Tank Position

[String]

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

Yes. Apeks TX-50 / DST first stage.

Happened when purging a reg slightly to top off a bag that someone else was filling at about 28m (~90ft) in 4c (39f ?) water shortly after theyd been serviced.

Ive also dived with people who have experienced freeflows under a few circumstances with a few brand regulators.

 

[John C. Ratliff]

String,

Those are all good reasons to be able to access the valve to turn it off. Thanks for the information. Just to tell people, even when wearing the tank lower, the valve can be accessed, but the technique is slightly different. Because there is slack in the shoulder straps, you can push up on the tank with one hand, while reaching around with the other.

Today's high performance regulators will produce enough air flow during free flow to lower the temperature of the first stage, and the second stage, below the freezing point of both fresh and salt water. Having the first stage isolated from the effects of potential freezing of water around it is one way of coping with the situation at the first stage. Some regulators have anti-freeze engineering in the second stage too. But when the regulator is out of the mouth, and being used to inflate a bag, as String states above, then the potential exists. This is a place to consider the Aqualung New Mistral for diving, as these situations would not result in a free flow freeze, as the second stage is also isolated from water.

I have experienced a slight freeze of the MR-12 second stage in the 1980s during a very cold dive in a fresh water lake. But this was not serious, and did not result in an extreme free flow. It did result in a small leak of second stage air during the dive.

SeaRat

 

[BurBunny]

Tank should be mounted so that you can reach the valves.

Don't mount the tank to adjust the trim, for if you are using an AL80 the trim will change as you consume the air. The AL80 is negatively buoyant when full, and positively buoyant when empty.

For instance: if you mount the tank high keep your trim head down, when the tank is near-empty it will actually start pulling your head up.

Tank should be mounted so that you can reach the valves, period.

Sorry, but I disagree. Given that yes, #1 priority is being able to reach the valve, and #2 not injure head, there's nothing wrong with adjusting the position of the tank relative to the BC to assist with trim. You never want to adjust to the point that you can't achieve the first two priorities, but other than that, why not use every tool you have to maintain an optimal body attitude in the water? And yes, it does change during the dive, but for example early in a dive with a full tank a slightly head down attitude is easily compensated for with buoyancy, whilst later in the dive, especially for many women, "floaty feet" become a serious issue on safety stops and before, requiring the woman to arch her back to stay prone. NOT comfortable. Slight adjustments with the tank can often make a huge difference there.

 

[Charlie99]

1. One should NOT adjust the position of the tank to adjust trim at either full tank or empty tank. Instead one should adjust the tank position, with respect to the wing or BCD bladder, such that the change in buoyancy of the tank from full to empty does not CHANGE the trim. Then use other, fixed, weights to get the trim right, or move both the tank and BC bladder or wing with respect to your body to adjust the trim.

The reason the adjustment is relative to the bladder is that at the beginning of the dive you will have air in the bladder to compensate for the weight of the air. At the end of the dive the tank will be lighter and you won't have air in the bladder when near the surface.

2. If you analyze the whole trim thing a bit more, you will see that for someone wearing a wetsuit that compresses, there is an ideal position of the wing/bladder with respect to the body such that trim changes the least as you descend and ascend. In this case, buoyancy of the wetsuit distributed throughout the body goes down with depth, and is replaced by additional air put into the bladder or wing.

I first figured the above 2 items out by trial and error, and only then figured out why. Hopefully, by looking at the problem in this way, others can dial in trim, and changes in trim, with less trial and error.

 

[DiverBuoy]

2. If you analyze the whole trim thing a bit more, you will see that for someone wearing a wetsuit that compresses, there is an ideal position of the wing/bladder with respect to the body such that trim changes the least as you descend and ascend. In this case, buoyancy of the wetsuit distributed throughout the body goes down with depth, and is replaced by additional air put into the bladder or wing.

??

I must be missing something more subtle here. The issue you are addressing is certainly buoyancy related of that I fully agree, but I must be missing the direct connection with trim/attitude in the water.

It's one thing to discuss the aspect of trim as it relates to diving dry. For example a dry suit diver following PADI's instruction method of compensating for buoyancy using solely the dry suit inflator while maintaining a fully empty compensation bladder (which is not taught by other agencies) might cause an overweighted diver to become a giant unstable bag of air moving all around themselves as their trim changes. A dry suit diver using this method who then gets oriented in a heads down position may struggle to right themselves in the water because any air in the dry suit travels toward the feet where there is no release valve, making correction much more difficult.

Regarding a wetsuit the compression of the cells in neoprene certainly impacts buoyancy but since these gas bubbles are held in place by the material and as such do not move freely about the body altering one's trim. Really other than minor changes in thickness in arm pits, crotch, and neck ... the wet suit is generally the same thickness (read: water weight displacement) all over, and I believe would have very little direct impact on your trim. Of course in some cases longer legs require ankle weights, these same legs in a shorty may not have the same issue.

