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Technology / Re: How much abuse can a domestic microwave oven tolerate?

« on: 29/01/2024 17:59:41 »

Most common thing is that the reflected energy in the cavity will, due to the turntable, eventually return energy back into the probe end of the magnetron, leading to a high voltage flashover between the metal cap on the end and the waveguide cavity. This results in a burst of ionised air, and a dead short at RF of the magnetron output, with a loud bang. This RF short, with the stored energy in the magnetron cavities, and the capacitor between cathode and anode (the main capacitor outside has little effect, RF wise it is at the end of a really long RF trap by the cathode) results in the internal copper strapping inside the magnetron being subjected to very high currents, as the circulating energy is rapidly dumped via the output short, and those straps overheat and burn open circuit. This then results in the magnetron no longer being able to reach full power, even though it does draw the same current in operation, but the damaged cavities, like a whistle that has had the casing broken, will no longer oscillate with the circular flow of electrons from the hot cathode to anode.

You open the magnetron, and you will, after you saw through the end seal by the RF side, that there will be melted copper rings that strap the cavities together, so all cavities contribute to the RF output, even though only one has the output coupling loop in it.

https://upload.wikimedia.org/wikipedia/commons/1/16/Magnetron2.jpg

https://upload.wikimedia.org/wikipedia/commons/thumb/b/b0/Magnetron_section_transverse_to_axis.JPG/800px-Magnetron_section_transverse_to_axis.JPG

This shows the rings clearly, you will find them melted with the magnetrons that have had arcing over. the heavy copper strap that is leading up is the output loop, coupling off of the one cavity, and the rings around the cathode (silver in the middle) are the straps that connect the cavities, and also act to reduce harmonics as they strap alternate cavities together so that they will all resonate at the same 2.4GHz frequency.

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Radio Show & Podcast Feedback / Food waste, from The Naked Scientists 2 August 2022

« on: 05/08/2022 17:18:29 »

Listening to this Tuesday's The Naked Scientist, on Friday 5 August, and am certainly doing my little bit to combat food waste. Just got in from the "last day" sale at my local branch of a large retailer, where I picked up a good chunk of vegetables in prepack, that are on the "Sell By" date. Very much reduced in price, and thus about to be tossed, but still good to eat for a while more.

So the mushrooms, chopped cabbage and stir fry vegetables are all in the freezer, because they do freeze well, and I can eat them later on, and use in stew or stir fry, with almost no difference in taste. The fridge is also full now, a pocket of oranges, grapefruit, onions, peppers, tomatoes and eggs, all of them reduced in price, from full price to 25p per pack. Well worth it, the fresh stuff will be used this weekend and during the week, the eggs are going to last a week or more in the fridge, and do taste nicer than the regular ones. Doing the best I can to eat healthier, so lots of vegetables, chicken and such, and cutting down on junk foods. Total was around 15 pounds, including some other items that I have been watching drop in price, so got some of them as well.

I decided not to give in to my sweet tooth, so did not buy any of those yummy cakes and puddings on sale as well. Will have to eat a grapefruit or orange instead, which I do really like, though the grapefruit outside is not showing any signs of bearing fruit this year, it makes sour grapefruit, which are really nice and tart. Even saved more money, walking to the shop and back.

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Technology / Re: What's the different between CR2 and RCR2 batteries?

« on: 21/07/2022 06:21:21 »

No, they just are marked that because they meet the dimensions set out by the IEC for the battery, which has a set of dimensions with a tolerance, and a very broad range of capacities, though they should meet the "typical" capacity spec, but that is not guaranteed.

4

Chemistry / Re: Why does austenitic stainless steel fail to burn?

« on: 10/07/2022 13:48:33 »

Plasma cutter works on all grades of stainless steels, using the available oxygen in the compressed air, and the electric arc, to cut it with ease. Lot faster than the grinder, and cheaper to run than the gas axe as well, just needs a lot of compressed air, and power for the arc. even works on other metals, including things like titanium and aluminium, and even copper, though the cut and finish can vary, it depends a lot on flow rates, cutting speeds and arc power.  Laser cutting simply provides the energy input using a laser of some form, generally IR to provide the energy to melt the material, and the air jet does the rest.

