As a diver descends into the depths, the water pressure increases. This causes nitrogen gas from the breathing air to dissolve into the diver’s blood and tissues.
Coming back up too quickly allows bubbles to form as the gas comes back out of the solution. This condition is known as decompression sickness or “the bends.” It can cause severe injury and even death.
To avoid this, divers must follow no decompression limits (NDLs). These indicate the maximum times divers can stay at various depths before having to make decompression stops on ascent. Understanding NDLs is essential for safe diving.
This comprehensive guide examines the top expert insights on what no decompression limits are and how to use them to dive safely.
What is No Decompression Limit?
No decompression limit defines the maximum amount of time a diver can safely spend at a particular depth before having to make decompression stops during ascent.
To make it easier to understand here are some pointers:
- NDLs are calculated to allow safe levels of nitrogen to build up in tissues.
- Exceeding the no decompression limit means a decompression stop is required before surfacing.
- Each combination of depth and bottom time has a specific NDL.
- Dives within NDLs are called “no-decompression dives.”
- Going over the limit requires adding decompression stops to allow excess nitrogen to off-gas.
NDLs provide divers with safe time boundaries for their depth and profile. Staying within these limits prevents decompression sickness. Exceeding them risks bubble formation during ascent.
How No Decompression Limits Are Determined
No decompression limits are calculated based on dive depth and breathing gas:
- At greater depths, higher water pressure causes more nitrogen to dissolve into tissues. This means shorter NDLs.
- NDLs also depend on the breathing gas. Enriched air nitrox allows longer no decompression times.
- Limits are set conservatively to reduce DCS(decompression Sickness) risk. Most tables have built-in safety buffers.
- Dive computers use complex algorithms to track tissue saturation and calculate personalized NDLs.
- tables/computers factor in various parameters like exertion level, water temperature, and ascent rates.
NDLs have been determined through extensive research on nitrogen loading and offgassing. While not foolproof, following established limits significantly reduces decompression sickness risk.
Common No Decompression Limit Tables
No decompression limits are commonly presented in standardized tables created by dive organizations and researchers. Some popular NDL tables include:
U.S. Navy Diving Manual Air Tables
- Based on Haldane’s calculations of nitrogen absorption in body tissues.
- Standard no decompression times are used worldwide.
- Provides limits for single dives and repetitive dives.
- Used in commercial and military diving.
PADI Recreational Dive Planner
- Created by the Professional Association of Diving Instructors.
- Conservative limits for recreational diving.
- Maximum depth of 40m for air dives.
- Adjustments for age, fitness level, fatigue.
- tables for air, nitrox, and repetitive diving.
You can download the planner here.
Divers Alert Network (DAN) NDLs
- Research-based tables from DAN dive medicine experts.
- Has air dive tables and nitrox tables.
- Provides limits for altitude diving as well.
- Used in DAN dive computers and apps.
There are various NDL tables catered to different needs. Most reputable training agencies publish standard tables for divers to use.
How to Use No Decompression Limit Tables
Using no decompression limit tables is straightforward:
1. Identify the dive depth – Find the depth you plan to dive along the left column.
2. Read across the row – Check the no decompression time limit in minutes at that depth.
3. Plan max dive times – Stay within the limit for a no-decompression dive.
4. Adjust for conditions – Reduce NDLs for cold water, high exertion, etc.
5. Monitor depth/time – Use a dive watch or computer to track the depth profile and times during the actual dive.
6. Ascend if approaching limit – Start ascent with 3-5 minutes of NDL time remaining.
Following the NDL table values will help you complete dives safely within no-decompression limits. Adjust them conservatively and monitor your exposures.
No Decompression Limits for Common Depths
No decompression times get significantly shorter with increasing depth.
