Wikipedia:Reference desk/Archives/Science/2012 July 12

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July 12

derivation of the kintetic theory equation

Hello.

My question is related to the derivation of the equation pv=1/3nMwC^2. Most of the derivations I have seen assume that the average time between colisions, t, is 2l/Vx when l is the length of the cube. When assuming this, you actually assume that each molecule travel from one wall to the other wall at constant velocity without any clashes with other molecules. this is an unlikely assumption, yet those derivations do succeed. my quastion is - why? and where can i find more rigorous and accurate derivation?

I am sorry for my poor english...

Thanks in advance, 94.159.137.4 (talk) —Preceding undated comment added 06:54, 12 July 2012 (UTC)

Isn't the assumption really that the collisions with other particles are elastic? If it does elastically collide with any other particle, it transfers momentum which that particle then in turn transfers to the wall after exactly the same amount of time has expired. This results in the same net effect for the wall, so the predicted pressure is exactly the same. 203.27.72.5 (talk) 07:41, 12 July 2012 (UTC)

I thought about this interpretation, but I don't think it's true. It would be true only when moving particle collides with a static particle. Think about collision between two particles with same mass and opposite X-component velocity - They both would be stopped. 94.159.137.4 (talk) —Preceding undated comment added 09:20, 12 July 2012 (UTC)

If they stopped, the kinetic energy would be destroyed; so they can't stop as long as energy is conserved. Imagine two billiard balls colliding head on. They don't stop, they bounce back in opposite directions. 101.173.170.146 (talk) 09:34, 12 July 2012 (UTC)

Yes, you are right, my fault. Yet I would be happy to see a rigorous derivation. 94.159.137.4 (talk) —Preceding undated comment added 09:51, 12 July 2012 (UTC)

On matters of mathematical rigour, I'm the last one that should be commenting, but is the derivation at Kinetic_theory#Pressure_and_kinetic_energy adequate? 101.173.170.146 (talk) 10:56, 12 July 2012 (UTC)

No, because of reasons I wrote above. You can see this thread in physicsforums.com, which explains the problem clearly (especially on "loom91"'s notes), yet no complete answer was given. 94.159.137.4 (talk) 11:32, 12 July 2012 (UTC)

A more accurate derivation can be found in the book: "Fundamentals of Statistical and Thermal Physics", by F. Reif, secion 7.13 page 278 and further. It starts with the heuristic derivation, followed by an exact calculation. Note that in the heuristic derivation the formula you get contains the square of the average speed, while in the exact calculation, you get the correct expression containing the average of the squared velocity. Count Iblis (talk) 17:33, 12 July 2012 (UTC)

Thank you! perhaps it's good idea to write in Kinetic_theory#Pressure_and_kinetic_energy that the derivation which described in the article (and in many academic textbooks...) is not an accurate one. 94.159.137.4 (talk) 06:22, 13 July 2012 (UTC)

It's not inaccurate. Simply assume that the Volume of the box is so small that the chances of a collision between particles happening before the particle reaching the wall is negligible. Then build a larger box by stacking the smaller boxes side by side and removing the walls between the boxes (Which has no effect in the pressure). Dauto (talk) 15:47, 13 July 2012 (UTC)

Oughterlony immunodiffusion

My question concerns the interpretation of Oughterlony immunodiffusion which I think it, the content in Wikipedia, can make people misunderstand some good points. For example Ag A contains two epitopes so called X and Y while Ag B contains epitope Z, if both Ags react with the mixed Ab which contains anti-X, anti-Y and anti-Z, the result should come out without any relationship (of identity, partial identity or non identity). Is that right? I think this technique was created before immunologist really understand about antigenic determinants. It originally is for study the relationship of each class of antibody. The content in the web can make people understand that the two Ags which are not relate to one another still give a result of non-identity. — Preceding unsigned comment added by 110.164.187.222 (talk) 12:05, 12 July 2012 (UTC)

You never posted an exact title as requested at Wikipedia:New contributors' help page/questions#Interpretation of Oughterlony's technique. Is this about Ouchterlony double immunodiffusion? I'm not qualified to discuss the topic. PrimeHunter (talk) 12:10, 12 July 2012 (UTC)

As explained at Precipitin, the antigen must be multivalent - if there's just one epitope per antigen, then it will react with an antibody, but it won't form a large gel-like complex; you'll have two dead-end epitopes stuck to one monomer antibody (four to IgE, ten to IgM - see antibody. I think F(ab')2 fragments are OK, but don't try to use Fab fragments. ;) ). Antibodies and antigens will ignore those that are not matches, so if you have two unrelated multivalent antigens, each should find its own favorite antibodies and form complexes with them somewhere or other, but ignore the other antibodies. That means that you could have multiple white lines, or even white lines crossing one another. Mix in a little bit of the right fluorescent antibody (or apply it to the gel afterward) and you could mark which is which in pretty colors if you want. Wnt (talk) 13:38, 12 July 2012 (UTC)

This is a minor point, Wnt, but where you mention the 4-valency of IgE, I think you meant sIgA. -- Scray (talk) 04:25, 13 July 2012 (UTC)

Thanks for spotting that! Wnt (talk) 21:12, 13 July 2012 (UTC)

looking for a chart

from organogenesis

hi! i'm a biology student, taking a course in comparative anatomy. i am looking for a chart [or something similar], which will: show the origin of the organs [what came from the mesoderm, ectoderm, etc.], and maybe even more- like what originated in the neural crest.

thanks! — Preceding unsigned comment added by 212.76.110.252 (talk) 16:10, 12 July 2012 (UTC)

You should read the relevant articles. Also check out embryogenesis and organogenesis, whic is a stub, but which has the picture I posted. You'll probably do better with a google image search. μηδείς (talk) 16:41, 12 July 2012 (UTC)

do you have any idea which key words i should use? i am looking for something with more details- like the somits.

