1

General Science / Re: Air conditioning condensate is thrown away?

« on: 24/02/2021 21:41:25 »

Isn't the idea of condensate that they cool it down just enough to become liquid - so it's warm water, and not so effective at cooling as cold water?

The condensate will be from the room-side evaporator coils, so actually will be cool water, but the temperature of the water doesn't matter a whole bunch, that energy is completely dwarfed by the latent heat of vaporisation.

I'd prefer any airborne bugs to be sent to the drain rather than recycled

It would be on the outside of the dwelling anyway.

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General Science / Re: Air conditioning condensate is thrown away?

« on: 24/02/2021 20:34:29 »

Right, but you can add biocides.

3

General Science / Air conditioning condensate is thrown away?

« on: 24/02/2021 17:15:11 »

Anyone happen to know why air conditioning condensate is usually thrown away?

I would have expected it to be sprayed over the condensing coil, to cool that off, and to evaporate the water away. Surely the lower the delta-t between the condensing coil and the evaporator coil, the better? Even if the air entering the condenser coil is at 100% relative humidity, it wouldn't be after having been heated, so there's always capacity for evaporation.

The physics is that you're spending (quite a bit) energy to condense that water (dehumidifying the room air), but if you evaporate it on the condenser, you get a lot of that heat of vaporization back by reducing the work load on the compressor.

Is the problem rust, or legionnaires disease or what?

Does anyone know?

4

General Science / Re: Is White Chocolate real Chocolate?

« on: 21/01/2021 04:07:35 »

White chocolate is made from cocoa, and hence is a type of chocolate. It contains as much cocoa as most milk chocolates. But it is a very different tasting product because it doesn't contain the cocoa solids. But it is still a creamy melt-in-your mouth type of chocolate because the melting point is below body temperature.

5

General Science / Re: Why is evolution considered a theory?

« on: 09/12/2020 21:36:31 »

Because it's only a theory. Like gravity.

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General Science / Re: Is Mathematics Falsified?

« on: 09/12/2020 21:30:13 »

You have to be really careful taking things to an infinite limit. There's an infinite number of ways to 'go to infinity':

lim_(x -> ∞) nx/x = n

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General Science / Re: Is Mathematics Falsified?

« on: 08/12/2020 00:48:57 »

Dividing an infinite number by infinity gives a result that is undefined; it could be any number at all, so you haven't accounted for the infinite number of other possible solutions to both side of your equation, and that is why you have an impossible result.

8

Physiology & Medicine / Re: How do you remove an excessive amount of petroleum jelly from the skin?

« on: 27/11/2020 18:51:09 »

If it was actual petroleum jelly, then to remove it I would suggest 70% alcohol hand sanitiser. I disagree with BoredChemist, in my experience soap doesn't dissolve petroleum jelly and other mineral oils well at all. Or Swarfega, that would work extremely well.

But there's zero chance that your skin still has petroleum jelly after five years, not only does skin shed, but petroleum jelly wipes off. Maybe if there was some kind of weird contaminant in the petroleum jelly or on your skin at the time it might cause some kind of long-term issue that might give the appearance; but petroleum jelly, if pure, is a very innocuous substance that is soon gone.

9

General Science / Re: Don't drink and dress !!!...(Plastic bottles make clothes !!)

« on: 24/11/2020 17:15:27 »

Loads. No pun intended. Very roughly a couple of hundred milligrams which is about a million microfibres per kg of washed clothes, and that's each wash.

https://www.nature.com/articles/s41598-019-43023-x

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General Science / Re: Can tea be made in a microwave successfully?

« on: 21/11/2020 01:44:21 »

I do microwave brandy though.

Do you have a spark proof microwave?

Do they exist?

I do put a glass over the top to help keep the brandy from cooling too rapidly,  if it did ignite it should soon run of oxygen and shouldn't burn for very long because it limits the oxygen.

11

General Science / Re: Can tea be made in a microwave successfully?

