Stuartv, thanks for your comprehensive response. Just for my own clarification, when you say you use your Oceanic as back up, do you only use it in gauge mode with a printed bailout table? Or is it so liberal compared to your GF setting on the Shearwater that you let it run in dive mode and if the Teric fails you just follow the Oceanic deco profile?
You also mentioned that the Oceanic was more liberal than the Seabear. Is that because the Oceanic was using the DSAT algorithm versus the PZ+?
[snip]
I completely understand your tech argument for planning your dive and diving your plan. Good advice. And you are correct, the biometrics of the Scubapro can be turned off if required. But as a previously bent diver I appreciate that these inwater adjustments are trying to make the dive safer and perhaps should be taken into consideration. If it is an issue of gas capacity; then carry more gas. When I started diving, I sucked air like it was going out of style. So back in 2001 I bought a 15 ltr Faber (I think that's 125 cu ft in North America). Since then my SAC has improved but I am still humping that tank on all my dives. My buddies either use 10 or 12 litres so by the end of a typical dive, I've got enough air for the both of us. Also, if I set my computer to be conservative, I can still match my buddy's default air NDL's by diving nitrox which pushes out my conservative NDL's. So with a bit of ingenuity, one can dive conservatively, have enough gas and match your buddies NDL's to surface together.
When I have used my Oceanic as a backup, I have carried a written plan that I could follow if needed. I have only used it as a backup on technical dives where the planned deco was pretty light. What I have observed is that, in those case, leaving it dive mode (instead of gauge mode) has never resulted in a violation. However, with a bigger dive involving multiple deco gases and longer deco times, I expect that doing that WOULD result in a violation. The thing that some people seem to not get about having a violation is that it doesn't lock you out. It just locks you INTO Gauge mode. So, even if that happened, with my written plan, I could still use it just fine (in gauge mode) to get out safely.
My Seabear (RIP) was a fully capable tech computer, just like my Shearwaters. I ran the Seabear with the same gradient factor settings as my Shearwater. Thus why the Oceanic was more liberal.
Regarding what you said that I bolded: The problem is how much more gas do you carry? When you plan a tech dive, you need to plan for all your contingencies and know how much gas you need for every contingency. If your computer is set to allow it to add deco time based on factors like your heart rate or skin temp, then I don't know how you could plan out ahead of time what the dive profile will be so that you KNOW you are carrying enough gas.
I suppose the ScubaPro planning software could have a "worst case" option for planning, where it would give you a profile based on the "most conservative" ascent that the computer would give you. But, that assumes that the computer's algorithm has known limits to what its worst case would be. So, does it? Does SP document anywhere what the worst-case limits would be? I.e. if it increases deco when your heart rate is at 150bpm, will it increase it further if your heart rate is 160? Is there a max heart rate where the deco would not increase if the heart rate increased any further? Is there a skin temp that would increase deco requirements? Is there a minimum whereby if you get even colder, the deco will not increase any further?
I don't know the answers to any of those questions. But, as long as they are unanswered, there is no way I'd do any technical dives with a computer that could be influenced by biometric factors.
