Breathing on a regulator 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

 

[GrumpyOldGuy]

There is a reason there is an ambient port on a 1st stage. It allows the pressure from the water to be compensated for, the IP is relative to depth. This is the major difference between a scuba first stage and a plain old regultor you might find in other (non-scuba) applications.

Of course, at 300' you would have a lot of other problems breathing air, like oxygen toxicity.

 

[mkutyna]

It is usually more correct to say that the IP is 90-150 psi above ambient pressure.

 

[SparticleBrane]

Both o the above posts are correct. The first stage supplies the IP above the ambient pressure, so whatever the water pressure is at 300ft, or 250ft, or 5ft, the pressure applied to IP is 90-150psi (usually ~140psi) above the surrounding water pressure.

It sounds like your instructor said "IP is 140psi" and left off "...above ambient pressure." It is a very important distinction...

 

[Steve SDI/TDI HQ]

The three postings above have nailed it.

The work of breathing at 90 metres and beyond from a decent regulator is increased compared to breathing it at say 20 metres. The density of the gas moving through the first stage is the modifying factor rather than IP. That gas is after all approximately ten times its density at the surface! This is offset considerably by the helium that will (hopefully) make up the largest percentage of the mix being breathed, but there is a difference. This is also compensated by using regulators designed and set up for deep diving... slight adjustments to cracking and intermediate pressure as well as tender loving care coupled with regular inspection!

 

[Rhone Man]

I seem to recall reading somewhere that when Nuno Gomez did his world record deep dive to 1,080 odd feet (nearly 300 metres) he had to have some specially made regulator to deliver a high enough intermediate pressure. But at 31 ATAs, I would bl**dy well hope so.