My first double hose dive

[halocline]

Also, has the RAM been rebuilt with silicone parts (and for that matter by someone who knows DH regs)? The parts will make a difference and tuning one is more of an art form than it might first appear.

Yes, the RAM has been rebuilt and tuned well; I did it. In fact, it breathes a little better than my PRAM, even did before I did the full restoration on it.

To elmerfudd, the position sensitivity of double hose regs is really disorienting at first. It's a huge change from single hose diving. Another big difference is that the air expands out of the 2nd stage about 12" from your mouth as opposed to around 1" for single hose regs. This gives the air flow a very different feel, one which can be a little discomforting at first as you miss that rush of depressurizing air, but with a little experience you will probably really appreciate. It's more natural and the air is warmer and less dry.

Think about how regs are tuned so that they'll flow with 1" or so of water pressure differential. Then think about putting your head down with a doublehose; you could be lowering the mouthpiece 6" or more below the cans, resulting in a big increase in inhalation effort. The position of the reg on your back is very important to getting the most out of dh regs, more important than the differences in fine tuning the reg itself. Your RAM is tuned about as lightly as you can get without it freeflowing in use. You should be able to hover vertically heads up without an uncontrolled flow, although if you take the mouthpiece out of your mouth in that position it will flow. Unfortunately, this does make it more sensitive to surface flows.

Finally, no matter what you do, or what doublehose fanatics will say, inhalation effort overall on a doublehose is going to be higher than on a high performance single hose reg, because of the position sensitivity. Those of us that like diving double hose accept this because there are other aspects, like no bubbles in the face, no drymouth, no mouthpiece fatigue, no excessive venturi shoving air down your throat, etc, that more than make up for it. And, we learn to spend much of the dive in a position that allows the reg to breathe easily.

 

[elmer fudd]

Thanks for the tips. I'm thinking of taking it out again today. What I like to do with new and very different equipment is dive with it several times in order to develop an understanding of it. It's easy to try something out once, not learn it's strengths and weaknesses, try to use it in the way you're used to and then write it off as junk. I think that's what happens more often than not with split fin and paddle fin users. They get used to one and then can't adapt in one dive to the radically different kicking style that works best with the different fin.

I've read Vance Harlow's Regulator Maintenance and Repair, and he chalks up the harder breathing to the compressibility of air and having that 18" hose between the mouthpiece and the diaphragm. There wouldn't seem to be much that could be done about that.

I think that on this last dive I had the tank much too high. I set it purposely very low at first, but as I was having free flow issues with it in the water, I asked my buddy to move it up a couple of inches and I think instead of 2" higher, he moved it up about 6" or into the same position I would normally dive with my single hose.

 

[John C. Ratliff]

Elmer,

A couple of things are worth mentioning before you go out on your next dive. Yes, the inhalation will be somewhat harder. This can be minimized by the regulator position; it should be between your shoulder blades with just enough hose length to get the hose to your mouth. If you have free-flow problems, simply move the mouthpiece underwater to stop it. The "compressability" is not what is happening in the inhalation phase. What is happening is that the air must travel down a longer tube, and because of that looses some of its velocity. The Aquamaster series has a very well-designed venturi, whereby about 1/3 of the air goes directly down the tube, and 2/3s of the air goes back into the regulator box. This is so that the venturi is not so violent that it does not stop when you stop breathing, but gently assists your breathing. Once you start the inhalation, your lungs don't do much until you exhale, which stops the air flow through pressure changes.

What is not talked much about is that the exhalation effort is much less in a double hose than in most single hose regulators. Because of the positioning, the exhalation is almost effortless in a double hose regulator. The inches of water that cause negative pressure inhalation will almost suck your exhalations out of your mouth, and physiologists tell us that much of the WOB (work of breathing) is in the exhalation phase, not the inhalation phase, of breathing. If you want completely neutral breathing, turn on your side so that the regulator is exactly in the center of your lungs on a vertical axis when you swim horizontally.

The comment was made by mattboy that "no matter what you do, or what doublehose fanatics will say, inhalation effort overall on a doublehose is going to be higher than on a high performance single hose reg, because of the position sensitivity." Well, this is simplified. If you take a diver in a vertical position, head up looking at the surface, you will get the same positional difference in the single hose mouthpiece that a double hose "sees" in water pressure, and the double hose will out-perform the single hose because of the positional difference. If you put the diver head-down, the single hose has the advantage on inhalation, but a distinct disadvantage on exhalation. If the regulator is between the shoulder blades, and the diver is slightly head-up, both regulators will perform about the same, as they both will be in the same plane and the same distance from the center for the diver's lungs, which is where this effort begins.

SeaRat

 

[elmer fudd]

So I just finished up a second dive about 40 minutes ago. This time I had the tank at the right height and it made a world of difference. I purposely inverted myself, went upright, rolled side to side and didn't notice any problems breathing. One thing I did notice though, there are some critters that let you get a lot closer when there aren't bubbles coming from around your face. I was within about 1.5' of a large striped perch and they usually don't want to get much closer than 5'. I also need to work some more on clearing the regulator underwater. I removed it at one point to spit out some phlegm and wasn't prepared for the amount of water I had to clear to start breathing again.

Prior to this dive, I also removed all the LP attachments. I kept the wing on my backplate just in case, but I would have had to manually inflate it. At the beginning of the dive I deflated it all the way and I never touched it again. I found I had great buoyancy out to about 20' and then as my suit compressed I became negative. That wasn't a big deal as I was solo diving today and wanted to stay in very shallow. In the future though I might want to strip off a bit more lead if I intend on diving deeper without a BC.

