Questions about pressure of regulator

[tbone1004]

so having the demand mechanism below your body makes the inhalation effort a bit less than it is with the 2nd stage and diaphragm behind your head. I don't find double hoses unpleasant to breathe on, but it is similar to breathing off of a 2nd stage while on your back, it's noticeably harder than on your belly. The 2nd stage regulator is only a demand valve and while it does reduce it to ambient, if it was not reduced initially from the high tank pressure, it would be VERY difficult to crack and almost impossible to control the flow. It also would have varying performance as the tank pressure drops. By dropping to intermediate pressure of nominal 125psi, the regulators will function with a pressure greater than ambient down to about 9ATA or 270FSW. After that point you have to raise the IP to overcome the ambient pressure. Breaking down the stages makes the process much easier to control, and much less dangerous. 125PSI coming out of a hose the size of the reg hoses is irritating at best and can cause bruising etc. 3000psi coming out of a hose like that can cut punch a hole straight through your hand. This is why part of why bare tank breathing has been forbidden from being taught by the agencies, although I still very much enjoy it and find it quite relaxing if done properly.

The first stage also acts as a manifold to distribute pressure to things like drysuits, BCD's etc. Now, also think about how heavy the first stage is. This is a necessary evil to contain all of that pressure. If a 1st stage was incorporated into the demand regulator side in your mouth, it would require that much metal just to keep from exploding, so now instead of your nice and light plastic second stages, you would have a hunk of brass weighing almost half a kilo hanging out of your mouth. Would be cause for a lot of problems with jaw fatigue. The same would go for the inflator mechanism on the BCD's and drysuits, they would have to be able to withstand that amount of pressure and would then be very very large and heavy.

 

[Dr. Lecter]

I'm going to sound incredibly stupid, but I have wondered the same thing, about why this has to be done in two stages. Is weight really the only answer?

As AJ notes, a single stage gas regulation system doesn't deliver great consistency downstream. One will encounter the same thing when airsmithing a paintball marker's gas system, though in that instance it's called shoot-down. Basically, two stages gives the first stage greater chance to keep up with pressure changes caused by downstream demand, creating greater consistency in gas delivered from shot to shot/breath to breath.

 

[raftingtigger]

The first regulators had one stage - located on the tank valve. A regulator senses ambient pressure - at the depth the regulator is at. With a single regulator (double hose) the ambient pressure measured was behind the diver's head. So if the diver is prone then the gas delivered is slightly less than ambient (like breathing through a long snorkel). Doable, but more work of breathing. If the diver is supine (laying on their back) then the gas is delivered at a pressure greater than ambient. This leads to a free-flow (and also a way to purge the mouthpiece). This same principle is one of the reasons re-breather divers are fussy about where the counter-lungs are.

With a final stage at the diver's mouth the ambient pressure is measured at the diver's mouth (give or take and inch) and there is less work of breathing.

 

[tech_diver]

Creating a single stage demand valve that can hold back 3000 psi yet breath easily is a heavy technical challenge. The Spare Air does this but will never get the delicate cracking pressures that modern second stages do. Also, it is very hard on the soft parts involved.

 

[redacted]

Creating a single stage demand valve that can hold back 3000 psi yet breath easily is a heavy technical challenge. The Spare Air does this but will never get the delicate cracking pressures that modern second stages do. Also, it is very hard on the soft parts involved.

I would think a balanced coaxial flow valve could get cracking performance in a usable range. But I don't know how one would overcome flow dynamics and seat imperfections to make it stable.

 

[PfcAJ]

Its important to note that double hose regs did eventually evolve to have 2 stages located in that round can thats right at the tank valve. It honestly operates the same way as a modern 2 stage reg, its just that the stages are REALLY close together and now there's a hose between them.

The old regs are pretty neato.

 

[tech_diver]

"I would think a balanced coaxial flow valve could get cracking performance in a usable range."

mmmm... very interesting idea.

The seating materials and stability would be an issue. I remember years ago when I was servicing Spare Airs, if anything bigger than a gas molecule went through the demand valve, the seat surface was compromised (scratched) such that it was very hard to keep it from leaking without seriously cranking down its cracking pressure. But that was a long time ago, designs and materials may have changed.

 

[GrimSleeper]

By dropping to intermediate pressure of nominal 125psi, the regulators will function with a pressure greater than ambient down to about 9ATA or 270FSW. After that point you have to raise the IP to overcome the ambient pressure.

Nice explanation, apart from the quoted bit. The first stage reduces tank pressure to +/- 125 psi (8.5-10 bar, usually, I'm mostly metric) over ambient pressure, and will do so at whatever depth you take it to (at least until you reach a pressure that exceeds the force the springs inside the first stage can exert). No adjustment required to exceed 270 feet - John Bennet took Apeks firsts out of the box and dived them to 1000 feet. On a slightly less dramatic scale, the only adjustment I had to make to Mares Abysses for a series of 350-foot dives was to detune them slightly by dropping the IP, as they were delivering too much gas at depth...

 

[tbone1004]

right, sorry was looking at an unbalanced reg on my desk when I was writing it and must have been thinking about how those function.

 

[dfx]

right, sorry was looking at an unbalanced reg on my desk when I was writing it and must have been thinking about how those function.

I don't think balanced vs unbalanced has anything to do with it...