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April 15

Radio propagation, part 2

Is it possible, in principle, for HF radio transmissions on a sky wave from a transmitter of the same power as that fitted to an Electra 10-E located at Gardner Island to be picked up on the 5th harmonic of the carrier frequency all the way over in St. Petersburg, FL? (Note that I am not asking whether Betty Klenck actually heard what she claimed to have heard, but only whether she could have received such broadcasts in principle.) 2601:646:8E01:7E0B:6D49:6960:EF64:7278 (talk) 04:21, 15 April 2017 (UTC)

A 15525 kHz signal might propagate that far if there is ionisation. The signal would not absorb too much in daylight, and there was probably daylight over most of the great circle path at the time to reflect down to earth. But you can try to find what is the great circle and what part is daytime, and what is post sunset. The worst time for propagation is around sunrise. The next issue is harmonic. An old transmitter back then could make a harmonic pretty easily, but it would be much weaker. A voice modulated signal would also be much harder to detect, but they probably used AM and a pure carrier might be more detectable. Morse code could also penetrate further. Lack of training is a serious issue for radios from this time, people could not just dial up a digital channel number or frequency. Several knobs would ahve to be optimally set. They probably did not even know exactly what frequency they were on! Graeme Bartlett (talk) 08:05, 15 April 2017 (UTC)

The great circle path is plotted on http://www.wolframalpha.com/input/?i=great+circle+St.+Petersburg,+Florida+to+Nikumaroro It is only a quater of the Earth and over water, so the whole would be in daylight and more likely possible at that frequency. If you want to explore further find the subspot number for the date, and then with that and date and time you can plot a map of highest useable frequency, and see if 15525 is below that along the path. Graeme Bartlett (talk) 08:14, 15 April 2017 (UTC)

Right, but the issue is, how much weaker would a 5th harmonic be than the carrier frequency? 2601:646:8E01:7E0B:6D49:6960:EF64:7278 (talk) 09:53, 15 April 2017 (UTC)

The only way to find that is to take a Measuring receiver (Spectrum analyzer) to a surviving Lockheed Model 10 Electra because the harmonic radiation depends on unspecified characteristics of the transmitter and aerial. This is a lead to a restored plane and here are mentioned other surviving planes e.g. in the National Air and Space Museum, Canada Aviation and Space Museum, New Zealand Museum of Transport and Technology and Science Museum, London. Blooteuth (talk) 13:05, 15 April 2017 (UTC)

So, no way to know for sure without testing? Thanks! 2601:646:8E01:7E0B:88C9:21D2:79C0:44A3 (talk) 01:13, 16 April 2017 (UTC)

The OP asks about whether a transmitter "of the same power"...[as the Earhart plane's radio]. So, first of all, let's establish that Earhart's transmitter had an output of fifty watts, as described here at the Tighar site. Akld guy (talk) 23:35, 15 April 2017 (UTC)

I see no further answers for some time, so I present the following hypothetical explanations. HF antennas on airplanes were grossly inefficient (far too short for the wavelength). Perversely, this meant enhanced performance at higher multiples of the fundamental transmitter frequency. At one of those higher multiples, the length may by chance have been near-optimum. There was normally suppression of the harmonics in transmitters due to antenna matching circuitry designed to optimize the amount of fundamental frequency power transfer to the antenna. Such suppression typically resulted in the 5th harmonic being 40dB down (one ten-thousandth of the power) relative to the power at the operating frequency, meaning that with her 50 watt transmitter there could have been 5 thousandths of a watt (5 mW) of 5th harmonic signal delivered to the antenna. Such low-powered signals from ground stations have been known, under very favorable atmospheric conditions, to span oceans. At great height, such as that of an airplane, with the antenna length happening to favour a harmonic, such a small signal could very well have travelled halfway around the world. So, yes, it is possible that a tiny amount of power and favourable antenna and atmospheric conditions could have resulted in the signal being heard in Florida. There is another explanation altogether, not related to your question: We are accustomed today to privacy in our continent-to-continent communications that use undersea cables and satellite links. "In 1927, radio-telephone communications opened on a 24 hour basis, US-UK. By 1949 there were 70 radio-telephone circuits for all 5 continents." - [information taken from an ITU Telecommunications textbook whose ISBN number I have lost]. If we assume that Earhart's radio signals were picked up at remote receiving stations and relayed via one of those public radio-telephone channels to the US control station, it's possible that the listener in Florida heard the relayed signal rather than the signal from Earhart direct. I apologise for the WP:OR nature of all this. I have a keen interest in 1930s-WW2 military aircraft radio equipment (AN/ARC-5 and SCR-183), am restoring several for use on amateur bands, and have experience in matching them into very short antennas. Akld guy (talk) 05:43, 16 April 2017 (UTC)