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Question of the Week / Re: QotW - 22.07.04 - Why can't I open a fridge door immediately after closing it?

« on: 04/07/2022 21:04:06 »

Normally when you open the door, all the cold air inside the fridge flows out, and then is replaced by warm air. Then when you close the door the warm air now has to radiate heat to the cold interior, and in doing so cools down and becomes slightly smaller in volume, needing to be equalised in pressure by outside air that slowly bleeds in via the vent tube for the condensate water. Thus the inside pressure is lower, and the force on the door from the difference makes it harder to open till the pressure inside equalises. For chest freezers the same, just there is a small vent instead in the seal, lots of small holes punched in the seal at places.

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Technology / Re: What Question Could You Ask To Determine Sentience Of An AI ?

« on: 01/07/2022 18:01:51 »

Today I encountered an x-ray machine exhibiting boredom and possible suicidal ideation. Halfway through a clinical exposure, it decided it had had enough and switched off. Rebooted, it cut off even earlier each time. Clearly fed up with studying human anatomy. Next week I'll pack a dead rat in my toolbox and see if a change of subject might perk it up a bit.

I think you will find out that the tube has decided that it is dejected, and is no longer interested in emitting electrons any more, and the high voltage generator is also tired of being dunked in oil as well. It wants a head transplant.

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Technology / Re: Are solar panels worthwhile?

« on: 18/04/2022 10:52:29 »

Depends on what you use to generate the power. Coal is cheap, and common in China, where 90% plus of all solar panels are made, though hydropower is also a good part of the energy mix there, so the energy mix is somewhat different. The advantage of solar is that they can last 25 years plus, and thus will recoup the energy in production, and are also recyclable with relatively little extra effort, as now you have the components easy to separate, if somewhat labour intensive, but still worth the energy 25 to 50 years down the line.

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General Science / Re: Is there a safer alternative to electroconvulsive therapy for erasing memory?

« on: 16/11/2017 17:10:03 »

As far as I know memory is not stored as a particular location, or a specific neuron set, but more like a hologram, with the memory being more created by the linkages formed between neurons, and then being kept by being refreshed  and with new memories creating additional pathways.

Thus I see that your method, while being able to erase particular memories, will also have a very unwanted side effect of also erasing or modifying the rest of the memories stored in this region, and thus will prove less than ideal in action. The action of thinking of a bad memory can also have some side effects, as association will also cause additional recollections which might be desired, and the erasure will more than likely destroy or modify them as well.

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Geology, Palaeontology & Archaeology / Re: Which rocks contain metallic flakes resembling silver and gold?

« on: 26/08/2017 09:19:12 »

Lot of granites do have quite high gold concentrations inside, but are not at all economical to mine because they are so hard, and the energy required to grind them to powder to enable the gold to be extracted is more than the value of the gold. Thus most gold comes from softer dolerites where it is easier to crush them, and we do not really mine seawater even though it has a similar gold content, as concentrating the gold is hard in that case. However a lot of the flakes in rocks are also various forms of mica, which can look like gold and silver flakes.

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General Science / Re: Why are builders using small bricks?

« on: 06/08/2017 05:04:36 »

A smaller brick is a brick that is cheaper to manufacture, as you have to fire it for a shorter time to vitrify the clay all the way through ( or at least deep enough into the clay to be usable), as well as having a better packing in the kiln used to fire them, along with less wasted brick after firing. Larger bricks require a much longer fire, as you have a fixed temperature in the kiln, and the brick is slow to vitrify, this is a fixed rate that it progresses into the surface, and the smaller brick has a larger surface to volume ratio.

As bricks tend ( at least these days ) to be delivered palletised, as opposed to the old way of bulk bricks being delivered in a tipper truck, they are fired shorter time, and are thus weaker, which is actually an advantage as there is built in crack stopping and expansion in the brick failing before the mortar joins, stopping any cracks from propagating all the way through the wall.