thanks again... — Preceding unsigned comment added by 212.76.110.252 (talk) 16:48, 12 July 2012 (UTC)

Well, to begin with, see somites... but admittedly, that's not the image you want yet... Wnt (talk) 18:07, 12 July 2012 (UTC)

You should really just fool around with searches like "organ mesoderm", etc., at google images. You will immediately get hundreds of choices to browse through, rather than one at a time from us. μηδείς (talk) 18:56, 12 July 2012 (UTC)

Lasers

Wavelengths of the visual spectrum

Is the blue-violet laser stronger than the green one ? ( We are talking about pointers )124.253.89.189 (talk) 16:52, 12 July 2012 (UTC)

According to laserpointersafety.com, a violet pointer is more dangerous than a blue one, which is in turn much more of a hazard than a green one. Clarityfiend (talk) 17:11, 12 July 2012 (UTC)

What about this one. Is it exactly blue or violet ? Last time this very vendor fooled me into buying a three AAA battery laser (green) which he claimed is powerful enough to light up a matchstick ( It doesn't, though it does produce a slight sensation of heat on skin ) — Preceding unsigned comment added by 124.253.89.189 (talk) 17:51, 12 July 2012 (UTC)

The page you linked says the exact opposite of that. They seem to be specifically talking about the danger of temporary blinding/distraction, not permanent eye damage, which is why they scale it by the eye's sensitivity to each frequency (it's most sensitive to green). And they are talking about lasers of the same power. -- BenRG (talk) 02:02, 13 July 2012 (UTC)

Clarityfiend, you are misinterpreting what that site says. Lasers with longer hazard distance are more dangerous than ones with shorter hazard distances. That said, I'm not sure on what they are basing that. The official standards for laser safety do not distinguish between violet and green light.--23.24.20.245 (talk) 16:59, 16 July 2012 (UTC)

On the colour spectrum, the higher-frequency colours have higher energy. Lower wavelengths means higher frequencies means higher energy. This is also why red is less harmful to your night vision than green. BigNate37_(T) 18:50, 12 July 2012 (UTC)

This isn't strictly true; red light does not affect your night vision as much because rod cells (responsible for low-light vision) are less responsive to higher wavelengths; the chart in that article shows they are most sensitive to blue-to-green wavelengths. -RunningOnBrains^(talk) 19:52, 12 July 2012 (UTC)

I stand corrected. BigNate37_(T) 22:21, 12 July 2012 (UTC)

Higher frequencies have higher energy per photon, not per laser beam. It's the power that matters, not the color. -- BenRG (talk) 02:02, 13 July 2012 (UTC)

If you want to get an answer that makes any practial sense, you will have to qualify what you mean by "strong". The energy of the light is highest at violet and lowest at red with everything else falling in between per the spectrum in the picture above. The power of the various laser pointers is not dependent on the colour of the laser, but rather on its design. The power output in watts may be stated on the laser pointer itself or on the packaging. If you're wanting to know which will heat something up the most (i.e. which will come closest to igniting that match head) that will be the one with the highest power output. My intuition is telling me that you're not going to find a battery powered pocket laser pointer that can acutally do that though. 203.27.72.5 (talk) 01:59, 13 July 2012 (UTC)

Timeline for noticeable adverse effects from 100% recirculated commercial airline cabin air

The post above ("Airliner cabin air") got me thinking (I don't know why, but I had just assumed that airline cabin air was recirculated, at least to a large extent, which I now realize is pretty daft).

Let's take for the sake of this example a modern Boeing 767-300 outfitted with a 2-class cabin, and a full passenger capacity of 269 people. Assuming that there is NO exchange of air, and that the 100% of the air is continually recirculated, how long would it take for enough oxygen to be depleted such that some passengers will begin to experience adverse effects from the lack of oxygen? How long would it take for the majority of the passengers to lose consciousness or even die due to the lack of oxygen? --Zerozal (talk) 17:06, 12 July 2012 (UTC)

As for how long before it becomes uncomfortable, that's just a few minutes. Between lower oxygen, higher carbon dioxide and other gases, high humidity, and high temperature, it's hard to know just what makes it "stale", though. The various factors can also affect each other, like high temp and humidity causing people to emit more body odor, making the air quality quickly worsen. StuRat (talk) 17:27, 12 July 2012 (UTC)

Talking of cabin pressure in aircraft and it's effect upon humans, is it possible to get rid of hijackers by suddenly lowering the pressure ? It will cause all humans in cabin to lose consciousness (without causing any permanent damage upon them). Later all can be revived easily by bringing pressure to normal. But by that time hijackers would be nicely bound and gagged, and their toys taken away...124.253.89.189 (talk) 18:09, 12 July 2012 (UTC)

It doesn't make much sense to me. Whatever people have to deal with the terrorist, would need a mask and oxygen tank. And the terrorists could also have a mask and oxygen tank (smuggled into the plane in the same way they smuggled the weapons). Besides that, cockpit doors are reinforced nowadays, and pilots are trained not to cave in. Any terrorist without a bomb won't cause much harm. 79.148.233.179 (talk) 18:20, 12 July 2012 (UTC)

Stu enjoys posting on this forum - pity he doesn't check his facts first.

During most of the flight, the passengers are at rest. According to http://ep.physoc.org/content/64/3/205.full.pdf the rest oxygen consumption of an average fit humnan is 0.263 Litres/min A 767-300 has a cabin volume of length ~49 m, 4.72 width, with a half-circle profile. Allowing for volume occupied by seats, luggage racks, etcx, the internal volume is approx 880 m³. 269 average humans, taking the density as roughly that of water, occipies a volume of 65 litres each, or a total of 18 m³. So the volume of air in the cabin is 880 - 18 - 862 m³. Air is 21% oxygen, and 767's pressurise to 80% of sea level value. so that's 145 m³ of oxygen. 269 passengers at 0.263 l/min gives a total oxygen consumption of 0.0708 m³/min. So the passengers consume in the order of 0.05% of the available oxygen per minute, or about 3% each hour.

