13 ms·
I’m terrified of making one of these ‘big picture’ mistakes. I’ve long been weary of the fact that there is a surprising amount of engineering going on without
by ggcdn 7y ago
I’m terrified of making one of these ‘big picture’ mistakes. I’ve long been weary of the fact that there is a surprising amount of engineering going on without scrutiny or peer review. And over reliance on computer modelling and increasing complexity makes it less and less easy to grasp the big picture.
Just yesterday I made a trivial error crunching some numbers for when a construction site could turn off their dewatering. Quite the opposite of this submarine, my structure would be 1000s of kips too buoyant and could potentially float up out of its excavation due to hydrostatic forces. A mistake like going unnoticed could easily be a career-ender.
- JohnJamesRambo 7y agoWary :)
- nemosaltat 7y ago¿Por qué no los dos?
- userbinator 7y agoAnd over reliance on computer modelling and increasing complexity makes it less and less easy to grasp the big picture. More importantly, this also diminishes order-of-magnitude/approximation thinking, which is a very important source of "common sense". I've graded physics/electronics exam questions, and it's astonishing/amusing just how far-off some of the wrong answers are. Some memorable examples: - Asked to calculate the current-limiting resistor for an LED in a low-voltage circuit, some students gave answers in the megaohms, others in the miliohms (correct answer was in the low kiloohms.) - A question about the engine power of a dragster resulted in wrong answers of over a GW (roughly a million horsepower!) and under 1kW. (Correct answer was around 1MW.)
- tigershark 7y agoWait, is the engine power measured in something else than KW nowadays? I think that the HP -> KW switch happened something like 20 years ago, at least in my country.
- freeone3000 7y agoEngines in the US continue to be measured in horsepower.
- tigershark 7y agoOk, makes sense then, thanks.
- userbinator 7y agoEven in Europe, where there's even a metric horsepower that's often used in advertising and such: https://en.wikipedia.org/wiki/Horsepower#Metric_horsepower_(PS,_cv,_hk,_pk,_ks,_ch) https://en.wikipedia.org/wiki/Horsepower#Metric_horsepower_(...
- walshemj 7y agoI worked on Audi websites a while back back and Ps was used even on the German sites
- ehnto 7y agoI still see Japan use PS, which is known as Metric Horsepower. I think it comes from the German words for Horse Power, "Pferdestärken". It's 1 PS == 0.98632 HP
- mhh__ 7y agoVehicle dynamics literature does seem (as an outsider) to involve a slightly weird blend of imperial and SI units, research nowadays is metric but old research being cited in textbooks (i.e. RCVD has a lot of imperial)
- gambiting 7y agoAnyone in the world measures engine power in kW? That's odd. What country is that?
- Symbiote 7y ago
- freepor 7y agoThis is a kind of problem where just knowing a lot of shit is a partial inoculation. If you know the population of New York City, and the weight of a Boeing 747, and the number of languages spoken in the world, and random shit like that... you catch these errors all the time.
- droithomme 7y ago> knowing a lot of shit is... > population of New York City 15 million? > and the weight of a Boeing 747 600 tons? > and the number of languages spoken in the world 15,000? How'd I do? I don't know any of these though, I just guessed. I think I'm probably within an order of magnitude but I'll let you decide.
- iambateman 7y ago8.6M 360 tons (takeoff) 6500 Your approximations are pretty good! Next time just divide by 2 and you’re good ;)
- freepor 7y agoYour estimates show that you know enough other shit to make good estimates.
- alexis_fr 7y agoHere you notice that engineering involves a lot of general culture, and making a good engineer starts at birth. When I was a baby I asked my dad to draw power outlets, we had Legos, we’ve visited dozens of factories, I did a dozen programming languages before 18, I’ve always gathered data about things (Ok a nuclear plant is 1GW, a plane has 20t of fuel, etc), and I’m an average engineer. Meanwhile my bigger sister has technical but never much about making things work, we went to the same engineering school but she ended up project manager because she can’t do the engineering job. So she manages people.