I also took out the Nemrod Max Vue today. It worked better than I thought it would, but the modern masks I've used really are better. I found the two separate panes of glass on the side gave a somewhat distorted view of things, it didn't seal that well and it took a LOT of air to clear. On the plus side, that big glass pane on the front was huge and very clear.

 

[halocline]

So I just finished up a second dive about 40 minutes ago. This time I had the tank at the right height and it made a world of difference. I purposely inverted myself, went upright, rolled side to side and didn't notice any problems breathing.

That's great, glad to hear it. Positioning does make all the difference. If you ever get interested in doublehose diving enough to invest in another piece of gear, I highly recommend the freedom plate. It noticeably improved the way my doublehose regs performed over my standard backplates.

I find clearing is easier if I roll a little to the left while exhaling. There's a book, I think it's called "Basic Scuba" by Fred Roberts, someone will immediately correct me if I'm wrong about the name, that's got a lot of excellent info about double hose diving.

 

[herman]

I didn't know it was yours Matt, I had no idea of what was installed in it or who had set it up. If it was yours I know he has one in good shape and set up like it should be.

Clearing one is not much problem, just roll on your left side and exhale. If you need a breath you can either hold the mouthpiece above your head with the opening down and it will freeflow, clearing the reg. An alternative method is to roll to your left side and milk (alternately compress and stretch ) the supply hose. If the mouthpiece check valves are doing their job, the water will be pumped out of the mouthpiece.

 

[Luis H]

I've read Vance Harlow's Regulator Maintenance and Repair, and he chalks up the harder breathing to the compressibility of air and having that 18" hose between the mouthpiece and the diaphragm. There wouldn't seem to be much that could be done about that.

That statement in Vance's book is totally incorrect. Air is very compressible if there is a reasonable pressure difference. The pressure difference in the second stage housing and the entire air hose loop is measured in inches of Water. This low pressure differential is insignificant from a compressibility flow calculation.

The air density in the second stage housing and hoses is constant down the length of the hose, it doesn't change with flow (it only changes when you change depth). Therefore the flow calculation is not considered compressible flow.

Compressible flow is when you get large enough pressure differential to cause a significant density change. That is what happens in the first stage valve orifice and the second stage valve orifice.

Note: As an engineer I have been doing gas flow calculations and testing for close to 30 years. I hope this makes sence... sometimes my explanations are not as clear as I wish they were.



BTW, John the flow velocity doesn't change down the hose/tube. Flow velocity only changes with change in cross sectional area change. What you are loosing due to friction is some pressure, but I have measure it to be almost insignificant in the standard 1 inch ID hose (under our normal breathing flow rates). Reducing the flow cross section makes a huge difference in the operational range we are working with.

 

[halocline]

The air density in the second stage housing and hoses is constant down the length of the hose, it doesn't change with flow (it only changes when you change depth). Therefore the flow calculation is not considered compressible flow.

What you are loosing due to friction is some pressure, but I have measure it to be almost insignificant in the standard 1 inch ID hose (under our normal breathing flow rates). Reducing the flow cross section makes a huge difference in the operational range we are working with.

Thanks for clearing that up, Luis. I suppose because there is a pressure differential between the highest point of the hose and the deepest point in diving, then the gas is denser at the deeper part of the hose route, or possibly the hose changes shape a bit (flattens out) to reflect the slightly higher ambient pressure. I'm sure these factors are mostly theoretical, not anywhere near enough to effect how the reg breathes.

I am interested in the air velocity, though. I have an idea that a big difference in feel for the double hose is due to the fact that the air expands into a much larger chamber (the hose) at ambient pressure, so you're breathing out of a large volume of ambient air. This really slows the air down. Compare this to a typical single hose, where the volume of ambient air is much smaller (only the 2nd stage under the diaphragm) and so it's moving much faster.

It took me a few dives to get used to that; there's plenty of air, but psychologically it doesn't feel like the same volume because it's moving so much slower. Hence the long slow breaths that everyone advises. Once you get used to it, IMO it is a more natural feel. It's easy to forget you're breathing from a regulator with a double hose.

I did find a single hose 2nd stage that I'm really excited about, my SP pilot. Sometime maybe you could have a look at yours and we can talk about it. Mine has been converted to a coaxial valve, but I'm planning on getting another one and leaving it stock.

 

[halocline]

I didn't know it was yours Matt, I had no idea of what was installed in it or who had set it up. If it was yours I know he has one in good shape and set up like it should be.

I know you didn't know, and it was a perfectly reasonable question to ask. The old stock diaphragms and poor tuning can really hurt the performance of a double hose. This one has all new silicone internals, new rebuild kit, new titan seat, polished orifice. The one thing I left stock was the exhaust side wagon wheel. I love Luis's new wagon wheels for the intake side, but I've found with the batch of silicone mouthpiece valves that I have, the stock wagon wheel seems to seal a little more reliably on the exhaust side. Since there's rarely any sort of exhaust effort, it's a good tradeoff IMO. I have the same set up on my PRAM.

 

[elmer fudd]

I'm going to give the benefit of the doubt to Harlow. It's simply going to take longer to open a valve via a diaphragm when you have a larger volume of air to contend with. It's pretty basic. Put a small cup up to your mouth and breathe in. Notice how easy and how little breath it takes to create a strong suction. Next try it with something big like a 5 gallon carboy. You'll have to inhale a lot deeper and the vacuum won't be nearly as strong. The same thing is going to happen with a regulator. When you inhale on the second stage of a single hose reg, there's very little dead space there and the diaphragm will be acted upon considerably quicker because of this. When you inhale on a double hose you've got a volume of air several times larger to contend with which will slow the speed with which the diaphragm can open the valve.

It has to do with the compressibility of air simply because this effect wouldn't occur with a non-compressible liquid. If the hose were filled with water instead, then the valve would open just as fast regardless of the volume.