Failure to recognise when an aerosol isn't a droplet 26

[LittleInch]

FacEngrPE dropped this in an obscure post and it reads very well.

Basically the whole epidemiological world though various dieses and viruses were spread by "droplets" which landed on surfaces and then infected people or were sneezed at you.

And a lot of times they are probably right.

But there was a magic 5 micron cut off between droplets and aerosols. Why? Read on

https://www.wired.com/story/the-tee...t-helped-covid-kill/?utm_source=pocket-newtab

So is this a disaster - Well you tell me.
But it goes to show that just because a lot of people write something, it doesn't mean that they originally got the wrong end of the stick and then the error repeats itself until it becomes fact.



Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[1503-44]

Hah! You think that's dangerous, try SCUBA diving the morning after cleaning your plate in a Mexican restrauant. Now that's what I call internal pressure. You can truely go balistic rising the last 10m. Oh no. Actually that's about the same. The pressure halfs in each case, so yeah. You're right, except for the enormous quantity of beans you get in MX.

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I take no responsibility for any damages or injuries of any kind that may result.

 

[Alistair_Heaton]

Did most of my 900 dives in a dry suit.

Standard was drop one put a bit of gas in it and then ask a mate to crack the zip. Which usually involved getting thier nose close to it when they cracked the first 5 cm open.

 

[TugboatEng]

On the exclusion of droplets below 5um, it is possible that a certain minimum droplet size is required to protect the virus from dessication and UV. In that case it would be correct to exclude droplets below that size.

 

[1503-44]

We just got rid of the 5um. Please don't resubstantiate it.

That explains why there are no dry suits in MX.

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I take no responsibility for any damages or injuries of any kind that may result.

 

[Alistair_Heaton]

well I had a 6 year kid glued to me for 18 hours coughing farting and running a fever. He tested positive. After 2 weeks and 3 nasal raping's the local health mafia gave up on me. I suspect I had it Jan 2020. but i am also O neg which apprently makes a difference.

They want blood plasma off me but that will kill me working for 3 weeks, 500ml of blood only kills me for 3 days so they can have that every 12 weeks.

 

[RedSnake]

I couldn't read the article but I know from before that certain very small virus can float around in the air.
That's why there is less infections from those when there is high humidity, and if you have a air humidifier.
The viruses get caught in the droplets and fall down much quicker to the floor.

Covid-19 have a soft "oil based" outer shell so it can be dissolved with soap and water.
Tugs fecal virus is like a golf ball you need other tuffer stuff to brake it apart, it has everything it needs inside so it can maintain itself longer floating around and do not dry out.

BR A



“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[TugboatEng]

I have read the article now and it's quite a bit different than I thought. A quick and dirty summary is that the sub-5um particles are the ones that can both linger long enough and penetrate our defenses to trigger infection. Ventilation is above all the most important protective action.

The paper that inspired this Wired article can be found here:

https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3829873

 

[Alistair_Heaton]

Covid aside I was reading about fungus spores in houses when designing my place.

They are 2-3 um and I must admit 3 years on we have zero issues with mold in the house with constant air changes with the heat exchanger ventilation.

 

[electricpete]

An interesting concept to think about when considering respiratory droplet transmission is Wells' curve (not exact but gives you some things to think about).

The starting point is to realize that small particles significantly reduce in size by evaportion as they fall. That may not sound intuitive if we think about raindrops from the sky or water droplets from our faucet, but smaller particles lose size by evaporation a lot quicker. Why is that? Intuitively I'd say it is the smaller particle has lower ratio of volume (4/3 * pi*R^3) to surface area (4*pi*R^2) (so the small particle has relatively larger surface area over which to evaporate). That's a little bit imprecise because that ratio is not unitless (has units of length). If I wanted to make it more dimensionally precise, we could multiply the denominator by some assumed-constant volume evaporation rate per surface area per time....then the ratio has units of time and we see the smaller particle has much lower time to evaporate it's entire volume.

Then going with traditional (not exact as we all know from reading op) assumption that particles above a certain size can remain airborn, we have to ask whether a particle will drop below that size between the time it is exhaled and the time it reaches the ground. If yes, then it can stay airborn (even if it were above the minimum assumed required size to stay airborne at the time it was exhaled).

Then we see a potentially critical role of relative humidity. During low realtive humidity the evaporation occurs faster. The lower the relative humidity, the larger the exhaled particle that can concentrate enough to become airborne before they reach the floor. The result in low humidity environment is more aerosolized particles and higher potential concentrations of virus among those aerosolized particles (because only the water leaves when the droplet shrinks, the virus remains).

There are also of course many other effects of relative humidity. Higher relative humidity tends to make the respiratory tract more robust (which again makes higher relative humidity preferable for stopping spread). Higher relative humidity above I think 80% can "kill" the virus faster (which moves things in the other direction). And probably more.

