Single vs two-hose image questions?

[Luis H]

Hi Akimbo,

I agree with your statements and this is a great subject. But to the typical recreational and even most tech divers (with dives measured in hours) the actual WOB is of a lot less importance than the perceived WOB.

The typical diver will not have any feeling of the actual WOB and we tend to get a lot more fatigued by the heat loss from being submerged than most anything else (unless we are swimming hard).



It is interesting what you are saying about drysuit and WOB. I would like to learn more about that. Do you have any references?

Vintage hard-hat divers used keep the "collapse plane" low enough to have zero differential pressure inside the upper part of their suit. At least that is my understanding (I have only dove a Navy MK-5 a few times, so I have minimal knowledge).

Several decades ago I worked with an engineer, who used to work in diving research at Westinghouse, near Annapolis, Md. He had a patent for a zero differential pressure dive suit. It was probably an idea ahead of its time and I have never heard about since then.

 

[Akimbo]

I agree with your statements and this is a great subject. But to the typical recreational and even most tech divers (with dives measured in hours) the actual WOB is of a lot less importance than the perceived WOB.

We are on the same page. I would say it is true for extreme rebreather dives in less than 150M as well — and probably deeper. It is pretty academic for open circuit recreational divers except for gaining greater understanding of diving physics and physiology.

 

[Nemrod]

Which is here, not up there :



James

 

[Mekotronix]

I appreciate all the information tossed my way. I confess I don't understand how 6" of pressure differential translates into a significant difference in breathing effort (it's not hard at all to sip water 6" through a straw), but I'm not arrogant enough to assume y'all are wrong.

 

[GJC]

I appreciate all the information tossed my way. I confess I don't understand how 6" of pressure differential translates into a significant difference in breathing effort (it's not hard at all to sip water 6" through a straw), but I'm not arrogant enough to assume y'all are wrong.

Sucking water with your mouth is a little bit different than inhaling air into your lungs.

Try this:

Go to a pool and breath at the surface with you body vertical and your head above water. Then try breathing with a snorkel with your head under the surface with your body still vertical. See if you notice any difference in difficulty inhaling. You may need to hang onto a ladder or something to keep from floating to the surface or becoming horizontal.

 

[Nemrod]

I appreciate all the information tossed my way. I confess I don't understand how 6" of pressure differential translates into a significant difference in breathing effort (it's not hard at all to sip water 6" through a straw), but I'm not arrogant enough to assume y'all are wrong.

We are not wrong. A snorkel will work for a test but a short piece of garden hose will be better. See if you can breath through it at 6 inches of submersion. You will not be able to draw a breath at 6 inches of water column but just barely and certainly not repeatedly as it would be exhausting. If a regulator had a cracking effort of 6 inches it would not sell very well to say the least.

But cracking effort is just one aspect of regulator performance, an important one to be sure but just as important is exhaust effort. And there is this one particular regulator with an S on the purge cover that is supposed to be adored by all and near blasphemy to disparage it and yet I will point out it has a tiny little exhaust valve and would never rate well consequentially on the ANSTI machine. But it is a pretty face. Sometimes we can overlook exhausting, exhaust effort , but cracking effort, not so much as we seem to really notice that! Especially 6 inches of differential .

James

 

[John C. Ratliff]

I appreciate all the information tossed my way. I confess I don't understand how 6" of pressure differential translates into a significant difference in breathing effort (it's not hard at all to sip water 6" through a straw), but I'm not arrogant enough to assume y'all are wrong.

The next time you're diving with your single hose regulator, get vertical, and look up for two minutes, while breathing. This will give you a very interesting demonstration of the difference of about 12 inches of water pressure. Then get horizontal and breath again. You'll notice quite a difference.

SeaRat

 

[John C. Ratliff]

We are not wrong. A snorkel will work for a test but a short piece of garden hose will be better. See if you can breath through it at 6 inches of submersion. You will not be able to draw a breath at 6 inches of water column, if you can, then just barely and certainly not repeatedly as it would be exhausting. If a regulator had a cracking effort of 6 inches it would not sell very well to say the least.

