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Frankly until you build a little time you can't really evaluate all the choices.
First take a look at *why* divers move to a BP&W in the first place.
Less clutter, easier to achieve horizontal trim, easier to vent while horizontal, less inherent buoyancy etc.
Using a doubles wing or a "dual purpose" wing with singles will reduce or negate many of the advantages of a clean, well designed, properly sized BP&W.Trying to save a few $$ now buy purchasing a dual purpose wing that isn't a good singles wing and is also very possibly the not the wing you need for doubles *either* is simply false economy.
BP&W's can be confusing, and there are lots of possible permutations, I'd be happy to walk you through the choices.
Tobin
Why do you need a system? Get the back plate you like the best, build a harness out of webing and hardware and get the wing which fits your dive style. Nothing says you can't put a diverite wing on a halcyon plate.
I'd have to say that the last line I've quoted here says the most about why the majority of divers that stay within simple recreational diving also stay with jacket style BCD. Walking a person through the choices online or over the phone is not the same as having someone diving with you to see what difficulties you may be having and help you make changes on the spot. The limits built into a jacket style BCD simplify the choices. Yes, this may be a less then optimal dive experience but one that may be easier for a less experienced diver to manage. A more experienced diver (The reason I included the first line of the quotes) on the other hand having a background to draw on may be better suited to manage the choices and judge the results to their dive experience.
I agree with you on the fact that a dual purpose wing may negate some of the advantages of the BP&W system but this too will be manageable to an experienced diver and give them the property they want. (A less costly overall investment with a wider usable dive profile range) Many other manufacturers understand that some divers want this versatility and have devised products that fill that area and lmit the negative effects. OMS and Dive-Rite just to name a couple. The users of these products have decided the trade offs of these products fill their needs. If there weren't plenty of divers that like these products it wouldn't be practical for manufacturers to offer them.
Scuba is strewn with products that appeal to the pre-purchase perceptions, not the post purchase reality.
I have no clue what your point is here. The choices that need to be made pre purchase such as plate size, plate material, wing capacity etc. do not require I eyeball the diver in the water. I've only made recommendations for a few *thousand* divers, with virtually zero returns.
Is there no place for a dual purpose wing? Maybe the experienced diver who will be undertaking a worldwide backpacking tour that involves diving doubles and singles needs on of these wings.
Of course these experienced divers don't splash about on SB asking about what they need in terms of wings. They *ALREADY KNOW* what compromises in terms of ease of use and performance they will be suffering by using such a wing.
Much of my day job is helping divers choose the gear that will give them the best possible experience in the water.
Selecting the correct doubles wing requires the diver know:
What cylinders they will be diving & The buoyancy of the exposure they will be using with these doubles.
How many divers considering the purchase of their First BP&W have any clue about which cylinders they might double up?
In my experience vanishingly few.
How many divers considering the purchase of their First BP&W even know what the buoyancy of their current suit, much less the buoyancy of the suit they will need with the extended exposure times typical of doubles?
In my experience this number is *ZERO*
Much much better to postpone the purchase of a a doubles wing until the diver is much much closer to actually diving doubles.
After they have a clue about what cylinders they will be diving.
After they owned a drysuit for a while and understand that longer dives = thicker undies.
~Once a week I have the eager new diver that wants to order one of everything (or two of everything if they have girl friend that dives) These folks assume they will have worn out single tank diving a couple months after BOW and will be in dubs in the very near future.
My standard advice is to wait to buy the doubles wing until they know more.
Who do you think the target market of the dual purpose wing really is? The handful of traveling divers for whom a second wing (2 lbs extra baggage) is just too much or the uninformed new diver who is certain they will need a doubles wing "very soon"?
Scuba is strewn with products that appeal to the pre-purchase perceptions, not the post purchase reality.
Tobin
There are other choices that with experience can be made prior to purchase such as how many d-rings, what type of weight system and how to add necessary accessories.
A new or inexperience diver may be better served by a Jacket style BCD and the limits of it's design. An experienced diver however may have knowledge of how extended dive time effect their exposure suit requirements whether in a wetsuit or drysuit. An experienced diver may have already used tanks of varied capacities and pressures and know the buoyancy characteristics involved. An experienced diver will know what accessories they will need for a given dive profile.
Maybe you are by your answers explaining why someone looking to purchase their first BP&W should be an experienced diver.
The newer diver doesn't need to dive for ten years in a jacket to develop enough experience to be able to buy the right BP&W, they just need a salesperson with a clue.
Tobin