I'm wondering, and it's a bit related to this topic, the following scenario:
- Perdix AI in OC REC, Transmitter, Singe Tank Nitrox 32%
- 40m deep watching a school of hammerhead
- NDL is reached so deco stop mandatory. GTR is OFF (as deco stop)
- what data should I monitor to "know" how long more I could reasonably keep enjoying the school at 40m ?
I was thinking of multiplying TTS by SAC and comparing it with T1.
(I just got a Perdix AI and didn't use it yet, now in the reading phase to understand it more. Used to dive with a basic Suunto D4i before that).
I understand what you are asking, but as has been said, the time is zero. Unless you planned to do a decompression dive, and are equipped and trained to do so, you shouldn't be doing that. As you know, the Perdix turns off GTR when you have a deco obligation. I think that's Shearwater's subtle way to tell you that you've overstayed your welcome at this depth and need to complete your deco obligation and get shallower. That's the correct answer.
Now, you can switch to OC tech mode during the dive, but you should have some familiarity with this before hand. Also, you can use the Dive Planner+ in Dive setup to plan a deco dive. Again, it would be advised to get training in deco diving before planning one. Plus, the time to plan a deco dive is not when you've exceeded the NDL. The time to plan it is well in advance.
I gather that your question was more around gas supply since you referenced GTR off. Gas supply is no longer the limiting factor on this dive, so is irrelevant provided you have enough gas to get you through your stops and all the way to the surface. GTR measures the amount of time you can stay at your current depth without getting into your reserve. So, as long as you had time here before going into deco, you should be good. But again, your time remaining to wait here is zero. You haven't planned as a deco dive, so shouldn't linger any longer.
In your second scenario, you qualified it with an issue with your buddy that you had to deal with. Again, same answer. Your time at that depth is expired, and need to make a proper ascent now.