Wikipedia:Reference desk/Archives/Science/2014 May 25

= May 25 =

Which is good AC or Fan for our health?
Zonex shrestha (talk) 04:32, 25 May 2014 (UTC)


 * Define "healthier". ←Baseball Bugs What's up, Doc? carrots→ 05:33, 25 May 2014 (UTC)


 * Is this about the idea that electric fans can kill you in your sleep? If so, please see Fan death about that urban legend. Sjö (talk) 08:07, 25 May 2014 (UTC)


 * A good AC system will filter out the air pollution. A fan will actally increase the amount of dust particles in the air. Count Iblis (talk) 12:48, 25 May 2014 (UTC)


 * People generally sleep better when the ambient temperature is more comfortable - and getting good sleep is well known to be important to good health. So, in some climates, there is likely to be an important health benefit.  Fans are less obviously good - they mostly help A/C to work effectively - but Sleep points out that white noise (such as produced by a noisy fan) is believed to help with insomnia. SteveBaker (talk) 13:34, 25 May 2014 (UTC)


 * Note that this depends significantly on climate, house design etc. A fan can make a big difference to the comfort level in somewhere like Malaysia where it's hot and humid but rarely gets hot enough that a fan will make it worse. AC helps more obviously but many can't afford the cost to run one all the time and will be able to comfortably sleep (at night at least) with a fan but often not so much without one. Definitely during a power cut I usually found it harder to get to sleep in Kuala Lumpur and I rarely used AC. (Of course some people have to do without either.) Having the windows open usually help a bit but not so much, probably for various reasons like house design and the presence of mosquito netting on windows.
 * Here in Auckland, most of the time if it's too hot, opening the windows is enough. Occasionally during the very hot days of summer it's still a bit hot. In those cases a fan often helps (and I generally find sufficient). A fan is also a great help if you're exercising in doors during the colder nights and don't want to open the windows either for the benefit of others in the house/room or for you when you stop. Few people would suggest you ever really need AC here for a residential building. It may be slightly nice to have, but really the main reason why people have one is because they want the heat pump functionality. And a big percentage of our heating uses electricity and combined with the generally considered mild climate, a heat pump makes a lot of sense if you do need heating.
 * So while a fan doesn't affect the indoor climate like the way AC does, it shouldn't be thought of as primarily something which helps AC to work.
 * Nil Einne (talk) 17:26, 25 May 2014 (UTC)


 * Certainly a fan is not pure "white noise", especially certain emplacements that make substantial low frequency, allegedly subsonic sounds; and white noise is a very poor substitute for the sounds of crickets and countless other small creatures which, by right, should be the performers of the nightly tune. But in my opinion even urban noises of people and passing cars are more relieving than the incessant, obtrusive, meaningless noise of these devices. Wnt (talk) 05:45, 26 May 2014 (UTC)
 * I've known folks who like those types of random sounds you're describing, and I've known some who hate the whoosh of a fan or air conditioner and require absolute silence. I find that a constant sound such as a fan is relaxing (as long as it's not too loud). Everyone's needs are different. ←Baseball Bugs What's up, Doc? carrots→ 10:55, 26 May 2014 (UTC)


 * Some arguments for fans:


 * 1) They use less energy, so the cumulative effects of energy use, like global warming from burning fossil fuels to generate that electricity, are reduced with fans. This can protect human health by slowing the spread of malaria into (currently) cooler climates, etc.


 * 2) Indoor air is not always good quality. Especially in a new home, outgassing from all the building materials can be problematic, as can household chemicals used for cleaning, etc.  Then there might be a smoker in the house, and cooking without a proper exhaust fan can pollute the air, too.  Fans placed in windows provide for better ventilation than A/C.


 * 3) A/C units can create their own health concerns. My window A/C units smell like mildew when I first turn them on.  With central air, there's Legionairre's disease to worry about.


 * I use both, myself. I use fans when the outdoor air is cool, dry, and unpolluted, and A/C otherwise. StuRat (talk) 19:28, 25 May 2014 (UTC)


 * Using air conditioning to deal with polluted air sounds like a futile spiral to me. If the air is really that polluted, a person should either flee or be willing to take part in the fate of the Earth.


