Wikipedia:Reference desk/Archives/Science/2014 November 27

= November 27 =

Availability of phase change material fabrics
Are their phase change material fabrics available to the public (in the UK) for use in domestic sewing projects? --78.148.108.62 (talk) 05:55, 27 November 2014 (UTC)


 * Ebay sells everything. £15 per yard. Plus a ridiculous amount for postage etc.--Aspro (talk) 18:33, 27 November 2014 (UTC)

Von mises
The article on Von Mises is very confusing. Can someone explain simply how you can use it to explain where and when yielding occurs on a material? If for example, I know the pressure on the material, and the material properties, how do I use those values to calculate when and where yielding occurs? — Preceding unsigned comment added by 194.66.246.71 (talk) 19:43, 27 November 2014 (UTC)


 * That's first year stuff in an engineering degree course, and I must admit it took me two goes to undertsand von mises, and I've now forgotten it again. As such, a post on a forum like this isn't really adequate. Can I recommend that you read JE Gordon's Structures, or things don't fall down? http://www.amazon.com/Structures-Things-Dont-Fall-Down/dp/0306812835 It should be compulsory reading for anyone who is interested in stress analysis. Greglocock (talk) 21:51, 27 November 2014 (UTC)

Amylase Inhibitors
I'm avoiding from eating Beans cause I'm kinda allergic to them... What other foods could serve me as a reasonable resource for natural Amylase Inhibitors? Something that has really lot's of these molecules? thanks. Ben-Natan (talk) 21:24, 27 November 2014 (UTC)
 * Dietary advice is a bit like medical advice and so I decline to say. Also, the FDA has no Indications for this protein group. In other words there is no evidence that it has a beneficial effect in vivo. Better to cut down on starchy foods. Udo  Erasmus is  a recognized expert on what to eat.   From Fat to Fit And how much of each type. Food Pyramid. Then consult your own health care professional.--Aspro (talk) 22:09, 27 November 2014 (UTC)

How does do he take the pure oxygen in the glass in his hands?
I saw in this page a picture of a guy who takes a glass that inside there's oxygen [you can see picture of the same idea (but not with a man who hold it) here in Wikipedia]. According to what I know about the oxygen, the oxygen can not be in the state of matter of liquid when it's up to -183. So I believe in this place it's not -180 degrees (you can understand it according to the wearing of this man). I think it should be broken by the higher level pressure that should created when this oxygen appears such as this temperature (unless the oxygen in a appropriate strong cylinder)5.28.173.111 (talk) 23:22, 27 November 2014 (UTC)
 * The oxygen is cold so it is liquid. It's warm outside so the liquid is boiling and evaporating, but it takes time to evaporate, so the man has time to check the liquid oxygen. Ariel. (talk) 23:26, 27 November 2014 (UTC)
 * Sorry to piggyback a question onto this, but this actually made me wonder. I understand the above, but I kind of get where the OP is coming from too. For example, the liquid inside a butane lighter immediately and violently evaporates if exposed to regular pressure, like if the lighter casing is broken, I have witnessed this first hand on several occasions. The boiling point of butane is only -1 °C, the boiling point of oxygen is -183 °C. Why doesn't the butane "boil" away slowly like the oxygen? If you cooled down a lighter below -1 °C, like in a freezer, and THEN broke the lighter would it also boil away slowly like the oxygen? Vespine (talk) 03:38, 28 November 2014 (UTC)
 * It's the mass-volume, temperature and pressure. Normally, we usually think of the liquid as being room temperature and coming to an equilibrium pressure (like LP gas).  Liquid oxygen is not that case, though.  It can be chilled while near atmospheric pressure to a liquid.  The dewar flask keeps it from absorbing heat and limits the external heat needed to boil it away.  The rate at which heat can be exchanged regulates the boil (and pressure).  If it's not in a temperature insulated tank, it will be a gas at very high pressure.  You can feel the heat exchange process by filling and releasing a compressed gas (expansion feels cold, compression gets hot).  --DHeyward (talk) 04:02, 28 November 2014 (UTC)
 * If you break a container that starts under pressure, the liquid is already well above its boiling point to begin with, and reaching an equilibrium based on pressure tends to be fast and forceful. Cooling the butane lighter should help.  However - I would have to look up to know for sure - I am suspicious that butane might have a lower heat capacity in liquid form than oxygen, which would make it evaporate faster (the lower the heat capacity, the more has to evaporate to compensate for the same leakage of heat through the insulation).  Still, the rate of heat leakage in a container will be proportional to the temperature difference, so it should still be much slower to boil than oxygen in the same container when they both start off at boiling point and atmospheric pressure. Wnt (talk) 05:37, 28 November 2014 (UTC)