Humans at rest are perfectly comfortable at an altitude of about 3,000m - at that height, the pressure is ~70% of sea level. So the comfort limit is reached after about 3 hours is the cabin is 100% recirculated.

However, its more complex than that. Each m³ of oxygen consumed is replaced by a m³ of CO₂. It happens that the mammilian breathing system automaticllay regulates, but our internal sensor does not sense oxygen, it senses CO₂. So the CO₂ breathed out will cause everyone to breath harder, without them necessarily noticing it. This will completely maintain blood oxygen at saturation level for much longer than the 3 hours calculated by oxygen consumption. This principle is used in gas fire suppression (eg Inogen) in computer and server rooms - when a fire is detected, a mixture of inert gasses and CO2 is pumped in, to force oxygen out and reduce it to about half the normal level - that causes most fires to go out. The CO₂ causes any people to just breath deeper, so they are not harmed.

It also happens that humans gradually starved of oxygen don't feel discomfort - they just get sleepier and sleepier, until they are unconscious.

The overall implication is that you needn't worry about aircraft fresh-air delivery failure. Cabin oxygen will considerably outlast the fuel. You will have landed long before it is a safety issue.

Sudenly dropping cabin presure to disable terroists is a bad idea. It can only work if you disable the automatic oxygen mask system. Humans will take some time, 10 minutes or more, to loose consciousness, but they WILL suffer all manner of medical problems from tissue rupture, getting hit be objects flying about, and cardiac arrest if they have any latent cardiac issues.

Ratbone124.178.45.153 (talk) 10:56, 13 July 2012 (UTC)

I never claimed that oxygen levels alone caused discomfort after a few minutes, I listed several factors. Also, you mentioned that people breath harder as carbon dioxide goes up. I agree, but also think this makes you feel uncomfortable, as do the temperature, humidity, odors, etc. Having been on a plane parked on the tarmac without any air circulation, I can tell you it gets uncomfortable quickly. StuRat (talk) 06:41, 14 July 2012 (UTC)

It also looks like you made a math error. 49 m length by 4.72 m width, with a half-circle profile (assuming the other half to be the luggage compartment, etc.), gives me a volume of 428 cubic meters, not 880 (LπR²/2 = (49π(4.72/2)²)/2 = 428), and this is before subtracting volume for seat, overhead bins, and, of course, people. StuRat (talk) 07:02, 14 July 2012 (UTC)

And you've also failed to take into account people with diminished lung/heart function, etc., who may become uncomfortable due to oxygen deprivation (hypoxia) far sooner than healthy individuals. There's even discussion of requiring higher air pressure in the cabin, in order to protect such people: [1]. StuRat (talk) 07:15, 14 July 2012 (UTC)

I think StuRat was still right that it would be uncomfortable after a few minutes though, no so much from the lack of oxygen, but more from the humidity and temperature building up causing it to feel stifling. I suppose there might be an unstated assumption that with no air exchange the air con is also broken though. If the passengers were aware of it, I'm sure many would feel uncomfortable instantly. 203.27.72.5 (talk) 20:19, 13 July 2012 (UTC)

Actually, if the cabin was just well air conditioned, it sounds like it would be better without outside air coming in. According to Ratbone there's no real negative side effects from the lower oxygen levels. Getting drowsy from the carbon dioxide would help you get to sleep and pass the time and it should keep annoying kids less active; kind of like a primitive hypersleep. In the event of a crash the LEL of fuel air mixtures would be higher. The pilot and crew would still need some extra oxygen from somewhere, but that seems a lot easier to do than the whole cabin. 203.27.72.5 (talk) 02:33, 14 July 2012 (UTC)

Carbon dioxide, if anything, would make you breath harder, then hyperventilate. I believe it's carbon monoxide which makes you drowsy, and, if it did, you'd be in danger of dying. Lucky for us very little CO should occur in the airplane cabin, unless there's a fire. StuRat (talk) 06:44, 14 July 2012 (UTC)

Let's look at Stu's claims in turn:-

1. Experince of plane on tarmac. This is not relavent to the OP's question, as he asked about 100% recycling of air, not complete shutdown of aircon. With no air moved past your face, you will notice distress a lot quicker even if temperature and humidity are the same. Note that some people have an irrational discomfort in enclosed places when things go wrong. I have experienced people claiming they are running out of oxygen after a 10 minutes or so in a stuck elevator - complete nonsence. I too have been stuck in a powered down airliner for about 30 minutes - air quality was not a problem, but being annoyed with the boredom and delay certainly was.

2. Math error. Yes, I forgot to divide by 2 to allow for the cargo hold not being part of the available air. This means that oxygen would be down to comfort limit in about 1 hour 20 min, not 3 hours as I said. But don't forget the raised CO2 will cause the passengers to automatically breather deeper.

3. Folk with diminished health. The experience of airlines over many years is that slow air quality change almost never causes problems. This is as you would expect, as humans have considerable lung over-capacity at rest, and CO2 drives breathing depth to maintain blood oxgen at stautation level. In other words, in stale air you breath deeper but still well below breathing capacity, and as blood oxygen is still saturated, the heart doesn't see any difference.

4. Rising humidity. I had some difficulty finding a simple definitive reference for human moisture evolution rates at rest (plenty of refrences for exercising sportspersons), but figures range from 0.01 kg/hr to 0.022 kg/hr for an averge sized human at rest at 25 C. We drink about 1 litre a day when not working, but most goes to make urine, the remainder breathed out and sweated. We can expect an aircraft with aircon working but with no fresh air induction to bring humidity down to 40% REL. Accoding to my stadard ASHRAE pschrometry chart, this means a moisture load of 0.0065 kg per kg of air, and a n air density of 0.85. This equates to 2.7 kg of moisture in the cabin space. Assume each person evolves 0.015 kg/hour, and a confort limit of 70% REL, which has a moisture load of 0.011 kg/kg. This means that the comfort limit with 269 humans will be reached in 30 minutes. But, wait, there's more: All those bodies will generate heat, as perhaps the sun shining on the hull. So teh aircon will remove this heat, and removing heat from air by means of passing it over a cold plate always means condensation. This will considerable extend the 30 minite time toi discomfort, perhaps even indefinitely.

