Planning (hypoxic) dives feedback

[Attilla de Groot]

Hi,

I have been planning my hypoxic dives for this week and I have some feedback/questions around this.

- In the planner there are IBCD warnings for my 80 and 100m dives, while we did account for it with the gas switches. According to the documentation the “rule of 5th” is used to base these warnings on. According to my instructor this was a rule that was use in the past, but these days the BSAC recommendation is followed in the tech diving community:

> The British Sub Aqua Club (BSAC) recommends that divers allow for a maximum of 0.5 bar difference in PN2 at the point of the gas switch. According to former BSAC Tech lead Mike Rowley, “The recommendation isn’t an absolute but a flexible advisory value so a 0.7 bar differential isn’t going to bring the Sword of Damocles down on you.”


GUE has a different approach where they wouldn’t switch to EAN50 (not practical imho), but in their explanation (https://gue.com/blog/isobaric-counterdiffusion-in-the-real-world/) they do refer to the above. Perhaps it would be an idea to update the warning based on the BSAC recommendation of 0.5, since it seems to be more up to date than the “rule of 5th” (and also more practical).

- The planner gives the option to set the last stop to 6m or not (then it becomes 3m). Multideco has an option where you could provide the depth manually if not 6m. This would be practical to have when doing deco with Oxygen. Most computers will give warnings when passing a PO2 of 1.6 and also warnings when not staying at 6m. What I understand is that it is common practice to do the last deco stop at 4 of 5 meter (after the 6m stop is finished). This would be more practical than 3m (certainly in open water), but it would result in a bit more gas usage that I would like to plan for.

- It is unclear to me when working with the planner and the descent. It is possible to set a descent rate, but that only seems to be applicable for the first stop (assuming “drop to first depth” has been enabled). In most dives you would need to account for work to be done when descending (e.g S-drill, placing cookies, running lines etc.). In multideco (not using it myself) the descent is calculated for every “step” during the descent. Ideally, I would want to plan a dive to 100m and need the following stops:

- 5m (S-drill)
- 21m (staging EAN 50 tank)
- 40m (placing cooking)
- 50m (switch to back gas)
etc..

Then I would only have to enter those depths and time I want to stay there. Subsurface would then add the descent time to get to the given depth.

- In the plan I also get the warning: "  — Warning: required minimum gas for ascent already exceeding start pressure of cylinder!”. This is for the intermediate gas I’m using to prevent a IBCD. It is a nearly full tank and not using much according to the planner. Not sure what this warning is based on.

- For training and certification is is needed to make backup runtables for failure scenarios (e.g loosing a gas or going deeper than planned). Perhaps it would be an feasible to have variables to do this and store the RTs for the failure scenarios.


Hope this is useful.


— Attilla

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