What are you carrying you gas in? Unless you are packing a 15 or 18 litre cylinder, you should look at the real gas laws. A 12l @ 300bar of 10/60 is a lot less than 3600l of gas.
For air or oxygen mixes the ideal gas laws are ok.
Mike
Deep Bailout Gas Choice for ~70M / 230FT
- Thread starter Peter L
- Start date
What are you carrying you gas in? Unless you are packing a 15 or 18 litre cylinder, you should look at the real gas laws. A 12l @ 300bar of 10/60 is a lot less than 3600l of gas.
For air or oxygen mixes the ideal gas laws are ok.
Mike
kwinter
PITA
Can you please explain this more to an idiot like myself using small words? I understand a little about why ideal gas laws like PV=nRT don't work for blending because of compressibility factors, with He being non-compressible. But once the cylinder is filled, isn't that already accounted for in the total volume? So why wouldn't the total be 360 liters? If anything, I would expect it to be more since as you breathe and the gas expands, the compressibility factor disappears.
My simple mind is confused.
The Duck
New Member
nigel's explanation is a good one. He is certainly someone more qualified than me to explain it.
Essentially the ideal gas laws are an approximation that gets worse the further away from STP (standard temperature and pressure 0degrees C, 1 bar) you get.
In the case of helium mixtures at 300 bar this can be a significant difference.
In the example given of 10/60 (at 20C) @ 300b the ideal gas laws would give you ~3600 litres at 1b. If you use the so called real gas laws you will find that you only have ~2860 litres at 1 bar.
This will cause an obvious problem if you were planning on breathing the "missing" 739l
incidentally for a cylinder of 50% the difference is only -146l so helium is the one to watch out for.
Regards,
Mike
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kwinter
PITA
Thanks. But Van Der Waals doesn't really address my question. The gasses do not behave like ideal gasses when we get far away from STP. But if we ignore the temperature portion since that is fairly stable, then it really comes down to the pressure. I can understand that the vessel under 300 bar pressure does not contain 3600 liters of gas because of the interaction of the molecules with each other (compression, attraction, or whatever you want to call it). But when released from that pressure, the gasses should behave more like ideal gasses. So why wouldn't that 2860 liters of gas expand back to 3600 liters when breathing it at anywhere from 1-10 bar ambient pressure? The number of moles is the same, but at STP the volume of a mole should be 22.4 liters even if it is different at 300 bar.
Still confused.
Still confused.
The number of moles in the 300bar 12l cylinder is the same as 2860l of 10/60 at 1 bar assuming the temperature is 20c.
If you take 3600l of 10/60 at ambient and try to compress it into a 12l cylinder you will end up with a pressure much higher than 300 bar.
This is why it is important to use the van der waals equations when calculating bailout volumes for helium mixes.
Mike
If you take 3600l of 10/60 at ambient and try to compress it into a 12l cylinder you will end up with a pressure much higher than 300 bar.
This is why it is important to use the van der waals equations when calculating bailout volumes for helium mixes.
Mike
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matt
Well-Known Member
Perhaps someone can tell me if this thinking is flawed and if so why?
If you have 3600 free litres of gas (any) at 1 bar in a big (3600L) container and then you reduce the container volume (perhaps with a powerful ram, but assuming no change to temperature) to 11L for a while (say overnight and then you return the container to its original size does the container still have 3600 litres in it and if not why not?
Matt.
If you have 3600 free litres of gas (any) at 1 bar in a big (3600L) container and then you reduce the container volume (perhaps with a powerful ram, but assuming no change to temperature) to 11L for a while (say overnight
and then you return the container to its original size does the container still have 3600 litres in it and if not why not?Matt.
matt
Well-Known Member
You must have typed this at the same time as I typed my question!
So your answer is yes, there would be 3600L but the pressure in my example would be much more than 300 bar (if the gas was 10/60), right?
Matt.
Right.
(Assuming that you maintained a reaonably uniform temperature while compressing the gas)
I will leave the calculation of exactly how much more pressure as an exercise for the reader.
Mike
matt
Well-Known Member
Right.
