Wikipedia:Reference desk/Archives/Science/2018 February 16

I bought a small portable humidifier for dry indoor air, it says in the manual not to put it closer than 4 feet from people. It seems like it would be more effective to put it right next to me or point the mist at me. Is there an explanation for why this should not be done? --User777123 (talk) 00:48, 16 February 2018 (UTC)[reply]

I suppose you don't want to humidify the humans, especially in winter, but the air they breath. --Hofhof (talk) 01:07, 16 February 2018 (UTC)[reply]

You could call the company that made it and ask why. Or you could try it and see what happens. ←Baseball Bugs ^{What's up, Doc?} carrots→ 02:53, 16 February 2018 (UTC)[reply]

It depends on your local climatic conditions (i.e., Relative humidity and temperature). The water needs time to evaporate. With very low RH and high temperatures this can be quick. Sit too close to a mister with mid RH and mid temperature and you'll get wet within 4 feet. Try it some day. So 'experiment' to see what suits you. If you live down far south, an addition of a little bit of Taquería lowers the surface tension of the spray droplets which promotes evaporation and works wonders. Do remove the worm first -as it may block up the spray nozzles.--Aspro (talk) 13:18, 16 February 2018 (UTC)[reply]

I assume you mean tequila (a distilled spirit made from agave) and not taqueria (a small restaurant that serves tacos). Worms are sometimes welcome in the former, never in the latter. --Jayron32 13:35, 16 February 2018 (UTC)[reply]

Just to add on what Aspro said above, humidity is water vapor, which is to say water in gaseous form. Humidifiers do not put water vapor into the air, they put tiny droplets of liquid water into the air; the small droplets evaporate and that adds to the humidity. If you put objects (including people) too close to the output of the humidifier, what will happen is the water droplets will stick to the THAT rather than evaporating, reducing the effectiveness of the humidifier and making the person rather damp instead. You need plenty of air space for the droplets to have sufficient time to evaporate and add humidity to the room. --Jayron32 13:39, 16 February 2018 (UTC)[reply]

• Actually, there are at least two radically different types of humidifier, steam and droplet (often called "cool mist") humidifiers. The first runs a current through water, causing it to boil inside an internal column inserted in the water tank. The second agitates water, causing droplets which, being drops, drop. There are a lot of tips to using a humidifier, but this is not an advice column. I use a Vics-brand steam humidifier (they are cheap and you do not have to buy their additives unless under medical advice to do so) religiously during the drier part of the year, but I take all sorts of precautions regarding location, condensation, and precipitation due to hard water which can leave a rime stain on exposed surfaces. Go on line and look for advice on problems with water damage and how to spread the humidity through the room, rather than have it drop on the floor in front of the device. μηδείς (talk) 20:46, 16 February 2018 (UTC)[reply]

Sounds like it could be a hot steam generating type. These can burn/cook parts of your skin over time when you are to close and additional unfavorable conditions like allot of microdrops in the steam and an airflow that blows the steam towards you come together. 4 feet is very close. Its 1,2 meters and according Beaufort scale "Light breeze"(2), which commonly used fans can easily produce, is already 1.6–3.3 m/s, so in theory the water/steam could be on you 0,5 seconds after boiling! It does not even have to be likely to happen. Manufacturers just add such warning to prevent getting drawn into court for failing to warning customers about potential unsave usage. --Kharon (talk) 22:39, 16 February 2018 (UTC)[reply]

That strikes me as silly, since the steam type ones don't usually have fans that steam cook you, as far as I know. The steam in mine floats upwards and condenses, which I discourage by having a fan blowing on it to lower the steam's vapor pressure and spread the humidity as efficiently as possible. Of course this means I have a bubbling machine and a white noise generator running simultaneously. But my electric bill is still quite reasonable. The only real drawback is if you suffer inadvertant benzodiazepine withdrawal, and hear Frank Sinatra singing out of your desk fan. But that's another forbidden issue. μηδείς (talk) 01:18, 17 February 2018 (UTC)[reply]

Could be worse. You could hear Yoko Ono singing out of your desk fan. --Jayron32 04:04, 17 February 2018 (UTC)[reply]

