Wikipedia:Reference desk/Archives/Science/2015 September 17

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September 17

Gains and losses at petrol pumps

I often see people vigorously tapping the nozzle of the petrol pump to shake off every last drop before replacing the nozzle at the pump. If we assume petrol costs just £1 a litre (it's currently a little more than that for unleaded, where I live, but this keeps it simple), what would be an approximate monetary value for what these people are gaining and impatient people like me are losing?

If we pretend that every second you're doing something else other than working, you're losing wages (yes, I know), if we assume the jiggling takes about 10 seconds and someone is paid at just the UK minimum wage of £6.50 per hour, how much petrol would they need to save in order to make it worthwhile? And what if they're paid at the average wage of c.£3000 per month? Based on these calculations, is there an hourly wage that could be calculated at which jiggling the nozzle becomes more worthwhile than extra time spent working?

Finally, is there a good scientific reason not associated with money for doing this jiggling? Would these drops of very flammable material present a significant additional fire [or other] risk, in an environment where there is already an inherent significant risk? Does it make a difference that it would be cumulative over time, or does the fast evaporation of petrol nullify this factor?

Thanks all.

The curious scienceandmathsidiot, --Dweller (talk) 10:19, 17 September 2015 (UTC)

I've long thought that people (who, after all, never actually see petrol) think it's much more viscous than it is (perhaps people imagine it's like ketchup, with blobs of petrol stuck to the inside of the nozzle that must be dislodged). In actuality petrol is about half as viscous as water. Once the pump is off, the only place there could be any is in the local minimum of the hose, so lifting the hose does yield a small mount (I've tested it while filling a can, and gotten about 5 or 10ml at most from this). -- Finlay McWalterᚠTalk 10:54, 17 September 2015 (UTC)

As mentioned above, I'm usually pretty lazy about the whole business, but I think I have sometimes seen more than just a few drops fly out of the nozzle as I remove it. --Dweller (talk) 11:22, 17 September 2015 (UTC)

The economic benefit isn't in conserving that last drop of fuel, but in stopping it getting on your car's bodywork. Gasoline works very well as a paint stripper (and even more so as a wax stripper), and if you let that stray drop run down the side of your car rather than tap it off, your car will streak very quickly, which will cost considerably more to repair than the nominal value of the time you spend shaking off drops. ‑ iridescent 11:02, 17 September 2015 (UTC)

That is precisely the kind of reply I was hoping for, and why this isn't on the Maths desk, where I was originally going to post it. Thanks. --Dweller (talk) 11:22, 17 September 2015 (UTC)

Incidentally, Diesel fuel is more viscous than water (roughly double) - and its heavier fractions don't evaporate after a spill. So it makes more sense to shake the Diesel dispenser than the petrol one. -- Finlay McWalterᚠTalk 11:06, 17 September 2015 (UTC)

There is an error in this question. The questioner makes an observation: People tap the nozzle when removing it from a car. Then, the questioner makes an assumption: People are tapping the nozzle because they want to get every last drop of gas out of the nozzle. Finally, the questioner asks questions based on the assumption being true. What if the assumption is false? I tap the nozzle when removing it from the car, but not because I want to get every last drop from the nozzle. I do it because there is usually a drop or two of gas on the nozzle. As you move it from the car to the pump, they can fall. If you not very careful, they can land on your leg or foot. Just one drop of gas on your pants and you will have a bad aroma follow you throughout the day. So, it is best to tap a couple times. It is the same as using a urinal. You want to tap a couple times so you get a drip on the inside of your pants. 209.149.113.66 (talk) 12:48, 17 September 2015 (UTC)

I never "tap" and it's never yet dripped on my clothes. --Dweller (talk) 14:12, 17 September 2015 (UTC)

Therefore, nobody else in the entire history of gas pumping has ever tapped to keep from having fuel drop on their clothing. You need to take part in more scientific studies since your practices are universal for all of humanity. It would really cut out a hell of a lot of time spent observing human behavior. All anyone has to do is as what you do. 209.149.113.66 (talk) 14:27, 17 September 2015 (UTC)

