Wikipedia:Reference desk/Archives/Science/2014 March 18

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March 18

The meaning of SMAC

What is the meaning of SMAC when it occurs in a list of blood test. (It's a name of blood test) 194.114.146.227 (talk) 06:39, 18 March 2014 (UTC)

Sequential Multi-Analysis - Computer, a system of automated multiple blood tests, a little more information at Comprehensive metabolic panel. Richard Avery (talk) 07:23, 18 March 2014 (UTC)

I bet all the people here who read the title immediately thought about Sid Meir’s Alpha Centauri Diwakark86 (talk) 09:35, 18 March 2014 (UTC)

Main Battle Tank

When I read the article of AMX-30 tank , I was shocked about its armor , what an 80 mm armor can sustain ? a bullet ? Tank Designer (talk) 10:33, 18 March 2014 (UTC) Maybe the French designers were very concerned about the speed of the tank but does that worth building a tank which can′t sustain anything but bullets ? — Preceding unsigned comment added by Tank Designer (talk • contribs) 10:39, 18 March 2014 (UTC)

After WWII, tank design philosophy shifted from highly armored heavy tanks to more maneuverable and versatile "main battle tanks." The somewhat newly developed high explosive anti-tank (HEAT) rounds could penetrate enough to negate almost all armor, making heavy tanks virtually useless overnight. So, France and other countries decided to go with the lightly armored, fast MBTs to provide protection. Justin15w (talk) 14:40, 18 March 2014 (UTC)

Thank you , I was thinking about this point but I was uncertain about it . however your armour is strong . you will find a missile that will penetrate it Tank Designer (talk) 14:55, 18 March 2014 (UTC)

You're welcome. Nowadays new armor technology such as composite armor, ceramic, etc. provide much more protection than the rolled homogeneous armor used back then. So the MBT has shifted back towards a highly armored (but still maneuverable) MBT. Many countries have switched from HEAT to kinetic energy penetrator rounds so as to better defeat new armor technologies. Justin15w (talk) 15:13, 18 March 2014 (UTC)

Also kinetic energy penetrators can′t be defeated by hard kill protection systems . Tank Designer (talk) 15:22, 18 March 2014 (UTC) French Tank designers were determined to have the fastest tank in the world even after they used composite armour technology in Leclerc Tank Designer (talk) 15:31, 18 March 2014 (UTC)

An armor thickness of 80mm RHAe will stop almost all non-armor-piercing weapons: bullets, antipersonnel shells, near-misses from bombs, and the like. --Carnildo (talk) 01:58, 19 March 2014 (UTC)

Sucrose solution freezing

When you add some salt or sugar to the water in order to lower its freezing point does it make the resulting ice any less firm? 195.94.247.195 (talk) 10:58, 18 March 2014 (UTC)

Sea ice has an intragranular porous substructure that consists of submillimeter diameter air bubbles and brine pockets, totaling 4-5 vol.%, arrayed in a plate-like manner parallel to basal planes. [1] Brine inclusions 3 vol.%, typical of first-year sea ice, lower the flow stress by a factor of two [Op. cit.] and increase the quasisteady-state creep rate by about an order of magnitude. 84.209.89.214 (talk) 12:53, 18 March 2014 (UTC)

However, features of sea ice are not necessarily shared by all salt-water ice. For example, air bubbles may be due to wave action and bacteria and plants below the ice releasing gases. StuRat (talk) 17:15, 18 March 2014 (UTC)

Anyone equipped with a home freezer with an ice cube tray can try an experiment to answer the question. 84.209.89.214 (talk) 10:06, 19 March 2014 (UTC)

How do they encase cheese in wax without bad stuff from the wax getting into the cheese?

I saw a wheel of cheese encased like a pill in wax. It looked as though it had been dipped into liquid wax because there was no seam at all. How is this (the encasing) done? I'd think liquid wax, before it had solidified, could seep into the cheese. 75.75.42.89 (talk) 12:00, 18 March 2014 (UTC)

Dutch Edam cheese is protected by a coat of food-grade paraffin wax E905 which won't hurt you, but whose colour tells a lot about the cheese.

