Blaiz wrote:Ok, this complete noob is going to ask a couple of dumb questions now.
Most (all) of my experience has been on a single tank (pony occasionally).
I understand that having doubles, well, doubles one's gas tank. Which is useful for...going deeeeeep? REALLY long dives?
I'm gathering from this thread that two tanks are required to balance trim on a sidemount rig. Could one, in theory, just have two AL40s and do a shorter, shallower dive? (not saying i want to trade my 95s/119s for that setup, just clarifying)
Is it possible to just stuff one tank on one side and go dive?
What does it mean to tether one's [stage bottles]?
Having two tanks has two significant advantages ... providing you with more gas (for going deep or staying a long time) and providing you with redundancy (in case you lose access to one tank) ... and so you choose your cylinders according to the dive profile. In any case, you're going to want cylinders that will allow you to maintain adequate reserves in BOTH tanks to get you and/or your buddy to the surface in the event of a failure. So for that reason I wouldn't recommend AL40's unless you're planning a simple, shallow dive at someplace like Edmonds UP, because you're going to breathe that tank down to your reserve level rather quickly. Also keep in mind that AL40's are pretty buoyant ... as are AL80's ... and you have to weight yourself accordingly. For cold water applications, I think steel cylinders are the best choice. If you want smaller cylinders, then something like an HP85 might be the way to go. On my two Gulf dives I used LP77's and they were very nice ... smallish and fairly lightweight, at least compared to the LP95's I've used on my other dives.
In theory you can dive sidemount on a single cylinder ... you just have to distribute your weights to offset the buoyancy weight of the tank. If you're going to go that route, I'd recommend stowing a backup second stage, just as you do with your backmounted singles reg. I wouldn't, personally, make this choice, because it removes much of the safety net that the sidemount rig offers ... but we all make our own risk assessments and decide accordingly.
Tethering means that you clip one or more bottles onto a small loop of rope (called a "leash") and clip it onto a D-ring so that it trails behind you as you swim along. When you need that bottle (or one of those bottles), you unclip it from the leash and bring it forward, clip it onto a shoulder D-ring where you can manipulate the valve, and use it according to your needs. This usually also means you'll be moving a spent bottle back onto the leash as part of the exercise. This is an advanced exercise ... typically done by people who are diving in places where multiple stage bottles are used to extend your bottom time. A stage typically has the same mix as your main cylinders ... or perhaps something intermediate between your main gas supply and your deco, for those going to very deep depths where the main supply has a very low oxygen content. In that case, it's generally referred to as "travel" gas. Anyone using that approach had better, before then, have become completely comfortable with moving bottles around and keeping mental track of which bottle they're breathing off of at any given time ... because deploying the wrong gas can end up killing you if you're not careful. So anyone contemplating the need for a leash needs to be well beyond recreational skills before that technique becomes practical to them.
For typical recreational applications, such techniques would never be needed. I've done possibly as many as a hundred dives in the 180 to 240 foot range and only once needed to use a leash (on the AJ Fuller) ... and even then it was more a convenience than a necessity.
Those would would contemplate such questions would be much better served taking a class from a qualified instructor (preferably one who actually does those sorts of dives) than asking a relative sidemount newb on an internet forum ... because although the complexity of such things isn't really any greater than with backmount, it is different. And even among those who are using them, there are different approaches and philosophies to mounting stages on a sidemount rig. Rob showed me two different ways, and explained the advantages and drawbacks to each. I decided that mounting them on top of the main tank worked best for me.
... Bob (Grateful Diver)
its just different styles of diving people.. like some people prefer touring bikes to roadster to sport touring.. to all his own..
