Planning (hypoxic) dives feedback

Robert.Helling robert at neu.atdotde.de
Tue Mar 22 04:14:26 PDT 2022


Attila,

thanks a lot for your feedback!

> On 22. Mar 2022, at 07:33, Attilla de Groot via subsurface <subsurface at subsurface-divelog.org> wrote:
> 
> 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).

I think the whole isobaric counter diffusion is not really well understood and adding yet another criterium used by one agency (which does not sound too well motivated to me anyways) does not help much. Maybe we could add this one day in the future but I don't think it has much practical relevance. Note that Subsurface not only implements the rule of fifth but also does an actual check if more N2 is entering the body than He is leaving (it does it in the deco calculation). But this triggers at unexpected times. I have written about this here:

https://thetheoreticaldiver.org/wordpress/index.php/tag/icd/

> 
> - 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.
> 
I am hesitant to add yet another adjustable parameter. The Subsurface user interface and the planner in particular is already quite baroque and it is very easy to get lost. Note that from a deco point of view, when you are doing o2 deco, the depth doesn't make any difference for the off gassing since at any depth the intert gas ambient pressure is 0 with O2. So, if you plan with a 3m last stop, you can as well spend it at 5m without any change to your deco schedule (except the final ascent takes a bit longer).


> - 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.

Indeed, the only descend that the planner automatically does for you is the initial one to the first depth. All the others have to be done manually.

> 
> - 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.

The planner does a minimum gas calculation for the end of the last manually entered segment (why do you enter this last segment manually and don't set a switch depth and let the planner handle it for you?). It calculates the the gas you need if at the end of that segment you stay at that depth the "problem solving time" and then do the ascent but all with a SAC increased by the "SAC factor" (by default 4 as it should include gas sharing and both divers have a double SAC rate due to stress).  For your 22/40 gas, you regularly use 63 bar in the ascent, multiply that by 4 and that is already more than what you started with.

> 
> - 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.

This has been discussed before and it has been found that to cater everybody's different needs would lead to an explosion of further user controlable parameters. There is some support for such planning though: You can mark a gas in the gas table as "Not used" by clicking in the last column. This allows you to quickly simulate gas failures. And you can turn on "Display plan variations" which allows you to compute changes in total runtime for different last depths and/or times of the last segement (assumend to be the bottom time). It's only the variation of the total runtime and you have to manually distribute it over the different stop depths but I think with a little bit of experience this can easily be done within the error margins all these calculations have anyways.

Best
Robert


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