The typical red wax coating denotes a young Edam produced for export. In Holland, the cheese is sold with a yellow wax coating. A black wax coating shows that the cheese has been matured for at least 17 weeks and therefore has a stronger flavour. Other colours of wax coating can be found, for example, a green coating generally indicates that the cheese contains added ingredients such as herbs or garlic. 84.209.89.214 (talk) 13:10, 18 March 2014 (UTC)

So is it dipped in liquid food-grade paraffin wax or are semisolid sheets wrapped around it with the seams smoothed out or something else? 20.137.2.50 (talk) 13:37, 18 March 2014 (UTC)

It looks like one can dip one's cheese into the liquid wax and then hold it until the dipped half cools and dries enough to hold it while dipping the other half, building up the wax little by little until you have a seamless wax-encapsulated cheese. 75.75.42.89 (talk) 21:12, 18 March 2014 (UTC)

The "food grade" part is key. You could eat the wax, if you wanted, although it would be quite tasteless. Lip balm also often contains such wax, and you obviously will end up consuming some of that, too. StuRat (talk) 13:23, 18 March 2014 (UTC)

Are there any harmful waxes? I mean, relative to the risk of cheese? They seem a pretty tame group. Food colouring, that's a different story. InedibleHulk (talk) 14:46, 19 March 2014 (UTC)

I'm not aware of any harmful waxes, unless there's a stinger in your beeswax, but they could always add some toxic chemical to the wax, like food colorings or preservatives. StuRat (talk) 19:59, 19 March 2014 (UTC)

Paraffin wax is also a rocket fuel. 75.75.42.89 (talk) 23:18, 19 March 2014 (UTC)

Isn't that a piscemutagen, causing you to grow a pair of fins ? :-) StuRat (talk) 17:03, 20 March 2014 (UTC)

Even there, it's the additives that give it the zip. But you're right. Molten wax isn't good for people. Especially if you're chasing it with peppers. InedibleHulk (talk) 17:34, 20 March 2014 (UTC)

The wax seems to separate easily from the cheese when it is opened. I expect someone here knows the process, but, if not, I can ask at the Hawes Creamery (near my home) where this is done on small Wensleydale cheeses. Dbfirs 12:19, 18 March 2014 (UTC)

Not even Wensleydale? shoy (reactions) 11:59, 19 March 2014 (UTC)

Waxes to cheese are applied by either dipping or brushing. Dipping occurs more frequently in industrial processes, while brushing occurs more often in home or small factories. It's just faster to dip. Justin15w (talk) 16:11, 18 March 2014 (UTC)

Most Dutch cheeses available in the UK are wax-coated. When I lived in Germany for a few months in 1984, my local supermarket sold English Cheddar, that came in a thick red wax rind (rather thicker than I am used to on Edam). I have never seen Cheddar so wrapped in the UK, and presumed that this was done specifically for export. --ColinFine (talk) 17:46, 18 March 2014 (UTC)

Colin, try and find Snowdonia cheeses: Cheddar cheeses in wax. They are yummy! --TammyMoet (talk) 20:09, 18 March 2014 (UTC)

And there's Black Bob Cheddar and a number of others, [2], [3], [4] and [5]; although I suspect this is a recent innovation and agree that this is not the way one normally buys cheese in Blighty. Curiously, black seems to be the colour of choice, perhaps to avoid confusion with the Dutch stuff. Alansplodge (talk) 20:56, 20 March 2014 (UTC)

Shelf life of electronics products?

Do modern electronics products have a "shelf life"? By that I mean a time after which a significant percentage (say 10%) of the units of a product will not work properly, even if they have been stored properly and have not been operated. Batteries and electrolytic caps are two things that degrade over time, but what are other ways electronic parts will fail because of old age? Do modern electronics products have a design shelf life? Assuming that they do, what are the design/observed shelf lives of common electronics products like smartphones, flat panel monitors, Wi-Fi routers, and battery-less(?) dongles like the Chromecast? --173.49.82.36 (talk) 12:24, 18 March 2014 (UTC)

Yes, and it's sometimes intentional. See planned obsolescence#Lifespan-limiting design. InedibleHulk (talk) 13:20, 18 March 2014 (UTC)

I've noticed that LCD TVs and monitors don't last nearly as long as CRTs did (they could last for decades). StuRat (talk) 13:48, 18 March 2014 (UTC)

I used to throw heavy (for a kid) rocks at those in the dump, and they'd often bounce right back. A Wii controller wouldn't stand a chance. My LCD TV is still flawless since mid-2007. Time will tell. InedibleHulk (talk) 14:41, 18 March 2014 (UTC)

