Underwater Trim

[rstofer]

I have been reading through my copy of "The New Science of Skin and Scuba Diving" and nowhere does it mention trim. It does mention 6" or 7" of breathing effort and that is clearly apparent in my pool sessions.

As 6" or 7" of cracking pressure FAR exceeds any modern single hose regulator, is it common to swim in a somewhat heads-up position to overcome this?

Or, is it simply a matter of "suck it up!" (pun intended)?

I don't really like the 'absolutely flat in the water' trim anyway. Among other things, I can't raise my head far enough to see things out in front of me. In thinking about my previous diving with a jacket BCD prior to my BP/W, I believe I was almost always in somewhat of a heads-up position.

Second item: I understand that with my conventional tank and single hose reg, I should be able to reach the valve. Well, I can't. But it's even worse with the DH rig because the tank is so much farther down the back and the valve handle is behind the can. Any thoughts on this? I had the idea that if I did have to reach the valve (or J valve), I would just loosen the straps and swing the tank around. After all, those quick release buckles come apart pretty fast.

Can anyone reach their valves?

Richard

 

[simonbeans]

Why do you think they invented J-valve RODS?

 

[rstofer]

Why do you think they invented J-valve RODS?

Yup! That part is easy although I don't currently have the rod. But there are a number of folks that state a diver simply must be able to manipulate the tank valve. I certainly don't have the flexibility to do that.

Now, for the doubles manifolds, it might be a little easier because the knob is above the reg and in the center of the manifold. But I haven't go to doubles yet.

Richard

 

[simonbeans]

I have never been able to reach back and activate the reserve lever. J-valve rods are available from VSS. Or, just buy some stainless rod or even brass at or local home center and bend your own. It definitely is not difficult. If those that tell you that a diver simply must be able to manipulate the tank valve can all do that, goodie for them. I can't, and I haven't stopped using a J-valve with a rod yet. Oh well, I see now that this vintage stuff is really dangerous. I best get a refresher from PADI and learn the right way. (Sarcasm intended)

 

[captain]

I have never had a reason to reach back to operate a valve and if I did have the need I would simply slip out of the harness and swing the tank around.

 

[herman]

If your reg is at 6 or 7 in water column cracking pressure, you need to have it looked at or you need to correct the way you are wearing your harness. At that amount you are having to suck really hard. 2 or 3" is more like it UW, all my double hose regs are in the .5 to 1.0 range above water and an educated guess, around 2 or less UW.

Never tried moving the valve when wearing an old BP and a DH but it's no problem on my modern gear and I suspect none on the vintage either. With no crotch strap, I just grab the bottom of the tank with my right hand and pull out and up on the tank which shoves the valve higher and into the center of my neck, then reach over my head with my left hand. Fairly easy that way. Reversing that should give you access the the reserve valve.....Gotta try that tomorrow just to see.

 

[INFIDELxx]

There was an in depth thread about this on VSS. I cant find it now but I remember reading and seeing pictures of the US Navy mounting the manifold on doubles with the lever upside down and no J rod. They would simply reach back and push the lever up. If they could not reach they would raise the tank by pushing up with one hand and actuating the lever with the other. By having the lever reversed it would prevent unintentional actuation.

Enlarge the picture and you can see the position of the lever.

 

[Nemrod]

I trim for as horizontal as possible. But you are right, it was never a subject in the old days like it is now, typically, a no BC diver will naturally trim slightly inclined, head up, especially when swimming.

There is no reason to reach a valve on a single tank rig or on non isolating, single outlet double tanks, what are you saving the air for? This "reach the valve" fuss began with techie concepts where a double outlet, isolating manifold is often used and the need to isolate for overhead environments. This is not an issue with an open water, no overhead diver. Turn your air on and leave it on.

The reason double hose regulators have the LARGE diaphragms, powerful venturi and high mechanical advantage is specifically intended to overcome the distance from the center of your lungs to the regulator diaphragm.

Twin hose rigs that had poorly designed levers (Healthways, Dacor) and small diaphragms like the Trieste have issues that require expert tuning to overcome and maybe a 21st Century silicone diaphragm for the Trieste.

