Why two stages?

[kouhoutek]

I've been diving for about a year now, and right now I am try to get a deeper understanding how my equipment works.

I've been researching regulators, and I have a decent grasp on how they work, but one thing has always escaped me...why two stages?

I know a SpareAir type systems use a single stage, so it is not impossible to do. Why are two stages better? I have some guesses, but I am not sure:

• Safety - two points of failure between your lungs and 3000 psi
• Engineering - two smaller pressure drops might be easier to engineer than one big one
• Flexibility - both divers and manufacturers can mix and match stages for their own purposes
• Reliability - a single stage is ok for a rescue bottle, but will not hold up to regular use
• Performance - you just can't get easy breathing across different depths from a single stage

All of these sound somewhat plausible, but I have not been able to find much to back them up.

Any ideas?

 

[Cyclops_Diver]

I'm no expert...and I have a lot of better ideas...but how about transport?

Be kinda silly hugging a AL80 tank so you could get your mouth on the single stage which includes the mouthpiece that is mounted directly to the tank like a spare air.

Then there is the hose issue....maybe they are not built to hol dthe sort of unchecked pressure an unregulated tank puts out so you need a stage at the tank to reduce that pressure. Then another stage to reduce the pressure even further so you can have regulated on demand air instead of a constant stream in your mouth.

Oh...I reckon I could go on....

 

[halocline]

Two reasons that come to mind are: 1. It's nice to have a valve right near your mouth so that there is minimal pressure differential between your mouth and the valve, and 2. It's easier to design a more sensitive demand valve if it does not have to deal with a huge pressure differential.

But, there are still some single stage old double hosers around, and they do work.

 

[Rhone Man]

It wasn't always so - in the old days of double hose regulators, there was just a single stage to bring the pressure down from 3,000 PSI to the 30-60 we breathe at depth.

Breaking it down into two stages just made engineering each stage easier, and made each stage last longer without needing servicing. It is also (in theory, but also I suspect in practice) makes breathing easier, and reduces liklihood of freeflow.

At the same time they moved the exhaust from back to front, but I don't think that the developments were related (except that the higher pressure probably made it too inflexible to route back around your head again).

I always remember a James Bond filme (Thunderball I think) where someone cuts Bond's regulator hose, and after killing the baddie, James then breathes off the severed hose. Film doesn't really address why the air is not coming out of the first stage at 500 PSI like a fire hydrant, but that is movies for you...

 

[mike_s]

the reason you have "another stage" at your mouth is for controlling/routing your "exhaled air".

When you breath in, the diaphram pulls forward on the 'spoon'/demand regulator and allows LP air to pass through the mouthpiece, until you stop breathing in. then it stops.

when you exhale, the exhaust diaphram (located below the mouthpiece, allows exhaled air to exit the regulator, without allowing water into your mouthpiece.

you then repeat the process again.

Thus the need for a second stage, to allow you to switch from "inhaling" to "exhaling".


Note: from what I understand, the old double-hose regs that didn't have a 2nd stage in the front still had a diaphram to inhale air from one hose and exhaust it into the other, stopping the mixing of fresh and exhaled air. (someone correct me if I'm wrong).

 

[Nemrod]

The Spare Air is a two stage. So are the majority of double hose regulators. What you mistake for a stage is not a stage but a combination LP stage and mouthpiece housing. Two stages can and are assembled together as one unit.

The first stage drops tank pressure to IP (intermediate pressure--around 140 psi) and then the second stage --ON DEMMAND-- further drops the pressure to ambient. The "pie in the face" doodad that your typical single hose regulator has does divide the duty, the first stage is on the tank, the second stage is at the mouthpiece but as has been noted, there are other ways to do it.

BTW, there have been single stage double hose regulators such as the original Mistral, it drops tank pressure to ambient in one drop--no IP--and because it has few moving parts and only one O-ring it is exceedingly reliable far beyond any modern regulator which is why Cousteu preferred them. No single hose regulator I know of is a single stage design.

N

 

[1_T_Submariner]

Having the IP pressure available is also a lot easier to control when inflating BC's, dry suits and other stuff.

A hose to carry up to 5000 PSI is not that hard to make. So I'm not sure if the hose really matters.

 

[nereas]

Those are all great answers!

In the olden days, we did not bleed off any of the intermediate pressure for a B/C, nor for a drysuit, nor an additional 2nd stage (they were called a "mouthpiece" in olden days). We learned "buddy breathing" in the olden days.

If you stepped down your tank pressure all the way to breathing pressure all in one stage, then two people sharing air off one tank would likely bottleneck each other's breathing pressure. And also, B/C-wing inflation would be much slower.

I think 150 psi as an intermediate pressure in a two-stage system works perfectly, especially for drysuit and wing inflation. I would not want a single stage system.

 

[captain]

Another reason for the two stages is eveness of inhalation effort. With a single stage the inhalation effort starts off high when tank pressure is at it's highest and gradually decreases as the tank pressure drops. Although the difference is not that great it does exist.

In the case of the two stage being tank pressure is first reduced to an intermediate pressure which stays relativly constant through out the the range of tank pressure, inhalation effort stays relatively constant.

As Nemrod stated because of the lack of O rings and few moving parts and no intermediate pressure that can change, the single stage is very very reliable. One of my favorite regulators is a La Spirotechnique Mistral single stage.

The single stage could be made to operate a BC or dry suit inflator by using a restriction orfice similar to that on an SPG, but an octo, though it probably could be done would not be as easy to do. All single stage diving regulators are of the double hose type.

 

[Nemrod]

If you were going to use an octapus on a single stage regulator then the octopus would have to be of a new design that would contain both the high pressure and LP stages in the mouthpiece. This could easily be done, the hose would carry HP air. This would not be an ideal circumstance because the HP hose could not have a restrictor orifice making a burst hose a really bad thing. I suppose there could be a HP fuse, a sudden and drastic drop in pressure would trigger a fuse that would shut off the air flow in the event of a burst HP supply hose. No big deal to design but until we return to single stage twin hose regulators--why?

N