Single vs two-hose image questions?

[Mekotronix]

@John C. Ratliff posted this image on the thread about scuba diving generations.

Being newly OW certified and having practically no historical scuba knowledge, it's not clear to me why the different orientations result in easy/hard inhalation/exhalation. It appears they are comparing the pressure at the center of buoyancy (in the lungs) with the pressure at the valve. Single hose regs have the (2nd stage) valve at the mouthpiece. I'm guessing a two hose regulator has the valve at the tank? (Did two hose regulators at the time have a 1st and 2nd stage?)

Assuming my understanding thus far is mostly correct, I understand the theoretical justification for easy/hard inhalation/exhalation. However, we're talking about pressure differentials across distances of ~6", or ~0.015 atm. That's a pretty small difference. How noticeable was the effect they are describing?

 

[tbone1004]

Double hose regulators can be either single stage or 2-stage regulators. Single stage regulators have a single valve that knocks tank pressure down to ambient pressure, 2-stage has the same valve setup as a single hose where the first stage drops tank pressure to intermediate pressure and the second stage drops the intermediate pressure to ambient but there is no hose connecting the two.
In both cases with a double hose regulator the valve assembly is attached directly to the tank.

Most regulators are tuned with an inhalation resistance of about 1.2" of water give or take depending on the regulator. The difference between 1" and 1.5" is noticeable in the water, 6" is like trying to breathe through a straw and is very noticeable. That said, the venturi is what matters and initiating a breath on a double hose does take a hair more effort, but it doesn't take much to keep it going. Very different than trying to breathe a single hose regulator upside-down.

What is not outlined in that drawing is that the diaphragms are intended to face "down" so the water pressure is assisting in opening the valve *why the single hose regulators freeflow if you put them in the water mouthpiece up and why they stop when you turn them upside-down*.

 

[Nemrod]

As Tom says, a 6 inch difference is huge! But the double hose regulators are deigned with greater mechanical advantage, the huge diaphragm and a powerful Venturi, so in actual diving use they work just fine. And there are different breathing techniques and swimming positioning not to mention gear and tank position to effectively dive with a dh regulator. Argonaut Kraken:



Yes, some dh regulators were a single stage like the Mistral. They drop from tank pressure to ambient with a single stage and have a powerful Venturi. The diagram is illustrating the differences in the height of the water column from the regulator diaphragm to your human diaphragm. Which in a dh regulator is located in the part affixed to the tank valve and in a single hose regulator is affixed to the second stage at your mouth.



My wife (circa 1980 at Ginny Springs) with a first generation USD single hose Conshelf. And just a few weeks ago in Bonaire to be complete:



A Kraken being prepared for assembly:




James

 

[Akimbo]

The actual WOB (Work of Breathing) is primarily generated by chest and diaphragm mussels that move gas in and out of the lungs. The average point is considered at the center mass of the lungs.

A demand regulator reacts to the differential pressure across the regulator's diaphragm. Demand regulators can be tuned to react at less than 1" of water/1.8mmhg of differential pressure.

As you can see from this diagram, the distance the demand regulator's diaphragm is from the centroid of lungs can have a greater impact on actual WOB than how well the regulator itself performs. The optimum position for any regulator is at the same depth as the centroid of the lungs, from the WOB perspective..

 

[Nemrod]

Yes, for cracking effort but whether double hose or single hose, once the Venturi is established the inhalation effort will decrease as the Venturi suction assist keeping the diaphragm pulled inward actuating the valve. DH regs may have a practical cracking effort as low as .6 inches compared to most sh regs at 1.2 inches. But the water column/pressure differential between the center of the lungs and the dh diaphragm is potentially large (compared to a sh) in a normal swimming position. The overall WOB is reduced by taking long and slow inhalations riding then Venturi and the exhaust effort for a dh regulator in swimming position is very low. So the average or overall WOB is comparable to a sh regulator but at the same time perceptually very different.

My new to me SP G250 second stages that I have been diving of late have really been impressive as they have a large diaphragm for good mechanical advantage and a very powerful Venturi and the VIVA vane seems very effective. I can see now why it is the most copied, most produced single hose second stage. And the Kraken in my photo is the most advanced dh regulator ever produced even over the AL Mentor (military regulator).

James

 

[Akimbo]

Here is an image of Jacques Yves Cousteau gearing up to test dive a "push-pull" demand regulator for the Conshelf 3 experimental saturation dive in 1965.

It allowed the expensive Helium-Oxygen breathing gas to return to the habitat for reprocessing. Note that it was a chest-mounted double-hose configuration.

The same concept is used today in sat diving except the demand exhaust regulator is mounted to the hat and the exhaust is routed through the bell and back to the surface for reprocessing.

---

 

[John C. Ratliff]

The actual WOB (Work of Breathing) is primarily generated by chest and diaphragm mussels that move gas in and out of the lungs. The average point is considered at the center mass of the lungs.

A demand regulator reacts to the differential pressure across the regulator's diaphragm. Demand regulators can be tuned to react at less than 1" of water/1.8mmhg of differential pressure.

