Atmospheric pressure, how strong it is (in absolute terms, ~15 psi/100 kN/sq m), and how little we notice. A rough demonstration:
You know those 15-lb (~7 kg) dumbbells? Ever lifted one? They’re heavy. (No bragging, please.)
Now, place your hand on a table, palm up, and rest the dumbbell on your palm. “Ow!”, right? That hurts! It feels like your hand is getting crushed! Get that thing off!
Well, you’ve only increased the pressure that’s normally on your hand by about 5%, plus or minus.
This is very misleading. Most of the discomfort would be from the hard table against the back of your hand, and this would be because of local pressure on specific points.
Pressure causes problems when there’s a big change in a relatively short time. Ears, for example, have a hard time with this, but you can equalize them by closing your nose and mouth and trying to blow out. Before I knew about this trick, I could never dive to the bottom of the pool. Now, no problem.
A more realistic example would be to bury your hand in a foot or two of fine sand. Does that sound uncomfortable?
In the sand example, it’s also important that the pressure is acting from all sides (top, bottom, left, right) so there’s no force acting to deform your hand.
We can handle a relatively large range of pressures, and there are other problems before you start causing mechanical damage from the actual pressure (lack of oxygen at low pressure, dissolved gas at high pressure).
This is very misleading. Most of the discomfort would be from the hard table against the back of your hand, and this would be because of local pressure on specific points.
Good point, but it feels about as uncomfortable if you use a padding over the table that eliminates the stress concentrations at your bones and knuckles. Especially if you double the dumbbell weight and recognize that it’s only a pressure increase of 10%.
Your thought experiment with the dumbbell is an incorrect way of thinking about ambient pressure. Ambient pressure pushes against an object from every direction. It does not work to deform or break, only compress from all sides.
Picture this: You have a hand-sized water balloon on a table. You place the two dumbbells on it; it breaks. You have another water balloon. You take this one, tie it to a dumbbell, and drop it into deep water. Do you expect it to break when descends to 3 feet (i.e. 10% increase in pressure)?
I would not expect it to break at all. When water and other non-gases are put under pressure, the bonds and repulsive forces within push back.
Don’t quote me on this part, but I would guess that to break a bone with just ambient pressure, you’d have to raise the pressure to about the compressive strength of the bone, around 100 megapascals. For reference, standard atmospheric pressure is around 100 kilopascals.
edit: changed 3 meters to 3 feet, per prase’s comment.
Not so sure about that, just dive a few meters under water, and the pressure gets up very quickly, roughly, every 10m you dive, you get an additional atmosphere of pressure, and people are known to be able to dive below 100m with training but without special apparatus. The problems arise mostly when the pressure changes quickly (or when it gets very high), but a pressure of 10 atmosphere, with sufficient preparation and adjusting time, doesn’t kill a human being.
Yeah; you notice pressure changes easily when you skin dive, but you notice them mostly because of the compressive load on your lungs and the various air spaces in your head, which are full of air at surface pressure. If you go scuba diving instead, you won’t notice the load on your lungs anymore—the regulator delivers air at ambient pressure, not at surface pressure. You do need to equalize the pressure in your ears and facemask frequently as you go up and down the water column, since they’re set to ambient pressure every time you do so and that changes as your depth does, but breathing itself doesn’t get much harder as you go deeper.
(There are various other pressure-related problems that can crop up, though—nitrogen narcosis is the most important one at recreational diving depths.)
Depends on the gas mix and the application. The gas mixes used at depth in technical diving are usually hypoxic, since oxygen toxicity becomes an issue with ordinary air at an ambient pressure of about six and a half bar or depths of around fifty meters; heliox, for example, is usually around ten percent oxygen. On the other hand, it’s fairly common for the gas mixes used during the decompression phase of a technical dive to be richer in oxygen than air is, since that helps flush nitrogen out of your tissues.
