At what point do you consider redundancy?

I own single and double tanks, and am happy diving either. I have a pretty good set of criteria for choosing one or the other . . . If the dive is below about 80 feet for any significant period, it's doubles. If I'm diving 25/25, it's doubles. If I'm diving with a new diver, or an unknown diver, it's doubles, even if the dive is shallow and familiar. If the walk from the car to the dive site is unmanageable in doubles (a couple of our local sites are, for me) I dive a single tank and don't dive with an unknown buddy.

For dives in the shallow range, with known buddies, it's single tanks all the way.

Caves, of course, it's doubles.

How do you sort it out?

NDL dives are in single tanks. Square profiles below 30mts become deco dives. All overheads are in doubles.

Well, you're addressing several related thoughts. Generally, I bring two or more of each piece of equipment to the dive site.

Redundancy is one diver carrying two of everything in the event that one is lost or fails. It is dive profile dependent as to what one should consider necessary redundancy - your buddy, and his/her equipment, is never counted as a part of your redundancy.

Simple redundancies: independent doubles, two cutting tools, two regulators, two masks (used for deep dives, in currents, etc.), two lights, two time pieces, two computers, two inflation devices, etc.

Technical dives require a different profile than that of non-overhead NDL diving and the definition of redundancy changes.

TSandM:

...How do you sort it out?

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For me, to double or not is usually not a redundancy issue at all, but rather is strictly a gas supply question. If the gas plan requires toting two tanks, I do - otherwise I don't. The exception is when I'm in a place where I can only get redundancy in regulators by using two tanks. Otherwise, a single tank with an "H" valve is fine for meeting my redundant gas supply criterea.
As for the question of "When do you require a redundant gas supply?" my answer is "always!" It's just that for most recreational diving, the surface provides that redundancy without any additional equipment beyond a standard Scuba kit... if I am going to remain in an envelope where I can always make a safe ascent to the surface in the event of some catastrophic gas failure, then the surface provides redundancy. If I'm going to introduce an obstacle to that safe ascent... depth, deco obligation, overhead, entanglement hazards, workload, etc... then a second regulator and a way to preserve enough gas for the other one to make a safe ascent is required.
Are there grey areas? Yes.
Example: in the 130 FSW range, in good conditions with no deco obligation, a good buddy kept close satisfies the redundancy requirement adequately for me...

Rick

TandSM, I am assuming 25/25 is Trimix. Do you dive doubles in this case because of circustances that would cause you to dive trimix? Like depth or is it because of the mix?

My current redudent air supply is on my partners back.

I don't dive doubles much at all (heck, I can barely afford one tank must less two plus a manifold). However, I always dive with at least a 19 cu ft pony rig for redundancy whether it is 5' or "all the way to the bottom."

Yes, my shallow redundancy is my buddy. If I can't trust my buddy, I provide my own redundancy.

25/25 dives are, by definition, dives at the deepest and longest limits of what I am trained to do; redundancy (beyond buddy's supply) seems wise then.

Rick, you are right that redundancy in regulators can be achieved without hauling doubles around. I like the idea that I can complete the dive on either TANK, as well.

If I can't have direct surface access, it's doubles. The ONLY time I'll dive singles is when it's a long walk and I have a trusted buddy.

TSandM:

I own single and double tanks, and am happy diving either. I have a pretty good set of criteria for choosing one or the other . . . If the dive is below about 80 feet for any significant period, it's doubles. If I'm diving 25/25, it's doubles. If I'm diving with a new diver, or an unknown diver, it's doubles, even if the dive is shallow and familiar. If the walk from the car to the dive site is unmanageable in doubles (a couple of our local sites are, for me) I dive a single tank and don't dive with an unknown buddy.

For dives in the shallow range, with known buddies, it's single tanks all the way.

Caves, of course, it's doubles.

How do you sort it out?

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To solve the walk-from-the-car problem, for a long walk, I will set up a tarp, and haul my tanks out to it, including a small wooden ladder that I have modified into a bench so I can heave my twin tanks up onto it, and then get into the shoulder straps that way. So the walk to the car does not need to be a critical deciding factor between singles or doubles.

I have not dived single tanks in a long time. However, if the beach is too steep to easily negotiate with twin tanks on, then I would consider a single tank.

It is just too nice having twin 72s on for the beach, and 2 of everything, like two first stages, two second stages, and manifold with an isolator valve, which is the main benefit of a twin tank system. (Twin 130s for boats, which are heavy monsters.)

On those rare occasions when I do dive with a single tank, I virtually always have a 40 cu ft stage with me filled with EAN 50. Thus, again, 2 of everything, just no manifold and isolator this way.

Depth is not really an issue for me. Depth only determines the available NDL time. With twin tanks and 2 of everything, virtually nothing can go wrong during an NDL dive at any depth, which cannot be easily resolved simply by shutting down the isolator valve.

Regarding unknown buddies, twin tanks won't help you much. The mostly likely problem with an unknown buddy is buddy separation, in which case a DPV to help you look would be best. And the worst case is a panicked buddy who takes both of your 2nd stages as well as your mask off. Twin tanks won't help you then either. The main prophylaxis for dealing with an unknown buddy is to make sure you are the stronger.

Once you get used to 2 of everything, you become spoiled, and there is simply no going back anymore. I am thinking about selling my single steel tanks, since I hardly use them anymore. I am guessing the local paintball crowd would be the most interested in them.

Additionally, my fiancee makes sure that I dive with my twin tanks anytime I dive solo. I explained to her how 2 of everything works, and therefore she holds me to it for solo diving. If I want to dive with a single tank together with a 40 cu ft stage, I need to find a buddy. She is not interested in scuba; we do other things together, like hiking, jogging, tennis, etc. But sometimes she sits on the beach in her beach chair under the beach umbrella and reads, while she waits for me to finish my dives. If I did not have twin tanks on, she would be too worried about me diving alone.

nereas:

Regarding unknown buddies, twin tanks won't help you much. The mostly likely problem with an unknown buddy is buddy separation. The worst case is a panicked buddy who takes both of your 2nd stages as well as your mask off. Twin tanks won't help you then either. The main prophylaxis for dealing with an unknown buddy is to make sure you are the stronger.

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WHAT!?!?!?!

If you're separated from a buddy, you NEED a redundant air source, since one will not be available. Twin tanks completely fit that, even you stated this when mentioning solo. This is really basic stuff, please stop misleading newer divers who might be reading this.