Often part will have a designed life. A switch may be good for 10,000 operations, flash memory is typically good for 100,000 program-erase cycles. Anything which moves, connectors, pots, motor brushes, will have a typical life which you can find in datasheets. They will also have designed limits for operating and storage conditions. Often 0°-70°C. Keeping/using items outside these ranges will shorten life. Some products come in normal and military grades, the latter generally have much wider ranges and cost much more. Other effects might be plastics degrading, and cosmic rays causing problems with memory.--Salix alba (talk): 15:42, 18 March 2014 (UTC)

• We're mixing up terminology here. The term "shelf life" is usually used for food, and it refers to the time that a product remains good if it is stored without being used. That's what the OP is talking about here. It doesn't really have anything to do with planned obsolescence. None of the responses above actually address the question that was asked. (I have no idea what the answer is.) Looie496 (talk) 15:47, 18 March 2014 (UTC)

Yes indeed. Think of a car. Rarely, someone will buy a new car, park it in a nice dry garage...and then drop dead. 30, 40 or 50 years later, some distant relative will open the garage, and to their surprise, find the car. (Often called a "barn find" in the classic car world). With nothing more than some new tires, a fully charged battery, a fresh tank of gas and maybe an oil change, it'll start right up and probably drive pretty well. On the other hand, if you buy a new car and drive it to work and back every day for 30 years - it'll be a useless pile of scrap by the end of that time. That's the difference between "shelf life" and "operational lifespan". For some things (like food and drugs) only the "shelf life" matters - and for other things (like cars), the shelf life is relatively unimportant compared to the operational lifespan. For electronics, there are a few factors that can result in a shelf-life limit - but mostly the damage is done by operating the equipment - so "operational lifespan" is mostly what matters. But for most electronics, the damage is done by repeated heating and cooling cycles brought about by having the machine turned on and off repeatedly. In many cases, leaving the machine turned on 100% of the time will greatly extend it's life. So talking about an amount of time is difficult without also specifying the nature of the usage and the way the item is stored. SteveBaker (talk) 19:01, 18 March 2014 (UTC)

Aye. I'm usually a better reader than this. InedibleHulk (talk) 20:26, 19 March 2014 (UTC)

I imagine the growth of tin whiskers might be one limiting factor on the unpowered lifespan of electronics. It's possible that the anti-lead provisions in recent regulations may exacerbate this particular problem, if manufacturers' other mitigations are not as effective as adding lead.

See also this for more on the aging of transistors themselves, although I'm not sure how many of these effects might be a problem in the absence of voltage and current. I'd also imagine that things like liquid crystals would be perishable over the long term. -- The Anome (talk) 18:50, 18 March 2014 (UTC)

• I own three Sony VHS-DVD combo dubber/burners. The first one cost me $450 and lasted for a year. The DVD tray driver belt has always been an issue. I ended up buying two new ones, for $150 and $75 dollars for my two residences. All three had developed faults within a year, inability to dub, to load a DVD. I kept two at one residence, one for recording and dubbing, the other for playback. The third required turning on and off to get it to load a DVD and then would freeze up when it got warm after an hour or so. When th machines worked perfectly they were great, since they had internal tuners. So I could run the cable through them, then into the TV with its own tuner, and be recording a movie on DVD on one channel on the combo set and be watching a separate channel on the TV.

Then the cable companies at both locations switched to wonderful new digital service which provided no better picture but did make the internal tuners useless. This is not to mention the fact that none of these players could play out-of-region disks. It's quite obvious that obsoletion and intentional sabotage like DVD region codes are an intentional strategy, abetted by the government. I now simply watch whatever I like on sites like tubeplus with no qualms and no cable. μηδείς (talk) 20:17, 18 March 2014 (UTC)

I think that counts as somewhere between forced obsolescence and bit rot, where interface changes render equipment useless, even when that equipment remains fully functional. -- The Anome (talk) 08:11, 19 March 2014 (UTC)

Electronic products don't normally specify a difference between an unused vs. used lifetimes. There are lots of things that can affect some electronics whether it's used or not but use often accelerates these processes. Light bulbs, for example, don't have an infinite shelf life but the shelf life is longer than useful life. --DHeyward (talk) 06:48, 19 March 2014 (UTC)

We actually have an entire article on failure modes of electronics. Most of them are related to failure in use, but some could happen on the shelf:

• Corrosion damaging the electronic packaging or PCB, which can be accelerated by high humidity or high ambient temperature
• Tin whisker formation
• Drying out or contamination in electrolytic capacitors.