Century old bricks were fired almost to be fully glass, so they would survive handling, but they then are both very likely to be somewhat distorted, and also the mortar join will be the weak spot. Hard fired brick does have a use as high strength wall, but in a regular house this is not needed with the building code being designed around a massive safety factor on the much weaker brick allowed and the lowest allowed mortar strength. The standard double wall construction method with brick is much stronger than any load the house will ever experience.

Now, if you are building with hollow concrete block you have a much weaker wall, simply due to the block itself being a very weak item, and with not much surface area to act as a mortar joint between them as well, meaning that wall had a much lower load rating despite being a much larger block.

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Just Chat! / Re: Your aged care nurse?

« on: 01/07/2017 11:59:52 »

I want kind, caring and able to listen, along with patience, ability to have meaningful conversation no matter how long or how much you ramble on, and most importantly of all without any qualms about changing adult nappies, me drooling and burping all over.

She works down the road at the frail care that looked after Mom when she was terminal.

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General Science / Re: What materials can eddy currents sort?

« on: 29/04/2017 10:11:51 »

You can use it to sort out things like copper and aluminium from a waste stream after you have pulled out the ferrous objects, as the eddy current separation will take these common metal types out as the rest of the stream follows a gravity fall, you can use this to change them to side chutes for separation. Then you can further sort them using a further eddy current sorter as they do have different conductivity, so you can increase the concentration of the metals in the streams.

After that you either smelt them again and accept the impurity level increase, or do more refining and get a pure metal back. For copper that is electrolytic refining, and for aluminium it is just resmelting in an inert atmosphere and skim off the impurities off the surface.

Of course you do need a waste stream high in metal, and all preferably shredded to a uniform size so particles are mostly purer metal. The scrap stream out is going to be mostly plastic and glass, so you can burn it in an incinerator to get power, though you will need a high temperature incinerator and precipitatorsand filters to get the more toxic gases, like dioxins, to either decompose or get filtered out of the hot gas.

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That CAN'T be true! / Re: Could satellite surveillance be used to follow an individual?

« on: 27/04/2017 07:36:58 »

A mirror that could image a person on the street clearly, would be around 10 Hubble mirror sized, and would have to be lifted to a geosynchronous orbit. You would need a few of them to track individuals, and your tracking would only be possible on absolutely clear, windless and cloudless days or, with a much larger mirror, in IR at night, with the same requirements for wind, cloud and added to this low humidity.

Active tracking would require megawatts of power, and that means either a good few square kilometers of solar panels, and a few tons of battery as well for the storage of power to keep it operating during orbital eclipses, or a megawatt of nuclear reactor, at around 30 tons mass for the core alone, without any shielding.

As current rocket technology can only get around 10 tons to GEO, rocket including payload, this would almost never be done. There are cheaper alternatives, I would estimate you would need around 200 of the cheapest 55 million dollar launches, per satellite, to just assemble it in LEO and then slowly, over around a year, move it up to GEO and put it into operation using the power plant to drive Xenon thrusters to provide enough energy to get up to that slow orbit.

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Plant Sciences, Zoology & Evolution / Re: Is it true puppies get the best out of both parent's genes?

« on: 22/04/2017 15:47:12 »

Can agree to the pavement special being the best, generally the sortaterrier is a good indoor dog, and the sortador is a good yard dog. They are sort of like a terrior and a labrador in each case, just they are close to a generalised nondescript brown mostly dog. Best you can get.

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General Science / Re: Should we ban pit bulls?

« on: 17/04/2017 07:50:02 »

They used to exist here, but it was finally realised that so few actually paid for them, plus the number of council employees that were there to enforce it, was costing many thousands of times more than the money coming in, plus it did not serve a purpose at all, so it was finally removed. There only exists a raft of laws about dog and cat inoculations and kerb laws, most of which typically are never enforced.

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That CAN'T be true! / Re: Does an empty hard drive or memory card weigh less than a full one?