5 Raised temperatures. Since the OP's question did not required aircon to be completely shut down, there is no reason for temperature to rise. However, an aircraft at crusining altitude is passing thru very cold air, so the issue is not critical.

6 Diference between CO and CO2. Both make you sleepy. Neither cause discomfort. Recovering from CO is a serious problem though.

Conclusion: While I made a calculation error, my opinion still stands: Stu is wrong, lack of fresh air in an airliner is not of any critical importance. Ratbone121.215.69.8 (talk) 11:34, 14 July 2012 (UTC)

1) You misread my response again. I did not say "complete shutdown of aircon", as you claim. I said there was a lack of "air circulation", not "aircon". It was actually quite a cold night, and the plane was ice cold to begin with, so no cooling was needed, at least not for the time we were on the tarmac. I've also had a similar experience in well insulated houses, where, without any air circulation, a roomful of people quickly becomes "stuffy". StuRat (talk) 20:20, 14 July 2012 (UTC)

You said "parked on a tarmac without any circulation". That implies a complete aircon shutdown. Such things happen sometimes if the aircrew decide to resolve a problem by a complete shutdown of all systems then progressively bring it all online again. You can't have aircon without air ciculation thru it. In other words, I interpreted your words in a way that made the most sense - giving you the benefit of the doubt. Do you mean that the plane's normal venting was sealed? Doesn't seem likely - not at all easy to do deliberately, and why on earth would they, and not a likely fault condition. How did you know anyway? Did the crew tell you? Sorry, Stu, you're just not believable. Ratbone58.167.227.14 (talk) 15:56, 15 July 2012 (UTC)

Now I see you've moved on to accusing me of lying. On the tarmac, they always seem to turn off the air exchange here. I assume it's to prevent exhaust gasses from taxiing planes, etc., from being sucked into the passenger compartment. Or it may just be a consequence of not running the engines, which are used to power the air exchange. Perhaps they do it differently there. With a normal take-off, you may not notice it getting stuffy inside while on the ground, but when there's a delay, it's highly noticeable. Notice that things were so bad here we needed an airplane passenger's Bill of Rights, and it still allows planes to sit on the tarmac for 4 hours before fines kick in: [2]. (4 hours without fresh air might actually kill people, so I assume they periodically fire up the engines and do an air exchange.) StuRat (talk) 18:39, 15 July 2012 (UTC)

Well, if the cap fits... Actually, I don't think you mean to lie - I think you just make initial posts without thinking or checking first, then have trouble accepting you have things wrong. You are unware of how airplanes are operated. When on the ground, without propulsion engines running, airliners are powered from Auxilary Power Units (http://en.wikipedia.org/wiki/Auxiliary_power_unit) these are small turbine-powered generators usually mounted in the tail with the exhaust clearly visible. Some manufactuers install them further forward exhausting to the side - in these cases a cover closes over the exhaust when teh APU is shut down. When at least one main engine is running, the APU is not needed and is shut down. Sometimes, if convenient, aircraft on the ground are powered from a Ground Power Unit - a generator set mounted on a trailer, or in some cases, a convertor run form the airport electricity supply (Mains electricity is either 50 Hz or 60Hz, for historical & technical reasons, aircraft use 400 Hz power, so a convertor or special genset is needed). It would be very unusual indeed for an aircrew to have an aircraft unpowered when passengers are inside, unless they are resolving a problem. I have been on lots of local and international flights. Each time the aircraft was powered down, the captain first went on the PA and told us he was shutting down to resolve a problem. Ratbone124.182.16.199 (talk) 03:53, 16 July 2012 (UTC)

You don't seem to understand some of the pressures a pilot is under, at least in the US. They often have a very narrow window for take-off, and if they miss it, they then have to wait on the tarmac until there's an opening in a packed runway schedule. They can't go back to the gate, there might be another plane using it, and, if they did, they'd never get a chance to take off when another plane misses it's window. So they sit and wait, far from any electrical outlet they can use. And, even if they could, they wouldn't want to, as they need to be ready to go at a minute's notice. At the same time, they don't want to use up too much fuel, or they will then have to go back and refuel, since they often carry the bare minimum required, to keep the plane lighter. So, they often just sit there, biding their time, with the passengers suffering. Hence the need for the Passenger's Bill of Rights, to prevent such abuses. Maybe it's better in Aussie. StuRat (talk) 07:33, 16 July 2012 (UTC)

3) You still seem to be talking about normal, healthy people when you say things like "humans have considerable lung over-capacity at rest". People with certain health problems are already right at the limit, with little over-capacity to draw on, and the link I provided shows a dramatic increase in deaths during flights, likely due to changing demographics leading to more unhealthy aging baby boomers on planes. It can be argued that such people shouldn't fly, or should have portable oxygen generators with them if they do, but people aren't always aware of their health problems until they have a crisis. StuRat (talk) 20:14, 14 July 2012 (UTC)

People that seriously sick that they would be at risk would already be under professional medical advice. You get people with latent/undetected heart disease, but as has been explained, unless lung function is crippled, they are under no risk that they don't already face anyway. Your point here has been disputed by others, as you are aware - so I'm not going to waste time on it. Incidentally, us baby boomers are fitter than the previous generation (better food and medical advice, no WW2 service), and probably far fitter than the next generation, due to their obesity and lack of outdoor activity. When I was in my teens and 20's, chaps at work in their 60's could not run to catch a bus. I'm now past that age, and I can (and do, if I'm caught in the rain) run flat out for 100's of meters. And there's nothing special about me physically. Ratbone58.167.227.14 (talk) 15:56, 15 July 2012 (UTC)

