OP
LI-er
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Very helpful.
Converting liters to cubic feet
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Very helpful.
First off most EU tanks called 12Lt are 12.2Lt. Check the water capacity on the tank. An Alli 80cuft is 11.1Lt about 10% difference irrespective of pressure, so you will get slightly longer dives.
It’s so much easier to work in metric that imperial (US), which isn’t always the same as imperial (U.K.); miles and gallons come to mind.
Also keep in mind that air is not ideal gas. The higher the tank's pressure gets the more air you need to further increase that pressure. And similarly any extra bar already in the tank will yield less than the "ideal" gas volume at 1ATM. Especially as you go over 200bars the difference between ideal gas and real gas volume becomes quite noticeable.
Actually at lower pressure like 40 to 80 bar around the ideal gas equation gives more content than a real gas.
Also keep in mind that a pressure gauge has a finite accuracy, mostly around plus minus 10% or maybe 5 % I believe.
For a practical application, as long you have less than around 230 bar the ideal gas equation should be good enough.
You can calculate your sac in terms of bar(psi) per minute and then just need to multiply with the factor for depth.
So you can then calculate am average ascent rate plus safety stop and maybe some deep stop to calculate the minimum pressure, for a given tank size, for a safe ascent to end up with a safety pressure on surface. In order to be on the safe aide you should multiply your sac by two, because you assume worst case scenario at that point (buddy out of air, stress and increased sac and so on).
Add like 5 bar for each deco minute (as long as you only have a short deco).
Then you know your latest point of ascent drom like 40m, 35m and so on.
Actually if you take the factor of two serious wirh an increased sac and leave 50 bar safety margin you should not descent to 40m at all with 12l...
Also keep in mind that a pressure gauge has a finite accuracy, mostly around plus minus 10% or maybe 5 % I believe.
For a practical application, as long you have less than around 230 bar the ideal gas equation should be good enough.
You can calculate your sac in terms of bar(psi) per minute and then just need to multiply with the factor for depth.
So you can then calculate am average ascent rate plus safety stop and maybe some deep stop to calculate the minimum pressure, for a given tank size, for a safe ascent to end up with a safety pressure on surface. In order to be on the safe aide you should multiply your sac by two, because you assume worst case scenario at that point (buddy out of air, stress and increased sac and so on).
Add like 5 bar for each deco minute (as long as you only have a short deco).
Then you know your latest point of ascent drom like 40m, 35m and so on.
Actually if you take the factor of two serious wirh an increased sac and leave 50 bar safety margin you should not descent to 40m at all with 12l...
....the less air you need to increase that pressure.
A 1 liter cylinder at 300bar, contains about 275 liters of air (at 1 bar). Assuming that cylinder has cooled down to ambient temperature.
The difference comes from the compressibility factors of each gas (mainly nitrogen and oxygen).
The compressibility factor of each depends on the pressure and temperature and is not linear.
Take the valve off a cylinder and fill it with water. How much water goes in?
That's the size of your tank.
When expressing that amount in liters, it makes sense. In cubic feet, it becomes zero point something, which isn't easy to work with.
A solution is to multiply that volume with the max rated pressure, so the numbers become easy.
But adding insult to injury, manufacturers came up with LP and HP tanks, increasing the confusion.
Now that you have moved from imperial system to the metric world, it will all become easier.
You will come across 7, 10, 12 and 15 liter steel tanks. Steel tanks are usually rated for 232bar, some are rated to 300bar.
Aluminum tanks (you'll often here aluminium in non-english countries) are rated to 207bar.
Calculations will become easier, just stop thinking imperial.
I find it convenient to think of a tank in terms of the volume of sea-level-pressure gas it can hold, so I can think of a tank in terms of how many at-the-surface minutes I'll get from it at my Imperial RMV. I can't do that if you tell me my tank is a 12 liter (by he way, the metric abbreviation for liter is l not L) tank, I also have to know what its pressure rating is, or I can't work out how many minutes I'll get from it if I fill it to max pressure. It's just a bit more algebra, however, so no big deal.
On the other hand, if you tell me my tank is 100 cuft, I'm good...I know immediately how many minutes I can get from it. That, to me, is the advantage of the "imperial system" but has nothing to do with the units, it ahs to do with how the szie of the tank is expressed.
So instead of telling me I have a 12 liter tank, just tell me I have a 2500 liter tank (or whatever), then I can divide by my metric RMV.
The issue is NOT just Imperial vs Metric units, it is also how tank size is expressed and used. The units thing is just a little calculation, of no consequence.
On the other hand, if you tell me my tank is 100 cuft, I'm good...I know immediately how many minutes I can get from it. That, to me, is the advantage of the "imperial system" but has nothing to do with the units, it ahs to do with how the szie of the tank is expressed.
So instead of telling me I have a 12 liter tank, just tell me I have a 2500 liter tank (or whatever), then I can divide by my metric RMV.
The issue is NOT just Imperial vs Metric units, it is also how tank size is expressed and used. The units thing is just a little calculation, of no consequence.
It has a consequence.
Your sac is based on the assumptions of a specific atmospheric pressure, your tank filling then HAS to be to filled to the rsted pressure. Later is not guaranteed.
In contrast for instance I know at 1 bar I use 12 liter per minute (ideal warm conditions) and 18 on bad conditions.
So its 1 to 1.5 bar per minute at surface or like a factor of 4 at 30m.
Then I know independently of the actual filling.
But ok its a matter of calculation and grt used to it.
I would as I pointed out above more use a latest point of ascent as a function of the pressure to memorize.
The gas usage I presented here would be for planning.
Your sac is based on the assumptions of a specific atmospheric pressure, your tank filling then HAS to be to filled to the rsted pressure. Later is not guaranteed.
In contrast for instance I know at 1 bar I use 12 liter per minute (ideal warm conditions) and 18 on bad conditions.
So its 1 to 1.5 bar per minute at surface or like a factor of 4 at 30m.
Then I know independently of the actual filling.
But ok its a matter of calculation and grt used to it.
I would as I pointed out above more use a latest point of ascent as a function of the pressure to memorize.
The gas usage I presented here would be for planning.
To re-state what others have re-stated, in Europe the standard is to specify the size of a tank in terms of volume if it were filled with water (that is, something incompressible), whereas in the US the standard is to specify the size of a tank in terms of volume if it were filled with a compressible gas that has been compressed to some standard "fill pressure," and you were to measure the volume of that gas when decompressed (1 ATA). In other words, the European standard specifies tank size independently of pressure, whereas the US standard takes fill pressure into account.
For example, I have heard it said that the volume of gas that could be released from a full Al80 into a 1 ATA space would roughly fill a phone booth (for those old enough to know what a phone booth is), which apparently has dimensions of length x width x height that would equal about 80 cf (though, as others have pointed out, an Al 80 really holds about 77 cf). A European 12 liter tank at its fill pressure could similarly release that much gas into a volume about the size of a phone booth (maybe one of those cool British red ones).
For example, I have heard it said that the volume of gas that could be released from a full Al80 into a 1 ATA space would roughly fill a phone booth (for those old enough to know what a phone booth is), which apparently has dimensions of length x width x height that would equal about 80 cf (though, as others have pointed out, an Al 80 really holds about 77 cf). A European 12 liter tank at its fill pressure could similarly release that much gas into a volume about the size of a phone booth (maybe one of those cool British red ones).
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