A decompression stop is a depth you hold for a set time on the way up. While you wait there, your tissues release gas, and none of them goes past the limit the model allows. When the stop time is over, the model allows the move to the next stop.
The model calculates a ceiling: the shallowest depth you can be at, at that moment, without a compartment exceeding its limit. The ceiling moves up as you release gas. Stops are set on a 3 m grid, at or below the ceiling. On the dive in this guide the ceiling at the end of the bottom time is 21.7 m, so the first stop is at 24 m, the next 3 m step below it.
This guide follows one plan from the surface back to the surface: 45 m for 25 minutes on trimix 21/35, with EAN50 for decompression from 21 m, at gradient factors 30/70. Every number comes from the DiveLogic engine.
Reading the schedule
A schedule lists the bottom, each stop and the gas switch in the order you reach them. For each row it gives 4 things: the depth, the time at that depth, the runtime at which you leave it, and the gas you breathe there.
Runtime is measured from the moment you leave the surface. Time is the length of that one row. Confusing the two is a common error when copying a plan onto a slate, so check which column you are reading.
| Depth | Time (min) | Runtime | Gas |
|---|---|---|---|
| Planning the dive… | |||
The descent
The runtime clock starts when you leave the surface. The descent to 45 m at 20 m/min takes 2 minutes 15 seconds.
The descent is part of the plan. Your tissues take up gas on the way down, and the plan counts that time.
Leaving the bottom on time
The plan allows 25 minutes at 45 m, so you leave the bottom at a runtime of 27:15. Every stop in the schedule depends on this time.
The ceiling, drawn in red, has moved down during the bottom time. At the moment you leave it is at 21.7 m.
The ascent to the first stop
The ascent to the first stop is at 9 m/min. The first stop is at 24 m and lasts 1 minute.
The ceiling is shallower than the stop, as it is at every stop. The stop depth is where you wait; the ceiling is the limit you must not go above.
The stops and the gas switch
Between stops you ascend at 3 m/min, so each 3 m move takes 1 minute. Hold each stop for its full time before you move.
At 21 m you switch to EAN50. Confirm the cylinder and the depth with your buddy before breathing from it. The diver’s cylinder changes colour at the switch. The stops get longer as you ascend: 2 minutes at 12 m, 4 at 9 m, 8 at 6 m.
The last stop and the surface
The last stop, 14 minutes at 3 m, is the longest. The ascent from the last stop to the surface is at 1 m/min, so the last 3 m take 3 minutes.
The model counts this slow final ascent as decompression. You surface at a runtime of 69:35.
Leaving the bottom late
The schedule is correct only for the dive it was planned for. If you stay at the bottom longer, your tissues take up more gas, and the plan you carry no longer applies.
Move the slider to replan the same dive with extra minutes at 45 m. With 5 extra minutes the engine adds stops at 21 m and 15 m, drops the 18 m stop, and lengthens the 9 m stop from 4 to 5 minutes, the 6 m stop from 8 to 10 minutes and the 3 m stop from 14 to 17 minutes. The time to surface rises from 42 to 50 minutes and the runtime from 69:35 to 82:35. The dive uses about 415 L more 21/35 and 170 L more EAN50.
- Original plan
- 21/35
- EAN50
Where to hold a stop
Hold each stop at its depth or slightly below it, never above it. The blue band shows the 6 m stop and the metre below it.
At the start of the 6 m stop the ceiling is 4.9 m. The compartment closest to its limit, the leading compartment, is at a gradient factor of 46% at 6 m.
Above the ceiling
Drift up 2 m at the start of the stop and you are at 4 m, above the 4.9 m ceiling. Above the ceiling, the leading compartment exceeds the limit set by the gradient factor.
The plan does not visit 4 m at this time, so the engine gives no gradient factor for that depth. The ceiling is the engine’s statement that the depth is not allowed.
Above the stop, below the ceiling
Later in the stop the ceiling has risen to 3.5 m. The same drift to 4 m is now below the ceiling, but it is still above the stop.
This is within the model’s limit, but it uses up the margin the plan keeps, and the rest of the schedule was calculated for a diver at 6 m. Descend to the stop depth.
Finishing the stop
Held at 6 m, the leading compartment’s gradient factor falls from 46% to 19% by the end of the stop as the tissues release gas. The ceiling rises from 4.9 m to 3.0 m.
When the stop time is over, ascend at 3 m/min to the next stop.
If you miss a stop
If you ascend above a stop or leave it before its time, follow the procedure your training agency teaches for omitted decompression. Procedures differ between agencies. In general they return you to the stop depth promptly, if that can be done safely, and add time to the remaining stops.
After a missed stop, end the diving for the day, watch for signs of decompression sickness, and seek medical advice at the first symptom.
What to remember
- A stop is a depth held for a set time so that tissues release gas without exceeding their limit. Stops are on a 3 m grid, at or below the ceiling.
- Read the schedule by depth, time, runtime and gas. Runtime is measured from leaving the surface.
- Leave the bottom at the planned runtime. A longer bottom time needs longer stops and more gas, so carry a contingency plan for it.
- Ascend at the planned rates: here 9 m/min to the first stop, 3 m/min between stops, and 1 m/min from the last stop to the surface.
- Hold each stop at its depth or slightly below it. Above the ceiling the leading compartment exceeds its limit.
Following a plan in DiveLogic
Plan the same dive, 45 m for 25 minutes on trimix 21/35 with EAN50 from 21 m at GF 30/70, and open the schedule in the Review step. The Contingency card adds overrun plans such as +5 min / +3 m, each with its own schedule to carry with the main plan.
Sources
- Bühlmann A. A. (1984). Decompression-Decompression Sickness. Springer, Berlin.
- Baker E. C. (1998). Understanding M-values. Immersed 3(3).
- U.S. Navy (2016). U.S. Navy Diving Manual, Revision 7. Naval Sea Systems Command, SS521-AG-PRO-010.