I see. And if the transmitter was at ground level, could the sky wave still have reached Florida? 2601:646:8E01:7E0B:4457:AEC3:D9F:5111 (talk) 06:57, 16 April 2017 (UTC)

Nothing can be discounted. But all sets of favourable circumstances would have had to come together at the very time when Earhart was lost. In my view, if her plane was on the ground on a remote island in the Pacific: - almost impossible that she was heard on her operating frequencies (on or near 3.105 or 6.210 MHz), highly unlikely that a harmonic of those was heard. For a very good analysis of the technical aspects and shortcomings of her radios, and bungling that may have occurred, see here. Akld guy (talk) 08:56, 16 April 2017 (UTC)

However there is a suspiscion that she did not know how to tune the antenna, so it might be more like 20dB down instead of 40, if it was tuned to the higher frequency. Graeme Bartlett (talk) 09:06, 16 April 2017 (UTC)

According to the reference in my first post, the transmitter was located in the aft section of the fuselage. She did not need to (and could not from the cockpit) tune the transmitter for maximum power. The only option she had was to change frequency (by turning a knob in the cockpit connected to the transmitter via a flexible cable like a car's speedo cable). The transmitter was already fixed tuned (before take-off) for each of the operating frequencies for the antenna specified. However, a possibly unauthorized modification of the antenna may have badly affected the pre-set tuning. Akld guy (talk) 09:22, 16 April 2017 (UTC)

Pressurised carbon dioxide cylinders

This question is about SodaStream, a home carbonation machine which uses cylinders of pressurised carbon dioxide.

Here in New Zealand, the "up to 30L of sparkling water", this is how they advertise them, gas cylinders provided by Sodastream have 270g of carbon dioxide per their own labelling. The "up to 60L of sparkling water" gas cylinders have 400g (not 2x270g=540g). This immediately seemed strange to me. The fact that the masses* in US (or I think much of the world) for the different size cylinders aren't in any way similar was another red flag. There the 130L cylinder has 935g/33oz of CO2, 60L has 410g/14.5oz much more similar to what you would expect and the 130L actually has a little more per volume of water. *Weights if you prefer, the distinction isn't relevant to this question I'm pretty sure.

I tried asking SodaStream about this [1] but the answer I received is I think irrelevant to my question. As far as I see, their reply only relates to the pressure and volume of carbon dioxide in their cylinders but that doesn't help me unless they give exact values for both cylinders in which case it may be possible to figure out an answer. All I care about, as I felt I indicated in my question, is how many bottles of sparking water I'm going to make from one cylinder, assuming I'm pressuring the water to the same level. The reasonable assumption is the 60L cylinders would give close to 2x the amount of sparkling water as a 30L cylinder but it seems likely it's not even close.

I believe this will depend on those factors Sodastream knows (but likely aren't public). If the internal volume of both cylinders is the same (i.e. they just pressurise carbon dioxide more), then there will be the same amount of "wasted" carbon dioxide when the cylinders are used up (carbon dioxide which is under too low pressure to be used for carbonation). Which means you have double with 2 30L cylinders being wasted compared to 1 60L. The 60L cylinder does seem quite a bit bigger but that could be (but is unlikely IMO) just thicker walls if it's under higher pressure, rather than a larger internal volume.

My questions to the RD are 1) Am I right SodaStream's answer is largely irrelevant to my question or have I missed something? 2) Since I don't know how long it will take to get a decent answer from SodaStream, does anyone have any idea how likely it is that I will get anything close to 2x 30L with one 60L cylinder assuming the quoted masses of carbon dioxide are correct (and pressurising water to the same level)?