Point taken. --Dweller (talk) 14:28, 17 September 2015 (UTC)

Actually, there is a slightly funny joke there that I didn't catch - wish I had. I assume you meant that you've never dripped fuel on your clothing, but the statement above also refers to dripping urine on your clothing. So, you could have just as well meant that you never tap at the urinal. 209.149.113.66 (talk) 14:54, 17 September 2015 (UTC)

I guess someone should actually crunch some numbers here. Suppose a 'drop' of gasoline were (guess) 2mm across...and let's cheat and say that these drops are neat little cubes of gasoline. A liter is 100x100x100 mm³ and our drop is 2x2x2 mm³ - so at 1 UK pound/liter - each drop is worth 0.0008 of a penny - and it takes 1,250 drops to be worth a penny.

So shaking off droplets to save wasting the petrol is entirely pointless.

Finlay asserts that 5 to 10mL can be obtained from the hose. Since a mL is 1/1000th of a liter - it costs 1/10th of a penny...so saving 10mL is worth a whole shiney penny! If emptying the hose takes you 10 seconds longer, then you're earning 3.60 UK Pounds/hour for your labor...which is almost certainly not a good use of your time either...and if the person before you does it, then you LOSE that penny. If everyone does this, then nobody earns a thing.

Another consideration is that if the petrol evaporates instead of ending up in your tank, it adds to global warming...but each drop is 1/125000th of a liter - which is under a millionth of a tankful...and even if you multiply by every car owner on the planet - it's still negligible.

I agree about not wanting petrol on paint/clothes/whatever - so there is value there - but saving the petrol isn't worth the effort.

SteveBaker (talk) 15:08, 17 September 2015 (UTC)

I don't tap the nozzle, but I have been known to lift the hose so it is above the inlet before I put it back. I have honestly no idea if that makes hundreds of mls of gas flow or nothing - I've heard something, but you know how noisy gas stations are, not to mention my greedy imagination. Wnt (talk) 15:47, 17 September 2015 (UTC)

Snopes did an article on the efficacy of various alleged methods of getting better value from petrol pumps (e.g. fill up in the morning when the petrol is cooler and so more dense). AndrewWTaylor (talk) 16:36, 17 September 2015 (UTC)

Chaps, you're all wonderful. Thanks. --Dweller (talk) 20:45, 17 September 2015 (UTC)

I find the dichotomy that am I either shaking drops off of my gasoline nozzle or earning hourly pay (at my salaried job) to be a false one. shoy (reactions) 14:08, 18 September 2015 (UTC)

I agree that there is no direct relationship here. However, you do need some kind of personal metric for "is this task worth doing?" - would you spend an hour to mow my lawn for me for free? Probably not. Would you do it for $1? $10? $50? If you have no feel for the value of your (non-working) time, then how can you know whether some otherwise pointless activity is worth doing? If you literally have nothing better to do with your spare time than to spend it on activities like this one - then I find that profoundly sad. My point is that if you're prepared to sacrifice some part of your time working for a living at $xx/hour - then why wouldn't you value the remainder of your time based on a similar metric? SteveBaker (talk) 13:57, 21 September 2015 (UTC)

Gas / petrol station attendants in Libya (where you very nearly always ask to fill up the tank) insist on rocking the whole vehicle near the end of filling process 'so as to get more fuel in the tank'. They will stop pumping to rock the car side to side and then 'squeeze' some more in. Drove me crazy.Hayttom (talk) 10:12, 18 September 2015 (UTC)

They do the same thing through Central America, but it isn't to get more gas in the tank. It is to help mix in the water. If they don't mix it well, your engine may quit before you get out of the station. Instead, you get a few miles away before you find they slipped a gallon of water into the tank. The only place I didn't have issues with the gas station attendants trying desperately to slip water into the tank was in a very arid area of Oaxaca where water costs about just as much as gas. 209.149.113.66 (talk) 14:23, 18 September 2015 (UTC)

Unidentified twig

Can anybody help identify this?