As you can see from this diagram, the distance the demand regulator's diaphragm is from the centroid of lungs can have a greater impact on actual WOB than how well the regulator itself performs. The optimum position for any regulator is at the same depth as the centroid of the lungs, from the WOB perspective..

Akimbo, the actual metric measurement of an inch is 25.4mm of water pressure. I'm sure you know this, but a lot of people may be confused. One inch of water pressure is equal to 1.8 mm of mercury (mmHg). Mercury, of course, is very heavy compared to water. It's actually:
1 inch water pressure = 1.86645 mm Hg = approximately 1.9 mm Hg

@tbone1004, the double hose regulators which have about 1.2 inches of water pressure cracking effort have been somewhat detuned, as mine have about 0.5 inch cracking effort. There is a reason for detuning them, in that the exhaust is not exactly on the diaphragm, as originally envisioned by Cousteau and Gagnan. If the duckbill exhaust is exactly flat against the diaphragm, and it is fitted so that the center of the duckbill is positioned directly over the center of the diaphragm, it will not leak air in a vertical or upside down position. But if it is not, then it will leak air at 0.5 inches of cracking effort. The Snark III has this characteristic, as it incorporates a huge mushroom valve in the top cover. This means that there is probably an inch of water pressure between the top of the mushroom valve and the center of the diaphragm when in the vertical position. But this is a very slight leak, and doesn't bother me when I use my Snark III. Some people put their duckbills into the case without this consideration, and a finely-tuned Aquamaster or Royal Aquamaster will leak in a vertical position.

Now, about the water column between the demand regulator and the center of the lungs, divers with both single hose and double hose regulators have to take this into consideration. Each has a different position that is best, and worst, concerning water pressure. We double hose divers wear our regulators on "kits" (that's a new term to me, but very much appropriate) where the regulator is situated between the shoulder blades. This is best for double hose regulators. If mounted too high, they will breathe hard, no matter their cracking pressure. The Venturi helps, in that with a good Venturi, once the cracking effort is overcome, the regulator almost breaths for itself (some actually do, and backpressure is needed to stop the flow).

SeaRat

PS, the above diagram titled "Two Hose Regulator Versus Single Hose" comes from Bill Barada, in his book published by U.S. Divers Company titled Let's Go Diving, in 1962 on page 29.

 

[tbone1004]

@John C. Ratliff my Kraken is tuned to about 0.7 right now after it has settled but it has a wicked venturi that @Luis H has been able to tame and it's incredible.

 

[Luis H]

I like those 1962 diagrams. They are very historic, but they only tell a little portion of the actual story related to Work of Breathing.

It is interesting that here in ScubaBoard the WOB conversation only involve the center of the lungs.

I have been to the Navy Experimental Diving Unit (NEDU) and I have seen the WOB "test dummies”. They don’t measure the work of breathing in the center of the lungs; they measure it on the back of the throat.

About 6 years ago, I had a lot of discussions with the lead research scientist at NEDU (he is now retired) and we talked about WOB and what I called the “perceived WOB”. The “perceived Work of Breathing” is actually a lot more important to human user/ diver. The lungs have no sensation and do not feel any Work of Breathing.

The NEDU has done a lot of research for the ideal location of counter-lungs in rebreathers, with respect to “perceived WOB”.

Modern, open circuit divers would have no reason to give the center of effort for perceived “Work of Breathing” any thought. But for rebreather divers it is important because of the location of the counterlung will affect performance and the perception of WOB.

Here is a quote from one of the papers that studied WOB for rebreathers.

However, in water,
because of the differential pressure in the
water column between the mouth and the
lung centroid, the eupneic centroid in
which breathing is perceived to be the easiest
immediately shifts because of pressures
experienced at the mouth . This explains
why a diver thinks that a
single-hose regulator breathes better than
a double-hose regulator in the upright position.

Here is a diagram from the same article.

In a comfortable swimming position (with a slight body slope, about 10 degrees), the elevation, in the water column, of the eupneic centroid can be level with the center of a double hose regulator, if the regulator is properly located, between the divers shoulder blades.

In a swimming position, the center of the lungs is several inches below a DH regulator. If we had any sensation and perceived WOB in the center of the lungs, a DH regulator with the demand valve that far away would have never been an acceptable location.


Another example that shows that the center of the lungs has no sensation is using a snorkel on the surface. You can not tell any difference in WOB with a snorkel on the surface in a horizontal versus a vertical position. I have intentionally tried to see if I can feel any difference and you can not.

 

[Akimbo]

Perceived vs actual WOB was a major topic of discussion when looking a sat dives in the 1200-1600'/365-500M range. Of course my clients were concerned with how much work divers could do during an 8 hour lockout without being exhausted in a few days.

Actual WOB is reflected in the daily caloric burn rate. I have read that drysuits can cause a lot of actual WOB variation, which only matters in surface dives since sat divers use hot water suits.

You would think that perceived WOB would be improved by lightweight hats with neck dams, at least partially.