Wikipedia:Reference desk/Archives/Science/2014 November 2

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November 2

D5W is considered as isotonic solution?

D5W is dextrose 5% in water, and I saw on this blog that the nurse called it "isotonic". is it right? (this solution is without any salt - electrolytes) 213.57.114.161 (talk) 01:07, 2 November 2014 (UTC)

I don't think Isotonic is defined well enough, in a medical sense, to judge. You can put dextrose in normal saline for intravenous fluids or in just water or you can put lactose in a potassium salt based solution. "Isotonic" is going to depend on renal, liver, and pancreatic function and I suspect any solution administered by IV that doesn't overload the system with either edema or dehydration is going to be considered "isotonic" in a medical sense. None of them match the diversity of salts found in blood so trying to equate sodium to potassium as a salt will be very dependent on the patient. What's neutral for one patient could be lethal for another. --DHeyward (talk) 06:49, 2 November 2014 (UTC)

DC choke

I need to make a 50mH choke to take 3A dc. Any chance I can do it using a gapped iron core? If so, where would I get a core suitable for gapping?--86.176.23.223 (talk) 13:23, 2 November 2014 (UTC)

See Inductor and magnetic core for our relevant articles. Farnell do one that's 50 mH at 2.3 A (here), and other suppliers have similar products. (You can, of course, use two 25 mH inductors in series instead). They all appear to use toroidal cores - if your inductor is for a DC application, saturation, the main reason for gapped cores in AC applications, shouldn't be an issue, as you're not worried about distortion and harmonics. This paper gives some practical information if you want to make your own toroidal inductor. Tevildo (talk) 18:14, 2 November 2014 (UTC)

Thanks but the Farnell choke is a common mode line choke--- it wont take anything like 3 A differentially. I know I need a gapped core but where to buy them?--86.176.23.223 (talk) 18:21, 2 November 2014 (UTC)

Ah, right. Farnell and RS will sell you ferrite cores, but I don't know of any hobbyist suppliers of iron cores. A Google search on "iron transformer core suppliers" will give you plenty of commercial suppliers - some of them might be happy to sell you a single core, but I can't recommend any particular firm. Tevildo (talk) 21:48, 2 November 2014 (UTC)

Used to buy a lot from these two companies. I don't know about today but back the they had very helpful technical department. If they could not supply me with what I was after, they could nearly always point me towards a company that could. So run your need past them. It may be that you can utilize an old wire coat hanger or some laminations from an old transformer. This solution would need access to a vice, hacksaw and some files but might be quicker in the long run. Of course, you will need some way of measuring Henries (or in your case -tiny weeny henries). If you don't have a LCR meter in your kitchen one can always try this: [1].--Aspro (talk) 01:51, 3 November 2014 (UTC)

Heat transfer

If you're trying to solve a heat transfer problem with a layer of conduction and 2 layers of convection, then can you simply add together the Q values after working out Q, the heat transfer, individually from each surface using the conduction and convection equations? Or is it more complicated than that? — Preceding unsigned comment added by 194.66.246.112 (talk) 14:23, 2 November 2014 (UTC)

You have to set the rate of heat transfer through each layer equal to each other, this yields equations for the temperatures at the boundary of each layer. Count Iblis (talk) 17:42, 2 November 2014 (UTC)

that would eliminate heat transfer from the equation though. That's what I want to find. — Preceding unsigned comment added by 194.66.246.26 (talk) 18:06, 2 November 2014 (UTC)

It actually allows you to solve for that, after you eliminate the intermediary temperatures. Count Iblis (talk) 20:16, 2 November 2014 (UTC)

how? I'll give an example below.

Layer 1 - convection Q=1000*0.103*80=8240

Layer 2 - conduction assuming time=1 and thickness=1 Q=35*0.0456*(80-20)=95.76

Layer 3 - convection q=10*0.103*20=20.6

How do I find net heat transfer from this? — Preceding unsigned comment added by 194.66.246.55 (talk) 10:48, 4 November 2014 (UTC)

If you bury a body, how long does it still generate heat?

Does it generate heat during decomposition? — Preceding unsigned comment added by Senteni (talk • contribs) 17:47, 2 November 2014 (UTC)

Strictly speaking, a living thing stops generating heat at the exact moment it stops being alive—more specifically, at the moment the last of its biological processes has ceased functioning. It will, however, continue to emit heat until it reaches thermodynamic equilibrium with the surrounding medium (in your case, dirt, or, if left unburied, the atmosphere surrounding it). The microbial action of decomposition will generate some heat. I have no idea how much, but I have to imagine it is negligible. Evan ^{(talk|contribs)} 18:28, 2 November 2014 (UTC)

Newton's law of cooling may be useful here. --Jayron32 18:34, 2 November 2014 (UTC)

If you bury the cadaver six foot down then it will be in an anaerobic environment. The only bacteria able to live will be the endothermic ones. They require heat and so will not produce any. So no. It will be colder that the surrounding soil. Err. May I ask why you ask?--Aspro (talk) 18:38, 2 November 2014 (UTC)