. It came off a tree in Yorkshire, England, but I don't know anything more than that.

It looks to me like a stalk of emerging fruit, but I don't know what; though it makes me think of Magnolia. --ColinFine (talk) 11:14, 17 September 2015 (UTC)

Looks to me like an acacia bud. How long is it? Ignore that; just realised it's almost certainly mistletoe if it came from a tree. ‑ iridescent 11:20, 17 September 2015 (UTC)

Hi, @Iridescent. Is that a positive identification, or based on my loose use of 'off'? I have not spoken to the person who found this, but I have no reason to suppose that it is not a scion of the tree, as opposed to an epiphyte or parasite. --ColinFine (talk) 11:52, 17 September 2015 (UTC)

Not a positive investigation, but a "looks likely to be". Do a google image search on "phoradendron buds" and compare. ‑ iridescent 11:57, 17 September 2015 (UTC)

Why would a twig be sampled, instead of a leaf or bark? One potential reason - people not familiar with mistletoe often think: "why are some of these twigs on this tree so different from the rest?" And then promptly sample one for further inquiry. Basically I agree that it looks very much like a mistletoe "twig", actually a spike in this case. Lacking any good alternative proposal, mistletoe seems like a good ID to me. SemanticMantis (talk) 14:28, 17 September 2015 (UTC)

Cone winding machine

Cone winding machine is called the science of language? — Preceding unsigned comment added by 39.45.188.189 (talk) 15:09, 17 September 2015 (UTC)

I added the title "Cone winding machine" to distinguish this question from the previous topic. But I don't understand the question. -- Finlay McWalterᚠTalk 15:18, 17 September 2015 (UTC)

Maybe you are asking "what do you call a cone winding machine in more scientific or technical language?" If so, let us know. Our most relevant article is Winding_machine I think. SemanticMantis (talk) 16:12, 17 September 2015 (UTC)

And the science of language is called linguistics.--Shantavira|^{feed me} 07:25, 18 September 2015 (UTC)

Botfly myiasis in the brain

Don't know if this qualifies as a medical advice question, so apologies if it does. I've been hesitant to do much research on this myself, mainly because it's gross as hell and it would probably give me nightmares about carnivorous maggots for months, but I've seen this black-and-white image on a lot of different sites, it's like the first google image result if you search something like "botfly brain", of what appears to be a huge botfly larva (or some other kind of maggot, I don't know) that made itself very much at home inside a (purportedly) human brain, and it raises a couple urgent questions that I hope someone that is either stronger of stomach or better versed in entomology (or both) than me can answer so I can sleep a little more soundly at night (or not):

1. Is the brain in the image actually a human brain? And is that actually a botfly maggot?
2. Is it common for botfly larvae of any sort to get into the brain of an animal or human? How could it get in there?
3. Would you know if you got a maggot in your brain (I'm left to assume this is more OR territory than I would like, though)? ^BLUSTER⌉⌊_BLASTER 16:49, 17 September 2015 (UTC)

Here's an article at Cracked.com that has a brief description of botflies and an image that claims to show one in a human brain.

I'm highly skeptical.

I numbered your questions. My answers are 1)I don't know, maybe 2)Absolutely not common, it could conceivably get in through the ear 3)most people learn pretty quickly when a larvae gets into their skin.

Most of the info in that cracked article is decent, but there's no attribution of any kind for that image. The image is posted dozens of places, and I can't find any with any attribution - so that's fishy. It doesn't even look that realistic to me, but I haven't seen many brains. Even the often-gullible redditor show some doubt and skepticism [1].