You are wrong when you state that "unless lung function is crippled, they are under no risk that they don't already face anyway". Hypoxemia is frequently caused by circulation problems alone, due to heart disease, etc., or due to anemia. And a person who doesn't exert themself or spend time at high altitudes may not even be aware they have a problem, until they take a flight. See Hypoxemia#Deficient_oxygen_in_the_inspired_air for the calculations on how blood oxygen level is reduced with altitude (one solution they propose is supplementing the low pressure air with pure oxygen, but that's risky, as oxygen tanks are a fire hazard). StuRat (talk) 18:53, 15 July 2012 (UTC)

This has been quite adequately explained (blood oxygen is held at saturation level unless under strenuous exercise or lungs are just about totally knackered) - and your opinion disputed by others, so I won't waste time here. Ratbone124.182.16.199 (talk) 03:53, 16 July 2012 (UTC)

What "others" ? I must have missed it. And they aren't "my opinions", I've provided sources. StuRat (talk) 07:38, 16 July 2012 (UTC)

4) You haven't included your sources or the details of your math, so I have no way to check your work this time. However, it looks like you admit that, without any air conditioning, humidity will become uncomfortable in 30 minutes. That seems about right. However, note that you don't always start out at the comfortable 40% relative humidity you used in your figures. If the plane just took off, it may very well have higher humidity, due to having had the doors open while loading passengers. And, if it then sat on on the tarmac full of passengers waiting for clearance, where they don't seem to run any fans or air conditioning, then moisture will have built up even further. So, you want to bring in fresh, dry air quickly, once in flight, to reduce this humidity level. StuRat (talk) 21:00, 14 July 2012 (UTC)

I included sources for basic data. In any case you could google or look it up for yourself - you would leran more then. You did not give any sources at all for your original opinions in this topic - you made nothing more than unsupported personal claims, which have been conclusively shown to be nonsense. Calculations stand on their own. I gave sufficient data for anyone with basic math ability to reproduce the humidity calcs - the O2 and CO2 calcs were given in detail.

Re the 30 minute calculation, I admitted no such thing. You did not read what I wrote. I clearly explained that the calculated times, and the 30 minute value especially, was just to establish a completely worst case solidly provable limit. It was a very conservative calculation, because it is based on a 70% RH limit taken from an aircon book (Trott & Welch) I have for people in England working in an office (not at rest) - most humans at rest will tolerate much worse than that at 25 C. You have overlooked that I pointed out that aircondioning means passing air over a cold plate, and this inherently and necessarily means that excess humidity will be condensed out - so the actual time to discomfort is in excess of 30 minutes, almost certainly well in excess of 30 minutes, and may be indefinite. Ratbone58.167.227.14 (talk) 15:56, 15 July 2012 (UTC)

Is this the chart you mean ? [3]. Google was able to correct your two spelling errors and find the "stadard" ASHRAE "pschrometry" chart. (You might "leran" something from Googling it yourself, like how to spell.) But this doesn't look like the chart you are describing. What inputs did you use ? I can't follow what you did from the scant details you gave. You've already demonstrated that you lack basic math ability, like being able to divide by 2 when appropriate, so I'm not going to just take it on faith that your calculations are correct. Show them or shut up. StuRat (talk) 19:19, 15 July 2012 (UTC)

Yes - that's the chart, but the one you posted have important scales missing. Find another with the right hand scale for moisture load in kg H2O per kg air, and the use should be obvious - find the intesection of the vertical 25 C line and the curved 30% rel humidity line, then read off on the righ hand scale: 0.0065 kg H2O per kg of air, and on the density scale, missing on the one you posted, 0.85 kg/m³. Find a metric one - its so much easier. The standard ASHRAE chart has all these things. You don't need to take my calcs on faith. They only need a primary school skill level to do. So do them yourself - be independent - go divide the H20 breathed out & sweated by the 269 humans into the water load for (say) a 30% rel humdity change read form the chart. Ratbone124.182.16.199 (talk) 03:53, 16 July 2012 (UTC)

5) You can't assume that flying through cold air significantly cools the plane interior, without any air exchange. An airplane, especially a large one, has a low surface-to-volume ratio, and they also tend to have insulation in the lining of the fuselage, as you don't want ice building up and periodically melting amongst the wires and such. Solar heating can also be significant, especially when flying above the clouds. StuRat (talk) 20:31, 14 July 2012 (UTC)

Are you trying to have it both ways, Stu? Note that if the plane does heat up, relative humdity will drop rather rapidly (see any psychrometric chart), so comfort is not as affected as much as you may expect. But it's irrelevant anyway - The OP's question is not about loss of aircon, its about zero fresh air induction. Ratbone58.167.227.14 (talk) 15:56, 15 July 2012 (UTC)

That would only be true if the total amount of moisture in the air was constant as the temp increased. Obviously, with planeload of people sweating and breathing heavily, this isn't the case. StuRat (talk) 18:26, 15 July 2012 (UTC)

You haven't grasped how cooling of air works, and cooling of people works: Cold air "holds" less H2O then warm air, as shown by the psychrometric chart. (More correctly, the vapour pressure of H2O falls sharply with temperature.) Cooling air by passing over a cold surface causes condensation, as the air on contact with the cold surface is brought to saturation. Thats why aircons have drains (and slinger rings it seems). As heat and moisture are added by the passengers, the aircon will remove both. While those folk are sweating away, and they shouldn't be sweating thatmuch, they aren't doing anything, if the temperature is increasing (which in the context of the OP's question it won't), the consequent increase in saturation vapour level will significantly oppose the increase in relative humidity. If temperature rises fast enough (and that's not very fast), relative hunidity will actually fall. You can see this on the psychrometric chart - for example plot a curve of intercepts for (say) an increase from 25 C to 30 C accompanied by a moisture load increasing from 6.5g/kg to 7.0 g/kg - you'll find this drops the humidy 5%. You will actually feel marginally cooler, as the body's ability to loose heat by sweating is dependent on relative humidity. Ratbone124.182.16.199 (talk) 03:53, 16 July 2012 (UTC)