 * A basic step in dealing with hot weather is to acclimate to it as much as possible, to be comfortable inside and out. But what mystifies me about air conditioning is that it seems to "addict" a room beyond that - take a room that is air conditioned, leave the thing off for an hour, and it seems intolerable compared to any neighboring room that never had one of them running.  The obvious explanation would be humidity, but they visibly dehumidify the air, often with constant dripping water outside.  So what's the reason? Wnt (talk) 04:56, 26 May 2014 (UTC)


 * Most of the population of China lives in areas where the air has particulate matter pollution at levels that the US EPA would consider unhealthy on more than 100 days a year. Central air with filters can appreciably improve indoor air quality and reduce the incidence of pollution related disease for these people.  As long as the filters are present, even running a central air system in "fan-only" can help to clean the air without necessarily having to use cooling or heating functions.  Dragons flight (talk) 05:22, 26 May 2014 (UTC)

Trillions of plastic fragments in Arctic ice
Re, given that most plastics are as inert as possible by design, what are the potential risks to ocean life from plastics? What are the likely risks? I remember from sophomore biology lab that rayon is a decent scaffolding for plant life in fresh water. How long will it take for solar ultraviolet to break these fragments down to indistinguishable ocean water components in the Arctic? 180.173.84.34 (talk) 12:32, 25 May 2014 (UTC)


 * Plastics are chemically inert - but not physically inert. For example, the plastic mesh used in fishing nets can get wrapped around the head of some fish or marine mammal - and precisely because it's so chemically inert, stays there until the animal grows to  maturity and is slowly and painfully choked by it.  Some animals see bits of plastic as food and eat it - since it doesn't break down in their guts, it can get lodged there and prevent other food from being eaten...the animal starves to death.  Our article Plastic particle water pollution describes consequences from the spill of small particles of "inert" plastics.


 * Perhaps there are a few minor benefits to a few, specific local lifeforms - but note that those lifeforms got on just fine without the plastic - so the benefits are likely to be minimal. Even if some specie does benefit from it, it'll boom in population and unbalance the local ecology - probably to the severe detriment of other species  in the area.


 * Solar ultraviolet is not very strong in the Arctic regions - and in any case, the UV is filtered by the water and/or ice - so only plastic that's floating at or near the surface would be affected. Some plastics are not much affected by UV light.  It's hard to say how long it would take for each of the myriad types of plastic to break down - but places like the three, known "Garbage patches" in oceans across the world suggest that even near the equator, this stuff has a very long half-life.


 * Our Great Pacific garbage patch article (and others linked from it) discuss in detail the harm caused by plastics in the ocean - and the mechanisms by which it may (or may not) be naturally removed. It's a deeply depressing read!


 * SteveBaker (talk) 13:29, 25 May 2014 (UTC)

What polymer is formed from phthalic acid and ethylene glycol?
Zonex shrestha (talk) 13:00, 25 May 2014 (UTC)


 * A Google search for "phthalic acid and ethylene glycol" will give you the answer in the very first link. (Since this has the appearance of a homework question, I am not going to supply it.) Looie496 (talk) 15:50, 25 May 2014 (UTC)

Fan housing
I have box fans I use as window fans. Since the fan housing is square, I've created a circular "mask" to go outside the area swept by the blades, to prevent back-flow of air in the corners. Is my assumption that the ideal mask should contain a hole exactly the size of the area swept by the blades correct, or should it be larger or smaller ? (It's not in the same plane as the blades, so there's no risk of them hitting the mask.) StuRat (talk) 18:28, 25 May 2014 (UTC)


 * This seems like a complex problem in turbulent flow dynamics. I can picture anything from air backflowing as you suggest to air being entrained and actually adding to the flow in the area.  Can you do an ad hoc test where you take a paper mask, cut holes in it, hang tissue paper in the holes, see which ones are really passing air backward? Wnt (talk) 19:44, 25 May 2014 (UTC)


 * I thought of that, and I could fairly easily determine the direction of airflow at each point, but it occurred to me that I might end up with more total airflow if I constrain it to blow through a smaller hole at high speed, rather than a larger hole at low speed, even without any back-flow. StuRat (talk) 20:18, 25 May 2014 (UTC)