 * The cryogenic dewar tanks are preferred for supplying heavy users because the tank doesn't have to be purged when it's refilled. Just the glass flask is a poor conductor so most of the heat to boil the oxygen is from air above and that can be controlled somewhat (note the duct tape).   The cryogenic storage tank will usually have various ports for high pressure, low pressure and liquid.  In a flask, the liquid appears to be boiling until it is gone.  For oxygen, it will go from the liquid Dewar flask to an evacuated high-pressure tank. Contaminants and catalysts create hazards in the cylinders (i.e. sea water in emergency O2 scuba rescue dive tank that is filled with pure, high pressure O2 ->salt water reacts with the metal much quicker than atmosphere.  Oil from a compressor/concentrator that accumulates in a gas cylinder can be a fire hazard - for those reason, Oxygen compressed air tanks are evacuated before being filled but the liquid storage tanks can be refilled continuously as needed).  --DHeyward (talk) 03:48, 28 November 2014 (UTC)

An interesting question. It's worth pointing out this is a high-resolution image. So far as I can tell, the way the man's gloves look makes me think that they are actually above freezing, with just bits of frost from having reached into someplace colder. The oxygen in the container seems to be boiling, as I'd expect. But there isn't very much vapor going around, even though this sure doesn't look like a Dewar flask. So the nature of the material of the container is very much open to question; I have a hard time thinking it is a single layer of glass based on the apparent insulating quality; nor would that seem especially safe... but I don't think many types of plastic would be a good idea... I have a wild guess it might be made out of silicone but certainly am not in a position to know that. Anyway, the loose foil seal at top is a dead giveaway that this is not under pressure, so the rate of boiling of the oxygen directly reflects the insulating quality of the container. Wnt (talk) 05:28, 28 November 2014 (UTC)
 * It's not hard to fill a standard glass beaker a third or half full of liquid nitrogen and get an effect similar to what's seen here once the glass has cooled down. On initial filling, there is somewhat violent boiling at the liquid/glass interface, but then it settles to a gentle boiloff. Can't keep it for long-term storage, but even simple glass must have enough insulating ability (poor enough thermal transport properties or whatever technical parameter) even without film boiling. DMacks (talk) 06:10, 28 November 2014 (UTC)
 * If humidity is low, so not much water condenses and freezes on the glass, then it is possible to look like this. It is either glass or quartz glas, because polymer and liquid oxygen together is bad idea. It is not double walled, so it is probably regular beaker glas and after the initial cooling stage (violent boiling) the boiling would be slow and the ice crystals were probably wiped away before the picture was taken. More interesting are the liquid nitrogen YouTube clips of man pouring it to his bare hands. Gergo vassilev (talk) 16:00, 28 November 2014 (UTC)
 * Leidenfrost effect/film boiling insulates both ways:) LN2 onto your hand is no problem, especially if your hand is not cupped deeply. Dipping your hand into it is more interesting. Or an ALS LN2 Bucket Challenge. Dipping your hand into molten lead is even more-so. Lest you think I'm making all this up, those last two are cited in the article I linked. DMacks (talk) 18:47, 28 November 2014 (UTC)
 * Ooooh. I just realized that this photo was taken at Eielson Air Force Base in Alaska, where temperatures can drop to 50 below.  Even if the room temperature is comfortable, the amount of humidity present might still be less than anything I am used to, so yes, there could be an unexpected lack of water vapor condensation!  So yes, it might be plain glass after all... though I'd still think someone might worry about what might get wet with liquid oxygen and explode if it breaks. Wnt (talk) 19:05, 28 November 2014 (UTC)