A conundrum

If a diver is using a full face mask connected to an air supply machine on a boat at sea level and the diver drops to -3 atmospheres (for example) how do u determine at what depth the surrounding sea pressure will overcome the hose's circular outward resistance created by the internal 1 atmosphere + the physical properties of the hose material (given you know that hose value) + properties of a cylinder and unified pressures?

Thanks,

Mark

Doesn't a full face mask still have the equivalent of a 2nd stage regulator attached to it, supplied with surface supplied air at an intermediate pressure of 140psi (or whatever)? Thus making the internal pressure of the hose ~9atm.

Is the "air supply" (compressor) supplying air at high pressure, and does the diver have a regulator? If so, the pressure in the hose is not one atmosphere. Nevertheless, the maximum depth that a hose having given properties can be used before it restricts airflow excessively is a valid question. I suppose the calculation is fairly complicated. That's all I can say on it. Sorry.

---------- Post added March 26th, 2014 at 01:12 PM ----------

Was typing while jcaplins posted. Yeah, same question.

markg2:

If a diver is using a full face mask connected to an air supply machine on a boat at sea level and the diver drops to -3 atmospheres (for example) how do u determine at what depth the surrounding sea pressure will overcome the hose's circular outward resistance created by the internal 1 atmosphere + the physical properties of the hose material (given you know that hose value) + properties of a cylinder and unified pressures?


Thanks,

Mark

Click to expand...

I guess someone else shares the same view of scubaboard as I do.

I'm already your fan Mark!

The question came up during a random conversation at the gym. This dive was off Bimini like 40 or so years ago ;-). There was no regulator, just the hose somehow connected to the mask--it did have a professional appearance. I suspect the only compression supplied by the topside machine was that which was necessary to move the air down the hose.

The idea at the time was that decompression was not necessary given you were breathing surface air. Now I'm not sure if that thinking was correct or we were just damn lucky.

Whatever the design, in order to keep the full face mask from sucking the divers face to the point of inability to breathe, the air in the face mask must be at approximately ambient (3 atmospheres in the OP's example). The only way for that air pressure to get there is via the air hose, so the air hose will be at ambient or greater and therefore will not collapse.

i.e. the assumption that the diver is breathing air at surface pressure is incorrect. Said another way, the pressure needed to push the air down the hose must be at least as much pressure as the diver is experiencing at depth

I think your error is assuming that the internal pressure in the hose is 1 atmosphere. This post should probably be in the Basic Forum. The pressure in the hose is generally at least 80 PSI higher than the bottom pressure. A pressure less than the diver’s depth would not allow any breathing gas to reach the mask.

This is handled in two ways. A typical Scuba demand regulator operates in the 130-150 PSI over bottom pressure range. Some masks, primarily used in the commercial diving industry, allow the diver to adjust the loading on the demand regulator valve to function with minimal breathing resistance with a supply pressure varying between about 80 and 200 PSI. That allows a low-pressure air compressor with a relatively fixed pressure output to be used.

Related to this method is some FFMs (Full Face Mask) use regulators that operate over a broader range than average scuba regulators. This works fine in shallow water most common in rescue and public safety work.

The second method is for the diving supervisor (the person operating the communications and gas supply system) to slowly change the supply pressure to the diver so the resulting pressure at depth is within the regulators’ operating parameters. This is much easier with high pressure supplies than on a low pressure compressor.

There have been a few commercial diving helmets with a first stage that regulated the pressure, but they are the exception.

Concern over collapsing the hose is a factor in Saturation Diving where exhaled gas is routed back to the surface for reprocessing. A negative-biased back-pressure regulator is used on the exhaust hose to prevent hose collapse after the demand-exhaust regulator mounted on the diver’s hat.

• markg2:

  If a diver is using a full face mask connected to an air supply machine on a boat at sea level and the diver drops to -3 atmospheres (for example) how do u determine at what depth the surrounding sea pressure will overcome the hose's circular outward resistance created by the internal 1 atmosphere + the physical properties of the hose material (given you know that hose value) + properties of a cylinder and unified pressures?

  Click to expand...

markg2:

Thanks,

Mark

Click to expand...

Akimbo answers this extremely well.
I'll just add that if your issue is surface supplied air related, this is typically a Brownies Third Lung type of an application--which can easily be used with a full face mask.
Brownies units are ideal in 20 to 30 foot deep water, and "can be used" to as deep as 70 or 80 feet by one or two divers...though the hose length and and drag at this depth becomes problematic if any current exists, and in the event of the air supply failing ( like a huge wake from a close passing yacht tips over the Brownie unit, and air supply ceases....) then you need a pony bottle on your waist belt with a necklace reg on it for a mostly uninterupted ability to breathe....though your air supply would only be enough for a rapid ascent, and this could be a DCS concern if you were using the Surface Supplied Air system to stay down longer than you normally would with a single 80 cu ft tank.

Deeper than 70 to 80 feet and the engine running the Brownie type system does not generate enough air pressure to keep two divers breathing easily...one is not so bad here..... Another option is having 2 or three tanks manifolded in the raft on the surface, with the hose coming down--this being the Tooka concept of Brownies....but again, it is a foolish direction for deeper dives.....
My personal feeling is that once you get deeper than 50 or 60 feet, surface supplied air should be the playground of the commercial guys that are actually trained and geared for this.
See:

• F390X Floating Direct Drive Hookah Diving Systems |
• Tooka, Snooka, Snookling,Hookah Diving Systems |
• Demo of non-diver using Tooka - YouTube How a non-diver can use the Tooka with an instructor....much easier than a Discover Scuba Class since no buoyancy or BC skills need to be taught.

Akimbo and Dan, you are overthinking this; the question was about a mask which had nothing other than ambient surface pressure, and whether the flexible hose would be crushed at 3 atmospheres (assuming this to be 66 feet of seawater, or 3 atmospheres absolute pressure). The answer is that yes, it would be crushed, and at that depth the diver's eyes would be sucked out of their sockets too! People tried this centuries ago, with disastrous effects. Here's an image from Diego Ufano, 1613 which illustrates this concept. But it is not possible due to water pressure. The condition is now called formally called a mask or helmet squeeze, and people have died from it.

If you hook it up to an "air supply machine" such as a compressor, it must pump air down with enough pressure to overcome the water pressure to get air to the diver, but then the diver would be at ambient pressure and subject to the decompression rules of breathing air at pressure, as Dan and Akimbo stated.

SeaRat

PS--I agree that this question should be in the basic scuba area.

John C. Ratliff:

Akimbo and Dan, you are overthinking this; the question was about a mask which had nothing other than ambient surface pressure, and whether the flexible hose would be crushed at 3 atmospheres (assuming this to be 66 feet of seawater, or 3 atmospheres absolute pressure). The answer is that yes, it would be crushed, and at that depth the diver's eyes would be sucked out of their sockets too! People tried this centuries ago, with disastrous effects. Here's an image from Diego Ufano, 1613 which illustrates this concept. But it is not possible due to water pressure. The condition is now called formally called a mask or helmet squeeze, and people have died from it.

If you hook it up to an "air supply machine" such as a compressor, it must pump air down with enough pressure to overcome the water pressure to get air to the diver, but then the diver would be at ambient pressure and subject to the decompression rules of breathing air at pressure, as Dan and Akimbo stated.

SeaRat

Click to expand...

actually, I think Akimbo and Dan both interpreted and answered the question quite well.

the dive off Bimini 40 years ago was a long time ago, and the details are probably a bit fuzzy, however, I'm sure everyone involved would have remembered someone's eyes getting sucked out of their sockets