Pressure Breathing
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As the whole problem of hypoxia is air pressure related, and FLs are defined using a constant pressure, it makes a lot more sense to define the oxygen requirements as FL than as altitudes.
Unfortunately in the US the transition altitude is well above the altitude (or FL, whatever you prefer) so it makes sense for the US to express the oxygen requirements as altitudes.
Unfortunately in the US the transition altitude is well above the altitude (or FL, whatever you prefer) so it makes sense for the US to express the oxygen requirements as altitudes.
It defines the oxygen requirements in terms of density altitude.
P
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DublinPilot
The FAA always talk altitude as FLs can vary substantially to the pressure of the day.
I am surprised they quote FLs with limitations for using or not using supplementary oxygen?
Pace
The FAA always talk altitude as FLs can vary substantially to the pressure of the day.
I am surprised they quote FLs with limitations for using or not using supplementary oxygen?
Pace
I was correcting your post in relation to UK requirements, not FAA requirements.
The details that I posted are from the UK ANO. Nothing to do with FAA.
dp
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It defines the oxygen requirements in terms of density altitude.
It makes much more sense to work in pressures. The Saturated Vapour Pressure of water is a pressure. The partial pressure of CO2 in the blood is a pressure. The "A-a" gradient is a pressure.
Google "Alveolar Gas Equation".
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DublinPilot
That was not pointed at you
and maybe its me that is being totally stupid 
Standard pressure is just that an artificial figure which if your lucky may coincide with an actual presssure of 1013 but is very likely to NOT!
It is the actual pressure which is relevant to Hypoxia.
There can be a huge variation from Standard high or low meaning the aircraft can be higher or lower than the standard pressure setting level showing on the altimeter.
Hence I find it hard to see how a flight level can be used with accuracy without reference to the actual pressure or at worst regional pressure setting
Pace
That was not pointed at you
and maybe its me that is being totally stupid Standard pressure is just that an artificial figure which if your lucky may coincide with an actual presssure of 1013 but is very likely to NOT!
It is the actual pressure which is relevant to Hypoxia.
There can be a huge variation from Standard high or low meaning the aircraft can be higher or lower than the standard pressure setting level showing on the altimeter.
Hence I find it hard to see how a flight level can be used with accuracy without reference to the actual pressure or at worst regional pressure setting
Pace
Last edited by Pace; 7th Mar 2013 at 09:46.
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Not really. The body only works at body temperature, so the air temperature in the lungs is body temperature, so pressure is proportional to density in the lungs. Not in the atmosphere, standard or otherwise.
It makes much more sense to work in pressures. The Saturated Vapour Pressure of water is a pressure. The partial pressure of CO2 in the blood is a pressure. The "A-a" gradient is a pressure.
Google "Alveolar Gas Equation".
It makes much more sense to work in pressures. The Saturated Vapour Pressure of water is a pressure. The partial pressure of CO2 in the blood is a pressure. The "A-a" gradient is a pressure.
Google "Alveolar Gas Equation".
Although it's still the human body, not the regulations, that have the casting vote.
P
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Pace wrote:
Pace, I know you know this
, but FL measures atmospheric pressure (albeit in units of feet), so it is exactly what the regs should specify.
It is the altitude which is less accurate, though as Backpacker said, altitude may be more practical if the TA is high.
Pilotage: agreed, the body can even over-rule the equations!
Hence I find it hard to see how a flight level can be used with accuracy without reference to the actual pressure or at worst regional pressure setting
, but FL measures atmospheric pressure (albeit in units of feet), so it is exactly what the regs should specify.It is the altitude which is less accurate, though as Backpacker said, altitude may be more practical if the TA is high.
Pilotage: agreed, the body can even over-rule the equations!
Last edited by 24Carrot; 7th Mar 2013 at 10:55.
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Pace, I know you know this, but FL measures atmospheric pressure (albeit in units of feet), so it is exactly what the regs should specify.
It is the altitude which is less accurate, though as Backpacker said, altitude may be more practical if the TA is high.
It is the altitude which is less accurate, though as Backpacker said, altitude may be more practical if the TA is high.
If I'm standing on the shoreline, at the time exactly halfway between high tide and low tide, then I'm at mean sea level. If the pressure at that point (at MSL) is 1000hpa and I set my altimeter subscale to 1000hpa then my altimeter would accurately read 0ft.
If I set it to flight levels (1013hpa) then it would read 390ft (13*30) which is FL003.9 (Not that you'd ever replort FL003.9 to anyone!). While using a standard scale is convenient for aircraft seperation, it is less accurate than altitude.
I'm standing at MSL (shore line half way betwen low and high tide) and altitude is reading 0, and FL is reading 390ft. So altitude is more accurate. In terms of the effect on the body using 0ft in your calculations must be more accurate than 390ft at that point.
I think that is the point that Pace was trying to make.
But of course in the UK an aircraft at that level will have their altimeter set to 1013 and the differences in effect on the body are likely to be small. The change is risk of hypoxia is probably considered to be far smaller than the risk of someone having a miss-set altimeter due to changing it to check the pressure altitude and back to FL.
dp
Last edited by dublinpilot; 7th Mar 2013 at 15:59.