This isn’t usually an issue for recreational divers, though, who generally don’t dive below forty meters and use pressurized air or, more rarely, enriched nitrox mixtures. At those shallower depths, you compensate for the richer breathing gas by breathing in a slower, more controlled fashion than you would on the surface, though this has more to do with conserving gas and controlling buoyancy than it does with oxygen issues.
I don’t see how this squares with Silas’ claim that a 15-lb dumbbell hurts your hand when you rest it on it.
OK, I just tested this by balancing the end of a ~21 pound dumbbell on my palm while my palm rested on a counter (surface area of dumbbell end looks to be about 0.60 square inches, making the pressure around 35 psi, or about 2.3 atmospheres?) It was a little painful, but I didn’t cry out or feel the need to get the thing off immediately. So in conclusion, I think Silas might have been exaggerating.
Resting my palm on a pillow instead of a counter, I don’t really experience pain anymore, just discomfort. Maybe that has something to do with the fact that deep diving doesn’t cause pain?
But shouldn’t you rather take into account the tolerances of pregnant women and people younger than the age when one can be trained? If they can’t live on the planet, it can’t be colonized.
I’m not an expert in diving (I only dived once about 10m deep during holidays), but AFAIK the training (and the trouble) is mostly to handle the change in pressure, much more than the high pressure itself. Going from 1Am to 10Am is dangerous if done without respecting many safety measures, but once you’re adjusted at 10Am, it’s not so much a problem. So a child born on higher pressure wouldn’t have too much troubles. Maybe 10Am is too much, but I don’t think 1.5Am or 2Am would cause any serious trouble, if the composition of the atmosphere is good enough.
Atmospheric pressure, how strong it is (in absolute terms, ~15 psi/100 kN/sq m), and how little we notice. A rough demonstration:
You know those 15-lb (~7 kg) dumbbells? Ever lifted one? They’re heavy. (No bragging, please.)
Now, place your hand on a table, palm up, and rest the dumbbell on your palm. “Ow!”, right? That hurts! It feels like your hand is getting crushed! Get that thing off!
Well, you’ve only increased the pressure that’s normally on your hand by about 5%, plus or minus.
This is very misleading. Most of the discomfort would be from the hard table against the back of your hand, and this would be because of local pressure on specific points.
Pressure causes problems when there’s a big change in a relatively short time. Ears, for example, have a hard time with this, but you can equalize them by closing your nose and mouth and trying to blow out. Before I knew about this trick, I could never dive to the bottom of the pool. Now, no problem.
A more realistic example would be to bury your hand in a foot or two of fine sand. Does that sound uncomfortable?
In the sand example, it’s also important that the pressure is acting from all sides (top, bottom, left, right) so there’s no force acting to deform your hand.
We can handle a relatively large range of pressures, and there are other problems before you start causing mechanical damage from the actual pressure (lack of oxygen at low pressure, dissolved gas at high pressure).
edit: grammar
Good point, but it feels about as uncomfortable if you use a padding over the table that eliminates the stress concentrations at your bones and knuckles. Especially if you double the dumbbell weight and recognize that it’s only a pressure increase of 10%.
I don’t agree with this.
Your thought experiment with the dumbbell is an incorrect way of thinking about ambient pressure. Ambient pressure pushes against an object from every direction. It does not work to deform or break, only compress from all sides.
Picture this: You have a hand-sized water balloon on a table. You place the two dumbbells on it; it breaks. You have another water balloon. You take this one, tie it to a dumbbell, and drop it into deep water. Do you expect it to break when descends to 3 feet (i.e. 10% increase in pressure)?
I would not expect it to break at all. When water and other non-gases are put under pressure, the bonds and repulsive forces within push back.
Don’t quote me on this part, but I would guess that to break a bone with just ambient pressure, you’d have to raise the pressure to about the compressive strength of the bone, around 100 megapascals. For reference, standard atmospheric pressure is around 100 kilopascals.
edit: changed 3 meters to 3 feet, per prase’s comment.
