Physics - surface interval discussion

[Kaltim Diver]

Thanks again. Suffice to say if I plan my dive and I don't surface with 70 or so bar in the tanks I look on it as a poorly planned or executed dive.

KTD

 

[Steve50]

Pressure does not open the first stage, it shuts it (that's what IP is) so the regulator would function right down to ambient pressure. The 2nd stage would be much harder breathing, that's for sure. But you can get air with less than IP.

Not that it matters, but 10 bar is just a teeny bit higher than most regulators' IP. Of course, if ambient was 10 bar then the IP would be closer to 20, because IP is always X above ambient pressure. Otherwise your reg would be very difficult to breathe at any depth.

not when there is only 10 bar in the tank

 

[halocline]

not when there is only 10 bar in the tank

What's your point?

 

[Crowley]

Feel free to give it a try!

Also worth metnioning that the absolute pressure at 100m is of course 11 bar - as is the absolute pressure in a scuba tank. There's one ata in an "empty" tank at the surface when your gauge reads zero therefore if you have a reading of 10, there is actually 11 ata in your tank.

Toodles

C.

 

[elan]

Feel free to give it a try!

Also worth metnioning that the absolute pressure at 100m is of course 11 bar - as is the absolute pressure in a scuba tank. There's one ata in an "empty" tank at the surface when your gauge reads zero therefore if you have a reading of 10, there is actually 11 ata in your tank.

Toodles

C.

Yeah that's a good point of clarifying what OP has actually meant by saying "got to 10bar" It can be understood as the tank actually has 10 bar inside or the gauge reads 10bar. If the gauge reads 10 bar then the pressure inside is in fact 21bar at 100m. And in the latter case OP will be able to breath the gas. Though it will not be as easy.

 

[knotical]

Hi elan,

you’ve confused me.

Yeah that's a good point of clarifying what OP has actually meant by saying "got to 10bar" It can be understood as the tank actually has 10 bar inside or the gauge reads 10bar. If the gauge reads 10 bar then the pressure inside is in fact 21bar at 100m. And in the latter case OP will be able to breath the gas. Though it will not be as easy.

My understanding is different. I always thought the SPG was connected to the HP side of the first stage and therefore reads cylinder pressure. The LP side of the first stage feeds the IP which attempts to maintain itself about 10 bar above ambient. But it can only do this as long as the cylinder retains 10 bar or more pressure above ambient. Once cylinder pressure drops lower than 10 bar above ambient the IP start dropping. So if gauge pressure shows 10 bar, the reg stopped providing an IP of 10 bar above ambient for some time, and the diver has already found it harder to breathe.

And in your earlier post,

I can breath through the reg when it is not connected to the tank and the cap is removed. So when the pressure is equal (in this case at 90m) you can definitely breath, so if you can create a pressure of 1 atm with your lungs you can definitely breath at 100m with 11atm ambient. To estimate I think the following rough model can be used, if you can hold a bar with a side of inch by inch with a weight of 14lbs approx with your lips using the suction power of your lungs you will definitely be able to breath in those conditions

I think not. Otherwise, I should be able to breathe through a long snorkel at the bottom of a 33 foot deep pool. There’s been lots of experiments where it was shown that even a few feet is enough to prevent that from working.

Not trying to pick on you, just trying to understand. Hopefully someone can ‘splain to me if I’m wrong.

k

 

[elan]

Hi elan,

you’ve confused me.

My understanding is different. I always thought the SPG was connected to the HP side of the first stage and therefore reads cylinder pressure. The LP side of the first stage feeds the IP which attempts to maintain itself about 10 bar above ambient. But it can only do this as long as the cylinder retains 10 bar or more pressure above ambient. Once cylinder pressure drops lower than 10 bar above ambient the IP start dropping. So if gauge pressure shows 10 bar, the reg stopped providing an IP of 10 bar above ambient for some time, and the diver has already found it harder to breathe.

As of my understanding, bourdon tube measures the difference between the incoming pressure and ambient pressure as the internal pressure tries to make the shape circular and the ambient pressure will work against it.

The only thing I was not sure about is weather the pressure of the water is transmitted to the inside of the gauge or not. If yes then so while the gauge will show 10 bar pressure at 100m it would be in fact 21 bar and on the surface it will show 20. If no then essentially it will be showing the pressure relative to the one in the gauge (1 atm)

And in your earlier post,

I think not. Otherwise, I should be able to breathe through a long snorkel at the bottom of a 33 foot deep pool. There’s been lots of experiments where it was shown that even a few feet is enough to prevent that from working.

Not trying to pick on you, just trying to understand. Hopefully someone can ‘splain to me if I’m wrong.

k

Initially I have assumed that by 10 bar the OP meant the absolute pressure of 10 bar in the tank. And no one cannot breath at 33 feet as it will be required for them to compensate the pressure difference of 1 atm. Phrase "so if you can create a pressure of 1 atm with your lungs you can definitely breath at 100m with 11atm ambient." was rather sarcastic

 

[knotical]

As of my understanding, bourdon tube measures the difference between the incoming pressure and ambient pressure as the internal pressure tries to make the shape circular and the ambient pressure will work against it.

The only thing I was not sure about is weather the pressure of the water is transmitted to the inside of the gauge or not. If yes then so while the gauge will show 10 bar pressure at 100m it would be in fact 21 bar and on the surface it will show 20. If no then essentially it will be showing the pressure relative to the one in the gauge (1 atm)

You've got to be right. I can’t believe I never thought of that. Typical SPGs are only air, not oil-filled. If they didn’t have some means of compensating for depth, they’d crush. Thank you.

edit: This video:
http://www.youtube.com/watch?v=djNB4Hluj1E
mentions a burst disk to prevent a gauge from exploding, but no mention of a way to “sense” ambient. So I guess I remain confused - or at least uncertain.

 

[elan]

You've got to be right. I can’t believe I never thought of that. Typical SPGs are only air, not oil-filled. If they didn’t have some means of compensating for depth, they’d crush. Thank you.

edit: This video:
YouTube - making a scuba pressure gauge or SPG
mentions a burst disk to prevent a gauge from exploding, but no mention of a way to “sense” ambient. So I guess I remain confused - or at least uncertain.

I was looking for some info whether the SPG sense the outside pressure or not but I could only find that it's sealed watertight, nothing about transferring pressure to the tube.

But if we think this way - it's not oil filled I believe. It has air inside and it's not compressible. To transfer pressure it would need air to compress thus we can make an assumption that the pressure is not transferred and it in fact shows the pressure relative to 1 atm. But then as you have mentioned it should be strong enough to withstand the outside pressure. Unfortunately I cannot evaluate this as I luck education on material strength. , May be some technician can clarify this.