Here are sample NDLs for popular recreational depths on air.
| 10m/30ft | 205 minutes |
| 15m/50ft | 109 minutes |
| 20m/60ft | 56 minutes |
| 25m/80ft | 36 minutes |
| 30m/100ft | 25 minutes |
| 35m/115ft | 19 minutes |
| 40m/130ft | 15 minutes |
As a rule of thumb, no decompression limits approximately halve for every 10m/30ft of depth. Dive profiles, gas mixes, and conservatism factors also impact limits.
No Decompression Limits vs. Decompression Dives
While a no-decompression dive is simpler and safer, decompression dive allows more bottom time:
No decompression
Shorter bottom times but faster ascents.
Decompression dive
Added decompression stops allow longer depths. Deco planning is a must, where software plans stops and gas mixes for stage decompression. Deco diving also requires more training and preparation for emergency procedures.
Note: Safety stops are necessary for any dive exceeding 30ft, even if within NDLs.
No decompression dives are best for beginners and conservative divers. Tech divers use decompression to expand bottom times for deeper, longer dives. Both utilize dive tables or dive computers for limited guidance.
Adjusting To No Decompression Limits
Several factors can alter no decompression limits, requiring more conservative dive planning:
- Age – Older divers have shorter NDLs due to slower nitrogen elimination.
- Obesity – If you are overweight, increased fatty tissues require longer offgassing.
- Cold water – Colder temperatures lengthen nitrogen absorption.
- Vigorous exercise – Harder exertion loads more nitrogen into muscles.
- Repetitive dives – Residual nitrogen absorbs faster on subsequent dives.
- Diving fatigue – Tired divers absorb more nitrogen.
Get your personal NDLs from an experienced instructor based on your age, fitness, and other variables. Stay on the conservative side when diving to limits.
No Decompression Limits on Nitrox
Breathing nitrox with added oxygen allows longer no decompression times:
- Less nitrogen – Oxygen displaces some nitrogen in the mix.
- More efficient – Less nitrogen loading for same equivalent air depth (EAD).
- Longer NDLs – Increased oxygen extends limits at a given depth.
- Absorption differences – Nitrox tables use equivalent air depth to determine NDLs.
- Gas planning – Match nitrox blends to planned dive depths for optimal benefit.
- Training required – Special training and certification needed to use mixed gases.
For example, nitrox 32 would allow over 2 hours at 20m compared to just 56 minutes on air. Consulting nitrox tables is required for safe limits.
Exceeding No Decompression Limits
Occasionally dives exceed planned limits because of some reasons like getting absorbed in underwater sights, currents pushing you deeper temporarily, forgetting to monitor depth and time, problems forcing you to stay down longer or your computer giving overly generous NDLs.
If you go over your limit:
- Don’t panic – Stay calm and think through next steps.
- Do safety stop – Stop at 5m for 3-5 min if not already required.
- Slow ascent – Ascend slower than 10m/min for the rest of the way up.
- Oxygen – Breathe oxygen at the surface to quicken offgassing if available.
- Call DAN – Consult Divers Alert Network experts for guidance.
- Observe closely – Watch for any DCS symptoms arising in the next hours.
While not ideal, minor NDL exceedances with proper response rarely cause any issues. Just learn from the experience for future dives.
Key Takeaways
Following a few key points on no decompression limits will help ensure safe, problem-free diving:
- Appreciate NDL tables/computers – They represent decades of data on nitrogen loading.
- Stay well within limits – Allow a cushion rather than diving to the edge.
- Adjust limits conservatively – Reduce NDLs for any adverse conditions.
- Monitor depth and time closely – Know where you are relative to limits.
- Make safety stops routine – Even if within NDLs for extra prudence.
- Ascend slowly – All ascents should be at a slow, controlled and gradual pace.
- Get trained on nitrox use – If you want to extend bottom times with enriched air.
- Analyze dives afterward – Learn from any exceedances or close calls.
No decompression limits provide science-based guidance for managing nitrogen exposure. Follow them closely and dive conservatively for maximum safety.