Decompression Planning: Dive Tables, Planning Software and Dive Computers

Dive tables are pre-calculated and pre-printed implementations of a decompression algorithm that divers can use to plan their dives. These were most popular before the 1990s among recreational scuba air divers wanting to plan simple dives, and their use was taught as part of all recreational scuba diving courses. The most frequently used items of information were the no decompression limits (see earlier) provided by these tables. Thus, divers could look up the allowable bottom time they could spend at any particular depth and still make a direct ascent to the surface without decompression stops. Even though no decompression was prescribed for dives ‘inside’ the no decompression limits, most training agencies advocated the use of a 3- to 5-minute ‘safety stop’ at 3 to 5 metres during the final part of the ascent, as an added precaution. Such stops are probably useful in reducing the incidence of DCS on routine no decompression dives.

The dive tables also provided a series of steps (with minimal calculation) for divers to account for the effect of inert gas accumulated but not yet eliminated after previous dives when planning further ‘repetitive’ dives. In air diving, this is referred to as ‘residual nitrogen’, and its presence, not surprisingly, has the effect of reducing the no decompression limit for subsequent dives.

The use of dive tables has declined dramatically with the rise in use of dive computers (see later), and some entry level diving courses no longer teach their use. Whether this is a good or bad thing is impossible to say. Tables were inexpensive and readily available, whereas in the early days this was not true of computers. More recently, however, entry level dive computers have become much more reasonably priced and have the advantages of avoiding calculation errors, thus ensuring that the diver who carries one is receiving accurate time and depth information, and most of these computers provide ascent rate alarms.

Planning software that runs a decompression algorithm (and often multiple decompression algorithms) can be purchased for use on desktop, laptop and tablet computers, as well as telephones. Such software is effectively an electronic dive table and is often used to generate tables that are transcribed onto underwater slates for specific missions. The advantage of such software is that the decompression plan can be tailored specifically to the diver’s equipment, gas mixes and decompression preferences (e.g. gas content model, bubble model, gradient factors). The multitude of combinations and permutations of circumstances that can be ‘run’ by such software would be very difficult, if not impossible, to replicate on pre-printed tables.

Dive computers that the diver carries underwater have become increasingly popular since the early 1990s and are now almost ubiquitous. These computers run software with one or more decompression algorithms programmed into them and with various levels of adaptability for decompression planning. They track time and depth exposures in real time and provide a constant display of parameters such as depth, dive duration, decompression ceiling and expected time to surfacing. These parameters are continuously updated as the depth varies and the dive duration lengthens, all with little or no effort on the part of the diver. Advanced computers allow the diver to choose the equipment being used (e.g. open-circuit or rebreather systems) and the gas mixes being breathed and to adjust decompression preferences such as gradient factors. Some computers can connect to the oxygen cells in a rebreather (see Chapter 62) so that they ‘know’ the inspired partial pressure of oxygen used by the diver and can incorporate this information in calculating decompression. Advanced computers that perform these functions and provide all relevant information in a head-up display constantly visible to the diver are now available.