Wikipedia:Reference desk/Archives/Science/2013 August 27

= August 27 =

mechanical seal
difference between balanced and unbalanced mechanical seal — Preceding unsigned comment added by 117.198.197.134 (talk) 05:34, 27 August 2013 (UTC)


 * Can you give us some context ? I doubt if we are talking about a mechanical marine mammal. StuRat (talk) 07:33, 27 August 2013 (UTC)


 * The terminology is well known to mechanical engineers, especially pump engineers, as googling "types of mechanical seals" will quickly show. I haven't actually answered this question as it sounds like a trade school homework question to me. 124.178.180.228 (talk) 08:53, 27 August 2013 (UTC)


 * Yes, and if this forum was only read by pump engineers, then omitting that they are talking about pumps might be acceptable. StuRat (talk) 12:06, 28 August 2013 (UTC)


 * A balanced mechanical seal stands up straight, while an unbalanced mechanical seal tends to tilt a bit. Hope that helps. :) ←Baseball Bugs What's up, Doc? carrots→ 13:19, 27 August 2013 (UTC)


 * I was really hoping to see those links piped to Paro (robot). Jessica Ryan (talk) 13:20, 27 August 2013 (UTC)
 * Fittingly, those are intended for the unbalanced. ←Baseball Bugs What's up, Doc? carrots→ 13:33, 27 August 2013 (UTC)
 * You rang? --   Jack of Oz   [pleasantries]  08:30, 29 August 2013 (UTC)

Battery powered water boiler
Well I've been thinking for a while about something to use the spare batteries, so I think that this is a good one. No, I don't want to use open flame because this is too dangerous. So here is my questions:


 * 1) Is it possible to power nichrome wire through a series of D batteries?
 * 2) How much D batteries are needed to boil a normal steel bottle filled until half?
 * 3) Will insulation help? What insulator are good for this (no, I don't want to use aerogel)
 * 4) Is it practical? 140.0.229.26 (talk) 15:29, 27 August 2013 (UTC)
 * Probably not practical. It takes 4.18 Joules/gram to raise the temperature of water 1 deg C.  That means it would take, say, 350 Joules to boil one gram of water from tap temperature (very approximate) or 350,000 joules for about a liter of water, a reasonable capacity for an electric kettle.  It would take a LOT of D-cells to generate that many joules of heat, even without taking into account inefficiencies in turning all that chemical energy to heat.  -- Jayron  32  15:47, 27 August 2013 (UTC)
 * The most energy I see for a D battery is around 75 kJ, so there is enough energy present in 5 of them for what Jayron32 needs for his liter of water. But how much energy can you actually get out and turn into heat that you can transfer to it? DMacks (talk) 16:35, 27 August 2013 (UTC)
 * If you have several dozen spare batteries, and nothing better to use them for, then yes, it's possible. I've got nichrome wire red hot with D cells, though for safety reasons you will probably want to adjust the current so that the wire gets less than red hot.  One problem is that if you wrap the wire round the steel bottle, then the steel will short out the wire.  The most efficient way of transferring heat is to immerse the wire in the fluid, but this can be very messy, and you are likely to get corrosion of the wire.  Expanded polystyrene would be the best insulator, but the hot wire will melt it, so you might be better using glass fibre (rock wool).  The numerous problems in implementing the project make it impractical, as Jayron said.  I suggest that you find an alternative use for your batteries, or try connecting say four parallel banks of eight cells in series and use them to power a 12v flask heater sold for use in cars.  Thirty-two batteries would at least warm the water in the flask, but I think it might take eight banks of eight to boil the water.   D b f i r s   16:30, 27 August 2013 (UTC)
 * It would be more efficient to run a heat pump to boil the water but it is still a pretty poor use for batteries. You're basically degrading the energy. Dmcq (talk) 16:46, 27 August 2013 (UTC)
 * You could use the batteries to power a radio while you are riding a bicycle-generator or building a solar oven... DMacks (talk) 16:57, 27 August 2013 (UTC)


 * Don't forget it takes an additional 2260 J/g just to boil water at 100C. Enthalpy_of_vaporization.  Saintrain (talk) 20:57, 28 August 2013 (UTC)


 * ... that's to turn all the water into steam. ("Boil" has two meanings, I suppose.)     D b f i r s   07:19, 29 August 2013 (UTC)
 * Wait. What? Doesn't everyone brew their tea in a still? Saintrain (talk) 20:23, 29 August 2013 (UTC)

Validity of employment tests
Hi all, I recently took some numeracy/reasoning tests for an application for a internal job application. I was quite surprised that I would have to take these tests due to the fact I have been doing a similar job for nearly 2 years and since I have a first class maths degree I feel I shouldn't really have to demonstrate that I am good at maths by jumping through some hoops. I was even more surprised to hear that they actually use these tests to gauge mathematical/reasoning ability (superceding qualifications). Ranting aside, I was wondering whether anyone has done proper scientific studies to gauge whether these tests actually accurately measure what they claim to? Or if they actually are in any way an accurate indicator of future job performance? I remember reading somewhere that Google had abandoned these tests since good scores were actually correlated with poor future job performance. Thanks! 80.254.147.164 (talk) 16:06, 27 August 2013 (UTC)