I understand all of that perfectly well, but none of it is at all relevant to what we are discussing here, which is your statement that "if the plane does heat up, relative humdity will drop rather rapidly". If it's heating up, then the A/C isn't working, so all that about A/C is irrelevant. And, I certainly understand that relative humidity would fall if you increase temperature WHILE HOLDING TOTAL MOISTURE IN THE AIR CONSTANT, but that clearly won't be the case on an airplane full of sweaty, panting passengers. You didn't seem to read or understand my post. StuRat (talk) 07:24, 16 July 2012 (UTC)

How about actually reading what I said: I covered the two separate situations: 1) air con running, which is what's relevent to the OP's question, and 2) the totally non-relevant case of aircon shut down, where both moisture load and temperature rise (if temperature rises - depends on the environment as previously explained), since you keep insisting on discussing it. You are too focused on your own view. Did you print out the Wikipedia psychrometric chart and plot the example I gave? If you did, you would have found relative humdity actually does DROP even though the moisture load INCREASED significantly - because rel humidity is very sensitive to temperature. And its rel humidity that matters. Ratbone124.182.45.47 (talk) 12:37, 16 July 2012 (UTC)

6) High carbon dioxide does cause discomfort, in that it creates the feeling that you need to breath. If you are already breathing heavily and still feel more of a need to breath, this is quite distressing. See "Use of carbon dioxide for euthanasia" (or animals), which states "research suggests that it is highly aversive and causes distress in many animal species" [4] StuRat (talk) 20:33, 14 July 2012 (UTC)

You reference is a pamphlet from the RSPCA, who does not enjoy a reputation for scientific rigor. But, more importantly, they are talking about deliberately sudden lethal doses - quite a different thing to what we have been discussing here. Gas fire suppression (Inogen being the main one) is often used in computer rooms, server rooms, and sometimes in telephone exchanges, where water could be of worse business impact than the fire. As I stated, these work by displacing oxygen with an inert gas mix, which puts the fire out, plus added CO2 so the humans automatically breath a bit deeper. There is no WP article on it, but you can go check it out. I have, as building owner's representative, witnessed a full test discharge on a new Inogen installation. Myself and colleagues walked around within the gassed room without any discomfort at all, though, as I was expecting it, I did notice I was breathing deeper than usual - but nowhere near as much as I would jogging or other physical exercise. Ratbone58.167.227.14 (talk) 15:56, 15 July 2012 (UTC)

As you put it, these are "nothing more than unsupported personal claims". StuRat (talk) 19:26, 15 July 2012 (UTC)

So, don't trust them - go research gas fire suppression youself. Ratbone124.182.16.199 (talk) 03:53, 16 July 2012 (UTC)

So why provide it at all? Why not just seal the cabin and use that air for the duration of the flight? It would have the advantages I listed above, plus reduce costs (according to Aspro in another thread), and according to what you said, would not cause any major issues. BTW, for what it's worth I don't think anything that StuRat said was relevant to what the OP asked, since the question was "how long until adverse effects?" which was further qualified by "how long until a majority of the passengers to lose consciousness or even die" and StuRat just talk about discomfort, so calling him out several logical leaps later when he's talking about a plane that was delayed on the tarmac seems a bit pointless. 203.27.72.5 (talk) 11:43, 14 July 2012 (UTC)

I took "how long until adverse effects?" to be a separate question from "how long until a majority of the passengers to lose consciousness or even die?", and considered discomfort to be an adverse effect. Note that discomfort can also lead to real medical problems, like panic attacks, hyperventilation, etc. StuRat (talk) 20:23, 14 July 2012 (UTC)

Good qestion. I'm not competent on the design/engineering of aircraft, however I do know a thing or 2 about aiconditioning of buildings. So, a few points: (1) It is quite difficult to completely seal things. In the aircon industry, there is a rule of thumb that 5% of air fed from aircon outlets should be fresh air from outside - this implies that 5% is leaving the building somewhere, otherwise pressure would rise. Nobody makes specific provision for this - leakage thru builing fabic and doors is generally very much more than sufficient. (2) I said that lack of fresh air is not critical. That does not imply it isn't desirable, especially on a long flight. That's a matter of other sorts of body odours as Stu said. Even though I am not a expert on aircraft, I do know that they do not fully pressurise (to save a bit of money on making a strong cabin). This implies a level of venting to outside anyway. Fully sealing the cabin might impoise structural safety risks if systems fail in flight. Ratbone60.230.211.9 (talk) 13:03, 14 July 2012 (UTC)

It may be of benefit to explain better how breathing works. The body breaths in response to CO2 as I said. However it is oxygen that the tissues need. We have evolved a breating system that in controlling CO2 maintains blood oxygen at saturation level - that is each red cell holds all the oxygen until reaching capillaries, becasue the lungs are ventilated at a rate a little faster than saturation requires. This has the following implications for persons at rest:-

1. If the level of oxygen in the air is reduced, CO2 remaining constant, you will feel distressed quite readily. It is a situation that evolution has not provided for.

2. If the level of CO2 (normally only about 0.03% by vol) is raised, all other gasses remaining the same, you'll breath deeper, even though you don't need to, and you won't have any discomfort up to quite high levels of CO2.

3. If oxygen level is reduced while simulaneously replacing it with CO2, you'll automatically breath deeper and blood oxygen levels will remain at saturation level, up to extreme levels where a limit is imposed by finite lung capacity.

4. If you are working physically very hard, the situation changes. With reduced oxygen pressure, your lungs may not have sufficient capacity to maintain blood oxygen at saturation level.

Do not confuse low oxygen with high CO2. The effects are different.

Interestingly, flushing the lungs with pure oxygen can kill. With near zero CO2 the chest does not move, and blood oxygen level will then fall.