I'm not saying this (particle concentration through evaporation) is the most important thing to be aware of in the complex field of respiratory dynamics (which is arguably better left to complicated computer simulations rather than attempts to visualize things). But it's one thing many people don't think about (I think most people assume respiratory particles remain at the same size they were when they were exhaled)


=====================================
(2B)+(2B)' ?

 

[RedSnake]

I had super problem at work after one day I was coughing the rest of the week and then it went away over the weekend.
The the safety engineer came by and made some measurements and came to the conclusion that the air was to dry 12-17 %RH and there where to much papper and to much dust.
So he sade buy a humidifier, easier said then done, I can buy almost every super technical thing I want at work but a simpel humidifier, was almost impossible I hade to order it from a subcontractor and then pay them.
Anyway the problem disappeared.
While waiting for it to arrive I tried to figur out what the optimal relativ humidity was and found this.

I decided to have it on 45% and it works.

This is also a reason why viruses spread more in the winter then in the summer here in the north, in combination with people being quite a lot inside and closer together.
Some viruses also protect themselves when it's cold buy pulling together(shrinking) and floats around much longer in the air.
When coming inside the body they go back to their normal size and do there dirty deeds.

Which by the way is a good song by AC/DC, which of course is most electricians favorit . ;-)

Best Regards A






“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[moon161]

Spheres of all sizes have the same ratio of volume to surface area...

Yes and that ratio V/A is r/3, so for large r volume dominates and for small r area dominates.

 

[1503-44]

Dominates what?

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I take no responsibility for any damages or injuries of any kind that may result.

 

[moon161]

The ratio area/volume is not constant as you stated but a function of the radius. This is the squared/cubed problem. for a drop of 1mm radius the volume/area ratio is .0003m³/m², for a drop of 10mm radius, V/A = .003 m³/m², 1m V/A = .33 m³/m² and so on.

For anything surface transport related- heat, vapor, etc, for the the relatively smaller sphere it will happen faster.

V= 4πr³/3
A=4πr²

4πr³/3*4πr²=r/3

To make it vivid, imagine I have a half liter of gasoline in a beer glass and then knock it over, and the evaporation surface increases a hundred times.

 

[RedSnake]

It would have been more vivid, if it hade been beer.

/A

“Logic will get you from A to Z; imagination will get you everywhere.“
Albert Einstein

 

[1503-44]

It happens faster not because of size. Heat transfer is dependent on surface area, but the ratio of surface area to volume, more specifically mass, is constant, so that cannot be the cause of the different behavior. Neither is heat adsorbed by volume, so volume isn't it either. But heat is adsorbed by mass, so there it is. The smaller mass requires less heat and time to evaporate.

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I take no responsibility for any damages or injuries of any kind that may result.

 

[miningman]

ThIS thread is all very interesting but it does not address the fundamental question of how to protect oneself. Does one intoduce UVC into ones house ??? Does one install better and more expensive filters on the furnace ?? Does one buy more expensive masks for when outside?? Improved ventilation is a no brainer but how effective is that in the middle of winter??

 

[moon161]

For a given volume, I think smaller drops will evaporate faster.

As for the volume/area ratio, I can't see it as anything other than as V/A = r/3 r is the radius and not a constant the range is (0,∞). Is there a mistake in my math?

 

[Compositepro]

As moon161 pointed out, the ratio of volume to surface area of a sphere is r/3. This means that if the sphere doubles in size (radius), then the volume increases twice as much as the surface area.

The ratio is not a constant.

This is an extremely important concept for all scientists and engineers to be fully aware of at all times and is key to scaling-up almost any structure or process. It is what limits how large animals or plants can grow and how tall buildings can be built.

 

[OHIOMatt]

The hardest hit facilities in the US have been retirement / nursing homes. In my area, it used to be (and may still be) common for owners and HVAC designers to use an exception in the building code to use operable windows for fresh air requirements. I have never been in a facility were any windows were open. During the lock downs, visitors were barred from entry, meaning that the doors were never opened. Basically the air was recirculated continuously with little fresh air mixing in. This discussion of particles combined with this fact makes me wonder if HVAC design did not contribute to the disaster.

 

[1503-44]

You just said, "volume to surface area of a sphere is r/3". Does that change?
V/A = r/3
3V = Ar
Ar/V = 3 I'd say 3 is constant, but I don't care.

What I am saying is that volume has nothing to do with temperature change or evaporation.
As you must know, that is
ΔT = Q x Mass x C_p , C_p = 1 F/Btu-lbm
It is about mass.
Volume is not in the equation.



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I take no responsibility for any damages or injuries of any kind that may result.