But then, can you spot a buried body with a infrared camera several days after it was buried? If the bacteria are decomposing it, it will have a different heat-fingerprint. --Senteni (talk) 18:50, 2 November 2014 (UTC)

One needs to be careful about interpreting CIS data. With shallow burials, oxygen might well assist the breakdown of leaking body fluids but the ground has also been disturbed and any organic mater in the soil will start to oxidize in on being exposed to atmospheric oxygen. So is thermal imaging really detecting body-heat or oxidation of freshly disturbed the soil? --Aspro (talk) 19:14, 2 November 2014 (UTC)

There are plenty of anaerobic exothermic reactions. Fermentation and methanogenesis are examples. As discussed at anaerobic digestion, an anaerobic digester can generate about 1000 kWh per ton of dry food waste and is seen as a source of renewable energy. I have no knowledge of what specifically happens to a body in a grave, but there is no reason to assume that an absence of oxygen would necessarily imply cooling. Most reactions useful as energy sources to bacteria are exothermic, and that's true whether the environment is aerobic or anaerobic. Some of the exothermic energy is captured to build large molecules (an endothermic process), but on balance, bacteria are almost always net generators of heat. Dragons flight (talk) 20:53, 2 November 2014 (UTC)

Don't think you actually read and understood the articles you quoted from!--Aspro (talk) 23:44, 2 November 2014 (UTC)

Shrug, if you want to continue being wrong, I can't help you. Aerobic composting is strongly exothermic (and easily gets hots). Anaerobic digesters are typically weakly exothermic, and because they are most efficient well above ambient temperatures (e.g. 30-60 C, depending on design) often require insulation and external heating to operate efficiently in cooler external conditions. Doesn't change the fact that many of the key processes involved are exothermic though. For example, methanogensis from glucose is:

C₆H₁₂O₆ -> 3 CH₄ + 3 CO₂ + 137 kJ / mole glucose

Or glucose fermentation:

C₆H₁₂O₆ -> 2 C₂H₅OH + 2 CO₂ + 74 kJ / mole glucose

Neither of these have reaction energies resembling that for aerobic respiration:

C₆H₁₂O₆ + 6 O₂ -> 6 CO₂ + 6 H₂O + 2546 kJ / mole glucose

However, all these processes are still exothermic. Dragons flight (talk) 00:32, 3 November 2014 (UTC)

• Are we suggesting that coffins are filled with helium? The corpse will generate heat as long as any bacteria are decomposing it, even if they are anaerobic. Laws of thermodynamics. μηδείς (talk) 20:24, 2 November 2014 (UTC)

See my comment above. The anaerobic's do obey the the laws the laws of Laws of thermodynamics because need to convert heat to help power their metabolism. Compost heaps (aerobic) are hot, sewage farm slug (anaerobic ) are cold! No amount of pseudo-scientific waffle is going to reverse that.--Aspro (talk) 23:44, 2 November 2014 (UTC)

• • The laws of thermodynamics do not make endothermic reactions impossible. I tried a general Google search on the exact phrase "endothermic bacteria", but did not find anything that looked like a reliable source. However, a search in Google Scholar for the same phrase produces one hit on a paper, with the excerpt "The existence of (anaerobic) endothermic bacteria, which consume heat rather than produce it, shows that quantification of metabolic rate in terms of heat production has already its limitations." (The paper is "Metabolic acceleration in animal ontogeny: An evolutionary perspective" by S.A.L.M. Kooijman, but as usual for these things, its content is only available for purchase.) Note the parentheses: Koojiman is not saying that all anaerobic consume heat. I know nothing about the ones actually involved in decomposition. --174.88.134.249 (talk) 20:48, 2 November 2014 (UTC)

• • • I am a little confused, User:Aspro. Are you saying that the buildup of lactic acid in muscles cools the body? Are you saying that in a closed system anaerobic bacteria convert heat to usable energy, and the system as a whole cools? I never did study bacteriology, but this seems to contradict everything else I did study in biology and the hard sciences. μηδείς (talk) 01:52, 3 November 2014 (UTC)

One thing that could be measured and converted into red ("thermal vision") or green ("night vision) is electrical resistivity and conductivity. It's something like fire, and changes according to how how fresh a body is (how much liquid is left). In neatly stratified soil, freshly turned graves (or innocent holes) also give off their own distinct pecularity. When looking for long-lost graveyards, this can pick up pooled water in a grave, long after the human liquid is gone. If the lost soul was buried in a metallic casket, piece of cake.

I don't think I have to tell you this, but never dispose of a body in a metal casket unless you're cool with people knowing about it. If you put that much planning and expense into it, you're better off chartering a helicopter in a fake name and dropping it off in a volcano. Then the pilot. Then the helicopter. Then, it's up (or down) to you. InedibleHulk (talk) 00:33, 3 November 2014 (UTC)

Gravehunting is still a science in progress, but they figure it, like all disciplines, has something to do with pigs and magnets. InedibleHulk (talk) 00:36, 3 November 2014 (UTC)

Thermographic cameras are really sensitive, a fraction of a degree difference is plenty to produce an image - and it may be that even when the body is not generating heat and has reached thermal equilibrium with the soil may be detectable.