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DP
That is my point and I am still trying to get my head around how an artificial setting of 1013 which bares no relationship to the actual pressure of the day can be used as a level limitation for avoiding hypoxia.
Forget 1013 lets make an artificial setting of 1040 or one of 980 why not use those? They would be as accurate as 1013? everyone could fly around in teh airways using 1040 or 980 with the push of a regulation pen.
You could be talking about a difference of 1000 feet from your flight level to your actual altitude.
Am I missing something here or being stupid?
A flight level doesnt measure anything and altimeter with 1013 set will show an altitude if the QNH happens to be 1013 but will show a FL which bares no relationship to an altitude if the pressure of the day is not 1013
Pace
That is my point and I am still trying to get my head around how an artificial setting of 1013 which bares no relationship to the actual pressure of the day can be used as a level limitation for avoiding hypoxia.
Forget 1013 lets make an artificial setting of 1040 or one of 980 why not use those? They would be as accurate as 1013? everyone could fly around in teh airways using 1040 or 980 with the push of a regulation pen.
You could be talking about a difference of 1000 feet from your flight level to your actual altitude.
Am I missing something here or being stupid?
Pace, I know you know this, but FL measures atmospheric pressure (albeit in units of feet), so it is exactly what the regs should specify
Pace
Last edited by Pace; 7th Mar 2013 at 17:01.
Well, if standard pressure was changed, the flight-level-limit would have to be changed to match. e.g. if standard pressure was 980mb rather than 1013, then the limiting flight level would be set at FL 120 rather than 130.
If a cosmic storm whisked away the earth's atmosphere, we'd still die at sea level because our lungs knew they were at flight level 10000, and this is the more valid metric to use for this purpose.
Or am I misunderstanding something too?
If a cosmic storm whisked away the earth's atmosphere, we'd still die at sea level because our lungs knew they were at flight level 10000, and this is the more valid metric to use for this purpose.
Or am I misunderstanding something too?
Last edited by abgd; 7th Mar 2013 at 18:32.
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I hope it's not me that's missing something!
I think a FL limit is appropriate for hypoxia because:
An altimeter is a barometer. It measures pressure plus how much you turned the sub-scale knob. If you always use the same sub-scale setting, you are only measuring pressure.
If everybody agrees on sub-scale setting to use, (which is 1013 by convention, but could be any agreed number), then everybody's altimeter measures the pressure in the same units. (feet, on a non-linear scale).
That means everybody can agree on an altimeter reading which was the limit for hypoxia, and it would correspond to an atmospheric pressure limit for hypoxia. Pressure is all that matters for hypoxia.
If the regulators specify a FL, they have specified an atmospheric pressure. It truly does not matter what the altitude is. The lungs do not care.
As an example, let us say we are at FL100, which is approx 700hPa if memory serves. That 700hPa is all our lungs care about.
Very roughly, 100hPa in the lungs will be CO2 and water vapour, 120hPa will be O2 and 480hPa will be N2. That adds up to 700hPa. The 120hPa partial pressure of oxygen is all that matters for hypoxia.
You will notice I have not mentioned the altitude yet, and that is because it doesn't matter, except as a rough indication of what the pressure might be at that altitude.
If the air below us is very cold, and QNH is very low, we could be at 8,000 ft. What do our lungs care? They are breathing in air at 700hPa. The O2 partial pressure is still 120hPa.
I think a FL limit is appropriate for hypoxia because:
An altimeter is a barometer. It measures pressure plus how much you turned the sub-scale knob. If you always use the same sub-scale setting, you are only measuring pressure.
If everybody agrees on sub-scale setting to use, (which is 1013 by convention, but could be any agreed number), then everybody's altimeter measures the pressure in the same units. (feet, on a non-linear scale).
That means everybody can agree on an altimeter reading which was the limit for hypoxia, and it would correspond to an atmospheric pressure limit for hypoxia. Pressure is all that matters for hypoxia.
If the regulators specify a FL, they have specified an atmospheric pressure. It truly does not matter what the altitude is. The lungs do not care.
As an example, let us say we are at FL100, which is approx 700hPa if memory serves. That 700hPa is all our lungs care about.
Very roughly, 100hPa in the lungs will be CO2 and water vapour, 120hPa will be O2 and 480hPa will be N2. That adds up to 700hPa. The 120hPa partial pressure of oxygen is all that matters for hypoxia.
You will notice I have not mentioned the altitude yet, and that is because it doesn't matter, except as a rough indication of what the pressure might be at that altitude.
If the air below us is very cold, and QNH is very low, we could be at 8,000 ft. What do our lungs care? They are breathing in air at 700hPa. The O2 partial pressure is still 120hPa.
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Back to the original topic- pressure breathing
Sufferers of sleep aponea use a CPAP machine which provides air to a mask at a raised pressure. The effect to the user is the same as described. ie you relax to breath in and have to use the diaphram to force air out. It feels most unatural at first as if the machine is trying to blow you up like a balloon.