« on: 19/11/2020 19:23:56 »

It's all Greek to me...

Next, someone will be wanting to microwave their retsina!

Hot retsina sounds double plus all kinds of bad.

I do microwave brandy though.

12

General Science / Can tea be made in a microwave successfully?

« on: 18/11/2020 04:03:42 »

Everyone knows that microwaving teabags in a mug is a crime against humanity, because it tastes disgusting, but recently Chinese scientists have said no this can be done successfully:

https://www.theguardian.com/food/shortcuts/2020/aug/05/tea-in-a-microwave-new-research-says-it-could-be-the-perfect-cuppa

The paper is here:

https://www.sciencedaily.com/releases/2020/08/200804111516.htm

Basically, they put a carefully designed metal lid on the glass which reflects the microwaves away from the top of the glass. Turns out that a cup full of water normally gets hotter in a microwave at the top than the bottom, but by shielding the top, the bottom gets the heating, and convection currents then even out the temperature.

But I figured that a suitable 'metal lid' can be trivially formed out of aluminium foil, which is traditionally, safely used in microwave cooking anyway, so I thought I'd try it out.

First I microwaved a 200g mug of water with a ceramic plate on top for a couple of minutes, and measured the temperature with an electronic meat thermometer probe. Turned out it was 95C at the top and 84C a the bottom. Bearing in mind the optimum temperature for tea depends on the type of tea, but is generally a much narrower range than that, these figures are disastrous for good tea!

(For optimum brewing temperatures see:
https://www.thespruceeats.com/tea-brewing-times-and-temperatures-1328730)

So then I fashioned a crude cover for the mug out of foil. I had to leave a gap for the handle but I figured it probably didn't matter that much, but otherwise it covered the top and went about 1/2 of the way down the side of the mug to reflect away microwaves from the top, so that the convection currents would instead mix everything up rather than stratifying. I then placed the ceramic lid on top to keep everything the same as much as possible and microwaved for the same time.

The results were: 77C at the top and 73C at the bottom.

Notably these figures were rather lower but also significantly closer together than the unshielded mug. The shielded mug clearly was heating somewhat slower.

To make up for the slower heating I microwaved for another couple of bursts and found:

91C at the top and 87C at the bottom. Again, much more consistent temperatures with the shielded mug.

Anyway, thought it was interesting. If you want to try this, the number of ways you could damage yourself or your microwave is surprisingly long. Among other things avoid superheating water at all costs- it's perfectly possible to get water above 100C in a microwave and then it can suddenly boil and scald you really, really badly. Oh yeah, and avoid getting the foil wrinkled or too close to the walls, itself or anything else metal to avoid arcing.

I also very successfully made tea this way, by just sticking the tea bags in the mug, covering it, and running it. Tasted consistently good.

13

Cells, Microbes & Viruses / Re: Are 85 Covid-19 deaths ON AVERAGE a day per 60 milllion a reason to PANIC?

« on: 13/03/2020 23:45:12 »

Quite a simple one this, yes or no ?

If so should we isolate ourselves from others ? Shun mass gatherings, stop trading ?

No, but the fact that about 100% of the world population is susceptible, and further that it's killing very roughly 1% of those it infects (so it could be expected to kill very roughly 70 million people, comparable or higher than the 1918 Spanish Flu) is.

14

COVID-19 / Re: What is the natural history of Covid-19?

« on: 13/03/2020 21:23:29 »

Unfortunately, it appears that an infected person can spread the virus before they show symptoms, which means the disease will be harder to control.

Not necessarily. If if's (say) one in a thousand, you may be able to stay ahead of it enough with quarantine measures. Like a thousand people have it, and you quarantine them, but one escapes quarantine because they didn't have any symptoms. Oh dear. But then you find and quarantine the few dozen that caught it off them, and then probably nobody catches it off them, so the number of people infected decreases to zero.

15

The Environment / Re: Are average wind speeds increasing with climate change?

