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Zitate aus der VDH-Gruppe:
It is not the position of the regulator and the mouthpiece, but the position of the regulator to the center of the lungs. Even when a diver is horizontal, his back is rarely horizontal. Our back form an "S" curve, and if we lift our heads up to see, normally we are not completely horizontal when swimming horizontal. What we are trying to do with the double hose regulator is to position the regulator box as close to the center of the lungs as possible.
Dropping the tanks allows the regulator to be positioned between the shoulder blades. Any higher, and the regulator will be off the surface of the back, and separated by several inches of water pressure from the back.
This consequently increases the breathing resistance.
If a diver is swimming slightly head-down, with the tanks completely horizontal, your case holds. But if the diver kneels, or even looks up, it does not.
The best way to visualize this is to take any photo of a diver and mark an "X" in the center of his/her lungs. This is the "ideal" position for a regulator, which none fits. Draw a horizontal line (with respect to the earth, not the diver), and measure the number of inches over or under that line where the regulator box is located. That is the positive (if under) and negative (if over) inches of water pressure that the regulator positioning exerts on the effort of breathing. It is only relavant to the mouthpiece if the regulator is located there too (as with a "single hose" regulator).
SeaRat, John c. Ratcliff:
The new Aqualung Mistral double hose regulator is revolutionary, in that the regulator position can be changed independent of the tank/valve positioning because of the hose to the second stage. This means that it could be chest-mounted, which would be a significant advantage. It also means that the tank position can be higher, to maintain contact with the valves, yet allow the regulator second stage to be positioned close to the ideal back position.
Longer hoses only work if the regulator's venturi system can handle them.
The regulator I used, the DX Overpressure single stage regulator, has a venturi hose within the intake hose, and allows a longer hose to be used without loss of venturi effect.
SeaRat, John c. Ratcliff:
The hose length was a manufacturer's decision. Some were too short (the original DX Overpressure Breathing regulator by USD, for instance). Some were quite long (Snark III hoses); this allowed them to be placed low on the back, but few divers did. The longer the hoses (with the exception of the DX, which had a venturi hose within the main intake hose) the worse the breathing characteristics. So it was a balance between performance of the regulator, positioning, and comfort. US Divers finally got it about right, but it took them awhile. Healthways did not; the Scuba Deluxe has hoses that are too short (the original Scuba regulator, with the Hope-Page mouthpiece, was about right). Dacor's hoses were also too short, in my opinion. A really nice combination was the Healthways Scuba Deluxe (with the venturi tube), and US Divers hose/mouthpiece system.
SeaRat, John c. Ratcliff:
The lower on the back, the better the double hose regulator will breath.
The placement is dependent upon your harness system, the length of the hoses, and the size of the tanks. You will notice that the "ideal" position is as close to the center of the lungs as is possible, and any deviation is felt as increased suction effort for either single or two-hose regulators.
I have about 7 (plus some parts) two-hose regulators, and one is a DX Overpressure single stage regulator by USD (the predicessor to the Mistral).
I put new hoses on it which were made for SCBAs (fire department self-contained breathing apparatus). These hoses are longer, and I've experimented with lowering the position of the regulator to the extreme. I found that the two-hose regulators breath best as low as you can get them. A reasonable compromise is to get them below the nap of the neck, and between the shoulder blades.
Backpacks will hold the regulator an inch or two off the back, and thereby increase breathing resistance by raising the position of the regulator. The old "military" harness for twin tanks is the ideal setup, as the regulators well nearly touch the diver's back. Cylinder size also affects the performance; the larger the cylinder, the higher the regulator, and the greater the water resistance for breathing with a two-hose regulator.
Once you get your double hose in great working order you need to get it placed properly on your back to allow the regulator to perform its best and provide you with the easiest breathing possible. As you can see by the pictures the regulator is much lower on the divers back than a standard single hose regulator would be. This is the optimum position for a double hose regulator.
The 2nd component is to get it as close to your back as possible without touching it. You may have to experiment with various backpacks as most of the ones manufactured today will place the regulator up much too high and too far away from your back. The original harness and early model backpacks are the best answer for this. Incorrect placement of the regulator is one of the main causes for divers condemning the double hose regulator and calling it hard to breathe.

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updated: 28.12.21