1At a glance
- Primary Function
- Calculates no-decompression limits
- Input Data
- Maximum depth, bottom time, surface interval
- Output Data
- No-decompression limit, repetitive group designation
- Key Principle
- Nitrogen absorption and elimination
- Safety Factor
- Conservative estimations for varied physiology
- Common Type
- Recreational Dive Planner (RDP) – table or wheel
- Alternative
- Personal dive computers
2How dive tables work for planning
Dive tables operate on algorithms designed to model how inert gases, primarily nitrogen, enter and exit the body's tissues during a dive. When a diver descends, the ambient pressure increases, causing nitrogen from the breathing gas to dissolve into the blood and tissues. The deeper and longer the dive, the more nitrogen is absorbed. Dive tables provide limits for depth and time that allow a diver to ascend directly to the surface without requiring decompression stops, thus avoiding excessive nitrogen supersaturation.
To use a dive table, a diver first identifies the maximum planned depth and actual bottom time of their dive. Based on these parameters, the table provides the no-decompression limit (NDL), which is the maximum time a diver can spend at that depth without needing decompression stops. For repetitive dives, divers must also account for residual nitrogen from previous dives by using a 'repetitive group' or 'pressure group' designation, which is then applied to subsequent dives to shorten their NDL.
The calculations often involve looking up a 'group' letter after a dive, using this letter to find a 'surface interval credit' after a specific surface time, and then using a new group letter to determine the adjusted no-decompression limits for the next dive. This methodical process ensures that nitrogen accumulation is managed across a series of dives, even when conducted over a short period, as is common on liveaboards.
3Dive tables vs. Dive computers vs. NDL
| Feature | Dive Tables | Dive Computers | No-Decompression Limit (NDL) |
|---|---|---|---|
| Calculation Method | Pre-calculated charts based on maximum depth/time | Real-time, continuous monitoring based on actual profile | Specific time limit at a given depth for direct ascent |
| Flexibility | Less flexible; worst-case scenario for a square profile | Highly flexible; accounts for multi-level dives | A value provided by tables/computers, not a planning tool itself |
| Conservatism | Generally more conservative, especially for multi-level dives | Can be less conservative due to precise tracking; adjustable settings | Determined by the conservative model used by tables/computers |
| Usage | Primary planning tool (historical), backup | Primary planning and in-dive monitoring tool | Critical parameter for safe dive planning |
| Error Source | Misreading charts, rounding up to next depth/time | Battery failure, user error in setting conservatism | Exceeding this limit without proper decompression |
4What dive tables mean for your liveaboard trip
On a liveaboard trip, divers typically undertake multiple dives daily, often for several consecutive days. This scenario, known as repetitive diving, significantly increases the importance of meticulous dive planning and nitrogen management. While most liveaboard divers utilise personal dive computers, the foundational knowledge of dive tables helps in understanding the principles of residual nitrogen and no-decompression limits, enhancing overall dive safety.
Dive tables serve as an excellent backup and educational tool. In the unlikely event of a dive computer malfunction, knowing how to plan a dive using tables can be invaluable. Moreover, divemasters often brief divers on dive plans that align with conservative table principles, even if individual divers are using computers, to ensure safety for the group.
Understanding dive tables also reinforces the importance of proper surface intervals and conservative dive profiles, which are particularly relevant on liveaboards where divers are keen to maximise their time underwater. This knowledge encourages safer diving practices, reducing the risk of decompression sickness and ensuring a more enjoyable and uninterrupted liveaboard experience.
5Types of Dive Tables and Key Parameters
| Parameter/Type | Recreational Dive Planner (RDP) | US Navy Tables | British Sub-Aqua Club (BSAC) Tables | Factors Considered |
|---|---|---|---|---|
| Origin | PADI (based on DSAT model) | US Navy Experimental Diving Unit | BSAC (developed from RNPL) | Ascent rate, depth, bottom time |
| Primary Use | Recreational diving worldwide | Military, commercial, scientific diving | Recreational diving in UK/Europe | Surface interval, repetitive dives |
| Format | Table, wheel, E-RDP | Printed charts, manual calculations | Printed charts | Safety stops, residual nitrogen |
| Conservatism | Moderate, suitable for recreational divers | More conservative, designed for occupational divers | Generally conservative, specific to UK conditions | Altitude, flying after diving |
| Altitude Diving | Specific rules/procedures apply | Dedicated altitude dive tables | Specific rules/procedures apply | Tissue half-times, M-values |
6Common misconceptions
Myth: Dive tables are obsolete because of dive computers. Fact: While dive computers have become the primary tool for dive planning and execution, dive tables remain a crucial educational tool. They teach the underlying principles of decompression theory and nitrogen management, which is fundamental knowledge for any diver. They also serve as an essential backup in case of computer failure.
Myth: Following dive tables guarantees no decompression sickness (DCS). Fact: Dive tables, like dive computers, provide guidelines based on statistical probabilities and models. They significantly reduce the risk of DCS, but no dive plan can guarantee 100% immunity from DCS due to individual physiological variations and environmental factors. Conservative diving practices, proper hydration, and fitness all play a role.
Myth: You only need to use the exact depth and time listed on the table. Fact: Dive tables require divers to always round up to the next deeper depth and longer bottom time listed on the table. This is a critical safety margin. For example, a dive to 18 metres (60 feet) for 27 minutes should be calculated as a 20 metre (70 foot) dive for 30 minutes.
Myth: Once you complete a dive, all nitrogen is gone from your body after a surface interval. Fact: Nitrogen off-gassing continues long after a dive, and the body can retain significant residual nitrogen for many hours, even a full day. Dive tables account for this with repetitive dive groups and specific guidance for flying after diving or multi-day diving. It's why surface intervals are crucial.
FAQ
What is the primary purpose of dive tables?
The primary purpose of dive tables is to provide divers with guidelines for safe ascents by calculating no-decompression limits, thereby helping to prevent decompression sickness (DCS) by managing nitrogen absorption and elimination.
Are dive tables still used by divers today?
Yes, dive tables are still used today, primarily as an educational tool to teach decompression theory and as a backup planning method. While most recreational divers use dive computers for real-time tracking, understanding tables is a fundamental part of dive training.
How do dive tables account for multiple dives in one day?
Dive tables account for multiple dives by using a 'repetitive group' or 'pressure group' designation. After each dive, a diver is assigned a group letter that reflects their residual nitrogen. This letter is then used with a surface interval credit table to adjust the no-decompression limits for subsequent dives.
What is a no-decompression limit (NDL) in relation to dive tables?
The No-Decompression Limit (NDL) is the maximum amount of time a diver can spend at a given depth and ascend directly to the surface without needing mandatory decompression stops. Dive tables provide these NDLs based on pre-calculated models.
Are dive tables more conservative than dive computers?
Generally, dive tables are more conservative than dive computers, especially for multi-level dives. Tables assume a 'square profile' (spending the entire bottom time at the maximum depth), whereas computers track actual depth changes in real-time, often allowing for longer bottom times.
What happens if a diver exceeds the limits on a dive table?
Exceeding the limits on a dive table without proper decompression procedures significantly increases the risk of decompression sickness (DCS). If an NDL is accidentally exceeded, a diver may need to perform mandatory decompression stops or follow emergency procedures as outlined in their training.