MK15diver
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Larger hoses

Sun Dec 10, 2017 4:23 pm

Has anyone ever put larger 1.5" rebreather hoses on a double hose and had it work ok? I know the AquaLung Mistrals that were actually 2 stages that came out in the early 2000's had larger hoses with a smaller 2nd stage and they seem to work. Curious about other regs though.
Someone told me that diving a double hose reg will kill me. I had no idea these things were dangerous.

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luis
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Re: Larger hoses

Sun Dec 10, 2017 5:44 pm

The Aqua Lung new Mistral had very little to no venturi flow assistance so it is considered by most DH divers to be a fairly poor performer. If you search a bit you can see how others felt about it. I have two of them (one is a special edition) and I have tried to tuned one of them to the max and it is still a poor performer.

In the case of rebreathers there is absolutely no pneumatic or venturi assist. The diver has to pull and push the breathing gas into and out of the counter-lungs, therefore it is necessary to reduce any potential flow resistance to the very minimum possible. That is why most rebreather hoses are about 1-1/4 inch ID (inside diameter). Some may be about 1-3/8, but I personally have not seen anything with a bigger inside diameter.

The flow area is a function of the diameter square, so just a bit bigger diameter hose makes a big difference in flow cross-sectional area (and hose buoyancy).

The tradeoff with having large ID hoses is that their flotation become more of an issue and the divers needs to add even more weights to the hoses.

I have done testing on the new Mistral hoses on some of my regulator designs (regulators with reasonably aggressive venturi flow assistance) and found that there was little (to no) improvement by using the bigger diameter hoses. The amount of improvements I could measure were so small that it would not be worth the tradeoff.

Again, this reduction in flow resistance could be important if you have a medium to poor performing regulator (or a rebreather) and even then, there is a diminishing return with going too big on the hose size diameter.

What I have concluded is that for a regulator with good venturi flow the ideal size is about one inch ID. And by observation, looking at the selection of hoses in the rebreather community it seems as they have optimized their hoses to about 1-1/4 inch (maybe a bit more).

Note: if you dive an Argonaut with the DSV mouthpiece and flow diverter, you can increase the venturi flow to be very aggressive. That will give you the most performance in any double hose regulator.
Luis

Buceador con escafandra autónoma clásica.

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Nemrod
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Re: Larger hoses

Mon Dec 11, 2017 12:43 am

The AL New Mistral is not a regulator to emulate. It was a kludge and a singularly poor performer. And it's ugliness only further justified it's low position on the scale of desirable regulators of any number of hose count.

There is no need for larger hoses on a proper DH regulator. The larger hoses would come with many negatives, weight in the bag, excess buoyancy in the water, more drag, more complication to fit a mouthpiece unit.

A well tuned Phoenix and the Argonaut, even if not well tuned, will actually push air to the diver once the Venturi is established. There is no advantage to larger diameter hoses, only disadvantages.

Nem

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SurfLung
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Re: Larger hoses

Mon Dec 11, 2017 10:31 am

What About the LENGTH of Hoses?
- I recently rebuilt/serviced a Snark III with original Nemrod hoses. These hoses were skinnier than USD hoses, but also they were about twice as long. When breathing thru these hoses and then switching to standard length USD hoses and mouthpiece, there was a dramatic difference... I had heard that was a fairly well known phenomenon but I didn't think it was SUCH a big difference. So, I was wondering if the extra LENGTH of the original Nemrod hoses also played a roll?
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SurfLung
The Freedom and Simplicity of Vintage Equipment and
Vintage Diving Technique are Why I Got Back Into Diving.

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luis
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Re: Larger hoses

Mon Dec 11, 2017 1:08 pm

The length would play a role, but a relatively minor role (in this relatively short length of hose). The length can make a difference, but it is not as critical as the diameter.

The new EPDM hoses that Bryan is offering are actually longer than the silicone hoses and I have not noticed any resistance difference (and haven’t heard anyone else make a comment). There may be a measurable difference if we took some precise instrumented measurements under very controlled conditions, but none of the divers have noticed a difference.

As I mentioned, the cross-sectional flow area is a function of the square of the diameter.

The area of a circle = diameter square x Pi/4 (Also expressed as the radius square times Pi).
A = (D^2)* (Pi/4)
or A = (r^2) * Pi

Therefore a small reduction in the hose diameter can make a big difference.

From my observation (and limited testing), the typical one inch diameter is about the optimal minimum diameter for a decent DH with a decent venturi assist. Going smaller can quickly increase the noticeable flow resistance and going bigger can help a bit, but the point of diminishing return is quickly reached.

Due to the area being a square function of the diameter, a small change can make a big difference in either direction.

Note: the inside roughness of a hose (or pipe) will also make a difference. Actually, the roughness can combine with the length to play a significant part in flow resistance, but the diameter is still more critical (especially in this short length of hose/ tubing).



Both questions about the diameter and length of a hose are very good question and I am the subject was brought up. They are both factors in fluid dynamics and my answers come are based on calculations/analysis, observations, and testing. But these are relative statements based on these flow systems, do not assume that they directly apply to other piping or fluid systems.
Luis

Buceador con escafandra autónoma clásica.

swimjim
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Re: Larger hoses

Mon Dec 11, 2017 5:36 pm

I've seen that reg and those aren't standard hoses. As Eben mentions they are twice as long. He's not kidding. It's like this reg was outfitted for Andre the Giant!

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