3 meters underwater is about 30% of atmospheric pressure added, not mere 10%.
Sorry, I forgot feet != meters. Ha.
Alternately, go swimming. The water adds roughly another atmosphere of pressure every ten meters. You will notice this.
Atmospheric pressure is HUGE. It’s enough that a variety of pneumatic systems use vacuum instead of high-pressure air.
What did you deduce from finding out about atmospheric pressure?
So colonizing a planet that has even a slightly thicker atmosphere than ours could be problematic?
I think colonizing any planet has way too many problems before you confront this one.
Not so sure about that, just dive a few meters under water, and the pressure gets up very quickly, roughly, every 10m you dive, you get an additional atmosphere of pressure, and people are known to be able to dive below 100m with training but without special apparatus. The problems arise mostly when the pressure changes quickly (or when it gets very high), but a pressure of 10 atmosphere, with sufficient preparation and adjusting time, doesn’t kill a human being.
Yeah; you notice pressure changes easily when you skin dive, but you notice them mostly because of the compressive load on your lungs and the various air spaces in your head, which are full of air at surface pressure. If you go scuba diving instead, you won’t notice the load on your lungs anymore—the regulator delivers air at ambient pressure, not at surface pressure. You do need to equalize the pressure in your ears and facemask frequently as you go up and down the water column, since they’re set to ambient pressure every time you do so and that changes as your depth does, but breathing itself doesn’t get much harder as you go deeper.
(There are various other pressure-related problems that can crop up, though—nitrogen narcosis is the most important one at recreational diving depths.)
One also needs a lower amount of oxygen in the breathable air.
Depends on the gas mix and the application. The gas mixes used at depth in technical diving are usually hypoxic, since oxygen toxicity becomes an issue with ordinary air at an ambient pressure of about six and a half bar or depths of around fifty meters; heliox, for example, is usually around ten percent oxygen. On the other hand, it’s fairly common for the gas mixes used during the decompression phase of a technical dive to be richer in oxygen than air is, since that helps flush nitrogen out of your tissues.
This isn’t usually an issue for recreational divers, though, who generally don’t dive below forty meters and use pressurized air or, more rarely, enriched nitrox mixtures. At those shallower depths, you compensate for the richer breathing gas by breathing in a slower, more controlled fashion than you would on the surface, though this has more to do with conserving gas and controlling buoyancy than it does with oxygen issues.
I don’t see how this squares with Silas’ claim that a 15-lb dumbbell hurts your hand when you rest it on it.
OK, I just tested this by balancing the end of a ~21 pound dumbbell on my palm while my palm rested on a counter (surface area of dumbbell end looks to be about 0.60 square inches, making the pressure around 35 psi, or about 2.3 atmospheres?) It was a little painful, but I didn’t cry out or feel the need to get the thing off immediately. So in conclusion, I think Silas might have been exaggerating.
Resting my palm on a pillow instead of a counter, I don’t really experience pain anymore, just discomfort. Maybe that has something to do with the fact that deep diving doesn’t cause pain?
People’s pain tolerances vary a lot.
But shouldn’t you rather take into account the tolerances of pregnant women and people younger than the age when one can be trained? If they can’t live on the planet, it can’t be colonized.
I’m not an expert in diving (I only dived once about 10m deep during holidays), but AFAIK the training (and the trouble) is mostly to handle the change in pressure, much more than the high pressure itself. Going from 1Am to 10Am is dangerous if done without respecting many safety measures, but once you’re adjusted at 10Am, it’s not so much a problem. So a child born on higher pressure wouldn’t have too much troubles. Maybe 10Am is too much, but I don’t think 1.5Am or 2Am would cause any serious trouble, if the composition of the atmosphere is good enough.
That is quite interesting. I’ll ask my friends in human physiology department what they think about it and get back to you.