Tank Position

[John C. Ratliff]

String,

I'm curious; have you seen or experienced yourself an uncontrolled free flow due to freeze-up? If so, what regulator and circumstances?

SeaRat

 

[DiverBuoy]

Now that I've been to a few dive parks and have observed other divers set ups I've been curious on if there is a correct way to mount your tank to your bcd??? I've noticed divers with there tanks mounted to were it looks as though the valve is half way down their back with the bottom of the tank is past their butts. The way they showed us in class was to keep the top of the bcd about even with the top of the valve. And if it worked out right you can put your BCD handle around the valve to hold it in place. Anywaze what are the advantages and disadvatages of the placement? Just seemed as thought there was no rhyme or reason to it.

David

Trim isn't taught at all in the program itself with PADI, in fact if anything their videos and written word indicate a more or less inaccurate position of your body in the water. But with this thread you've really opened one of the more passionate subjects on ScubaBoard, as this forum is the first place many divers have ever even learned of the concept of trim/attitude position in the water. This allows you to move your head up or down in terms of being viewed from the side by another diver. This gives you tremendous control over your movement and control in the water.

One of the significant adjustments for trim that can be made is to change the height position of your tank(s) in the tank band(s). PADI does teach to have the tank valve positioned at the top back of the BC, let's call this a starting point, thought it's actually wrong for many divers. It should be stated that this effort is about establishing a "more" horizontal position in the water, which some agencies and many divers recognize as the proper position of the divers body in the water.

In addition to figuring out your trim (gravity and center of balance) by moving the tank up and down, you can also add small weights to other places on your rig, to also aid in this effort. Some have found some advantage to tank neck weight rings or weight disks that may be mounted to the bottom of your tank (or under the tank boot). Typically most divers associate these only with neutralizing the buoyancy of their scuba cylinders but these can also play a role in establishing proper trim. Especially with a single scuba cylinder because of this being symetrically positioned already down the center line of their bodies.

You can of course play with these things on your own, but having a buddy assist with feedback can help as well. You can find some videos on www.gue.com which show divers with this "more beneficial" trim position. The thing about your new awareness that proper trim was not taught to you in your open water course is this becomes a first step to a whole new door of knowledge you were not exposed to. All I can offer is don't let your questions stop there.

 

[DementialFaith]

In order for students to learn how/where to position their tanks in respect to their bcs is, as people have mentioned, a process they need to learn through experience since everyone is different. However, here we teach students to keeps their tanks to a somewhat low height to avoid having their head hitting on the first stage because, frankly, it really hurts to hit the soft part of your head on a block of aluminum alloy. I personally used whatever height the bc back handle gives me because it was the easiest to set up and I 've grown to that position by now and I'm fine with it.

 

[DiverBuoy]

In order for students to learn how/where to position their tanks in respect to their bcs is, as people have mentioned, a process they need to learn through experience since everyone is different. However, here we teach students to keeps their tanks to a somewhat low height to avoid having their head hitting on the first stage because, frankly, it really hurts to hit the soft part of your head on a block of aluminum alloy. I personally used whatever height the bc back handle gives me because it was the easiest to set up and I 've grown to that position by now and I'm fine with it.

That's reasonable, unless they cannot reach their tank valve over their shoulder.

 

[PerroneFord]

Hit your head? You mean you don't rest your head on your regulator? Like an aluminum pillow?

 

[bradshsi]

actually i was thinking about changing it to " im getting whiplash from my burps"

I've had evenings like that. I found that drinking more than 8 pints of Newcastle Brown Ale tended to exacerbate it...

Oh to be a student again

 

[Mustangfreak]

I tend to have it a little low so I dont keep banging the back of my head on the 1st stage.

 

[Solitude Diver]

I've noticed divers with there tanks mounted to were it looks as though the valve is half way down their back with the bottom of the tank is past their butts.

Probably people heading into/out of the water, where tanks typically hang low due to the weight. Not necessarily where they ride when in the water.

As indicated above, it does happen that a DM or boat crewman turns off a diver's air with the result that about 3 breaths later they suddenly get nothing

What the heck is a dive master doing turning off someone's air? That seems bizzare to me, and bordering on sabotage.

I don’t think it is intentional shut-offs that are the concern. I’ve read a few threads now that included posts with people who have experienced boat crew members “accidentally” turning off air (thinking) they were actually turning it on.

In any event, people have entered water with valves off, and it is good to be able to turn on if necessary.

I'm curious; have you seen or experienced yourself an uncontrolled free flow due to freeze-up? If so, what regulator and circumstances?

During intentional free flows (student demonstrations, drills, etc.) I have had my Mark25/S600 reg’s freeze up a few times and turn into "uncontrolled" free-flows. At those times, using back-up/buddy air and shutting down reg to thaw can prevent total gas loss and end of dive.

 

[pt40fathoms]

I teach my students to start with the valve in a position where they can reach the valve while in the water. The question is often raised, "won't I hit my head on the valve"? My answer to this is "yes, if you suddenly jerk your head upwards to look at the giant octopus, or Captain Nemo's sub, otherwise be mindfull of the fact that it is behind your head, and learn not to jerk your head backwards."

Tank position does effect trim, and when positioned properly, it will go from negative to positive with little to no change in moment (the point at which it pivots). Positioning of your weights on your belts or placing non-ditchable weights on BCD's that provide for this option, or even directly on your tanks is the most effective means of adjusting trim, but tank positioning does play a role in trim as well.

In short, keep the tank valve within reach, and practice your valve drills *AFTER
* you have been properly instructed in how to do it safely. Don't just go out and start messing with your equipment while under water, that's how people get hurt!

 

[StSomewhere]

Just to show some others one of the reasons I continue to wear my scuba low, here's a 1969 photo of one of our parascuba jumps..

I'll remember that the next time I airdrop from 50 ft off of a helicopter.