For most of the 20th century, divers planned decompression with printed tables. A table lists a schedule for each combination of maximum depth and bottom time, calculated in advance from a decompression model and, for the major tables, tested on divers.
Since the 1980s, dive computers have calculated decompression during the dive from the measured profile, and planning software now calculates schedules and gas requirements for any dive on a desktop or phone. The models changed less than the tools: the tables, the computers and the planners all use the tissue compartments described in the tissues guide.
Navy tables
In 1908 Boycott, Damant and Haldane published the first staged decompression tables, calculated with Haldane’s compartment model, and the Royal Navy adopted them. The Haldane guide describes the method.
From 1912 the US Navy tested Haldane’s tables in a programme led by Chief Gunner George Stillson, and its own tables followed. The Navy revised them several times, including in the 1950s, when it also introduced a procedure for repetitive dives, and in 2008, when Revision 6 of the US Navy Diving Manual replaced the air tables with tables calculated with Thalmann’s model.
Recreational tables
Recreational divers first used navy tables, which were designed for working divers with a chamber on site. In 1976 Merrill Spencer published shorter no-stop limits based on bubbles detected by Doppler ultrasound after dives. In 1988 PADI introduced the Recreational Dive Planner and the British Sub-Aqua Club introduced the BSAC 88 tables, both designed for recreational no-stop and repetitive diving.
The layout of a table
A decompression table has a row for each maximum depth and a column for each bottom time. Each cell holds the schedule for that dive: no stops, or the stops and their times.
In the US Navy definition, bottom time runs from leaving the surface to leaving the bottom. It includes the descent. The last column, NDL, is the no-stop limit for each depth.
Finding the depth row
The diver enters the table at the exact or next deeper depth. A dive to 25 m is read from the 27 m row.
The rounding is always towards the more conservative schedule. A table cannot interpolate between rows because the schedules are not linear in depth.
Finding the time column
The diver then uses the exact or next longer bottom time. A bottom time of 27 minutes is read from the 30 minute column.
Reading the schedule
The cell where the row and the column meet gives the schedule for the dive. Printed tables also give each cell a letter, the repetitive group designator, which indicates how much nitrogen remains in the body after the dive.
For a second dive, the diver uses the letter and the surface interval to find the residual nitrogen time: minutes added to the bottom time of the next dive to account for that nitrogen. This illustrative table has no letters.
Try it
Choose a maximum depth and a bottom time. The highlighted cell is the one a table diver would read. The second value is the exact dive planned by the engine with the same model, without rounding up to a row and column.
- Table cell
- …read as 24 m for 40 min
- Exact dive, engine plan
- …22 m for 35 min
- Dive as dived
A multilevel dive
Few dives stay at one depth. On this dive the diver spends 10 minutes at 30 m, then 15 minutes at 18 m and 15 minutes at 12 m before ascending.
A depth gauge and a watch record only the maximum depth and the total time. A dive computer records the whole profile.
The table’s assumption
A table is entered with the maximum depth and the bottom time. It therefore treats the dive as a square profile: the whole bottom time spent at the maximum depth.
For this dive, the table reads 30 m for the whole bottom time, although most of that time was spent at 18 m and 12 m, where the tissues take up much less nitrogen.
The cost of the assumption
Planned with the same model, the square dive requires decompression stops. The multilevel dive requires none. The table is safe for this dive but gives the diver no credit for the time spent shallower.
Multilevel methods for tables, such as the Wheel that PADI introduced for the Recreational Dive Planner, gave some of this credit. A dive computer gives it continuously, because it calculates the tissue loading from the measured depth throughout the dive.
Dive computers
Mechanical decompression meters, such as the SOS meter made in Italy, were used before electronic computers. They represented gas uptake with a single mechanical element, in effect one tissue.
The Orca Edge, released in 1983, was one of the first electronic dive computers sold widely. It calculated tissue loading during the dive from its pressure sensor. Later computers moved to the wrist, and technical computers added gas switching, trimix, rebreather set points and user-selected gradient factors. The dive computer guide describes what they measure and display.
Planning software
As technical diving grew, desktop planners calculated decompression schedules for any depth, gas and model before the dive, which tables could not do for trimix and multiple decompression gases. Planners then added gas planning: the gas required for the dive, turn pressures, reserves for a failed regulator or lost gas, and oxygen exposure.
Divers who use a planner still dive with a computer. The plan checks in advance that the dive is possible with the gas carried; the computer follows the dive that actually happens.
What to remember
- Printed tables give a schedule for a maximum depth and bottom time. The diver uses the exact or next deeper depth and the exact or next longer time.
- Tables treat every dive as a square profile at the maximum depth, so a multilevel dive gets no credit for time spent shallower.
- Repetitive group letters and residual nitrogen time let a table account for nitrogen left from an earlier dive.
- Dive computers, from the 1980s, calculate tissue loading from the measured profile throughout the dive.
- Planning software calculates schedules and gas requirements before the dive, for any depth, gas and model.
Planning in DiveLogic
The DiveLogic planner takes a dive as a list of levels, so a multilevel dive is planned as it will be dived, with the same engine that drew the figures above. It also calculates the gas, turn pressures and oxygen exposure for the plan.
Sources
- Boycott A. E., Damant G. C. C., Haldane J. S. (1908). The prevention of compressed-air illness. Journal of Hygiene 8(3): 342-443.
- Naval Sea Systems Command (2016). U.S. Navy Diving Manual, Revision 7. SS521-AG-PRO-010. (Chapter 1, history of Navy diving; Chapter 9, definitions and rules for the air tables; Appendix 2B, Navy Dive Computer.)
- Workman R. D. (1965). Calculation of decompression schedules for nitrogen-oxygen and helium-oxygen dives. Research Report 6-65, US Navy Experimental Diving Unit.
- Spencer M. P. (1976). Decompression limits for compressed air determined by ultrasonically detected blood bubbles. Journal of Applied Physiology 40(2): 229-235.
- Hamilton R. W., Rogers R. E., Powell M. R., Vann R. D. (1994). Development and validation of no-stop decompression procedures for recreational diving: the DSAT Recreational Dive Planner. Diving Science and Technology Corp., Santa Ana, CA.
- Gerth W. A. (2010). Thalmann algorithm decompression table generation software design document. NEDU TR 10-09, Navy Experimental Diving Unit.