I bought Double Fantasy on my allowance and was in the emergency room with a broken toe when I heard Lennon had been shot. The fuckers let him bleed to death. Had they sent him to Presbyterian he'd still be alive. [Further reading of indepth accounts say that the damage to the various arteries in the hearth area was so severe, no treatment was possible. I had read in a source I cannot find that the delay in treatment by taking him to St. Luke's-Roosevelt allowed him to bleed out. The matter seems somewhat muddled, since even who the head physician of record is disputed, and lennon's body was cremated. μηδείς (talk) 15:39, 20 February 2018 (UTC)] μηδείς (talk) 03:13, 18 February 2018 (UTC)[reply]

Had Mr & Mrs Chapman used an effective means of birth control – John wouldn't have been shot in the first place. I blame the parents. --Aspro (talk) 14:38, 18 February 2018 (UTC)[reply]

Yeah and the same for Lennon's parents. Nil Einne (talk) 23:18, 18 February 2018 (UTC)[reply]

For that matter, the same goes for the parents of J.D. Salinger, whose book Chapman was obsessed with. ←Baseball Bugs ^{What's up, Doc?} carrots→ 23:24, 18 February 2018 (UTC)[reply]

If compressed air were passed through a water flow sensor such as this one, would it be reasonable to expect it to detect the flow (I simply want to know whether there is flow or not and not measure the amount). If there's a reasonable chance of it working, I could buy one to test at my intended flow rate but if there's no way it could work with a moving gas for some reason then I won't bother. Thanks! --129.215.47.59 (talk) 12:16, 16 February 2018 (UTC)[reply]

Reynolds number is what makes the difference.

This is not a precision instrument. It will work equally badly for water and compressed air. It depends on what you mean by "compressed air" - it would measure the sort of airflow used by an air tool or even a robot actuator, but not the pressures and flows of just blowing into it.

These sensors are just a simple paddle wheel. They are slightly unreliable, very badly made, and completely non-linear in their response (their wheel speed is not accurately proportional to the flow rate). They also fail after a few years, especially if exposed to sunlight, heat, some solvents or even the oil in some compressed air. I would also be careful when first pressurising it, as I wouldn't be surprised if the case simply blew apart (they're a two part moulded plastic case, held by small screws) - I'd put a couple of safety tie-wraps around it as well. Electronically they're quite reliable, as they mechanism is pulsed, not analogue, and the electronics are separate from the fluid.

I'd use it. They're cheap. Andy Dingley (talk) 12:31, 16 February 2018 (UTC)[reply]

It may work as alternative but its no part i would recommend if its function and measurement is or can become important. Such parts are ofcourse constructed and optimized for their documented purpose only and similar parts for air are shurely constructed (maybe just slightly) different! --Kharon (talk) 16:38, 16 February 2018 (UTC)[reply]

• Andy Dingley, any chance for a development of your opening sentence? My speculation below does not involve the Re at all, except to say we do not care because it is much larger than 1.

My starting point would be to conveniently assume the drag equation gives us a good idea of how things behave.

Suppose that the spec given for minimum water flow (0.3L/min) corresponds to a minimum value of drag below which the paddles will not move, and that the drag is proportional to ${\displaystyle Q_{v}^{2}*\rho }$ (where Q_v the volume flow rate (L/min) and ρ the density (kg/L)) for both water and air (the real assumption here is using section-averaged volume flow rate instead of local speed to compute drag - I believe it is roughly correct for high Re, but I may have forgotten about a sneaky hypothesis about compressibility somewhere). The flow rate spec translates into a threshold of ${\displaystyle 0.09kg/min/L}$ (water = 1kg/L).

If we assume air is X times less dense than water (about right at atmospheric pressure, depending on where you live), the volumetric flow rate must then be sqrt(X) higher than the spec. For instance, for X=900 (about the value for atmospheric air), you need a flow of at least 9L/min of air to move the paddle. For X=400 (compressed air at 2-3bar), it is 6L/min. Depending on your setup, such values may seem way too high to reach, in which case I would not buy the thingie. Tigraan^{Click here to contact me} 20:02, 16 February 2018 (UTC)[reply]