Honking Scubapro A700? Your experience?

[divin'dog]

Just came back from Honduras where I took my brand spankin' new A700 and MK17 for a dive vacation. Love the A700, breathes great but some noticeable quirks. When above water, it honks. Makes breathing a little more difficult to get a smooth breath while getting ready to jump in or using it on the surface. Have also noticed that if I'm horizontal (face down), the breathing is great, but if I pick up my head to look ahead or look up, it seems that the draw is just noticeably more difficult than when I'm looking down.

Anyone have the same experience? I was told the honking is normal for the A700...? Isn't there something that can be done to eliminate that? And why would it be slightly more difficult to draw a breath when going from horizontal to vertical? Other than that, I'm happy with my new setup.

 

[halocline]

There are two things you're referring to. The honking is just some resonance between the diaphragm and the regulator case. It's too bad this is happening on the A700; I always kind of assumed it was limited to plastic case regs. I've never experienced it on my old metal case SP 2nd stages, but they have a rubber purge cover that probably dampens the diaphragm quite a bit. Anyhow, it's harmless, it won't happen in the water, don't worry about it. I know its a bit annoying, but so many things in life are...

The looking up and experiencing the increased resistance is due to something called case fault geometry, and it has to do with the relative difference between the depths of the diaphragm and the exhaust valve. It's complicated to fully explain it, but think of this; the diaphragm is responsible for keeping the dry side of the regulator equal to ambient water pressure. If the internal pressure drops (like when you inhale) the diaphragm collapses, valve opens, and the reg fills with air.

The exhaust valve is responsible for not allowing the internal pressure to go above ambient water pressure; when it does, the exhaust valve lifts off the case and allows some air to escape.

When you're looking down, the exhaust valve is shallower than the diaphragm. This means that the internal pressure in the reg is slightly lower than the ambient water pressure at the center of the diaphragm. This makes it easier to inhale; pressure is a little lower. When you roll over and look up, the exhaust valve is deeper, and so you're working against higher pressure in the dry area of the reg, which means more pressure pressing against the diaphragm, more pressure to overcome to collapse it and open the valve sending air your way.

There are other factors that at least one very smart person believes; that's the difference between the reg depth and the depth of your mouth (or lungs) is responsible for this. As much as I want to agree, I can't. That's because my D series regs, which have a coaxial diaphragm/exhaust valve, do not breathe any more difficultly when I'm in the position that would normally cause this to happen. The only thing that's different is that the design of the reg puts the exhaust valve and diaphragm center at essentially the same depth in any position.

 

[sterlingMarineService]

honking is normal above water because its not wet.....

 

[DA Aquamaster]

As matt boy indicate both are "normal". The honking diaphragm above water can occur in any diaphragm second stage and has to do with residual stresses on the diaphragm. Normally, if you remove the cover reseat the diaphragm and replace the cover it will go away. It often also goes away over time on its own.

A way to prevent it is the wet the dipahragm when you seat it, but that is not recommended as it increases the potential for the diaphragm to become unseated during assembly.

I have epxlained Case Fault Geometry a few times over the years in some long and detailed posts - but not in the last year or two. Do a search of my posts (posts, not threads) for "Case Fault Geometry" or "Case Geometry Fault" or perhaps "D400" and you can probably find them. Then maybe have a mod sticky the most useful one.

I have also encountered the "position of your mouth relative to your lungs" comment from, as Mattboy decribes, a very smart person and it has bothered me that he persists in thinking that way as the physics of the argument just do not work.

 

[cbrich]

Both of my A700 honk above water, but goes away after it gets wet. I have not notice breathing effort while I am looking up. I had to get both of regs adjusted after my first dive, I like a little resistance.

 

[Big John]

Mine honks like a goose on a spring morning, but i wouldn't trade it.

 

[Belce]

I use the A700 with the Mk25 and did just a fab dive on a wall here on the island. Did big parts of it swimming on my back and it was perfect.

 

[MJJ]

Oh, hey. I have made zero dives (well, 2 in a pool) so I'm what you would call "long on theory, short on practice." But already I'd observed what I considered to be a positional difference in breathing and concluded it was the difference in depth between lung and regulator. I see there's disagreement. This isn't the thread to chew on this, but I want to. Can you guys point me to the thread where we are debating this?

 

[halocline]

There have been several, if you do a search (you might need to get a little creative about search terms) you'll find them.

If you are vertical in the water, heads up, and then go vertical, head down, you'll probably find very little difference in how your reg breathes compared to the difference when horizontal facing down and horizontal facing up. Yet, the two vertical positions represent the largest difference in lung/regulator depth.

