensive than dive tables and require batteries. They can also be more complex to use and understand. Furthermore, divers can become overly reliant on their computers, potentially neglecting fundamental dive planning skills.
Ultimately, the choice between dive tables and dive computers depends on individual preferences, diving style, and budget. Many divers use both, using dive tables for pre-dive planning and dive computers for real-time monitoring. Regardless of which method you choose, it's essential to understand the principles of decompression theory and dive conservatively.
For more information, learn more about Divemaster and our services.
Safety Considerations for Both Methods
Regardless of whether you use dive tables or dive computers, safety should always be your top priority. Here are some essential safety considerations for both methods:
Dive within your limits: Never exceed your training or experience level. Start with shallow, easy dives and gradually progress to more challenging dives as you gain experience.
Plan your dive: Always plan your dive in advance, considering depth, bottom time, gas supply, and potential hazards. Share your dive plan with your buddy and ensure everyone understands the plan.
Check your equipment: Before each dive, thoroughly inspect your equipment to ensure it is in good working order. Pay particular attention to your regulator, BCD, and dive computer (if using).
Monitor your air supply: Regularly check your air gauge to ensure you have enough air to complete the dive safely.
Ascend slowly: Ascend at a safe rate (typically 9 metres per minute) to allow for nitrogen off-gassing. Use your dive computer or depth gauge to monitor your ascent rate.
Make safety stops: Perform a safety stop at 5 metres for 3-5 minutes on every dive, regardless of whether your dive computer requires it. This allows for further nitrogen off-gassing and reduces the risk of DCS.
Stay hydrated: Dehydration can increase the risk of DCS. Drink plenty of water before and after diving.
Avoid strenuous exercise: Avoid strenuous exercise before and after diving, as it can increase nitrogen absorption and the risk of DCS.
Be aware of your physical condition: Factors such as fatigue, illness, and alcohol consumption can increase the risk of DCS. Avoid diving if you are not feeling well.
- Consider altitude: Diving at altitude requires special considerations due to the lower atmospheric pressure. Consult altitude diving tables or use a dive computer that is designed for altitude diving.
By following these safety guidelines and using dive tables or dive computers responsibly, you can minimise the risk of DCS and enjoy safe and rewarding diving experiences. Remember to consult with a qualified dive instructor for proper training and guidance. You can also find answers to frequently asked questions on our website.