P.S. Can we try and keep this question ontopic, and not digress into SodaStream's reputation and ethics, using paintball or home brew cylinders instead, or the merit of carbonated water or soft drinks. Also although I've effectively linked to my real name, I'd appreciate it if people don't refer to it onwiki.

Nil Einne (talk) 05:18, 15 April 2017 (UTC)

I removed 1 as thinking about it more, I decided I was definitely correct and also realised the simplest way to respond to SodaStream. Also I guess the key question is how much carbon dioxide is actually wasted. If it only tends to be 30 g (random example), then I don't see how the 60L cylinders can come close. Nil Einne (talk) 10:57, 15 April 2017 (UTC)

I think the cylinders will be partially filled with liquid carbon dioxide with gaseous CO₂ filling the "void".[2] At 293.15 K (20.00 °C) this will require a pressure of 5,730 kilopascals (56.6 atm).[3] As the cylinder is used up a small amount of what is left of the liquid will evaporate to keep the gaseous space in the cylinder at the same pressure. Only when the liquid is all used up will the pressure drop (very rapidly). There is no point pressurising the cylinder higher because the liquid is just about incompressible and no more will fit in. If the two cylinders are about the same size (are you suggesting that they are?) all I can suppose is that one is roughly half full of liquid and the other is more nearly full. If so pretty much all of the CO₂ will be usable in both cylinders. In the light of this I have no idea of the relevance of the Sodastream reply. This makes me think I am wrong but this is a good place to find out! I think the amount of carbonated water you will be able to get will be very close to proportional to the weight of CO₂ in a cylinder. In the UK 425g CO₂ is supposed to treat "up to 60L".[4] Thincat (talk) 13:54, 15 April 2017 (UTC)

I now see Sodastream saying the pressure in a cylinder is 57.3 bars (5,730 kPa) at 20 C[5] so this confirms that we are dealing with liquid CO₂. Thincat (talk) 14:12, 15 April 2017 (UTC)

At 5730 kPa, 20 C, liquid CO₂ is 773.4 kg/m3 so a 1 litre volume (for example) would weigh 773 g; at 101 kPa (1 atmosphere) gaseous CO₂ is 1.84 kg/m3 so a 1 litre volume weighs 1.84 g.[6] That might be a rough guide to how much is "wasted". Thincat (talk) 15:35, 15 April 2017 (UTC)

Apologies my wording was unclear. I don't think the internal volume is the same, I actually suspect both cylinders start off at a similar level of pressurisation given the size of the "60L" looks like it could easily be double the "30L". However I also can't rule it out. (Of course the internal volume of the "60L" cylinder could be significantly smaller than the "30L" one but that's starting to be an extreme situation and it may not matter anyway.) Nil Einne (talk) 16:17, 15 April 2017 (UTC)

One obvious missing piece of info is that they don't quantify how carbonated the water will be. Thus, they can honestly say that the two cylinders will carbonate those respective amount of water, to some degree. The marketing people probably decided on the actual claims to make. The "up to" fudge further protects them. StuRat (talk) 14:03, 15 April 2017 (UTC)

I guess I should have added an additional clarification. I'm well aware their claim is unclear, hence why I took pains to stress "same level of carbonation" in my question to them. But whether or not they have justification due to the weasel wording used is something which is also besides the point of my question. (Such issues will be best dealt with the humanities or misc anyway.) Nil Einne (talk) 16:18, 15 April 2017 (UTC)