- adonese 7y agoWe had this unspoken of law back in school where you better off write any answer than leaving the question empty. This is quite possibly drives some of these answers (at least here in my case)
- dmos62 7y agoThat's ironic, because in science nothing is usually better than nonsense.
- askvictor 7y agoThe education system has some major problems with it... That said, a simple fix for this particular issue is to assign negative marks for a wrong answer (compared to zero for leaving it blank) - some multiple choice tests do this to minimise guessing.
- mjevans 7y agoFor me it was always show your work, and hope for partial credit for where you got to before things went off the rails.
- anonsivalley652 7y agoPsychology and political majors can get away with that but it's not how safety/life-critical should engineering work. They original company failed to review, at multiple levels, fundamental calculations including density. They should have to recheck other calculations, such as hull strength and those related to propulsion as well.
- sethammons 7y agoI had a few teachers that gave negative points for wrong answers to prevent guesswork. The idea being that empty answers were feedback to them on what they might not have taught well.
- sitkack 7y agoI love this. Perhaps answers could be a tuple of confidence and the answer. Guessing is important, being unsure of your answer is also important.
- bsder 7y agoTo be fair, until I got an electric car, I would not have known intrinsically that it takes about 10-30 kiloWatts to keep it moving under normal freeway conditions or that 120kW is a pretty solid acceleration.
- droithomme 7y agoBut did you know that it was over 1 kW and under 1 GW? (And if no, were you enrolled in a Physics Electricity & Magnetism course at the time.)
- metaphor 7y agoIt's a meaningless question if the person answering it simply doesn't have some qualitative frame of reference to gain intuition from...and gaining intuition on EM phenomena grounded in reality isn't easy; there'd be a lot more EEs and physicists out there otherwise. Besides, if you're walking into a time-constrained exam of toy questions, I fail to see what incentive a student may have to corroborate toy answers against a reality that it may or may not be an approximation of.
- skissane 7y agoA lot of household appliances have power measured in watts (microwave ovens, electric heaters, etc). And megawatts and gigawatts are commonly used in discussion of electric power plants. My electricity bill is in kilowatt-hours, and that's what my electricity meter says on the front as well. Isn't that enough background knowledge to guess that an electric car would likely be more than a kilowatt (my microwave oven is more than that) but less than a gigawatt (a decent sized power station?)
- bsder 7y agoNo, not offhand. If someone, at some point, had said that kiloWatts and Horsepower are the same order of magnitude, I might have been a little better about that. Gigawatt would have set off my "Huh?" detectors as I have heard that in correspondence with power plants. However, given that diesel generators sometimes power buildings, it would not have set off too much worry.
- bsder 7y agoThe error here was 70 tons out of 2200. That's about a 3% error. You're not likely to catch that with simple order of magnitude rules.
- userbinator 7y agoIt's only 3% of the total but the characterisation of the error as a "decimal point in the wrong place" suggests that something was 10x heavier than it should be.
- metaphor 7y agoAnd how would you propose one gain intuition on what individual pieceparts of a build-to-print submarine ought to weigh, especially given the scale (and relative weight) of the thing in error can't easily be gauged by any individual? 10x error is hardly familiar territory if the thing in question is, say, trace heavy metals in a custom alloy. If I gave you volumetrically equal pieces of 6061 and 7075 aluminum--whose difference in density is ~4%--would you be able to intuitively sniff out that something was off? What if those pieces were several tons a piece and your calibrated industrial weighing equipment had a reasonable +/-5% margin of error?
- TheSpiceIsLife 7y agoIs +/-5% really a reasonable tolerance for industrial weighing equipment?