« on: 16/04/2017 14:32:56 »

Hard drives might lose mass due to outgassing of the volatile components of the actual materials used to make them, but this is going to happen wether or not data is stored on there, it is strictly a function of temperature, time and if the parts are spinning for the lubricants. The actual data itself will not change in mass much, easily a few orders of magnitude below the changes caused by adsorbtion of atmospheric water into the material that make it up, like the printed circuit boards, the epoxy packages and even the paint film. Even the aluminium casing will have changes in mass due to hydrogen from casting diffusing out of it. and for the newer helium filled drives this will also diffuse through the seemingly solid case in any case.

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General Science / Re: How does the ISS detect the approach of a foreign object?

« on: 14/04/2017 20:26:51 »

They rely on the US satellite tracking system, that uses a few large ground based radar units to get information on all the objects over 10cm in diameter in orbit, which is stored as a time, position and speed, so you can calculate position of an object within a block of space with a fair degree of accuracy. This then is used as a computer simulation of objects that approach within 10km of the ISS, and these objects that are predicted to approach within these limits are then considered for further orbital path determination, so that they can get a better accuracy of the orbit.

These predictions are not too accurate long term, but are good for around 24 hours as a rough test if something will be in the same place as the ISS at the same time, so that they can plan if it will miss, will come close ( the accuracy is not that good to say it will hit) and thus they either will move the station or simply put the crew in safe confinement in the Soyuz capsules in case it hits.

In general there are multiple hits per orbit, mostly small flakes of paint from older missions, tiny things that were released in use as the plastics in spacecraft sunshields disintegrate and other stuff. Speed and relative energy of these particles varies from almost zero for stuff in the same orbit and for stuff shed from the ISS and the supply craft, to almost orbital velocity of 22kps for stuff in orthogonal orbits, which can cause some damage if they hit, but where the actual particle is very small, so only makes a scratch or a tiny dent.

But no, there is no detection on the ISS, just a lot of work on the ground.

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Technology / Re: Is the speed of a hard drive invariant?

« on: 18/03/2016 16:50:25 »

Speed is very specific to the particular drive. While the platters inside are rotating at a constant angular velocity, the modern hard drive tries to record data at a constant data density per unit length of head travel along the track, so the data rate varies as the head moves from inside of the disc, where the lowest block numbers are, to the higher numbered blocks that are at the edge. The data rate further is going to be varied by the encoding applied to the data so it can be recorded reliably, with forward error correction and spectrum spreading data added, along with the encryption if used on the drive, so the rate will be varying around a small value that is compensated for by the buffer built into the drive, typically something like 64k, 128k or 256k, depending on the drive type and desired application.

Then you get variations caused by the drive having to move the heads, and after moving it has to check it is on the correct track, by reading some sector data, then writing when the right sector is below the head, and then reading the next sector data before the write. Also complicating things id the drive remapping bad sectors, so that you can have the next logical block to be read or written not being actually in the next physical sector, but it has been relocated to a spare block ( interspersed through the drive surface during manufacture and hidden, like so many of the internal operations, from the outside, so the drive appears as a perfect drive while in reality it is very unreliable, relying heavily on error detection and correction to get the data back and show it to the outside) so there has to be a head movement and then a few cycles to get the correct track. Tracks are so close together that the only way to get the correct one is to go to the approximate position and then move slowly while reading to get the correct track and then wait for the right block to go under the head.

Reading can be worse, as the drive often has to use error correction to get the data back despite noise, or do multiple reads to reconstruct the data from best guesses from the reads. Too many and it ( secret sauce again) will decide to reallocate the data on the block to a spare track when idle, and mark the block internally as relocated and not usable. Thus a drive which appears as perfect can go from working to unreadable very fast as the spares are used up, and the drive no longer can swap out growing defects.

But to the original question, so long as the data is coming in at a lower rate than the worst write ability the drive can keep up and write it, if it comes in the drive will buffer to the point where the buffer is full, then simply discard some data ( mostly the last lot) and return an error code of it not being able to write the data. Reading the data rate will be set by the drive, requesting faster will simply result in the drive returning as not ready until the buffer is filled with data, and the read will stall.