Recreational divers love and loathe dive tables in equal measure. To some divers — in an age of affordable wrist-mounted diving computers — they seem like an anachronism, a relic from a bygone era. Other divers consider dive tables a cherished souvenir of their diving coming-of-age, a nostalgic link to the past, and the holder of mathematical concepts that lurk behind the whirring algorithms of their recreational dive computers. But do we really need to learn how to read dive tables in this day and age?
What are dive tables?
In 1908, Scottish scientist John Scott Haldane advanced his pioneering decompression theory. His calculations were initially formalized in the first-ever dive table — a decompression table for the British Admiralty. The dive table then, as now, demonstrated the relationship between depth, time and the absorption and release of gas under pressure from solution in the body.
Haldane’s calculations were just that — calculations. He based his findings on experiments with goats and his theory that the human body comprised of five theoretical tissue compartments. However, through the ongoing march of scientific progress, and the crucible of war, his calculations were refined to produce the U.S. Navy tables. Fledgling recreational scuba divers often used these tables to plan dives as the sport blossomed in the 1960s and 1970s.
With scuba’s exploding popularity in recent decades, training agencies recognized that divers came in all shapes, sizes, ages and genders. A table that was reasonably safe for a hyper-fit navy diver may not necessarily work for all. Thus, recreational dive tables were developed; the most famous is Dr. Raymond Roger’s ubiquitous blue and white DSAT/PADI dive table upon which millions of new divers learned their craft. The mathematics, however, traces its roots all the way back to Haldane’s work, which forms the backbone of modern computer algorithms that help keep divers safe.
The Computer Age
As computing technology developed on land, so it did in the water. Dive computers providing real-time readouts of depth, bottom time and nitrogen exposure that could be worn as part of a standard scuba set-up entered the recreational market. However — much like cell phones — dive computers were initially the preserve of the wealthy. Now, as technology has accelerated and prices have fallen, most divers wisely purchase, or at least rent, a computer every time they dive in open water.
Many recreational agencies have now gone as far as making dive table usage purely an option within open water training. Dive tables appear to be slipping into obscurity, but should they go quietly into the night? Or is there still a place for dive tables in modern scuba diving?
Drawbacks of dive tables vs. computers
When we consider whether dive tables are still relevant, here are a few of their drawbacks versus computers:
- Traditional dive tables facilitate dive planning on a ‘square profile.’ This means that when planning a dive to 60 feet (18 m), for example, the dive table calculates as if you have descended directly to 60 feet and stayed at that depth for the entire time, with the associated nitrogen exposure. This means that, particularly on deep or repetitive dives with air, the planned dive time is considerably shorter than the multilevel profiling of a diving computer would allow, provided the diver intends to remain within no-decompression limits.
- There is also the associated knock-on effect on a diver’s surface interval due to the increased calculated theoretical nitrogen exposure. The table doesn’t give you a shorter time at depth — it simply assumes you spend your whole bottom time at the maximum depth planned.
- This type of square profile leaves little room for flexibility in dive planning without having pre-planned multiple options on a slate. If you accidentally exceed your planned maximum depth, even if only for a short period of time, the table doesn’t facilitate simple contingency planning as easily as a computer.
- Tables provide only depth and time information. The remaining parts of the dive planning and management remain firmly in the hands of the individual diver. Computers, conversely, often present the diver with a whole host of information in real time that allows for decision making and contingencies.
Benefits of dive computers over tables
While more expensive than a table, a dive computer is relatively inexpensive these days and has several benefits over a dive table in many situations:
- Firstly, and perhaps most importantly, a computer calculates the diver’s precise profile in real time, sampling every few seconds. It provides up-to-the-minute data on the diver’s individual profile and theoretical nitrogen exposure. This allows the diver to effectively multi-level a dive and, while the allowable time at maximum depth will be very close to that of a table (on the first dive of the day), the computer’s live calculation allows the diver to buy back dive time by multi-leveling the dive. For example, a diver using a computer may begin the dive at 100 feet (30 m) and then, as the computer counts down to five minutes from no-decompression time, ascend to a shallower depth of 60 feet (18 m). The computer would then recalculate and offer the diver more allowable time, if gas consumption allows.
- The computer provides data at the diver’s fingertips during the dive. Many modern computers not only offer depth, time and exposure information, but also supply ascent rate information, safety stop information, emergency decompression information, gas consumption (if linked with a transmitter), surface interval and repetitive dive planning, and have a compass.
- Many dive computers allow gas switching so that, as your diving progresses into extended range, nitrox, or technical diving, the computer can change between different gas mixes mid-dive at the press of a button.
- Computers also monitor no-fly-time at the end of a dive trip, offering precise information to the minute based on the individual’s profile.
- Computers log dives automatically and, in some cases, offer Bluetooth connectivity, making logging dives a paperless process. Some computers may even upload your full dive profile, depths, ascent rates and water temperatures, allowing divers to analyze their dives in detail and drill down into the data to potentially improve technique.
Drawbacks of dive computers versus dive tables
So, if dive computers offer all the benefits and information listed above, why bother with tables at all? Many divers remark on not having picked up a dive table since certifying and, even if they did, wouldn’t remember how to use one. However, dive computers — despite their functionality and features — are not infallible.
- Dive computers, while based on Haldane’s original theory, often vary slightly from one another in design, user interface and in the individual algorithm and model that calculates dive times. This means a group of divers may have considerably different allowable profiles if using a different computer brand, model and algorithm. This can lead to confusion in the water at a dive’s deepest point where one or more diver’s computer is more conservative. If all divers plan the dive using the same dive table, there will be no confusion regarding turn points or which computer’s data triumphs.
- The reliability of dive computers has improved exponentially over the years. However, as with any piece of technology, dive computers can fail through either malfunction or battery life. If you’re mid-dive and your computer fails and you haven’t planned a contingency with a table, timing device and depth gauge or, alternately, a second back-up computer, your dive is over. And, unless your computer failed during the first few minutes before you descended to depth, you’ll prudently likely sit out the next dive, even if you managed to borrow another computer. The unknown nitrogen exposure of that part-dive means the readings you’d get if immediately returning to the water with another computer would be inaccurate. A table, despite its drawbacks, never has a malfunction or requires a new battery.
- Dive computers are convenient and easy to use but relying on them exclusively and turning your back on tables can leave divers with a gap in their knowledge. No-decompression time and the relationship between depth and time often seems an abstract concept to those divers who have solely used computers. As divers increase skills and experience, they may begin to move into mixed-gas or technical diving. Part of conducting technical dives or use gases such as trimix is the ability to execute depth and dive-time calculations based on dive tables and SAC rates. Working knowledge of basic air tables prepares divers for planning more challenging dives.
Becoming a lost art
Dive computers should never replace a diver’s brain. Use your computer as an additional layer of planning and safety alongside other means; not as a replacement for sound judgment, good planning and reasonable conservatism.
If you haven’t used your dive table in a while, take a look at it. Familiarize yourself with it. Even if you don’t use it regularly, knowing the theory behind its creation is important. One day it may even keep you from aborting or missing a dive.