Sufferers of sleep aponea use a CPAP machine which provides air to a mask at a raised pressure. The effect to the user is the same as described. ie you relax to breath in and have to use the diaphram to force air out. It feels most unatural at first as if the machine is trying to blow you up like a balloon.
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An altimeter is a barometer. It measures pressure plus how much you turned the sub-scale knob.
If you define the hypoxia air pressure (assuming there is one) as a flight level, you can simply set the subscale to 1013, read the flight level and see if you're in the danger zone or not. Whereas if you define the hypoxia danger zone as an altitude, you need to correct for the subscale setting, but backwards from what we're used to. So it makes a lot more sense to define the hypoxia/oxygen limits as flight levels instead of altitudes.
Going along with Carrots example. Suppose for a moment that 700 hPa is the start of the danger zone. If the air pressure is 700.0 hPa or lower you run an hypoxia risk, if the air pressure is 700.0 hPa or higher you're fine. (I know it's not such a black and white issue, but let's go along with that for now.)
We have no way of knowing the actual air pressure in the aircraft unless we bring an actual barometer. But, hey, our altimeter is a barometer too, just with a scale that works the other way round (increasing air pressure leads to a reduction of the readout) and its datum is adjustable.
Flight Levels are defined against a "QNH" of 1013. In other words, FL000 = 1013 hPa, FL001 = 1010 hPa, FL002 = 1007 hPa and so forth. And since that relation is not entirely linear and since I don't have the ISA atmosphere definitions to hand, let's assume FL100 = 700 hPa. This means I can use my altimeter to find out whether I'm in the danger zone or not: Set the subscale to 1013, see if I'm above 10.000' on the dial and I'm in the danger zone.
But as soon as you start setting a different subscale, because you insist on flying altitudes rather than flight levels, 700 hPa air pressure no longer corresponds to a reading of 10.000' on the altimeter. It corresponds to something that may be a few 100s of feet lower or higher. So you need to correct for that correction again.
As an example, suppose the QNH is 1007 and you have that set on the dial. This means that zero feet on the dial now corresponds to a barometric pressure of 1007 hPa. 100 feet corresponds to 1004 hPa, 200 feet corresponds to 1001 hPa and so forth. And the 700 hPa level no longer corresponds to the readout of 10.000'. Instead, the 700 hPa hypoxia danger level is now found where the altimeter reads 9.800 feet.
So on a low pressure day you may be in danger of hypoxia even below 10.000 feet altitude. Conversely, a high pressure day will give you a few hundred feet above 10.000 feet where you will not suffer from hypoxia. (Again, assuming it's black and white, which it isn't.)
Now try calculating that when you're already suffering from hypoxia.
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Pace you could see what it feels like by getting a fat burd to sit on your chest.
Opps got the pressures the wrong way round but you can try it anyway
Opps got the pressures the wrong way round but you can try it anyway
Last edited by mad_jock; 7th Mar 2013 at 19:01.
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MJ
I do not want a fat burd sitting on anything only go for slim ones
So getting this right? If you have a fat Burd sitting on you you will know at what level hypoxia will set in? Does that work in a Cessna 150?
Pace
I do not want a fat burd sitting on anything only go for slim ones
So getting this right? If you have a fat Burd sitting on you you will know at what level hypoxia will set in? Does that work in a Cessna 150?
Pace
Last edited by Pace; 8th Mar 2013 at 07:57.
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well it will be breathing under pressure 
Oh and my take on this is...
you have two things.
1. the required partial pressure to allow oxygen transfer across the cellular wall in the lungs. Its not binary but a none linear curve.
2. The up take of oxygen in the blood and transfer to the organs.
Pressure breathing only increases the partial pressure and puts you up the curve for number 1. Smokers etc will require higher to get the same transfer.
Oh and my take on this is...
you have two things.
1. the required partial pressure to allow oxygen transfer across the cellular wall in the lungs. Its not binary but a none linear curve.
2. The up take of oxygen in the blood and transfer to the organs.
Pressure breathing only increases the partial pressure and puts you up the curve for number 1. Smokers etc will require higher to get the same transfer.
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I think a FL limit is appropriate for hypoxia because:
An altimeter is a barometer. It measures pressure plus how much you turned the sub-scale knob. If you always use the same sub-scale setting, you are only measuring pressure.
An altimeter is a barometer. It measures pressure plus how much you turned the sub-scale knob. If you always use the same sub-scale setting, you are only measuring pressure.
dp
Look guys, pressure breathing is based upon cabin/cockpit pressure - everything else is irrelevant.
The aeroplane I flew was officially cleared to 60,000ft, but has been higher.
In the event of pressurisation failure the oxygen regulator would automatically feed pressurised oxygen to the mask - all that was required was to pull down the mask toggle which would pull the mask very tightly to the face.
Now, the pressure suit and helmet and suit were something else.
Try this little lot.......
The aeroplane I flew was officially cleared to 60,000ft, but has been higher.
In the event of pressurisation failure the oxygen regulator would automatically feed pressurised oxygen to the mask - all that was required was to pull down the mask toggle which would pull the mask very tightly to the face.
Now, the pressure suit and helmet and suit were something else.
Try this little lot.......