 * It's true that any solution would be iterative, since changing the overall shape of the mask could change the airflow in any given test hole. I'm at least hoping it ought to converge, though. Wnt (talk) 22:11, 25 May 2014 (UTC)


 * Yep, and I may very well try that approach out tonight. Just wanted to see if anyone had any insight before I went the trial-and-error route. StuRat (talk) 03:31, 26 May 2014 (UTC)


 * I'd re-ask this at the math desk and see if you can open the frame and put triangular styrofoam blocks in the corners. That might be better for the problem User:Wnt mentioned. Sounds like something rom a Clancy novel. μηδείς (talk) 20:06, 25 May 2014 (UTC)


 * This is a fluid dynamics problem, which falls squarely into the realm of applied science. StuRat (talk) 20:15, 25 May 2014 (UTC)

The presence or absence of backflow is going to be significantly influenced by the nature of the space you are blowing air into. If the room in question is mostly sealed, you can expect the excess pressure created by the fan to result in a lot of backflow at those corners, but if there are other natural exits (such as vents, open windows, or other large air leaks), then I wouldn't expect to see much backflow and you might even see some entrainment. To get the best possible air flow you need to keep in mind both how the air enters and how it exits. Dragons flight (talk) 04:05, 26 May 2014 (UTC)


 * Coincidentally - I have the exact same problem right now! My wife runs a couple of laser cutters in a spare room - these machines generate quite a lot of wood smoke - which is sucked out and vented through a chimney in the roof - but a small amount of smoke inevitably leaks out into the room.  We have a dual-fan, window-mounted device for that reason - but yesterday I did some upgrades to make the laser cutters quite a bit faster and when they are both running at full speed, enough smoke particles build up in the air in sufficient quantity to set off the smoke detector in the adjoining room! (The are incredibly sensitive - they go off when you can neither see nor smell the smoke).


 * Increasing the flow rate out of the window would be a very good thing - and a huge box fan seems like it would have a higher flow rate than the smaller window fans. But just like our OP, I need to make a mask so that on windy days, the air isn't blown back into the room around the edges of the fan.


 * My best guess is that the hole should be a little smaller than the fan blades because there will presumably be Wingtip vortices that would pull outside air back in again...so my first guess is to make a hole that's maybe a half inch smaller than the diameter of the fan. (Another consideration is that it's much easier to change my mind and make the hole bigger later on than it is to make it smaller!)


 * At any rate, I'm very interested in the results...so perhaps StuRat would care to get in touch via my email (it's on my User: page) - and we can collaborate on the research and report back. SteveBaker (talk) 13:27, 26 May 2014 (UTC)


 * For one thing, I suggest you put box fans in all windows where they will fit, with an equal number pointing in and out, although I'd avoid windows which are much wider than the fan. In your case, I'd blow air into the adjacent room from outside, and blow it out of the room where the smoke is generated.  Try not to have an intake fan on the same side of the house as an exhaust fan, so it won't suck the bad air back in.  In addition to the mask, you also have to worry about the gaps around the fan housing.  I find just jamming something flexible in there, like my curtains, is one way to seal it fairly well.  Oh, and remove any feet on the box fans that they add for stability, as those introduce gaps.  Also, you can put the fan in the window sideways, if the controls keep it from fitting properly upright. Close the window on the fan tightly.  The mask should be on the intake side of the fan, as it will tend to be sucked towards it, that way.  I also am using a mask a bit smaller than the area swept by the blades, for the reasons you mentioned.  I used a flexible material, similar to linoleum, but I think something a bit stiffer might have worked better. StuRat (talk) 04:49, 27 May 2014 (UTC)


 * Fortunately, I have a laser cutter...the salvation as well as the cause of problems in this case! I can easily make a 'mask' out of acrylic plastic that's the exact shape of the fan/window and with bolt-holes in all the right places.   So once I know what I *want* - it's not going to be tough to make it.