I'm not sure where all the facts here come from, and sometimes you have to double check to see if anything is misinterpreted. I found the sizes here but not the weight values. I found some support for 400 g in random places [7][8]. I could wish for a better source, but so far this is moderately confirmed - the second source says 400 g in 0.605 liter internal volume = 660 kg/m³ which falls right at the highest part of the two phase region at room temperature. Perhaps the two phases help to insulate against unexpected events (sudden dents/crushes)? Now the next question of course is what is carbonated water. This works according to Henry's law with a constant K_H = 29 L atm / mol. If we take 400 g / 60 liters that gives us 6.7 grams of CO2 per liter of soda. (I don't know why this doesn't line up with the carbonated water article's figure of 0.2-1.0%...) Divided by 12+16*2 = 44 gram/mol that is 0.15 moles of CO2. At STP that would take up 22.414 L per mole, per ideal gas law without correction anyway, so that's 3.4 liters of CO2 gas when depressurized. Anyway, in water we take 29 L atm / mol * (0.15 mol) / (1 L) = 4.35 atm. Apparently soda cans or bottles typically hold less than 3 atm of pressure [9]. But, as always, there's a bit of a catch: the CO2 isn't simply dissolving into water, but is in equilibrium with carbonic acid. An acidic soda recipe (and how many aren't?) would presumably drive this reaction more toward the carbonic acid, bringing the CO2, however temporarily, out of the calculation for pressure. That was gibberish, let me try again: CO2 in water is in equilibrium with carbonic acid; the carbonic acid is in equilibrium with bicarbonate and hydrogen ion. A more basic soda recipe (sorry) would take up hydrogen ion and allow carbonate to enter the solution. That said, the recipe shouldn't be all that basic because it would be easier to just put in a bit of acid or acidic buffer to avoid having to dissolve in extra CO2 at the end. FWIW a pure carbonated water would be in an equilibrium [H+][HCO3-]/[H2CO3] = 2.5E-4 M, i.e. the pKa1 is 3.6. So if there's nothing else in the soda and we suppose it's 0.15 M H2CO3, x^2/(0.15 M - x) = 2.5E-4M; x^2 + 2.5E-4M x - 0.15M*2.5E-4M = 0; x= (sqrt(2.5E-4M^2 + 4 * 0.15M*2.5E-4M) - 2.5E-4M)/2 = ... 0.006, which gives pH 2.22. The numbers check out, unless I did the wrong calculation. And it makes sense that pure carbonic acid should be well above the pKa, which is what the pH would be for a half-and-half mix of carbonic acid and carbonate. But this disagrees with this study finding higher pH figures and blaming citric acid, which has a pKa of 3.13. Hmmm... Wnt (talk) 19:17, 15 April 2017 (UTC)

Thanks for your reply. The weight or mass for NZ and Australian cylinders come from the cylinders and packaging themselves which I've seen and even have photos of. I'm lazy to dig these up but you can see them in these photos from the SodaStream website "30L" [10], "60L" [11].

The US/ROW cylinders are a little more complicated. You can see the "130L" cylinder here [12] [13] a little low resolution but the details can IMO be made out. But you can find third party retailer images that are clearer e.g. [14]. Unfortunately the SodaStream US websites images for the "60L" cylinder that I found don't show the relevant part. But you can find third party images for the "60L" cylinder, e.g. [15] [16] which do show the details. The SodaStream HK website also has the same details "130L" [17], "60L" [18].

Note that as implied by the SodaStream HK website, the cylinders in AU and NZ (possibly South Africa too since the cylinders I get generally mention South Africa, but I'm not sure) are different, at a minimum they have a different attachment. (This comes up when people to want to attach third party cylinders.) I have seen some odd cylinders like [19] which list a weight of 425g in UK/NZ/Australia and 400g in South Africa, but the cylinders I've seen in NZ aren't like that. In any case, while 425g makes a different, it's still not enough to make up for the difference from the "30L" ones by itself. (The "30L" don't seem to be available in the US or the UK or HK.)

Nil Einne (talk) 07:45, 16 April 2017 (UTC)

BTW, SodaStream devices can only make carbonated water. If you want soft drinks, you need to add syrup after the carbonation process. If you try to carbonate something besides water, I've heard you can get some quite spectaculate results, I think on the order of Mentos+Coke. I've never tried since amongst other things, I don't want to get sugar etc all through the SodaStream parts. Nil Einne (talk) 11:23, 16 April 2017 (UTC)

From experience, attempting to carbonate anything which has non-dissolved components (e.g. an emulsion or suspension) will result in the whole thing fizzing up spectacularly (and you having to explain to your parents why you are cleaning pineapple juice off the ceiling). Solutions are often OK, so long as they don't have a viscosity significantly different to water. MChesterMC (talk) 13:05, 18 April 2017 (UTC)

Physics, Overview of Halliday and Resnick

I want to find some clear comparative overview (maybe as a table) of all (or most) textbooks published by Halliday and Resnick (+Walter or Krane). How do they overlap regarding exercises, target readership, content and so on? --Hofhof (talk) 11:22, 15 April 2017 (UTC)

Here's a great review bibliography: Chicago Undergraduate Physics Bibliography. As its author states, this thorough book review was "written by snotty undergrads..." but the reviews are "useful at least, for their recommendations on what not to read."

