2nd Stage Diaphragm Size

[jakehbk]

I've read and heard from multiple places that the size of the diaphragm in the 2nd stage is a big part of its performance. I was wondering if anyone could explain it fully with some kind of numbers involved with it. I hear that the diaphragm is more responsive, that a larger diaphragm means less effort to suck in to open the valve and more.

I own a Scubapro G250 and an S600. Both of them are well tuned and fantastic breathers in my experience. I don't know that I could tell the difference between the two underwater tho.

Or are the differences so small as to be negligible? Maybe they're so similar that a quick knock on a boats dive bench is enough to change it one way or the other? Or is it far more about the case itself?

Does a G250HP outperform the Gen1 S600? Is it because of the diaphragm or because of the case?

Hoping some science minded peeps can edu-muh-cate me! Thanks

 

[tbone1004]

think of a thumb tack.
The diaphragm is the top of the thumb tack, and the mouthpiece is the tip of the pin.

Mouthpiece size is fixed, in Scubapros case a smidgen over half a square inch, 0.5625 sqin
If the diaphragm is say 4" in diameter that has an area of 12.57, then there is a 22.35x increase in pressure.
If however that diaphragm is say 5" in diameter, then the increase is 34.9x
There is also the distance from the mouthpiece to the diaphragm which has a lot to do with the calculations, but that's assuming that the distance is the same.

So, because your lungs have to collapse the diaphragm, when the ratio between the mouthpiece and the diaphragm is bigger, less effort is required, but it is also able to flex farther which in the case of the Scubapro style second stage, means that the lever is going to depress farther which opens the valve more as the venturi kicks in.

In the specific case of the G250 and the S600, the case design has been modeled so that the work of breathing is essentially the same. The S600 has a smaller diaphragm, but it also has a smaller case volume, and also a smaller distance between the diaphragm and your mouth, once all of those variables are factored in, the total work of breathing becomes close to the same.

The diaphragm itself though will make a regulator more responsive, but in the case of second stages, most people aren't going to be able to notice the difference, especially when breathing the way you are supposed to be breathing.

In terms of the G250HP and gen1 S600 and specific performance, I do not have their specific work of breathing curves readily available. Some of the scubapro gurus like @DA Aquamaster may have them laying around and can comment on that specifically.

 

[Couv]

If I'm not mistaken, the S600 may have a smaller diaphragm diameter-but it is more cone shaped than the G250. The cone shape gives it more surface area than it would have if it were flat.

 

[tursiops]

If I'm not mistaken, the S600 may have a smaller diaphragm diameter-but it is more cone shaped than the G250. The cone shape gives it more surface area than it would have if it were flat.

I'm not sure the surface area is what counts (just the diameter)...but the shape will change the elasticity.

 

[jakehbk]

Wouldn't the ever increasing diaphragm size on our theoretical 2nd Stage mean that it would move inward less for a given suck, so to speak?

Is the more responsiveness just going to be at the start of inhalation? I can see that the extra force would make just cracking the valve a little bit easier, but would it then take more effort comparatively to open the valve the same amount?

 

[tbone1004]

Wouldn't the ever increasing diaphragm size on our theoretical 2nd Stage mean that it would move inward less for a given suck, so to speak?

Is the more responsiveness just going to be at the start of inhalation? I can see that the extra force would make just cracking the valve a little bit easier, but would it then take more effort comparatively to open the valve the same amount?

it is able to deflect more, and the venturi helps to move it the rest of the way. The venturi in the regulator body is critical to getting everything to function as well as it can

 

[lexvil]

Just for fun take the mouth piece off of both, fill each with water and measure the volume.

 

[Yawana-Dive]

The S600 has a smaller diaphragm, but it also has a smaller case volume, and also a smaller distance between the diaphragm and your mouth, once all of those variables are factored in, the total work of breathing becomes close to the same.

Ah! This makes sense.
I usually dive with an assortment of Scubapro regs that circulates between staff, rentals and myself. Last year, during a busy period when all our gear was rented out I dug up an old set of Apeks Flight I had lying around. A regulator I dislike for a bunch of reasons I won’t go into here, it has a small second stage with a small diaphragm, but I was pleasantly surprised that it was a very easy breather. Obviously the diaphragm to case volume ratio is important.

 

[Nemrod]

The basic equation is:

P=F/A

Where P is the pressure, F is the Force and A is the Area.

A larger diaphragm will have more Area and will generate more Force for a given Pressure. This is why a double hose regulator for example has a huge diaphragm as the regulator needs the mechanical advantage being located above the lungs rather than slightly below the lungs as is a single hose regulator. Of course, there is more to the design of a good second stage. The lever advantage, balanced or unbalanced, case volume, Venturi efficiency.

N

 

[jakehbk]

I guess I always keep thinking about no free lunch. The larger diaphragm sucks in with greater force but consequently moves a smaller distance, like with the hydraulics in a brake system. That's kind of why I brought up the G250HP and the Gen1 S600 because I believe their guts are similar if not identical.