« on: 18/02/2020 22:07:20 »

They should be increasing slightly because the equator is warming, but the poles (until they melt entirely) are being kept cool by ice at roughly constant surface temperature, so there's a greater temperature difference which should drive faster winds.

16

Physics, Astronomy & Cosmology / Re: Do rockets work due to conservation of momentum theory or imbalanced forces?

« on: 06/02/2020 04:36:50 »

Yes but the imbalanced force is created by the gas pushing on the front of the rocket. It is because the exiting gas is not pushing on the rocket is what causes the imbalanced force.

Of course there are no imbalanced forces in physics. That's Newton's Third Law. You may have heard of it. The force that pushes on the rocket (from the gas pressure times the cross-sectional area it acts over) causes an equal and opposite force on the exhaust which accelerates the exhaust in the opposite direction; and so the rate of change of the momentum of the rocket is always equal to the rate of change in momentum of the exhaust.

17

Technology / Re: Could regenerative braking help with air pollution from car brakes?

« on: 31/01/2020 22:56:45 »

Brake dust apparently makes up 20% of pm2.5 pollution:

https://www.dailymail.co.uk/health/article-7864855/Metal-particles-vehicle-brake-dust-damage-immune-diesel-pollution.html

Certainly brakes on vehicles with regenerative braking tend to last many fold longer, so you would expect a lot less pollution to be released.

18

Technology / Re: Would it be possible to 3D print housing using waste plastic?

« on: 31/01/2020 22:41:10 »

Tarmac pretty much occupies that space just fine. Plastic roads sound to me like a terrible idea- they would wear and all the ground-up plastic will end up getting washed out to sea. Building plastic into a permanent structure is at least the opposite of single-use plastic.

19

Technology / Re: Would it be possible to 3D print housing using waste plastic?

« on: 31/01/2020 00:19:32 »

So far as I can tell, the compressive strength is similarish to normal concrete, but it has one key advantage- it actually has some tensile strength without further reinforcement, so it's quite robust and not brittle, and the other obvious advantage is that unlike normal concrete it sets off very quickly.

There were some caveats I found, you possibly can't just use any thermoplastic, it has to actually stick the aggregate together, so it probably can't be (for example) polyethylene- polyethylene has a really slippery, "low energy" surface. IRC polyamide worked, those 'glue sticks' work really well for obvious reasons. I tried polycaprolactone and that worked OK because it gets sticky when hot. That was fun because you can mold it with your hands.

Although it's probably significantly more expensive than normal concrete, you probably don't need as much of it because you can put it right where you need it, and you can corrugate it and print it with spaces in the middle.

The real problem is whether there's a killer app for it. I mean you could easily print domes and things with it. Is that something people need though?

Oh yeah, and UV is an issue. Mixing in a tonne of carbon black and painting it would help a lot though.

Edit: graded aggregate is a very, very good idea, but perhaps not in the way you'd initially think- it minimises the amount of plastic you need, because you just have to fill in the gaps between the bigger aggregate, but with graded aggregate most of that is filled with smaller aggregate. Plastic is actually one of the more expensive materials in constructions so anything you can do to reduce it is super helpful.

20

Technology / Re: Would it be possible to 3D print housing using waste plastic?

« on: 29/01/2020 02:21:01 »

I mixed up some thermoplastic polymer concrete a year or two ago. To make it I found all you have to do is just heat a thermoplastic, stir in some sharp sand and then let it cool. I tried a few different polymers and they seemed to all more or less work. It seemed to be potentially 3D printable, although it looked like it was going to be quite abrasive on nozzles. Polymer concretes are actually quite strong.

If the waste plastic is cheaper than portland cement, then it could be quite good; normally plastic is relatively expensive. What kind of cost per kg can you get?

The upsides are that polymer concrete is usually waterproof, but the downsides of concrete is that it's a good thermal conductor; in most cases you want poor thermal conductors for structures, and the flammability is an issue, but with the right additives or polymers might not be an issue.