(Assuming that you maintained a reaonably uniform temperature while compressing the gas)
I will leave the calculation of exactly how much more pressure as an exercise for the reader.
Mike
In my example the pressure would read 472 bar when the volume is reduced to 11L. Agree?
Matt.
Can you throw your maths up please? Wondering if there is a tipping point in terms of HE content aka would you get much more doing 15/50 or 18/44 where I could use a smaller bottom vessel and a larger intermediate tank.
matt
Well-Known Member
I would if I did any but I cheated and just found the answer using Nigel's program
Ah the youf of today.............. :sarcy:
matt
Well-Known Member
Simon Mitchell
Well-Known Member
Hello,
Matthew asked me to comment on the significance of maximum voluntary ventilation measurements MVV numbers a while a ago and I forgot all about it... sorry Matt. I probably should have made this clearer when I first discussed these numbers earlier also.
The MVV measurement experiments I have referred to previously have been performed at increasing pressures during air breathing on a low resistance experimental breathing circuit. The key variable in this setting is gas density. The figures you could achieve in a real dive are almost certainly a lot lower, and they will be context sensitive, where the potential contributors to "context" were quite nicely summed up by Mark Chase:
You could add a few more potential variables to this list, but the net effect is that real world diving MVV is likely to be considerably lower than experimental MVV, and it could not be sustained for long anyway. I'm not sure any attempts have been made to measure it, and if they were, the circumstances of the experiment would need to be very carefully defined for it to have any meaning. Moreover, there would be a large number of potential combinations and permutations of experimental conditions that might or might not approximate the circumstances under which we as individuals dive.
I don't feel quite as negative about MVV as Mark; the numbers do indicate an indicative absolute maximum. But he is right in that they are not an ideal basis for planning bailout gas requirements.
Where does that leave us? Still in the dark really.... but I think that one excellent thing to come out of this thread is an awareness that gas consumption during bailout from a CO2 hit is likely to be higher than most of us have previously anticipated.
Simon M
Matthew asked me to comment on the significance of maximum voluntary ventilation measurements MVV numbers a while a ago and I forgot all about it... sorry Matt. I probably should have made this clearer when I first discussed these numbers earlier also.
The MVV measurement experiments I have referred to previously have been performed at increasing pressures during air breathing on a low resistance experimental breathing circuit. The key variable in this setting is gas density. The figures you could achieve in a real dive are almost certainly a lot lower, and they will be context sensitive, where the potential contributors to "context" were quite nicely summed up by Mark Chase:
You could add a few more potential variables to this list, but the net effect is that real world diving MVV is likely to be considerably lower than experimental MVV, and it could not be sustained for long anyway. I'm not sure any attempts have been made to measure it, and if they were, the circumstances of the experiment would need to be very carefully defined for it to have any meaning. Moreover, there would be a large number of potential combinations and permutations of experimental conditions that might or might not approximate the circumstances under which we as individuals dive.
I don't feel quite as negative about MVV as Mark; the numbers do indicate an indicative absolute maximum. But he is right in that they are not an ideal basis for planning bailout gas requirements.
Where does that leave us? Still in the dark really.... but I think that one excellent thing to come out of this thread is an awareness that gas consumption during bailout from a CO2 hit is likely to be higher than most of us have previously anticipated.
Simon M
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On the topic of forgotten questions, I asked you elsewhere "One question for you as a diver: How do you personally plan and calculate your bail out gas given your knowledge? It's kinda like asking a Dentist if he flosses..........
"
matt
Well-Known Member
Thanks Simon, interesting confirmations.
I think we've come full circle (or to a dead end!) in that we know that planning a deep dive with a (unreasonably) high RMV is not feasible due to physical limitations and not desirable due to extra stress. At the other end of the spectrum we could simply say we do not plan for CO2 problems and plan OC bailout only for a "normally" stressed RMV.
I'm still feeling that an RMV near 30 falls more into the "no CO2 plan" category driven by the feasibility argument rather than anything else. I think we all need to be realistic when we are making such plans and dives and cut the back-up plan accordingly.
Cheers
Matt.