Ratbone58.170.170.23 (talk) 15:33, 14 July 2012 (UTC)

Let me add some clarifications (note that I didn't say that Ratbone is wrong and frequently posts wrong answers):

"Interestingly, flushing the lungs with pure oxygen can kill", but not at sufficiently reduced pressure. At a low enough pressure 100% oxygen is fine. It's the partial pressure of oxygen in the air which is important, and that depends both on the oxygen concentration and the overall pressure.

"With near zero CO2 the chest does not move, and blood oxygen level will then fall." If you mean zero CO₂ in the air, then that's not true, since it's carbon dioxide in the lungs which matters, and that is the sum of the CO₂ inhaled from the air and that produced by the body. StuRat (talk) 20:08, 14 July 2012 (UTC)

In this case, what I said is correct, and what Stu said is mostly correct. Note that I did not say flushing the lungs with CO2 will kill, I said it can kill. If the subject human is performing significant exercise, he/she will produce more CO2 and that will generally be sufficient to maintain the breathing reflex even with pure oxygen. In hospitals, the masks etc are designed so the patient never gets pure oxygen. However, astronauts during the "space race" got pure oxygen, as their metabolic rate during takeoff acceleration made it both safe and necessary - they had extreme pulse rates and were close to their limits. Why did I say Stu is mostly correct? Well, he is correct in that no CO2 in the surrounding air is not relavent, however I am not sure where in the body the CO2 sensor is, but I don't think it is the lungs. Presumably it's actually the blood CO2 level that is critical to stimulating breathing. Reducing oxygen partial pressure will not change CO2 partial pressure - therefore it seems that reducing oxygen partial pressure will not stimulate breathing and will only make things worse. Ratbone58.170.179.193 (talk) 14:06, 15 July 2012 (UTC)

In what circumstance is "With near zero CO2 the chest does not move, and blood oxygen level will then fall" a correct statement ? Where is this near zero carbon dioxide ? In the blood ? How do you figure oxygen levels would fall in the blood without carbon dioxide increasing ? That statement just makes no sense at all. Or, as you so rudely put it "you made nothing more than unsupported personal claims, which have been conclusively shown to be nonsense". StuRat (talk) 18:22, 15 July 2012 (UTC)

Middle Eastern climate

Why is the Middle East so hot and dry even in areas that aren't in a rain shadow? --146.7.96.200 (talk) 18:38, 12 July 2012 (UTC)

Much of North Africa and the Middle East was once covered in forests. These were killed in Neolithic times by slash-and-burn farmers, and the soils were destroyed by overfarming and overgrazing by goats. See Deforestation#Prehistory. See also Cedars of Lebanon for a tree which was once quite widespread in the Middle East. With the soils destroyed, little local surface water remained to support a local rain cycle. The eastern US, for example, is very far from the Gulf of Mexico and the Pacific where its weather often originates. But there is enough retained local surface water that evaporation that occurs in Pennsylvania can fall as rain in New Jersey. Iraq is not going to get much rain that originates as evaporation in Jordan. μηδείς (talk) 18:52, 12 July 2012 (UTC)

This effect is also reinforced by the fact that the Middle East is in the Horse latitudes, where large-scale atmospheric subsidence (sinking of air) caused by the Hadley circulation suppresses large-scale rainfall. It's a major reason why much of Australia is desert as well.-RunningOnBrains^(talk) 19:58, 12 July 2012 (UTC)

It's a bit lumpy, though. The Azore High (semipermanent) really hits Saragasso Sea-NW India hard with the desert stick. Parts of Eurasia east enough get a respite, though (the monsoon, nearby ocean, and Himalayan "lid" probably helps, though). Miami, Cuba, and Taiwan would be Saharic on the other coast. Sagittarian Milky Way (talk) 22:56, 12 July 2012 (UTC)

Large continents, at that latitude, will tend to be dry. Much of the change in the climate is just the continuation of a geological cycle, where we've been slowly coming out of an ice age. The forested version of the Middle East was only possible at the tail end of that ice age. StuRat (talk) 02:58, 13 July 2012 (UTC)

The difference between Joshua Tree National Park and the Negev, of Central Asia and the Iowa plains, or of the Dust Bowl and the current drought, is soil quality, not rainfall. Of course rainfall matters hugely. But without overgrazing and the loss of soil the greater Middle East would be retain water and be greener than it is today. The connection between goats and deforestation is widely remarked upon. See also our article on deforestation during the Roman period]. μηδείς (talk) 04:07, 13 July 2012 (UTC)

why are IAU constellations biased towards the West?

Why do they not include any Asian, Arabic, African or Native American constellations? The Arabs advanced astronomy while Europe lingered in the Dark Ages, yet none of their culture is credited in modern astronomy. The Chinese constellations are extensive, more so than the Greek ones. Why are modern astronomers so racist? 76.104.28.221 (talk) 20:00, 12 July 2012 (UTC)

It depends on who you listen to: In India, for example, a lot of people use the traditional names (as do I). At the end of the day, "A rose by any other name would smell as sweet". Lynch7 20:13, 12 July 2012 (UTC)

See Astronomical naming conventions#Names and boundaries of constellations. The short answer is probably that when the scientific community adopted a consistent naming system for constellations, it was dominated by 'the West'. Note that as currently defined, the entire sky is divided into constellations, and adding 'new' ones would involve redefining the boundaries of old ones. In scientific terms, constellations are arbitrary and meaningless anyway. Regarding the Arabic influence on more meaningful concepts within astronomy, the traces of it are prominent however in the naming of stars: See Aldebaran, Altair, Algol, Betelgeuse, Deneb, Rigel, Vega etc - or simply see List of Arabic star names for them all. When you look into the night sky, if a star has a recognised name at all, the name was most likely first given by a speaker of Arabic. As for the suggestion that modern astronomers are racist, I suggest you do a little research before making such unfounded assertions - astronomy is a science practised worldwide by people of all nationalities and ethnicities, contrary to the implications of your posting. AndyTheGrump (talk) 20:35, 12 July 2012 (UTC)