 * Here's three relevant science refs, , , ranging from 2008(first) to 1981 (last). The abstract of the most recent says: The authors review criticisms commonly leveled against cognitively loaded tests used for employment and higher education admissions decisions, with a focus on large-scale databases and meta-analytic evidence. They conclude that (a) tests of developed abilities are generally valid for their intended uses in predicting a wide variety of aspects of short-term and long-term academic and job performance, (b) validity is not an artifact of socioeconomic status, (c) coaching is not a major determinant of test performance, (d) tests do not generally exhibit bias by underpredicting the performance of minority group members, and (e) test-taking motivational mechanisms are not major determinants of test performance in these high-stakes settings.
 * Here's some news/blog coverage of Google's thoughts on the topic, . As you recall, Google VPs are pessimistic about the value of such tests. However, the authors of the paper above conclude that tests are generally valid predictors of performance. There are many scientists at Google, but I'm not sure how scientific their HR analysis is in this case. Hope that helps, SemanticMantis (talk) 20:46, 27 August 2013 (UTC)


 * The recent Google policy decision was that brain-teaser style questions - which are categorically not standardized tests - were poor predictors of candidate quality. Brain teaser questions test creativity, problem-solving, and out-of-the-box thinking; and candidates' ability to successfully answer such questions did not correlate to the candidate's ability to develop into a high quality, productive employee.  On the other hand, standardized test questions - tests that ask questions about word-analogy and vocabulary, and tests that measure candidates' ability to correctly solve a large number of simple arithmetic calculations - correlate very well to later job performance.  Perhaps this is surprising, perhaps not.
 * It is my opinion that many detractors do not like standardized testing because it is so objective; but it is not generally accurate to criticize standardized tests for being ineffective. These tests are specifically designed to correlate to specific performance capabilites.  Modern tests are continually evolved by psychologists, using the scientific method, to improve the test's predictive power.  Nimur (talk) 20:54, 27 August 2013 (UTC)


 * OP here. As usual the Wikipedia science reference desk has excelled itself. Referenced, scientific and complete answers, thankyou! 80.254.147.164 (talk) 08:59, 28 August 2013 (UTC)


 * As for using a math degree to predict success, the problems there are numerous, as standards may vary by institution, or within each institution over time and by instructor. So, you achieving a certain GPA by no means can be taken as evidence that somebody else with the same GPA in the same course of study possesses equivalent skills. StuRat (talk) 12:00, 28 August 2013 (UTC)


 * Google's style of interviewing with those "brain teasers" is one that we use where I work (and interview) when interviewing people where creativity and problem solving is a big part of the job. The idea isn't to predict anything using the score on the test.  It's a way for a good interviewer to see how the candidate goes about solving them.  We always start out the interview by explaining that to the candidate: "We aren't interested in whether you get the answer right - we're only interested in the approach you take."...and I think it's valuable.  It's very instructive to see whether the candidate slowly and methodically works through all of the possibilities - or whether they take wild stabs and try things out - or whether they switch strategies when one doesn't produce results...whether they try to "game the system"...whether they ask questions of the interviewer.  Some of out problems have loopholes and ambiguities built into them specifically to try to provoke those kinds of responses.  They are tools to allow the interviewer to see the candidate work - they aren't test score numbers to punch into a spreadsheet.


 * A classic Google-style question is this one: "You've been magically shrunk until you're one centimeter tall and dumped inside a blender - the switch is going to be turned on in 30 seconds. QUICK!  What do you do?!"...clearly there is no right answer here.  One of ours (which we've now retired because too many people already know it) is: You have eight balls which look identical, one is slightly heavier than the others, using nothing but a balance beam with two scale pans, figure out which one it is using the least number of weighings.  Most computer programmers jump to the answer '3' - we tell them that the correct answer is '2' and invite them to tell us how to do it.  Remember - we don't care whether you solve it or not - it literally doesn't matter.


 * The problem (as I've heard it from Google employees) is not with the questions - but with the interviewers. It takes a certain amount of skill to extract the information about how creative a person is from those questions.  If you have to interview a lot of people - it's tough to maintain a standard.  Numerical score tests are easier to administer.


 * In our case, we use questions that have "right answers" but are not easy to solve under pressure. Many people get jobs with us after getting entirely the wrong answers - and some people who get the answers right don't get jobs here because we don't like the way they solved them.  We also judge people on how well we think they'll fit into a particular team that we're putting together - maybe we already have a wild lateral-thinker in the team - and we want a methodical thinker instead...but maybe it's the other way around.  Also, the "problem solving" part is only the second of three interviews - the first is a chatty "getting to know you" thing and the third is a more hard-core computer programming test (find the bugs in the following 10 programs - write me a program to solve some simple problem - describe an algorithm to do such-and-such...that kind of thing). SteveBaker (talk) 14:21, 28 August 2013 (UTC)