 

[Charlie99]

DiverBuoy, look at what happens as one descend while wearing a thick wetsuit.

On the surface you have buoyancy distributed all over your body from the wetsuit. As you descend, that buoyancy is reduced as the wetsuit is compressed. To stay neutral, you replace that buoyancy by putting air into a BCD bladder. To not have any effect on trim, that air bubble in the BCD needs to have the same center of buoyancy as did the distributed buoyancy of the wetsuit that it is replacing. While the air in the wetsuit stays in place when it gets compressed, that buoyancy needs to be replaced. It gets replaced by air in the bladder or wing --- so in one sense it is the same effect as if the air in the wetsuit migrated over into the BCD bladder (It doesn't physically do this, but the net effect is the same as if it did.)

Do this simple mental experiment ------ imagine that the BCD bladder was down at your feet. Now assume that you are at 15', neutral, and properly trimmed horizontally with no air in the bladder. What happens when you descend and put some air into the bladder to compensate for wetsuit compression? Pretty clearly, you would have a floaty foot problem.

Repeat the mental experiment with the bladder up on your head --- pretty clearly you would have a floaty head/foot down problem.

Somewhere between those two extremes is the right position to minimize trim change. That's why that standard BCD or BP/wing is designed with the floatation on the upper torso.

 

[Phil TK]

For instance: if you mount the tank high keep your trim head down, when the tank is near-empty it will actually start pulling your head up.

Well not really, unless you are using liquified air. The tank's centre of bouyancy stays near enough the same throughout the dive. It is the overall centre of gravity that I think you are thinking of -but that hardly moves either -just a fraction nearer the tank and further from the divers own c of g.
For the best valve access, mount your tanks upside down -and use specially configured hoses to suit.
Phil TK

 

[MoonWrasse]

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

The best way is the Slim Pickins position. Large tank in front, so that you can ride it on the surface

 

[John C. Ratliff]

...For the best valve access, mount your tanks upside down -and use specially configured hoses to suit.
Phil TK

This has been used in the past. I own a UDS-1 scuba from U.S. Divers, which is a three-tank system with a unique manifold/first stage combination. It is mounted in a yellow fairing, and the valve position is on the bottom. It initially had buoyancy problems, in that the valves were heavier than the tank, and the top of the tank tended to ride up off the shoulders. I corrected that with six pounds of weights, and I now really like the setup.

SeaRat

 

[DiverBuoy]

DiverBuoy, look at what happens as one descend while wearing a thick wetsuit.

On the surface you have buoyancy distributed all over your body from the wetsuit. As you descend, that buoyancy is reduced as the wetsuit is compressed. To stay neutral, you replace that buoyancy by putting air into a BCD bladder. To not have any effect on trim, that air bubble in the BCD needs to have the same center of buoyancy as did the distributed buoyancy of the wetsuit that it is replacing. While the air in the wetsuit stays in place when it gets compressed, that buoyancy needs to be replaced. It gets replaced by air in the bladder or wing --- so in one sense it is the same effect as if the air in the wetsuit migrated over into the BCD bladder (It doesn't physically do this, but the net effect is the same as if it did.)

Do this simple mental experiment ------ imagine that the BCD bladder was down at your feet. Now assume that you are at 15', neutral, and properly trimmed horizontally with no air in the bladder. What happens when you descend and put some air into the bladder to compensate for wetsuit compression? Pretty clearly, you would have a floaty foot problem.

Repeat the mental experiment with the bladder up on your head --- pretty clearly you would have a floaty head/foot down problem.

Somewhere between those two extremes is the right position to minimize trim change. That's why that standard BCD or BP/wing is designed with the floatation on the upper torso.

No no I think you are missing my point. The discussion is about trim. Depth alone causes you to add more compensation air to your bladder, ultimately the tiny (and I emphasize) tiny effect of losing buoyancy in a compressing wetsuit will have a very limited effect on how much air you are adding to your bladder. And all of this still deals with pure buoyancy issues. You still haven't demonstrated how wet suit compression relates to trim as a unique contributing factor.

Of course the "air bubble" as you put it in your compensation bladder effects trim in that it changes your center of gravity but that will occur as you go deeper with little regard to the miniscule amount of gas added to your bladder to compensate for the effect from the compression of your wetsuit. The point is that neoprene is not like an unstable rolling air source and is generally designed to resist compression, though inevitably it does compress. In fact besides arm pits and trim areas the wetsuit is generally a uniform thickness over a large surface area which as it compresses will not move around in an unpredictable way changing your head up or down position or as we say your trim. I still just don't see the relationship, sorry.