Trivia Question: Where Would You Be At 1/2 Ata?

[Conor]

I have read through most of the posts, although I got lost in the maths of some. I sense another question may not have been addressed (or maybe it has been and I missed it).

How does altitude affect ascent rate ?

At sea level a lot of dive professionals say that a lot of injuries occur in coming up the last 10 meters, as the gases double in volume through this transition, contributing to bubble formation and lung over expansion injuries. (Hence Safety Stop procedures and SAFE).

At 18,000ft would this transition stage occur in the last last 5m(ish) instead ? My thinking is that if at the surface you are at 0.5atm then adding the weight of 5M of water would bring you to around 1atm, equivalent to sea level. This would mean as you ascend the last 5M you would be doubling the volume of any gas exposed to ambient pressure, i.e. lung contents, bcd contents, blood gases and airspaces.

Also does such altitude changes affect the safety or required specification of cylinders? Considering most are spec'ed to hold 230 Bar at 1atm external pressure, how does having an external pressure of half of that affect the safe operating pressures of cylinders ? Do they drop as well ?

Apologies for my tuesday morning ponderings

Conor

BTW I'm a lapsed chemist although I would call all of this PhysChem.

 

[pufferfish]

Conor once bubbled...
I have read through most of the posts, although I got lost in the maths of some. I sense another question may not have been addressed (or maybe it has been and I missed it).

How does altitude affect ascent rate ?

At sea level a lot of dive professionals say that a lot of injuries occur in coming up the last 10 meters, as the gases double in volume through this transition, contributing to bubble formation and lung over expansion injuries. (Hence Safety Stop procedures and SAFE).

At 18,000ft would this transition stage occur in the last last 5m(ish) instead ? My thinking is that if at the surface you are at 0.5atm then adding the weight of 5M of water would bring you to around 1atm, equivalent to sea level. This would mean as you ascend the last 5M you would be doubling the volume of any gas exposed to ambient pressure, i.e. lung contents, bcd contents, blood gases and airspaces.

Also does such altitude changes affect the safety or required specification of cylinders? Considering most are spec'ed to hold 230 Bar at 1atm external pressure, how does having an external pressure of half of that affect the safe operating pressures of cylinders ? Do they drop as well ?

Apologies for my tuesday morning ponderings

Conor

BTW I'm a lapsed chemist although I would call all of this PhysChem.

Conor those are great ponderings even for a Tuesday morning. I think you are correct about the pressure changes and ascent rates at 18,000 feet.

The tank issue is an interesting one. Maybe someone else will know that one, but yes if the external wall pressure drops to half of normal one does wonder about the tank's ability to still hold its normal internal rated pressure.

Now tell me after diving would we be able to make a cup of tea at 18,000 feet?

 

[jrmy_1]

cnidae once bubbled...


Lets set one thing straight. This class was not a NDL class; all dives are considered decompression dives by GUE. This class was taught as a minimum deco class. Run your profile again using 32 opposed to 30/30 and see what you come up with. It should be somewhere around 3 minutes of stops. What’s happening with what you ran is the software is penalizing you for the helium when it shouldn’t be. I'm not a deco expert but what I think is happening is the software algorithm considers the helium to dissove imediately when it does'nt. Now take the software and make it so that your stops are at an 80 pause, 50,40,30,20,10 this is more realistic to what we did. As far as the altitude stuff nothing I stated came straight from the instructors mouth so I would'nt quote it when asking them questions.

When I ran the 32% dive, it came up with 40 seconds at 10 ft. It seems like the program penalizes you greatly for the helium. I have heard that the same is true if you add helium to your deco gas as opposed to nitrox. I have ran dives with different mixes and have found that helium helps to a certain percentage, but once you continue to add it, it starts to increase the length of the deco. It has to have something to do with the differential equations programmed into the software.

 

[Conor]

pufferfish once bubbled...


Now tell me after diving would we be able to make a cup of tea at 18,000 feet?

You would but on't be surprised if you have to make it brew longer, I'm pretty sure that the change in pressure would affect the rate of diffusion.

 

[pufferfish]

Conor once bubbled...


You would but on't be surprised if you have to make it brew longer, I'm pretty sure that the change in pressure would affect the rate of diffusion.

I was thinking more at what temperature would the water boil at 18,000 feet?

 

[pufferfish]

I found the answer here.
Altitude vs boiling point

Looks like for every 1000 feet in elevation the boiling point drops about a degree centigrade or for every 500 feet it drops about a degree fahrenheit. So at 18,000 feet our water temp would be about 82 C or 176 F. I guess you would just let the tea steep a little longer, eh?

 

[Conor]

Good point.... It would boil (i.e. form bubbles of steam etc.) at a lower temperature than at STP which would ruin your cup of tea as this would affect your rate of diffusion as well.

The boiling point should be calculable from (stolen from a website so I won't swear to it)

Temperature = -3200 / log(Pressure/1700) - 459.7

Temperature in degrees Farenheight; pressure in PSI

I have had more ponderings as well.

I reckon talk of depth is a bit of a red herring becasue it causes us to try and understand all gas behavious according to the depth we are at when actually it is the ambient pressure that is important, as has been said previously.

There are two main differences
1. Pressure transitions, i.e. those changes in gas behaviour related to changing pressure causing expansion/compression. i.e squeezes, Lung expansions, bubble formation. Changing the starting (surface) pressure for a dive affects the whole scale for these and as such anyone who thinks you can ignore altitude is a nutter.

2. Static pressures, i.e. changes in behaviour due to the current ambient conditions, such as partial pressures, gas consumption, on gassing etc. So Nitrox gets a wider range and your air consumption at a given depth should decrease.

So essentially you will ongas slower at altitude for a given depth, but gas will come out of solution faster for a given ascent rate, this rate may mean that the bubbles come out in the blood stream or joints causing bends.

By the way all the above is open for debate, I am not stating any of them as facts, just opinions/thoughts.

 

[Conor]

Glad you found that list, the formula was confusing me lots.

Maybe thats why the English have more than there fair share of Mountaineers....more used to sitting around waiting on a cup of tea

 

[pufferfish]

Conor once bubbled...
Glad you found that list, the formula was confusing me lots.

Maybe thats why the English have more than there fair share of Mountaineers....more used to sitting around waiting on a cup of tea

Sounds plausible about the mountaineers, but what about all the hooligans?

 

[Conor]

They are the ones without the patience for tea