Hi Robert, > On 22 Mar 2022, at 18:14, Robert.Helling <[email protected]> wrote: > > Attila, > > thanks a lot for your feedback!
No problem, > >> On 22. Mar 2022, at 07:33, Attilla de Groot via subsurface >> <[email protected]> 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/ I wasn’t suggesting to add a criterium from a single agency. Exactly because of the reason that you mentioned. However, GUE does have nice blogs with good references. In this case it is a recommendation from BSAC that is neutral in relation to GUE or any other org. As said, the “5th rule” from what I understand is outdated. Seems logically to update Subsurface according to recent research on the topic. > >> >> - 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). That does make sense. Having a “last stop depth” variable (default 6) seemed a small change to me. >> - 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. Wouldn’t it be useful to add something for all other stops as well given the use-case I mentioned? >> - 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. Ok, good to know this. I should have added the 22/40 with a deco stop at 40m. >> - 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. That is exactly what I’m doing now, luckily I’m not in certifications anymore and with two tri-mix computers I don’t have to write down all the failure scenarios anymore. So a problam for someone else and I get that you don’t want to clutter the interface anymore than necessary. — Attilla _______________________________________________ subsurface mailing list [email protected] http://lists.subsurface-divelog.org/cgi-bin/mailman/listinfo/subsurface
