Why two hoses originally?

[Ron]

Hey everyone,

I was reading up on how Cousteau and Gagnon developed the double hose regulator today. One of the things it mentioned it that when Cousteau originally went in the Marne River with his first design, which only had one hose, the regulator free flowed. So they added the exhaust hose to prevent that. I understand that the suction action of inhalation is what causes the diaphragm to action on the levers and cause compressed air to flow. Why does the exhaled air have to go back behind your head via another hose in order for the reg to not free flow?

[luis]

The exhaust has to be at the same level in the water column as the intake diaphragm.

If the exhaust is ever higher (in any diver position) than the sensing diaphragm, the differential pressure (in the water column) from the diaphragm level to the exhaust will cause a free flow.

The same thing happens when we take our mouthpiece out of out mouth and raise it higher than the diaphragm. The diaphragm is at a higher external pressure then the mouthpiece (or you can say the mouthpiece is at a lower pressure than the diaphragm, same as a relative suction).

As you know, every inch of water column increases the pressure by the same amount (see note below). The vertical column of air inside a semi-rigid hose on the other hand has basically the same pressure since the weight of air almost insignificant in this situation.


I hope this is clear.


Note:
We actually use inches of water column (inWC) as a unit of pressure for very low pressure differentials, since it is a constant. 1 inWC = 0.036 psi

[luis]

The same thing happens when we take our mouthpiece out of out mouth and raise it higher than the diaphragm.

Note that we sometimes used this feature to purge the mouthpiece, since we don’t have a purge button. This work just as well. You just put the mouthpiece in your mouth while it is free flowing.

It may not be as convenient as a purge button; because it doesn’t work in every orientation…you have to raise the mouthpiece and then reach it with your mouth, but actually work very easily. It you need it (as if you are out of air to blow the mouthpiece clear), it works very easily.

[luis]

BTW, the same physics apply to a single hose regulator. The limiting factor on how light you can adjust some high performance regulators (single or double hose) is not the mechanism, but the distance between the exhaust and the center of the diaphragm.

If you adjust a conventional single hose to breath too easily it will free flow when you are head down since the exhaust t can be higher than the diaphragm by as much as an inch. I was experiencing that last weekend with my stage regulator (a good reason for an adjustable second stage).

A regulator like the Scubapro D400 can be adjusted to breath easier than others because the exhaust is in the center of the diaphragm.

The way I tend to tune my Phoenix RAM and my round label RAM, it requires that the openings in the duckbill be very close to the center of the diaphragm or it free flows when I am vertical or I roll with my left side high.

[Bryan]

A rarely known bit of info…….Cousteau being French possessed an impeccable sense of style. Therefore he added a 2nd hose so his divers would look uber cool and not like a big….

[Ron]

Luis,

Excellent explanation, thanks!

Bryan,

Lucky I don't have that problem. I have a two hose regulator....

[Ron]

So I would imagine that a two hose regulator without a duckbill would free flow then, wouldn't it? There is always going to be a slight differential between the pressure that the diaphragm sees and the pressure of the exhaust right? This is why when you turn over your single hose it may free flow depending on how it is adjusted. This is also why my Atomic regs have a adjustable knob on the second stage, so you can tune them close to free flowing without having them actually do so by adjusting the knob on your octo or on the surface. Does the duckbill provide enough of a restriction to keep a double hose from free flowing if it is adjusted correctly? Bernoulli's principle states that when an object enters an area of restriction, speed increases and pressure decreases. This means that the duckbill would lower pressure in the hose, correct?

[Bryan]

Bryan,

Lucky I don't have that problem. I have a two hose regulator....

There are folks that could stand on the deck of the Calypso while it’s anchored in Truk Lagoon with a Spaco in one hand and a black rivet label Aqua-Lung the other while Dumas was helping them put on a set of Rene triples with a chrome handlebar mouthpiece CG 45 mounted to them and they could still be the biggest tool in the box…. I am just saying it’s possible and I’ve seen it

[Ron]

Well by that logic if I am a tool....then I'm something benign like an o-ring pick or a jeweler's screwdriver. I'm definitely not the kind of wrench that could take apart a tank....that's for shizzle.

[usddude]

WOW!! All I can say is...WOW!! I am like completely blown away...fried pie

[Nemrod]

Regulators like the Omega II and Tekna in particular did not have much differential between the exhaust valve and the tiny diaphram.

Yeah, Cousteu had it right, there is something to be said for symetry.

Nem

[Bryan]

Well by that logic if I am a tool....then I'm something benign like an o-ring pick or a jeweler's screwdriver. I'm definitely not the kind of wrench that could take apart a tank....that's for shizzle.

We all have our days in the tool box.....Ask Rob about his old fins.

[simonbeans]

But some of us, when it comes to being a temporary tool, are precision micrometers.

[YankDownUnder]

The demand valve was already in common use when Cousteau approached L'Air Liquide to make him a compressed air breathing apparatus. (His father-n-law worked for them.) The Commeines diving apparatus predated Cousteau by 2 years (GC43) and solved the problem with an adjustable exhaust restrictor on a full face mask. However, it needed manual adjustment. It was already used by the French Navy. It was a modification of the company's fire fighting breathing apparatus.

Cousteau was sent to Gagnan, who already had a demand valve. It was designed to feed cooking gas into motor vehicles. Gasoline was in short supply. When Gagnan was told by Cousteau what he wanted, Gagnad took out the demand valve from his desk and asked:" It this what you are looking for?"

The real contribution Gagnan and Cousteau came up with, was the return hose. Drager solved the problem by breathing back ito the regulator, but that resulted some CO2 build up, like in the pendulum rebreather.

The important thing is, that the exhaust is at the same level, or deeper than the diaphragm to prevent free flow. If deeper, there is exhaust restriction. A duck bill per se, is not needed, as can be seen in the Sea Lion regulator, which has none. However, water can get in the hose whithout it. The Heinke horn regulator used horn exhausts to get around the CG45 patent. Later Heinke went to a cart wheel type mushroom valve in the exhaus hose, near the regulator. (eg. MkIII Venturi Jet model)

You may have read that divers found small scuba apparatus inside M4DD amphibious Sherman tanks, which sunk on D-Day at Normandy. They were similar to a SpareAir, but had a short corrugated hose to the mouth piece. The exaust valve was inside the demand chamber, just like a single hose regulator of today. They were intended as an escape apparatus.

They were traced back to the American company "MSA" (Mine Safety Apparatus). The company refered the HDS researchers to an early employee, who was in his 80's. He said they were an 'on the shelf' device' and were produced in quantity, when the US Government approached them in WWII.

Cousteau's contribution was really in application and promoting scuba diving with ground breaking film, like the Silent World. Gagnan was the gas engineer who was responsible for the developement side of early double hose scuba.

[Fibber McGee]

on the temporary tool front: at least when you posted a micrometer pic it was a STARRETT....which negates any bad conotations!


On a note about the escape breathers from the tanks...does anyone have any information on those used to escape from submarines? ...these would have been intended to get a person up from, say 100ft or maybe more so I would guess they were a little more sophisticated...I also know they didn't always work.