N

 

[Nemrod]

One other thing before I vamoose for a bit. The breathing technique with a double hose is to take long slow inhalations and long slow exhalations, all most to the point some might accuse you of skip breathing (but you are not). The silly "panting" that most single hose divers do and most are taught is not efficient, especially with a double hose. Yeah, they are told to breath "naturally" but most people pant with shallow breaths. You can get away with that with a single hose plastic fantastic but not with a twin hoser. It will wear you out, trust me, if you pant with a double hose.

This style of breathing is very effective with a twin hose, you only have to overcome the "cracking" effort once in (X) seconds instead of maybe three or four times in (X) seconds if you pant. The twin hose with help from a powerful venturi will easily flow lot's of air once the flow is initiated. This concept coupled with the easy exhalation will reduce your effective WOB into the range of the best plastic Apecxo junk they make.

N

 

[DA Aquamaster]

A variant of this one seems to get argued about once a week on one vintage board or another with all sorts of theories coming into play as to why a double hose reg seems to breathe harder under water in some orientations.

With a single hose reg is is a matter of the relative position in the water column between the center of the diaphragm and your mouth. Since that is basically zero the limiting factor is the case geometery fault characteristics of the second stage that limit how lightly the valve can be adjusted before it freeflows under water due to the realtive positions of the diaphragm and exhaust valve.

A double hose reg has the diaphragm at one level and the exhaust valve at the same level, so as long as the exhaust valve is properly located, there is no significant CGF issue. Where it is different with a double hose is that you take the air out of the hose somewhere in the middle of that loop and that point may be higher or lower than the location of the diaphragm.

It is that difference then between the depth of the center of the diaphragm in the water column (which determines the pressure of the gas in the whole loop) and the depth of the mouthpiece where you are sucking the gas out of the loop that determines the suction needed for the initial inhaltion effort. If that position is 2-3" lower in the water column you will have to exert a suction force equal to that 2-3" of water plus the .5" to 1.0" of water pressure needed to activate the valve itself and initiate the air flow.

So...if you were perfectly level in the water with your faced angled forward, the diaphragm would be maybe 6-7" inches SHALLOWER in the water column than the outlet in the mouthpiece. So to get air to flow, you'd have to create suction equal to 6-7" of water to begin drawing the air out at that level. That would really "suck" to have to do that for the entire dive.

Conversely if you were in perfectly horizontal trim but rolled over so you were face up and on your back, the diaphragm would be 6-7" deeper than the mouthpiece and the gas in the loop would be at a pressure 6-7" higher than the pressure at the mouthpiece, so it would feel like it was trying to inflate you.

However in the normal swimming positon if you are trimmed slightly head up, it tends to place the diaphragm deeper in the water column relative to the mouthpiece and the air in the loop consequently is at a pressure a few inches of water greater than before. So you have to suck less to start the air flowing.

Now, the greater the distance along the axis of your body between the diaphragm and the mouthpiece, the smaller the trim angle needed to get the diaphragm deeper in the water. So rather than wearing the reg way up high behind your neck or even the lower portion of your head as is the fashion with single hose regs (and where you could reach the valve), you wear a double hose reg much lower on your body - down between your shoulder blades with the top of the can near /slightly below the base of the neck. That position allows you to place the diaphragm within an inch or two of the mouthpiece and reduces the cracking effort to something in the 1.5-2.0" of water range.

And as Nemrod states, once the flow is initiated you get the benefit of the venturi effect of the air being shot directly down the inlet hose that in turn "sucks" the diaphragm in for you and maintains the flow with little effort on your part.

So in effect a low mounted can and a slightly head up trim position are essential elements to get good cracking efforts from a double hose reg.

The need for slightly head up trim and a low mounted can is one of the major reasons I can't really take my double hose reg cave diving. Technical divers using rebreathers would encounter the same issue, except the counter lung is located so that it is at the same level on average as the mouthpiece in a level trim positon so that the inhalation effort is reduced. A needed approach as it already takes a certain amount of suction to pull the gas through the loop and the scrubber.