I strongly agree with Andy. The conventions themselves aren't racist; both the constellation names and the star names reflect the prior dominance of different cultures in the subject. AlexTiefling (talk) 21:31, 12 July 2012 (UTC)

Racist? I wasn't aware there was a constellation called Judeus Rapax or a star called Delta Labiorum Nigri. (Most major star names are in the Arabic language of course.) Perhaps the issue here is not respect for tradition but euphemism, neologism, political correctness and the narcissism of youth? μηδείς (talk) 22:09, 12 July 2012 (UTC)

As others have noted, the constellations are an anachronism, their status reflected by their anachronistic naming system that was set some centuries ago. Star names, where they are significant enough to have proper names, are almost all Arabic. Only the lesser stars that didn't warrant names have binomial Latin titles. Modern star catalogs use alphanumerical designations, which can't have "racist" connotations at all (assuming the use of the Latin alphabet and Arabic numerals is satisfactory). Acroterion (talk) 15:29, 13 July 2012 (UTC)

I'll also note that in modern times the naming of astronomical bodies has become much more diverse: Makemake (dwarf planet), Haumea (dwarf planet) and List of geological features on Titan provide examples. A Latin grammar and terminology has been retained, and since there haven't been any imperial Romans for some time, few have objected. Besides, they were running out of Roman and Greek deities, Shakespearean characters and the like. I'll also note that many features on the far side of the Moon have Russian names, since the Soviets sent the first probes that saw the far side. They therefore had dibs. Star Names: Their Lore and Meanings is an interesting read, as long as you don't take Allen's discussions of Chinese and Mesopotamian naming too seriously. See here [5] for a critique and more bibliographic material. Acroterion (talk) 15:39, 13 July 2012 (UTC)

The names of the constellations are simply traditional, not anachronistic. There's no difference between that and the names of the continents, the elements, or the days of the week for that matter. Renaming them would be ridiculous, and is the sort of things that happens when you get dictatorial regimes, such as with the French Revolutionary calendar. How often would they have to be renamed to keep current? With every new graduating regime, or every new graduating college class? Leave if open to unending RfC? μηδείς (talk) 22:46, 13 July 2012 (UTC)

Are anachronistic and traditional mutually exclusive? I didn't think he was suggesting that we update the names either, just that their use now is anachronistic (and a result of tradition). 203.27.72.5 (talk) 23:43, 13 July 2012 (UTC)

My choice of words could have been more clear: as others note, it's more like a tradition that's been maintained for an astronomical subject that nowadays has little meaning when you can just program coordinates into your telescope and have it find the feature on its own, rather than needing to recognize the patterns of the stars to find things in the night sky, so anachronism and tradition can certainly coexist. Acroterion (talk) 01:29, 14 July 2012 (UTC)

Constellations are useful for more than aiming a telescope though. Not everyone depends on constant GPS positioning to navigate. Rmhermen (talk) 04:46, 14 July 2012 (UTC)

archaism or even atavism might be closer to what I imagine you mean than anachronism. —Tamfang (talk) 05:24, 14 July 2012 (UTC)

By the way, List of Arabic star names has a lot of what looks like junk in the "Meaning" column. There's a page somewhere, isn't there, to ask for help from someone who knows Arabic to clean it up? —Tamfang (talk) 05:24, 14 July 2012 (UTC)

No, none of these other terms than tradition or convention has the appropriate sense. Anachronism is used for a representation of a more modern fact in an inappropriately ancient context, like painting Sodom as if it were a renaissance Dutch city. An atavism is an either morally or biologically primitive avatar in modern times, like cannibalism, a person born with a human tail; but there is nothing inherently primitive (as opposed to simply old) about greco-roman culture. They had republics, democracies, atomism, evolution, and everything else we have except electricity and nuclear physics. Archaism implies the intentional use of a dead form. But there is a constant tradition of greco-roman mythology in the West. This is not the adoption of some style that has been dead for centuries,, but the retention of a tradition that has been alive in film and literature as recent as Kevin Sorbo and Lucy Lawless. Everything in our language and culture has a past, even if it is Lady Gaga referencing Radio Gaga. The word the has a past. Should we make up a new language from scratch every year because it was used by our dead relatives, and is therefore from the last Saeculum? μηδείς (talk) 05:30, 15 July 2012 (UTC)

Dark Energy as the Quantum Error Correcting Code of the Universe

Given the Holographic principle and that information can not be removed from the Universe as a whole, the "boundary" of the Universe must be smooth and expanding. If there is always exactly enough Dark energy to maintain the Universe as a Flat space then the dark energy is acting as a Quantum Error Correcting Code to pad the history of the Universe onto its "boundary".

If this is correct, what sort of test could be made for the adjustment of dark energy to "fit around" the mass content of the Universe? Hcobb (talk) 23:15, 12 July 2012 (UTC)

I gather from the use of terms in quotation marks that you might be aware that the definitions of the words you are using might not be clear to readers. I am not sure of your hypothesis so I can't help you test it. 71.212.249.178 (talk) 01:07, 13 July 2012 (UTC)

Well one of your hypotheses is that space is flat. You can test that. See if a cube with a side ten times longer has a thousand times the volume. Graeme Bartlett (talk) 11:40, 13 July 2012 (UTC)

The dark energy has a constant density in all cosmological eras. It doesn't adjust to the distribution of other matter. The continued spatial flatness of the universe is a prediction of general relativity, independent of the exact nature of the mass/energy. We don't know anything about the boundary of the cosmos (except that, if you believe ΛCDM, its future boundary is a 3-sphere—but that's a spacelike surface that can't expand or otherwise change with time, and it's probably a bad idea to believe ΛCDM). Quantum error correcting codes have nothing to do with your question. -- BenRG (talk) 15:26, 13 July 2012 (UTC)