Improve end-pressure UX in cylinders table

[pabdakine]

I suspect there's an ideal/real gas mixup when computing the remaining pressure. Using this scenario to reduce round-off effects and changes in the Z-factor:

Fresh water, sea level
Standard 10L bar air cylinder at 232 bar
SAC of 20 L/min
Descent/Ascent rate of 1 m/min
OC dive, Depth: 1m, Time: 1 min (2 min total)

The average depth is 0.5 m, and the calculated usage of 42 L is correct.

The Cylinders | Pressure info indicates a drop of 8 bar (232->224 bar), which I believe is incorrect.

There is 2195 L at 232 bar, which is a ratio of 9.46 L/bar. Removing that 42 L should have dropped the pressure by 4.4 bar. (The difference cannot be explained by a reduced Z-factor, since that would make the pressure drop even smaller. Obviously, this is a small error in this situation, but I worry it may be larger in others with larger Z-factors.)

[Rolf-Smit]

It is actually correct, but the UI is confusing, that is something I think requires some work.

Why is it correct?
The pressure drop shown (232 > 224) is a combination of all gas required (used + reserve), in this case the reserve gas is likely 42 21 liter (if panic SAC rate is 40 20 L/min,), and the used gas is 42 liter. So in total the pressure drop is based on 84 63 liter of gas. Which is about 8 bar.

I have been thinking about not showing the pressure drop at all, but rather 2 delta's, one for used (about 4 bar), and one that includes the reserve (about 8 bar).

[pabdakine]

Panic SAC was set equal to the normal SAC of 20 L/min in this case.

I don't understand why any part of the reserve is counted, and that could be very confusing. Many people use the remaining pressure as an ascent indicator, so having it lower than it should be (i.e., a longer dive time) seems odd. Or perhaps I have misunderstood you.

I do know that I don't understand the "reserve" concept at all and can't puzzle it out from the descriptive text in the settings or screens. I initially thought it would be the minimum gas at the stressed SAC, but I don't think so. (Perhaps a different topic, though.)

[Rolf-Smit]

Panic SAC was set equal to the normal SAC of 20 L/min in this case.

Ah yeah that makes sense, if I calculate the plan with 20 it matches your example (updated my previous comment). Regardless the pressure information (232 > 224) is still not wrong, a total of 63 liter of gas is required for the dive to complete with a worst-case contingency (bring a panicked buddy that is out-of-air to the surface), and that makes, using real-gas calculations, a 8 bar pressure delta from 232 bar to 224 bar.

Just to clarify a few things:

• Worst-case: The scenario that requires the most gas to bring a panicked buddy back to the surface. Usually the moment during the dive with the highest TTS.
• Reserve: The extra gas you need to bring an out-of-air panicked buddy back to the surface, from the worst-case scenario.
• Used: Your own normal gas requirement for the whole dive (a dive without worst-case gas contingency)
• Start > end pressure: The start pressure of a cylinder, and the end-pressure after the worst-case out-of-air scenario.

I don't understand why any part of the reserve is counted, and that could be very confusing.

The reserve is part of the in the pressure delta (232 > 224) as that would be the most conservative thing to do. The end-pressure shown is what you would need in a worst-case scenario.

Many people use the remaining pressure as an ascent indicator, so having it lower than it should be (i.e., a longer dive time) seems odd. Or perhaps I have misunderstood you.

I don't see why people would use the end-pressure (remaining pressure) as ascent indicator, that would be very weird, and dangerous to do (your ascent could be the biggest part of the dive, if you have a bunch of decompression to do). You plan a dive, and this gas plan shows what you need to execute that plan (if your planning parameters match reality). If you keep diving till the end-pressure is reached you are over staying, or you are using way more gas then you planned for.

[pabdakine]

Thanks for the clarification! Very helpful. I think the top priority is seeing what my remaining gas is (e.g., to plan a second dive, estimate refill costs, whatever).

I understand wanting to see there is enough for the reserve, but the remaining pressure after the reserve is used seems superfluous. The reserve is either there or it's not.

Suggest displaying:
start pressure > normal ending pressure [when normal + reserve < full]
start pressure > normal ending pressure (reserve not met) [when normal+reserve > full, perhaps in red font for emphasis]

[Rolf-Smit]

I'm leaning towards something like this. Just a start and end pressure column, where the end pressure shows both end values (after normally finishing the dive, and after a worst-case out-of-air situation). The challenge really is space and usability. Unlabeled numbers are hard to understand, but labeling everything takes up so much space that on small phones with huge accessibility font sizes things start to be cut off. I think this strikes a good balance.

[pabdakine]

I think the best interfaces prioritize the 99% case with minimal extraneous information while having sufficient info to identify the edge case. The current label of "Pressure" seems quite clear, and having "start > normal_end" is very straightforward and far less cluttered.

In the less common case, the inattentive user will see the unambiguous info: "Reserve not met" in red. Yes, the columns would need to squeeze closer if any cylinder needs the warning; alternatively, a second row with that warning would also be easily interpreted.

The above alternative (2 end pressures) forces them to parse more numbers and mentally translate a number to the concept of "reserve not met". Although, I'm not sure what you'd put if that were the case -- "empty"? The shortage as a negative?

Lastly, I simply don't see the value in knowing that pressure if we have to share. What would I ever do with that specific number? It fits firmly in the "extraneous" category for me. The important concept to communicate is that sharing will fail. The clearer that can be the better, IMHO.

All that said, having the normal end pressure somewhere addresses my confusion. I'd prefer less clutter, but you're the boss!

[pabdakine]

Perhaps a distraction, but on this thought

I don't see why people would use the end-pressure (remaining pressure) as ascent indicator, that would be very weird, and dangerous to do

You're absolutely right, and I misspoke. I was thinking of the lowest pressure prior to the ascent such that the reserve remains intact. Conveniently, you've labeled that on the reserve graphic. The dive ending pressure is most definitely not that value. Sorry for any confusion.

[pabdakine]

I noticed the CCR/BO pressure display matches my intuition (reflecting just my usage). Consistency between those modes would be another reason to do the same for the OC case.

After running a scenario with insufficient reserve for sharing, I'm pleased to say it's quite obvious that sharing will be short. Nice job on that UI aspect! If I have a buddy, I would immediately change the plan -- rendering the ending pressure after exhausting the reserve irrelevant. If I am solo, that pressure is also irrelevant.

I guess I'm saying that I've realized there are even more reasons than I originally thought for making the end pressure be full - my_usage.