We have articles on Myiasis and botfly. The Myiasis article does mention a possibility of entering brain through ear. The larvae normally grow in or just under the skin (e.g. subcutaneous), not the brain. I don't think the larvae could penetrate the skull. It's often not even a big deal to get a botfly larvae in you. Many people (e.g. [2]) have decided to let the larvae emerge just for the fun of it - as long as the site isn't infected, it's not that much risk to the host. The US army [3] even recommends letting them naturally run their course. Botfly are parasites, not parasitoids - there's a big (deadly) difference! Here's a not-that-gross article about one guy's experience with botfly in National Geographic [4]. Anyway, even if it has happened a few times that a botfly got into a human brain, such an occurrence is freakishly rare- many animals and humans get botfly larvae under their skin and do just fine. SemanticMantis (talk) 17:29, 17 September 2015 (UTC)

• Sorry, should have gone straight to google scholar - at least two cases of myiasis in the human brain have been reported as of 1989 [5]. Here's [6] a 2007 article that discusses a few more cases, but I don't have the time right now to read it carefully. But even though we now have good scholarly evidence that it can occur, my above info still stands - this is freakishly rare, and botlfly larvae are often no big deal. SemanticMantis (talk) 17:33, 17 September 2015 (UTC)

Thanks for the thoughtful answer; my skin crawls a little less now... At least these friggin' things are only come as far north as Mexico if the WP article on the human botfly is to be believed. ^BLUSTER⌉⌊_BLASTER 17:59, 17 September 2015 (UTC)

No problem, I learned something too! SemanticMantis (talk) 22:42, 17 September 2015 (UTC)

Reminds me of The Wasp Factory, the incident that drove Eric mad... Ssscienccce (talk) 00:04, 18 September 2015 (UTC)

I hate to possibly disrupt your sleep, but there are other parasites that are more likely to wind up in your brain. One of the most significant is the pork tapeworm: see cysticercosis, complete with a rather unpleasant MRI image. So don't eat any pork that isn't well-cooked. Amoebiasis can also affect the brain. --71.119.131.184 (talk) 12:16, 18 September 2015 (UTC)

Gravimeter in weightlessness

Reportedly gravity as a curvature of spacetime becomes less as you go farther from the mass but it does not become zero and that at extreme distances, the gravitational force between two objects becomes negligible, say 1/100000000000000000 Newtons of force. This entails that negligible gravity is still there in weightlessness. Now according to gravimeter, the most sensitive ones can show 1 nanogal, or 10⁻¹² of the Earth surface gravity. What such or similar gravimeters would show in weightlessness? That is, the average gravity reading in weightlessness? Brandmeister^talk 17:20, 17 September 2015 (UTC)

The relevant article here is noise floor. The output of the instrument would measure its local transient acceleration, overlaid on the electrical noise from the transducer. In a weightless environment, both of these factors would be larger than the gravitational field from a distant object, so couldn't be measured. Tevildo (talk) 20:52, 17 September 2015 (UTC)

Possibly more relevant is that up in orbit, even though you are "weightless", you are not anywhere near outside the major influence of the earth's gravity field. What's happening is that the forces are actually "cancelling each other out" so that you "experience" weightlessness, and according to relativity where there is no privileged frame of reference, it is my understanding that there is no difference between "apparent" weightlessness and "real" weightlessness. So even if you were deep in the void of space, millions of light years away from anything, you might be completely surrounded by microscopic gravitational fields in all directions, but given a random distribution, the already miniscule forces would quite probably almost completely balance out in all directions. Vespine (talk) 22:41, 17 September 2015 (UTC)

[7] The gravity's up to several millionths of a g inside a large modern satellite. Because every atom wants to make slightly different orbits around the Earth but are connected to the rest of the space station so they can't. Sagittarian Milky Way (talk) 23:40, 17 September 2015 (UTC)

Yep, and the gravimeter would measure the difference between the suspended mass inside and the spaceship, so could be positive, negative or zero, depending on its exact location aboard. To measure that gradient, gravity gradiometers are used: they measure the difference between two gravimeters at fixed distance from each other. The GOCE satellite used accelerometers 50 cm apart, both capable of measuring forces as small as 10^-13 g (they use electrostatic suspension to keep a mass in place, measuring the amount of force needed). The combined sensitivity is 10⁻⁶ g.

In deep space, it wouldn't measure anything either. Ssscienccce (talk) 03:31, 18 September 2015 (UTC)