 * The room has just one window and one door. The vent fans on the two laser cutters are both sucking air out of the room (via the insides of the machines) and out of the roof - and with the window shut, you can feel fairly rapid airflow through the doorway.  If you push the door nearly shut, the airflow pulls it open again!   But for all that, smoke is somehow getting out of the room and into the rest of the house...sufficiently to trip the smoke detectors in adjacent rooms.   I suspect that the air that's leaking from the machines (despite the negative pressure inside them!) is warm and drifting upwards and out of the top of the door while fresh air is coming in at the bottom.


 * Using an inlet and an extractor fan on the same window seems kinda silly - sure the inlet fan will bring in fresh air - but I'm concerned that it's going to just get sucked right out again - and most of the power of two fans would be consumed in uselessly circulating a toroidal volume of air into and out of the window without much affecting the bulk of the air in the room.  So my thought is just to get as much air out of the room and into the outside world as possible though the window and out through the roof vent.


 * The roof vent is a 6" diameter tube - the window is a 3'x5' hole - so I'd expect to be able to get more air out through the window - hence sticking a box fan in there and filling about half of the area with fan. Using two box fans, one on top of the other to more or less completely fill the window is impractical because that would require me to remove the glass (these are vertically sliding sash windows).  So really the only thing that seems reasonable is to fill the bottom half of the window with the biggest, fastest fan I can reasonably find. SteveBaker (talk) 16:03, 28 May 2014 (UTC)


 * I'm concerned that if you put a powerful exhaust fan in the window of the laser cutter room, that will lower the pressure there, and suck smoke back out of the laser cutter. Also, the smoke blown out of the window of the laser cutter room might get sucked back into the windows of the adjacent room(s), so exhausting to the roof is a better option.


 * Instead, I suggest leaving the laser cutter room window closed, and blowing fresh air into the adjacent room(s), using window box fan(s) there. That should create positive pressure there and some positive pressure in the laser cutter room, as well.  This should hopefully help the laser cutter's own exhaust system out.


 * You could also blow air into the laser cutter room using that room's window for a box fan, and that should help push smoke up the laser cutter's exhaust, too. However, I'd be worried that any smoke that does still escape would then be more likely to be blown into the adjacent room(s).


 * I agree that stacking box fans on top of each other is impractical, and you'd also have a diminishing return on two fans in a window versus one.


 * A 6" exhaust tube seems adequate to me, provided it has a powerful enough exhaust fan mated to it. You might need to upgrade that fan, perhaps with one on the roof, to supplement the one by the laser cutter.  Also, the fans need to be started up before the laser cutter, to establish the air flow pattern first, and kept on quite a while after, to vent any residual smoke.  StuRat (talk) 16:50, 28 May 2014 (UTC)


 * If you are pulling enough air to move the door, then it seems unlikely to me that smoke would slip around the door. You say the main vent goes through the roof.  The first thing I'd check is whether there is a leak in the vent pipe.  Perhaps you are dumping some smoke into your attic and it is moving back down from there?  If not, then I might tend to worry that you are pulling smoke back into the house from outside.  You might try putting an intake fan in a window as far away in the house from the vent area as possible to see if you can ensure the air entering the house is as clean as possible.  Dragons flight (talk) 04:20, 29 May 2014 (UTC)


 * If the smoke is all released at the roof, it seems unlikely to get back into the house from there, unless Steve's house is in an area of downdrafts. And just because air is blowing in from one adjacent room doesn't mean it might not then blow back into another adjacent room. StuRat (talk) 15:30, 29 May 2014 (UTC)