But cracking effort is just one aspect of regulator performance, an important one to be sure but just as important is exhaust effort. And there is this one particular regulator with an S on the purge cover that is supposed to be adored by all and near blasphemy to disparage it and yet I will point out it has a tiny little exhaust valve and would never rate well consequentially on the ANSTI machine. But it is a pretty face. Sometimes we can overlook exhausting, exhaust effort , but cracking effort, not so much as we seem to really notice that! Especially 6 inches of differential .

James

James,

You've nailed it here. The current single hose regulators will not come close to the effortless exhalation of the Scubapro Pilot or A.I.R. I regulators. For these two regulators, the entire exhaust diaphragm is the exhalation valve.

Akimbo mentioned work of breathing on the rebreathers. My understanding is that there is no Venturi concept, as the diver is breathing off a rebreather bag, through inhalation and exhalation valves. That means these are close to the equivalent of the early diving regulators without Venturi assist (DA Aqualung, for instance). The Scubapro Pilot has such a huge Venturi that it needs to be turned toward the case to cause turbulance, otherwise there could be a problem with an overpressure situation in the lungs. Here is a thread that discusses the Scubapro Pilot, and has some posts by Tony Christianson, its inventor. One of the prototypes had an adjustable Venturi, but that was shelved when the power of the pilot valve/Venturi was really understood.

Now, about double hose regulators, the Venturi on most later double hose regulators was such that they would almost breath for themselves, and needed backpressure to turn off (stopping of inhaling initiates the backpressure), as mentioned in an earlier post. This means that it is mostly the cracking effort that is affected by the positioning. I sometmies dive my double hose rgulators with my Dacor Nautilus CVS (Constant Volume System), and this puts the positioning at a disadvantage, higher and off my back by about 4 inches. But other than the cracking effort, it is not too noticeable. Actually, I can negate that disadvantage by positioning myself head-down in the water when horizontal too. In this photo, I'm diving my Dacor Nautilus CVS with a Sportsways Hydro-Twin regulator. Notice the mouthpiece; it is a Healthways double hose mouthpiece that I've removed the nonreturn valves and wagonwheels from (but left the Gold Label deflector plate in place), and this configuration will allow the Hydro-Twin to free-flow once inhalation is initiated if it is removed from the mouth. Note that the non-returns add some resistance, and therefore impede the Venturi effect to some degree.

SeaRat

 

[Akimbo]

Akimbo mentioned work of breathing on the rebreathers. My understanding is that there is no Venturi concept, as the diver is breathing off a rebreather bag, through inhalation and exhalation valves. That means these are close to the equivalent of the early diving regulators without Venturi assist (DA Aqualung, for instance).

There is no "powered" assistance at all in rebreathers. Your lungs have to push and pull all the gas through the CO2 absorbent chemicals and the cumulative resistance in "the loop" (hoses, breathing bags, mushroom valves, etc). That WOB and increased dead air space in the mouthpiece disadvantages are compensated for by the fact that there is no reason to control respiration to save gas or for buoyancy control like an open circuit diver.

In fact, rebreather divers should be trained to intentionally breathing deeply to minimize CO2, partly due to the WOB and because they often are diving deeper (higher gas density). Open circuit divers tend to train themselves to breathe too shallow for optimum CO2 removal, which exacerbates Nitrogen Narcosis.

Rebreathers also have a thermal advantage because body heat in exhalations is recycled in addition to a small contribution by the exothermic reaction of the CO2 absorbent.

I have worked with experimental prototype rebreathers with electrically powered circulation in the loop for very deep sat diving. I would not be surprised to see the concept added to recreational rebreathers as battery technology improves. The main disadvantage is the noise, which is very noticeable on a rebreather because they are so quiet.

 

[Akimbo]

But to the typical recreational and even most tech divers (with dives measured in hours) the actual WOB is of a lot less importance than the perceived WOB.

I was thinking more about this after our phone conversation last night. Perhaps the most important reason that perceived WOB is more important in inexperienced recreationally divers is panic. It is well know that respiratory resistance and constriction is a panic-inducing stressor.

That applies to the breathing circuit as well as constriction by poor fitting wetsuits or suit-squeeze by under-inflated drysuits.