Breathing below 300'

[jim2386]

Hey guys,

I have a question that I brought up in my divemaster class last night and we weren't really sure of the answer. I thought I might try posting it here and see what kind of responses I get.

We learned that the first stage of a normal regulator setup reduces the tank pressure (assume full aluminum) from ~3000psi down to an intermediate pressure of 125-145psig. For sake of argument, lets say it's on the high end and supplies the second stage with 145psi pressure air.

Now, the whole point of the second stage is to supply air to you at a pressure equal to the ambient water pressure so your lungs can function properly.

Running the math:

1ata = 14.7psi

so 145psig/14.7psi = 9.86ata

Assuming salt water 1ata = 33fsw

So 33fsw * 9.86ata = ~325fsw 33 for atmospheric pressure = 292fsw. This is the depth at which the pressure of the water is equal to the maximum pressure the first stage regulator can supply.

So after all that, here's my question. If the first stage can only supply a maximum of 145psi and you're at 292fsw, you should be able to breathe because the pressure on your lungs and the regulator supply pressure are equal. However, does this say that at 300ft I wouldn't be able to breathe off the regulator because the water pressure on my lungs would be higher than the first stage could supply?

I swear I've heard of guys going deeper that 300'... maybe I'm mistaken. Doesnn't this say they'd drown?

Maybe I opened Pandora's box, but I would be interested to hear different explanations.

Thanks
Jim

 

[Chernish]

You heard right, and the reason is simple: 1st stages do not give 145 psi, they give 145 psi *above the ambiant pressure*.
There is a mechanism in it just for this.
Else your regulator would have piss-poor performance well before 300 fsw.

Cheers!

 

[vixtor]

the first stage of a normal regulator setup reduces the tank pressure (assume full aluminum) from ~3000psi down to an intermediate pressure of 125-145psig

over the ambiental water pressure.

shouldn't the divemaster training include some basics of how a regulator works? not to mention that probably there was a fully qualified instructor in that room.

 

[Steve50]

Was there an instructor in the room?

 

[wedivebc]

You should probably take your training from an instructor who has actually been below 300ft

Most regs are depth compensated which means the IP is a constant pressure above ambient.

 

[halocline]

You should probably take your training from an instructor who has actually been below 300ft

Most regs are depth compensated which means the IP is a constant pressure above ambient.

Not to nit-pick, but ALL regs are depth compensating for IP or they would not work.

To be fair, I've known some excellent instructors that had a pretty poor understanding of how regulators work. Asking all instructors to really understand the working of regulators is a little like expecting reg techs and engineers to understand dive teaching. To the OP, it's good question to be thinking about. Here's how it works: IP is the amount of pressure it takes to close the first stage valve. So, unpressurized, the first stage is open. There's a big spring that is pushing either a piston or diaphragm to keep it open, and the chamber that holds the spring is open to ambient pressure. (On non-sealed regs; sealed regs do the same thing but with an additional step) Pressurize the reg, and air flows into it until a different chamber that contains the ports to the second stage builds up enough pressure to force the piston or diaphragm to move, which closes the valve and compresses the big spring. When you take a breath, you lower pressure in that chamber, the valve opens a little until the pressure builds back up, shutting it off, etc.

Because water can get in the chamber with the spring, it also pushes, along with the spring, against the piston/diaphragm to keep the valve open. Therefore, as the water pressure increases (depth) it takes more pressure on the other side to close the valve (IP).

 

[wedivebc]

Not to nit-pick, but ALL regs are depth compensating for IP or they would not work.

.

Sorry but you are wrong. I have a couple of regs here that are not. Rebreather divers using mass flow systems like the KISS use non-depth compensated regs, that is why I said most and not all in my former post because this board is full of nit-pickers.

 

[DA Aquamaster]

Matt is on the mark. Roughly speaking you add .5 psi per ft - but that increased ambient pressure is added to the intermediate pressure by the action of either the diaphragm (in a diaphragm reg) of the pressure acting on the back side of the piston in a piston reg.

So the intermediate pressure will be 145 psi above ambeint whether you are at the surface or at 300'.

In the extreme, if you dove to about 6000 ft, the pressure in even a full 3000 psi tank would be at only ambient pressure and you would get no useable airflow.

At 300' the absolute pressure (the ambient pressure including the 14.7 psi atmospheric pressure plus intermediate pressure) inside the intermediate spaces of the reg would be 293 psi on a reg operating with a 145 psi IP. So the viscosity of the gas would begin to increase and that will reduce the flow rate. But with any reg that you want to take to 300', there is enough reserve flow rate to ensure that you will still get enough gas. And at 300' you will also be using a helium mix and they flow through regs about 20% easier anyway so it is really a non issue.

The whole thing does however make you seriously question the general knowledge and reasoning skills of the instructors involved.

Sorry but you are wrong. I have a couple of regs here that are not. Rebreather divers using mass flow systems like the KISS use non-depth compensated regs, that is why I said most and not all in my former post because this board is full of nit-pickers.

That is indeed nit picking.

In an open circuit reg, the goal is to keep the inhalation effort constant by keeping the Ip relative to ambient pressure constant, so it must be depth compensated.

In a rebreather, the IP is not compensated as the IP and/or orifice size is adjusted to keep the amount of gas flow constant and in the general ball park needed to match the amount metabolized by the diver in an average swimming condition to make it easier to manage the PO2.

It is an apples and oranges comparision.

 

[wedivebc]

Ummmm, he called me out.

That is indeed nit picking.

Since if I had said all regs are depth compensated someone else would have found a nit to pick so I used the word most, and still get accused of it. Have fun boys but this is getting tiresome.

 

[ljdiver]

just as everyone said, it would be 145 above ambient pressure

i just wanted to point out (because nobody has specifically mentioned it yet) that the psig units you correctly used in your original post actually mean 'relative to surrounding pressure'

straight from wiki: Psig (pound-force per square inch gauge) is a unit of pressure relative to the surrounding atmosphere.