 * The airflow through the door implies that you have a decent negative pressure in the room which is good, because your goal is to keep smoke from escaping it anywhere but through that vent pipe. I disagree with the suggestion to raise the pressure in the room by reversing your window fans - even if it does help push more up the pipe it will also help whatever doesn't make it up the pipe go out the door instead. How does air get into the machine? Although a large intake far from the smoke source seems like a good idea, it means you're likely to have stagnant air and vortices outside of the main airflow path, where smoke levels can rise and sneak out of seams. A loosely sealed box that takes air in from seams all around it will have more even and continuous airflow throughout it, and help keep pockets of smoke from developing. That's the technique we use for some of the air-cooled equipment here. If you need to improve airflow, I recommend upgrading the exhaust fans. Those are what keep the pressure low inside the box, which is how you keep the smoke from escaping in the first place. If you can't get enough airflow with a standard fan, rooftop blower units are an option. They also keep the fan noise (which gets loud with high-volume fans) out of your house, and you can install a muffler if it is too loud. K ati e R  (talk) 17:58, 29 May 2014 (UTC)
 * Sorry StuRat, I missed that your suggestion was to close the window, and put intake fans in a different room. On my first take I read it is pulling air in the laser cutter room window. That's not a bad suggestion, although hopefully upgrading the exhaust fans and thinking about (and improving if needed) the airflow in the laser cutter will do a good enough job that the setup isn't dependent on external fans. K ati e R  (talk) 18:06, 29 May 2014 (UTC)
 * All these fans sound hellish from here. It sounds to me like you're trying to make a fume hood.  I don't know whether you really need one (is the smoke toxic, or do you just need to tone down the smoke detectors?) but in any case you should be looking at setting up the work area for laminar flow as best as you can, and having as small an enclosed area for the smoke to spread in as possible.  Enclosing the area also should help reduce what I assume is a not inconsiderable risk of one of the lasers getting reflected out of the work area and blinding someone.  Fume hoods are common enough for everything from chemistry to cooking and you should be able to find many specific suggestions for design and testing. Wnt (talk) 23:02, 29 May 2014 (UTC)


 * Hmmm - the "pro/con" lists in Ducted fan is educational. SteveBaker (talk) 13:36, 26 May 2014 (UTC)

Reverse tomography for 3D "printing"
There's been a lot of good discussion of the issues with 3D printing here recently, so I'll ask: why don't the devices doing this take a tomographic approach? There's a minor precedent with the gamma knife, and the idea of simply reversing the data collected from tomography seems straightforward enough. Just have a rotating ring with lots of individual emitters, which software programs to focus microwaves at various points within the sample. Wherever the tightly packed powder filling the target container gets hot enough, it sinters together and solidifies. Potentially you could do multiple passes, allowing heat to dissipate from some regions you don't want to solidify after you've sintered others in place. Excess powder in cavities would be poured out from small holes left in the finished product. I think you might even be able to do it by having gamma sources around a ring and irradiating a metal powder that would resist microwave heating, shouldn't you? The result would not have a visible pixel structure. Wnt (talk) 20:01, 25 May 2014 (UTC)


 * I believe that those medical scanners cost around a million dollars each, and that's a bit pricey, even by 3D printing standards. StuRat (talk) 20:23, 25 May 2014 (UTC)


 * That's the medical racket - nothing in medicine has sane pricing; the modalities for recovery are held for ransom. True, I don't picture a very free market emerging in radioactive sources emerging in any case, but at least there isn't as much upward pressure on the price. Wnt (talk) 22:09, 25 May 2014 (UTC)


 * I don't think it's medical inflation only. Those medical scanners have big moving parts that can be controlled with great precision, something like astronomical telescopes which rotate as the Earth does, to keep a steady aim at the same spot in the sky.  Such devices are expensive. StuRat (talk) 03:29, 26 May 2014 (UTC)


 * The difficulties are things like:
 * These machines have to be large enough to scan an entire human. Most hobby-level 3D printers have around 6"x6"x6" build volumes - we'd need 24"x24"x80".
 * They have to be reasonably fast. Sick people don't want to spend an hour in those scarey, claustrophobic, noisy conditions - and a typical 3D printer takes an hour just to make a 2"x2"x2" cube.
 * They have to move considerable weight. The print head of a 3D printer weighs less than half a pound - the "business end" of most body scanners is so heavy that it's easier to move the patient and the bed they lie on than it is to move the scanner.
 * They have to be approved by the FDA with all of the animal and human trials processes.
 * It has to be possible to keep them clean and sterile
 * The patient has to be protected from the machine itself (liquid helium, the radiation source or the big magnets)
 * Sales volumes are small - so R&D costs have to be shared between a few thousand machines rather than tens or hundreds of thousands.
 * ...the list goes on. Sure, you can find cheap solutions to some parts of this - but engineering with ALL of those constraints isn't cheap.
 * The one thing that ISN'T hard to deal with is precision! FMRI machines only manage a precision of about 2mm, I know that the table that moves the patient through various other kinds of scanner only moves in either 1mm  or 3mm increments - so we might expect them to have only about 1mm precision elsewhere.  Most 3D printers, CNC routers and laser cutters built by hobbyists are easily capable of sub-millimeter precision.
 * The intersection of precision, size, speed, cost and weight is what causes the biggest issues for body scanners - you can move something small with great precision - but something large and heavy is much harder. Moving something very slowly and with great precision is also not difficult - but doing it rapidly is much harder. SteveBaker (talk) 12:46, 26 May 2014 (UTC)