More sincerely, just go grab the books and read their respective prefaces. Physics authors almost universally address their audience in the book preface: they outline what you should already know, what you should expect to learn, and how many hours, days, or months you might reasonably expect to spend with their book. For example, since I just cited Krane a few days ago, I have it handy: it has a wonderful front-piece with clearly-marked sections describing the audience, background, emphasis areas, ... "This work began as a collaborative attempt with David Halliday to revise and update the second edition of his classic text... This text is written primarily for an undergraduate audience, but could be used in introductory graduate surveys of nuclear physics as well. It can be used specifically for physics majors as part of a survey of modern physics, but could (with an appropriate selection of material) serve as an introductory course for other areas of nuclear science and technology, including nuclear chemistry, nuclear engineering, radiation biology, and nuclear medicine."

You will not find a more qualified person to explain the contents of such technical books than the book's own author and editor!

If you want a third-party review, you can search the specific author and text at, for example, APS: each APS newsletter usually contains a book review.

So what I'm getting at is, when you reach a certain level of study in physics - or anything else, like chemistry or even postmodernist deconstructive/reconstructive hermeneutics - the most important skill is to learn how to navigate knowledge within that knowledge domain. In complicated scientific fields, there is no better resource than the textbooks themselves; and as a secondary source, review articles in common industry or academic journals (like APS) can help point you toward the right books, papers, and websites. Learning how to use those resources, in itself, is a skill that is probably as important and as difficult as any of the specific mathematical techniques you pick up along the way. One of the greatest fallacies I see, repeated over and over and over again, is the misconception that you can just dive right in to the deep end of advanced physics without spending several years formally building toward it. It's a fallacy that is perpetuated by many of the lesser-quality popular-science writers who are driving their focus toward the community of science-enthusiasts who lack any actual scientific experience. That's just ... ridiculous! It's like asking how to run before you know how to walk. One can not, generally speaking, make headway learning nuclear physics until one has studied regular physics to a great deal of depth. Equally, one cannot read the original writings of Victor Hugo before one learns fluent and proficient use of French. That's how far off the mark we are. It does not matter if we link to the actual explanation in the very book you seek. By the time you are ready for us to cast our light on that explanation, you already know where to find it. As long as you are still lost looking for it, ... we can't help you find it - not even if we point right to it!

Nimur (talk) 17:04, 15 April 2017 (UTC)

Fantastic answer, thanks. Hofhof (talk) 17:15, 15 April 2017 (UTC)

Temporarily leaving a profession.

How easy is it to return to a profession like engineering if you temporarily leave it for a year to gain experience of another sector like business project management? 82.132.232.213 (talk) 15:37, 15 April 2017 (UTC)

Are you asking about a Career Break or a Sabbatical? Aspro (talk) 15:54, 15 April 2017 (UTC)

not really. Those are leaving to do something completely different. I more meant doing for example a project management role in a large business consultancy before returning to engineering, just to strengthen your general project management and business skills. 82.132.232.213 (talk) 16:02, 15 April 2017 (UTC)

I don't understand your issues with Aspro's suggestion. What you mean falls perfectly under the definition of sabbatical: " "sabbatical" has come to mean any extended absence in the career of an individual in order to achieve something. In the modern sense, one takes sabbatical typically to fulfill some goal". It's up to you to decide what goals you want to fulfill. Planning with care and getting feedback from your present and prospective employers might be advisable (see [20] for some literature about. --Hofhof (talk) 17:12, 15 April 2017 (UTC)

