DH with no exhaust hose - what would happen?

[DaleC]

I know it's a strange question but...

I've been sidemount diving when going modern these days and this has tweaked a curious eclectic vintage question for me. What would happen if one removed the exhaust hose from a DH so it would effectively become a single hose? I am thinking of a DH sidemount configuration off the right side.

Technically, assuming one would put a duckbill (or something) to prevent backflow into the mouthpiece, does the exhaust hose flow play a part in the reg function or would it still work without one (adjustments for position etc... not withstanding).

In this video from 00:09 to 00:18 I am holding my DH almost like sidemount and it breathed fine but the hoses are jumbled. I would like to try it with one hose.

http://www.youtube.com/watch?v=QpsR3EBy ... hQ&index=6

[luis]

At first I thought that this was a trick question…

You need to read a bit about the very first dive Cousteau did with a demand regulator and his follow up discussions of the experience with Gagnan.

The original Cousteau-Gagnan patent was not for the demand valve (demand valves have been around) it was for the exhaust hose.

It is all about the differential pressure in the water column. If the exhaust is higher than the cracking effort on the demand valve, it will free flow.

The simple answer is that the exhaust needs to be next to the demand valve sensing diaphragm to avoid a free flow.

In a double hose regulator it is important to place the exhaust right in front of the diaphragm, if you want to fine tune it. The limiting factor to fine tuning a Phoenix HPR (and now the new Argonaut) is the size of the exhaust valve. The distance from the center of the diaphragm to the top edge of the valve has to be less than the cracking effort (measured in inches of water column) or it will free flow.

When you take the mouthpiece out of your mouth and you just raise is a bit higher than the diaphragm (even if the mouthpiece is facing down), the regulator starts to free flow. The same violent free flow will happen if you remove the exhaust hose from the mouthpiece and you mouthpiece is just a bit higher than the regulator diaphragm.

In a single hose regulator, the distance between the exhaust and the diaphragm is refer to as “the case fault geometry”, and it is the limiting factor as to how sensitive you can tune a single hose regulator (in order to avoid a free flow when you are up-side down).



If you want to try side mounting with a double hose regulator, I recommend using an Aqua Master and attaching an LP hose to the hookah port (replace the first stage with a plug). Mount the regulator on your back with a hookah harness. You can then use a high flow quick disconnect to hookup to the first stages mounted to the side mount tanks.

Each first stage should also have a conventional second stage as a means to share air and as a pressure relief. If you are using two tanks, you can put quick disconnect attachments on both first stages so that you can swap the tank air source for your DH.

[DaleC]

Luis, you are the man!

Thanks for taking the time for the excellent reply. It makes perfect sense to me now. But damn... I thought I was really on to something there for a moment

Interesting idea re: the sidemount back position hookah.
I'm still working on a front mounted DAAM for my Hawaiian pack project. As you suggest it will be supplied via a remote first stage on the tank.

[luis]

Dale,

I was concern there for a moment and I thought this was a trick question. Then I remember that they don’t teach basic diving physics like they used to or maybe I just had a particular good instructor. Of course it helps that I am engineer and my hobbies involve applied physics, but I first learned the basic in diving physics from Harry Hauck when I was 14.

I just did a quick Google search about “Harry Hauck Puerto Rico”, which reminded me of how lucky I was to have him as my first diving instructor.

___________________________________________


I put together a set up for a chest mounted DH and I have tested it a few times.

One important feature that I added was a shot-off valve in the LP hose. If you don’t add that, every time you take off the mouthpiece from your mouth you will have a violent free-flow.

Again, the same physics about the pressure differential due to the water column applies.

When the mouthpiece is in your mouth everything works fine, since the exhaust loop takes the exhaust back to the demand valve diaphragm. BY having the mouthpiece in your mouth you close the loop.

The only issue is that the pressure in you mouth in this situations is now just a bit higher than the demand valve. At first you only notice it slightly, but after a long dive you really notice the fatigue in your jaw from having to hold on to the mouthpiece. If you relax your bite the mouthpiece will slip out and just very so gently blow out of your mouth.

Warning:
This is definitely an overpressure breathing system. The pressure differential is so small that I can’t imagine it being a problem with anyone with healthy lungs, but you do it at your own risk.

My next experiment is to try it with one of my vintage full-face masks. I have a Cressi FF mask that I put a HopePage mouthpiece to use it with a double hose and I also have an old US Divers mask that looks like the old Pacifica with the mouthpiece pod that can receive a DH or a single hose regulator. Neither one is a very comfortable mask, but with the over pressure feature it may help relieve any pressure spots from the rubber skirt (I am probably just wishful thinking).

After trying this set up, I can see why the Cousteau divers used the mouthpiece pod when they used the chest mounted regulator in their “Hydrodynamic Kits”.


The picture below is from the Flashbackscuba museum.
http://www.flashbackscuba.com/museum/

You can clearly see in this picture the hoses coming from the chest pod where the DH regulator is located. You can also see the mouthpiece pod they used. As far as I know, the pod was originally intended to work with underwater communications, but the communications equipment never worked properly. I am guessing that they continue to use the pods because the pods are help against your mouth with straps behind the head.

[DaleC]

Thank you Luis.
I have had the experience of a slightly too de-tuned DH pushing air into my lungs at the end of a dive before. Very odd feeling. I felt I was hyperventilating because, as soon as I exhaled, my lungs refilled again. There was no slow inhalation period. I also have used a positive pressure FFM and it was interesting to have the face mask want to push off my face instead of snug down onto it. With all of these things I test in a pretty benign setting first but... I've never let convention hold me back either. This is part of the fun of diving for me.

I've had the pleasure of talking to Ryan and looking at his hydrodynamic reproduction rig. Very interesting story behind its creation. I would like to try one of those pods to see if I can talk into it (like a vocal chamber). I use a GoPro camera that picks up sound fairly well and it might capture the sound. It would be great to be able to narrate as I was diving.

I should get on with the chest mount project. I've been stalled with it because I'm waffling over de-chroming my cans so they eventually have the same patina look as my brass Hawaiian pack but I suppose I could do the mock up to see how the whole thing breathes and worry about looks later.

Thanks again.
Dale.