 * Right, just like measuring mass within a gram isn't hard for a kitchen scale, but would be a serious challenge for a truck scale. StuRat (talk) 12:56, 26 May 2014 (UTC)


 * Yep - my point exactly. SteveBaker (talk) 13:03, 26 May 2014 (UTC)


 * The devil is in the details with these kinds of machine. Heating up powder is tough because most powders conduct heat - so getting one "voxel" (a 3D pixel) hot enough to melt it will make the adjacent voxels fairly hot too.  When you zap a bunch of voxels that are near to each other - but not touching, the heat contained in the volume has to be conducted away somehow.
 * If the material conducts heat poorly, then the voxels that you melted will stay liquid for a long time - and then unmelted powder nearby can float around and move in the parts of the object that you melted.
 * If the material conducts heat well, then areas that you didn't want to melt that are adjacent to areas that you did want to melt will get hot enough to melt in turn - and the finished object won't have the precise shape that you wanted.
 * With plastic extrusion machines, getting the temperature just right is the biggest problem. It has to be high enough to allow the plastic to be extruded through a very thin nozzle - and high enough to allow new layers to fuse with the layer below - yet not so high that the object will slump into a puddle or that the new layer will completely melt the layer beneath - and slowly enough that the thermal expansion of the material doesn't cause warping.  Computer-controlled fans are one answer to this - cooling the material after it's been extruded.  This is hard enough when the material is placed only where you need it and you can apply cooling airflow.  In the machine you have in mind - it would be virtually impossible to control that.
 * The other problem is that this machine is going to be spectacularly dangerous. Spewing focussed microwaves in all directions is not nice!  It's just not the kind of thing that a bunch of enthusiasts can easily build...the spectacular decrease in the price of 3D printers (they used to cost $35,000 - the "Peachy" printer was Kickstarted for under $100!) is because amateurs can easily tinker with them. SteveBaker (talk) 13:03, 26 May 2014 (UTC)


 * For the OP, "the idea of simply reversing the data collected from tomography seems straightforward enough". It's true that one can reverse the 3-D DFT transform relationship between an array of intensities or voxels and the element amplitudes and phases of a surrounding array of r.f. detectors or emitters. The limit to resolution due to Discretization error is understood; in short one will need a very large number of emitters each with accurately controlled amplitude and phase to create a typical complex 3-D object. The same math is used in the fields of Beamforming and X-ray crystallography. But I think the OP's idea founders on a barrier that does not exist in those fields: the printing powder initially has a homogenous Refractive index but almost certainly not when parts solidify. Internal reflections can create an intractable optimisation problem. At worst, some shapes such as a solid voxel within a hollow solid sphere will be impossible to print; at best there will be hard-to-predict standing-wave error structures that may resemble the 2-D blemishes on an over-compressed jpg image. 84.209.89.214 (talk) 15:01, 28 May 2014 (UTC)

Professional athletes
When professional athletes stop training in between seasons, do they stop training completely or do they maintain some light activity? — Preceding unsigned comment added by Clover345 (talk • contribs) 21:38, 25 May 2014 (UTC)
 * They'll typically exercise but at a lower impact allowing their bodies to get the necessary repairs. Note that when they exercise normally, they will increase the intensity of that to the point where it becomes unsustainable on the long run. They then suddenly power back causing their fitness levels to greatly overshoot the level they would be able to attain when sticking to a constant exercise routine. This is damaging to the body on the long run, so they need to give their bodies some rest for the next season. But what is rest and low impact exercise for athletes would still be extremely heavy exercise for most normal people. Count Iblis (talk) 22:56, 25 May 2014 (UTC)