Consider that there is:

• A difference in thermal properties between soil that's been dug up and replaced and soil that's been compacted for a long time.
• A change in thermal conductivity of a cavity a foot or so beneath the surface causing heat from the sun to be conducted down into the soil at different rates.
• That surface vegetation is growing differently due to changed moisture content from the water and other nutrients from the decomposing body.

You can easily come up with enough changes compared to the surrounding ground to produce a tenth of a degree change that a camera might pick up even a year or two after the body was buried.

I spent many years doing simulation of military FLIR (forward-looking infrared) cameras - and these are quite incredible devices. There is an anecdote about the blockade of oil shipments from Iraq during the first gulf war when the Iraqi's were painting over the Iraqi flags on the bows of their oil tankers in an effort to sneak them past the blockades...but the thermal cameras on Apache helicopters were sufficiently accurate that the difference in the thickness of the paint and the differing properties of the different colors of paint in the painted-over flag was sufficient to allow them to see it quite clearly in the infrared imagery.

When you consider that - you can understand how finding a body in a shallow grave - or the massive disturbance caused by the burial in a deep grave - would be quite visible in the infrared. We're not talking about a ten degree temperature difference, a tenth of a degree might easily be enough.

So even if someone went to a lot of trouble to screen the body, just the digging of the hole might be enough to find it. Burying the body deeply in a recently ploughed or soon-to-be-ploughed field might be the best bet...but even then, I'm kinda skeptical that you could hide it from a careful search.

SteveBaker (talk) 14:27, 3 November 2014 (UTC)

Don't know if Steve would agree with me here, soil is not very heat conductive. Heat from a body would not leave as large a thermal signature on the surface as would the petrochemicals in the soil oxidizing. Some soils have so much that they can spontaneously catch fire when exposed to atmospheric oxygen.. [2]. Even the Romans (that loved Garum), left the vats of fish guts in the sun, because at low temperatures the enzymatic action came to a halt. So the intestines and stomach of the copse is not going to generate much heat either. CSI: Crime Scene Investigation is fiction! They don't let science get in the way of a good story. So don't get your inexactitudes from this trash.--Aspro (talk) 21:16, 3 November 2014 (UTC)

I can't say what secret military capabilities exist, but the case of Eric Frein illustrated that police-grade IR was not meeting expectations when it came to finding a live body. Being able to detect a tenth of a degree difference in soil is all well and good... if you're searching a landscape that doesn't have big differences of temperature naturally. But where would you find one of those?

The thing is that you're not just measuring the temperature at one spot - you have a camera with live video. So if an area 6' long by 4' wide is slightly warmer (or cooler) than the rest of the area, you'll still see a ghostly rectangle showing through the noise caused by small plants, rocks, etc. Sure, there is a lot of noise, but when a strong feature is present, even with variation far less than the noise, you can still see it. People who know how to use IR cameras for this kind of task will be continually fiddling with the gain and offset knobs (like contrast and black-level) to try to pull out the kind of detail they want from the image. If you know you're looking for something that's roughly the size and shape of a shallow grave, then it gets a lot easier. Of course the bad guy could always dismember the body and bury the parts in separate holes a hundred feet apart - and then it would probably be impossible to spot this way...but criminals are mostly idiots - so competence is not always to be assumed here. SteveBaker (talk) 16:57, 5 November 2014 (UTC)

Still, I admit a fondness of disposal schemes that involve toilets or septic tanks. Just as a way to show you really care. :) Wnt (talk) 21:53, 3 November 2014 (UTC)

Toilets deposal? Do you mean in the sense of Aunt Agnis, When she dumps her dead brother’s ashes down the privy and then sits down to pee on them, to show how much she cared for him.[3]. Good film. If so, I'm glad I'm not one of your relatives ;-)--Aspro (talk) 18:46, 4 November 2014 (UTC)

I don't know how far into the IR the modern FLIR cameras go. At very long IR wavelengths, earth can be almost transparent. A a body is composed of mostly water and fatty acids etc. It thus, would have a higher specific heat capacity than dry soil. So a cadaver in a very sallow grave (as opposed to six foot under as I said ) could be warmed by sunlight, so would be cooler by day and warmer by night. At long IR wavelengths you can see a hot water pipes through a 9 inch think brick wall. As murder victims are probably often buried in sallow graves, this technique may well work in 'some' situations. Putrefaction though, is a reduction process, so won't generate heat.--Aspro (talk) 18:37, 4 November 2014 (UTC)

Note that coffins have a fair amount of air in them, in addition to the body, so bodies so buried may undergo aerobic decomposition for some time, giving off heat as a result. If they are embalmed, that should slow the decomposition time dramatically, but the same total amount of heat would still be generated, in a completely sealed system, when it does completely decompose, however long that takes. StuRat (talk) 12:40, 5 November 2014 (UTC)