I've also seen a few mentions of degradation or failure due to diffusion at the contact points of differently-doped regions or metal-semiconductor contacts. But the concerns seem to be mostly in a couple specific types of transistors, and is probably extremely slow in most devices at room temperature. What the actual shelf life would be would likely be strongly dependent on the quality of the manufacturing. Mr.Z-man 15:27, 19 March 2014 (UTC)

Thermal Imaging

does the thermal sight work at day or it only works at night ? why if yes or no ? Tank Designer (talk) 17:28, 18 March 2014 (UTC)

If Thermographic_camera is what you are asking about , then it works at day. It just measures infrared light. Star Lord - 星王 (talk) 18:44, 18 March 2014 (UTC)

Yes, Infrared cameras work just fine during the day. HOWEVER, many people confuse infrared cameras with night vision goggles and/or passive image intensifiers. They most certainly don't work during the day - and may well be damaged if exposed to daylight. SteveBaker (talk) 18:49, 18 March 2014 (UTC)

Thank you Tank Designer (talk) 21:07, 18 March 2014 (UTC)

However, thermal imaging requires a temperature difference between the subject and environment. If you are trying to spot people, and the ambient temperature happens to be body temperature, then they will be harder to spot. Depending on the location and time of the year, those temperatures may be more likely to occur during the day, making a thermal image less useful then. For example, if the enemy is hiding behind a boulder, you might see a thermal glow coming from behind it, on a cool night, but not on a hot day. StuRat (talk) 21:24, 18 March 2014 (UTC)

Thank you very much StuRat that what I was thinking about , the human being temperature is about 36°c -I don′t know how exactly- , and in summer the earth is too hot ,I like those who understand what is behind the words of the question .Tank Designer (talk) 21:58, 18 March 2014 (UTC)

You're quite welcome. StuRat (talk) 02:11, 19 March 2014 (UTC)

This Indian Government page describes the thermal imaging sights fitted to its T-90 tanks. These "can be used to search, detect and identify targets by day and night under normal and adverse conditions" and "can be used to detect and identify targets having temperature difference of up to 2°C." Alansplodge (talk) 18:35, 19 March 2014 (UTC)

I think they mean "with at least" 2°C difference, not "up to". And yes, if you're looking right at the target, this is true, but detecting the subtle glow from behind an object requires more of a difference than that. StuRat (talk) 19:55, 19 March 2014 (UTC)

I thought that was what they probably meant too - a case of Indian Minglish. Alansplodge (talk) 08:51, 20 March 2014 (UTC)

The sensitivity of infrared cameras is very variable though. Some of the modern US military units have precision down to some small fraction of a degree C. The actual sensitivity is typically classified - but one anecdotal story is that during the blockade of Iraq (during the first Iraq war), some of the oil tankers had painted over the Iraqi flags painted on the sides of the ship in an effort to slip through unnoticed. A standard US helicopter's IR camera was able to spot the flags by seeing how the different thicknesses of paint had altered the conductivity of ambient heat into the interior of the ship. Modern military helicopter cameras are said to be able to spot the residual heat from a vehicles tires warming up the ground for many minutes after the vehicle has passed by. It's hard to guess how much sensitivity this requires, but for sure it's vastly better than 2 degC. Of course India's IR camera technology may not be up to modern military standards...but even so, I'd guess that 2 degC was far behind state-of-the-art sensitivity for a tank gunsight. SteveBaker (talk) 19:33, 21 March 2014 (UTC)

I'd expect quite a bit more than 2C in residual heat from where a running vehicle had just sat. And it's not just an issue of the sensitivity of the camera, but the signal-to-noise ratio of the infrared signal itself. Parts of the landscape will be slightly hotter or cooler depending on when they went into shadow during the day, if an animal recently slept, urinated or defecated there, the color of the object, etc. So, picking out the crouching person from a landscape full of all these false positives would be tricky, unless the person is much brighter (hotter). StuRat (talk) 01:32, 22 March 2014 (UTC)

Thank you so muchTank Designer (talk) 19:34, 19 March 2014 (UTC) But we have a new problem my friends , everyone know about thermal camouflage like Russian Nakidka as an example , when you look at the thermal images taken for vehicles covered by that camouflage , you can see how difficult or impossible to discover them .Tank Designer (talk) 20:05, 22 March 2014 (UTC)