- metaphor 7y agoSure if the industrial weighing equipment leveraged is, say, a weigh-in-motion system with an acceptable GVW tolerance of +/-10% as described in NIST HB 44[1]. In any case, this toy detail was intended to snuff out the grandparent's presumption that 10x error in any arbitrary parameter is obvious. The difference between 6061 and 7075 is offered as a simple thought experiment in that they're both common alloys in industry, yet difficult to discern one from the other despite the latter having a zinc composition that's 20x greater in the best case (6061 = 0.25% max; 7075 = 5.1-6.1%; 20 ~= 5.1 / 0.25) as specified by ASTM B209[2]. Fact of the matter is a contractor was responsible for a build-to-print submarine. A particular drawing specified something-or-other with an off decimal value that escaped human detection during multiple engineering design reviews. Contractor had something-or-other manufactured to the exacting detail specs in said drawing and no doubt was able to provide supporting documentation in the form of material certificates of conformance and lot testing results...but all the precision instrumentation and manufacturing quality control in the world amounts to nothing if the basis of truth is fundamentally flawed...and it certainly didn't help that the only effective opportunity to detect catastrophic design error (not that it's a build-to-print contractor's responsibility to identify such subtle faults) was spot validation of a system-level key performance parameter that wouldn't have been possible until the entire system was fully assembled. [1] https://www.nist.gov/system/files/documents/2019/12/03/00-20-hb44-web-final.pdf https://www.nist.gov/system/files/documents/2019/12/03/00-20... [2] https://www.astm.org/Standards/B209.htm https://www.astm.org/Standards/B209.htm
- samatman 7y agoAn old engineer (marine) who was a mentor of mine, lamented the end of the slide rule, for exactly this reason. A slide rule can tell you the numbers; it can't tell you the magnitude, you have to know that going in. He claimed this taught engineers an intuition for figures that calculators don't.
- ajb 7y agoThere's should be a way build a calculator that did that, for for pedagogical purposes. For example, you could do everything in floating point and zero the exponent after each operation. I wonder if that's the same as the effect of slide rules? (Goes to check wikipedia...)
- apricot 7y agoYou could take a normal calculator, put it in scientific notation, and stick a piece of cardboard over the exponent.
- perl4ever 7y agoMaybe, but my parents came from the era of slide rules, and when I asked what the answer was to a calculation, I would be told "the answer is ... but I can't tell you where the decimal place goes"
- iso947 7y agoWas it 1.21GW? Maybe the student wanted to go back in time to before the test to revise more.
- bluenose69 7y agoI find that students routinely divide instead of multiplying, especially if they are asked to solve for something and then calculate the result. For example, $f=ma$ gets transformed to $a=fm$ instead of $f/m$, yielding a large error unless $m$ happens to be near 1kg. Checking units is good way to check for incorrect formulae, but I've found that students who mix up division and multiplication are also uncomfortable with units. The use of calculators cannot explain lack of understanding of units, so I think the problem is more basic. I've known school teachers, and am somewhat familiar with curriculum. Both are basically fine, and neither explains the problems I see with students in university. It seems to me that the root problem is that students are never told, in serious terms, that getting the right answers is important. The way we signal importance is with grades. Students who have received passing grades for incorrect work through many years of school can hardly be expected to show attention to detail in university. America (et al.) may be throwing away a generation of talent, by pursuing what seems to be a policy of unconditional approval. Luckily, there is some hope: my foreign-trained students hand in correct work, with appropriate digits and units. Quite often, they also show how they have checked the work. I don't think it's because their first language was French, or German, or Mandarin; I think it is because they went to schools where learning was still the focus, as opposed to the congratulation for simply being, not accomplishing.
- CraigJPerry 7y agoIt’s interesting that it’s been common to see this exact same opinion (a lack of conscientiousness on the part of students) expressed in Germany about German education over the past 10+ years yet you cite German students as a good example. In Germany it’s commonly stated that things were better previously and that there’s a ongoing decline. My understanding in the German case is that it’s not supported in any measures of student attainment - grades, earnings after 10 years etc.
- radiator 7y agoIt is not necessarily a contradiction, it is possible that German education is now clearly worse than 10 or 20 years ago, but still not as bad as the current situation in grandparent's country.
