POP QUIZ: How deep do you gotta be....

[roakey]

Originally posted by NetDoc
take away one atm for the atmosphere

Nope, by stating "3000 psi" implicitly we're talking gauge, not absolute pressure.

If 3014.7 psi was stated, then you'd take away one atmosphere.

Ignoring temperature and elasticity of the cylinder, I'm coming up with 6,735 fsw too.

Roak

 

[MikeS]

My guess is that this is a trick question. At whatever depth you opened the valve something would happen. The gas in the tank, oxygen and nitrogen (assuming air in the tank), would flow out until the same partial pressure was achieved on the outside as inside, just as nitrogen flows from your lungs into your body (mostly water) as pressure increases. If the valve were opened at a point were the internal and external pressure were equal, gas would leave the bottle until the partial pressure of each gas inside the bottle reached the partial pressure in the water. Because of the reduced pressure in the tank as a result of the escaping gas, water would flow in to equalize the pressure.

 

[Dee]

WAY deeper than you'll ever find me!

 

[AllenG]

And if you're in Florida, you would need to adjust it down to 2800 PSI to adjust for the infamous "Florida Fill" factor.

 

[Walter]

You've all made an incorrect assumption, one so basic to what we've been taught and what we teach that it's easy to see how you could blindly follow it. We do not increase pressure by 1 ATM every 33 ft in sea water and every 34 ft in fresh water. It is close to that, but it's not exact. When dealing with normal ranges, it's close enough. It's actually 33.075 ft in salt and 33.92 ft in fresh water. When dealing with extreme ranges the tiny error gets larger. The other factor is salinity varies from place to place and even day to day. Rick already mentioned temperature. Ignoring the differences in salinity and temperature and taking an average weight of a cu ft of water gives us 62.4 lbs/cu ft of fresh & 64 lbs/cu ft of salt. Pressure will increase by .4444 for every foot we descend in salt water and by .4333 in fresh. We'll reach 300 PSI at 6750 ft in salt & 6923 in fresh water. That comes to 2057.4 meters in salt and 2110.15 meters in fresh water. HTH.

 

[Rick Murchison]

Originally posted by Walter
You've all made an incorrect assumption, one so basic to what we've been taught and what we teach that it's easy to see how you could blindly follow it. We do not increase pressure by 1 ATM every 33 ft in sea water and every 34 ft in fresh water. It is close to that, but it's not exact. When dealing with normal ranges, it's close enough. It's actually 33.075 ft in salt and 33.92 ft in fresh water. When dealing with extreme ranges the tiny error gets larger. The other factor is salinity varies from place to place and even day to day. Rick already mentioned temperature. Ignoring the differences in salinity and temperature and taking an average weight of a cu ft of water gives us 62.4 lbs/cu ft of fresh & 64 lbs/cu ft of salt. Pressure will increase by .4444 for every foot we descend in salt water and by .4333 in fresh. We'll reach 300 PSI at 6750 ft in salt & 6923 in fresh water. That comes to 2057.4 meters in salt and 2110.15 meters in fresh water. HTH.

Ah, Walter, I'm surprised at you! Expressing the depth per atmosphere of salt water to three decimal places isn't possible with any accuracy, for it varies all over the world, from day to day in any given locale due to variable salinity [and therefore density]. The Mediterranian is much denser than the Caribbean, which is much denser than the Andaman sea... The polar seas differ from the tropics. You can be very precise for fresh - meaning pure - water at 4 degrees C - that, after all is a standard. But for salt water the best you can really do is say "about" 33.075 feet... so your very precise figure is just a very precise average figure, and probably doesn't exist with that exactitude in much of the ocean at all.
It's been fun watching the exercise...
Rick

 

[Walter]

I agree with you completely! I'd already stated why the numbers couldn't be exact, I was ignoring differences in salinity and temperature. Perhaps I should have been more clear on that point. Perhaps we should also stress, "Don't try to field test this problem." You gotta admit the excercise was fun.

 

[wetone]

Yep that it,,, 500 feet.

I was so narced, I couldnt tell that anything happened.

wetone

 

[roakey]

Originally posted by Walter
You gotta admit the excercise was fun.

WAS? I haven't seen why TexasMike claimed that JustAddWater was "Very close, but not quite correct."

That's what I'm waiting for...

Roak

 

[MikeS]

In order for the first stage regulator to work, isn’t a pressure differential require? Some physical force is required to operate the valve. Isn’t the first stage regulator just a sophisticated version of the regulator on my barbeque grill? All of the gas never comes out of the tank because a pressure differential is required to operate the valve in the regulator.

If this were true, the point that the air became unusable for breathing would be somewhat shallower than the point at which the pressure was equal.

Perhaps the question should be, at what depth, will the pressure differential between the air in the tank and the ambient water become insufficient to operate the first stage regulator.

Does anyone know what the required pressure differential is for a first stage to operate? Is it different for different makes and models?

And what about the second stage? Is the partial vacuum created by “sucking air” enough to operate the valve assuming the pressure was equal otherwise?