Suggestion: Find a place where you can be a working manager on an engineering project. That is, you would manage the project and work on it directly, as an engineer. That way you gain experience in both fields simultaneously. StuRat (talk) 16:10, 15 April 2017 (UTC)

The effect of taking a one-year break from an engineering profession depends on how much value is placed by employers on up-to-date competence. The best answer could come from someone working in the particular engineering discipline (chemical? electronic? mechanical? other?) if asked "Would it matter if all your experience of developments, components and methods were out of date by a year?". If you seek a job with a new employer, your lack of a recent relevant reference letter may also put you at a disadvantage against competing candidates. Blooteuth (talk) 19:12, 15 April 2017 (UTC)

Thank you for the answers all. There are many engineering opportunities in the organisation I want to join though in a business management role though. So maybe an internal transfer after a year or so would be easier. 82.132.232.213 (talk) 19:29, 15 April 2017 (UTC)

In my direct experience, dead easy. Not quite as easy if you want to return to your old job but I know people who have made an art-form of that as well. Perhaps on a simpler level I used to work with a contract drafty who used to work a contract for six months and then leave for a six month skiing holiday. If we had a job for him when he came back, great, if not he worked somewhere else. Greglocock (talk) 00:02, 16 April 2017 (UTC)

That's even worse. I can't imagine him gaining any applicable skills from skiing holidays to any job. I guess it depends on demand and supply too. 2A02:C7D:B95F:F700:D8F8:ECD8:94E7:FE66 (talk) 00:21, 16 April 2017 (UTC)

Given the well-documented "leaky pipeline" of women in STEM fields, there is quite a bit written about returning to e.g. engineering after a career break. Yes, engineers give birth too, and with evolving laws regarding maternity leave (and to a lesser extent paternity leave - both those phrases redirect to parental leave), companies have had to devise ways to re-integrate returning workers. The Women's Engineering Society talks about returning to work after children. Here's an article by the founder of Women Returners. The Society of Women Engineers suggests "6 Ways for Engineers to Update Skills after a Career Break". The Institute of Engineering and Technology offers advice on returning to work that doesn't mention parenthood; as is sometimes the case, changes aimed at benefitting one group can have a spill-over positive effect on another. Workers leave their profession for all sorts of reasons; smart employers look beyond the gap on the CV to see the skills the employee offers. Carbon Caryatid (talk) 16:44, 18 April 2017 (UTC)

Edible disinfectants ?

There's ethanol, but that's a bit hard to take in high enough concentrations to be effective. Any other options ? I'm asking as far as possible inclusion in mouthwash. Many contain hydrogen peroxide, but I'm not convinced that a concentration that would be effective won't cause burning there, either. StuRat (talk) 21:02, 15 April 2017 (UTC)

In principle, probably most any disinfectant, since the dose makes the poison, though many common ones wouldn't be that great for you to ingest in any sizable amount. Silver salts are one bet. Maybe thymol or lactic acid, as well. --OuroborosCobra (talk) 21:11, 15 April 2017 (UTC)

Doxycycline. Count Iblis (talk) 22:39, 15 April 2017 (UTC)

That's an antibiotic, not a disinfectant. Two different things. -Nunh-huh 02:11, 17 April 2017 (UTC)

Cetylpyridinium_chloride. SemanticMantis (talk) 23:13, 15 April 2017 (UTC)

And urine [21]. SemanticMantis (talk) 23:21, 15 April 2017 (UTC)

If the use is primarily as a mouthwash, the list gets quite wide. Chlorhexidine gluconate is a common one, and I find it doesn't have much of a burning sensation. It isn't recommend for long-term use though. --OuroborosCobra (talk) 16:40, 16 April 2017 (UTC)

Transgender suicide rates

I'm looking for high quality secondary sources on transgender suicide rates. Benjamin (talk) 22:39, 15 April 2017 (UTC)

• Haas et al., "Suicide and Suicide Risk in Lesbian, Gay, Bisexual, and Transgender Populations: Review and Recommendations", Journal of Homosexuality.
• Clements-Nolle, Marx & Katz, "Attempted Suicide Among Transgender Persons", Journal of Homosexuality. SpinningSpark 23:24, 15 April 2017 (UTC)