There might be something to the difference in depth between mouthpiece and diaphragm; actually the part of the diaphragm where it contacts the lever. If you ever try diving a doublehose reg, you'll immediately notice that the relative depth of the mouthpiece and diaphragm has a strong effect on how the reg breathes; in fact one way you purge a doublehose is to simply lift the mouthpiece up and it will start to flow.

I used to think that the buoyancy of the air in the reg played a role as well; think about it, you have an air bubble in the reg case, which is pushing in all directions equally as gas does, but there is also the net buoyancy of the air. In a fixed container this would not matter, but the diaphragm moves in response to changes in pressure so it's not fixed. I honestly don't know if this plays a role or not. But I'm convinced that case fault geometry plays a role, there's really no way it could not once you really look at the principles behind it, and my experience with the D series regs vs conventional 2nd stages (G250, balanced adjustable, etc) proved it to me.

It's an interesting topic.

 

[DA Aquamaster]

In simple terms (or as simple as I can make them):

First, how it works...

In short, water is pressing in on the diaphragm and it will continue to move inward until air stops leaving the exhaust valve and causes the pressure to equalize inside the second stage. How much air leaves the exhaust valve and in turn ow far in the diaphragm moves depends on the differences in depth of the center of the diaphragm and the top of the exhaust valve.

Consider your second stage in a normal swimming position - the purge cover will be facing 20-40 degrees down. Then visualize the differences in the water column. In this position the "average" water pressure acting on the outside of the diaphragm will be equal to the pressure acting right in the center of the diapharagm. (And this will be true in any position.) Now, compare that position to the upper edge of the exhaust valve and consider what it means.

On the average second stage the height of both of these points in the water column are fairly close (i.e. they are at the same depth). Depending on the angle, the exhaust valve may be a bit higher than the center of the dipahragm but it makes little difference as it takes some pressure to open the exhaust valve - in this case we will say it takes about .5" of water, but more on that later.

So in this position, this is what happens: As you finish exhaling, air continues to leave through the exhaust valve until the pressure on the highest point of the exhaust valve is equal to the pressure acting on center of the exhaust valve. Since there is little difference, the diaphragm does not really move in anymore than it woudl on the surface, so no extra pressure is placed on the lever. The result is a "normal" inhalation effort as all of the effort needed to press the lever inward and open the valve comes from the diver creating suction on the diaphragm. For the rest of the example, we will assume the seocnd stage is tuned to have an initial cracking effort of 1.0" of water (very good to excellent by most standards).

Next, consider your second stage facing straight down. The diaphragm is level in the water (all of it at the same depth) and is in fact as deep as it can get relative to the highest edge of the exhaust valve. In this position, the upper edge of the exhaust valve is about 1.0" to 1.2" higher than the center of the diaphragm. This means the water pressure outside the diaphragm is at a higher pressure than the water outside the exhaust valve. If the distance is 1.0" of water, then there is 1.0" of water's worth of pressure.

Since the exhaust valve takes about 0.5" of water pressure to open, there is about 0.5" of water difference in pressure remaining that must be equalized. To do this, air continues to leave through the exhaust valve until the pressure in the case is equalized. By the time this occurs, the diaphragm is pressing inward just like it normally does with the diver creating 0.5" of water worth of suction. This effectively pre-loads the lever and means that rather than having to apply 1.0" of waters worth of suction, the diver only has to apply 0.5" of suction - and from the diver's perspective, the reg breathes easier.

This also sets a lower limit on how easy the reg can breathe. If you set it too much below 1.0" of water, the diaphragm can press in enough to slightly open the valve and cause a slight freeflow.

Finally, consider what happens when you look up. The exhaust valve is now almost level in the water and is at it's maximum depth relative to the center of the diaphragm. The pressure in the case is actually greater inside the case than outside the diaphragm, so the diver has to apply additiona suction to overcome this slight positive pressure as well as the normal amount needed to open the valve. So the reg breathes harder in this position.

Now, where are the lungs in all this? They don't matter in terms of the pressure needed to open the valve. In any position the second stage is in, the lungs have stopped drawing in air when the pressure in them was equal to the pressure in the case of the regulator when it stopped delivering or exhaling gas.

Physiologically, it also does not seem to matter. Consider standing neck deep in a pool. The pressure at the center of your lungs is much greater than the pressure at your mouth, but you don't really notice it and you don't really notice it until you are under water a foot or so (setting the upper limit of the lenght of a snorkel.