- siscia 7y agoOf course it depends on several situation, but during an exams people are under stress and mistakes happens. What changes is the attitude of whoever is grading them. I know one of my professor wouldn't give me full marks, but if the reasoning behind the computation was correct I would still get something. On such cases I simply wrote a note next to result saying that I understood that the results doesn't make sense and that there must be some computational mistake around but I didn't had the time to find it. Other professors wouldn't accept nothing short that the correct answer, in such cases I know that whatever note I would leave would be pointless.
- lopmotr 7y agoI used to work in a school system where numerical answers weren't required in physics. Usually, the only reason you'd calculate a numerical answer was to help you check it for reasonableness. If you calculated a wrong numerical answer and didn't explain it, then your grade would be lower than if you had just stopped at the formula. If a question asked you to compare two machines to see which produced the most power, you'd get a top grade writing a symbolic inequality and pointing out in words that some factors are common so they don't affect it, but one of them has an x^2 while the other only has an x, so that for large values of x, that one produces more power, while for small values of x, it's the other one. You'd get a lower grade for calculating two numerical results and identifying the bigger one.
- mncharity 7y agoA feel for reasonable values is rarely an explicit curriculum objective. Perhaps in quantitative introductory biology. When a popular physics text had an ideal-gas-law chapter problem, with numbers describing solid argon, for multiple editions and many years, it was because everyone, checkers, instructors, assistants, and students, was mindlessly plug-and-chugging, not because there was a desire for students to validate simplifications or reflect on the reasonableness of results. There's like more than half a century of professorial letters to the editor, across multiple sciences, complaining of their graduate students lacking a rough-quantitative feel for the field. Maybe physics or engineering could use an effort like https://bionumbers.hms.harvard.edu/ https://bionumbers.hms.harvard.edu/ .
- chiph 7y agoWould dimensional analysis have helped? Specifically the factor-label approach? https://en.wikipedia.org/wiki/Dimensional_analysis https://en.wikipedia.org/wiki/Dimensional_analysis
- de_watcher 7y agoWhat do you expect students to do? Write "I don't think that my answer is correct"? Put no answer in? Or put in a plausible answer that isn't backed up by the calculations?
- drdeadringer 7y ago> there is a surprising amount of engineering going on without scrutiny or peer review I've worked on testing DoD satellites prior to launch. There were comment-level spelling errors [with "no real impact" to actual testing or functionality, honest] which passed through multiple reviews involving 3-4 people each review before being caught -- and even though they were in comments, required a version-uplift for the change. And these were comments for Humans. Imagine if I or someone else buggered up on actual executable software functions. [Hardware bugger-ups are a different similar stories]
- bigmattystyles 7y agoThe mistake is not that one might make an error, everyone will make errors. The mistake is the system that enables an individual contributor to make such an error without any accounting for these normal errors to occur. Second, blaming the person is just scapegoating and a further failure of a flawed system trying to cya.
- spookthesunset 7y agoExactly. If all it takes is one person to misplace a decimal or fsck up a calculation and your entire end product is fucked... brother that ain’t you, it’s the system that is fucked. For any system that matters, you have to prevent a single (or multiple) failures from sinking it. Any new failure that happens should be evaluated in a blameless manner and solved by improving the system so mistakes of that class cannot happen again.
- jessaustin 7y agoI don't know the specifics for thread parent, but on many construction projects there is only one engineer, and even that engineer may be a sub to an architect. In my experience, architects are offended at the suggestion that they check the work of engineers, and engineers are offended by questions from clients. So, entirely due to the emotional preferences of the "one person", that one person is really totally responsible for a great deal.
- bigmattystyles 7y agoI know it’s easier said than done but a good system should recognize and mitigate those personalities.
- MR4D 7y agoMakes it impressive how SpaceX can iterate so fast on designs and yet have few of these types of errors. It seems they understand their failures before they happen.
- 7y ago
- pauljurczak 7y agoover reliance on computer modelling In this case, reliance on computer modelling could have detected the error. Knowing the mass and volume of the sub, modelling the buoyancy is simple. Keeping the model updated and verifying that it can still float should be required for a project of this nature.
- btrettel 7y agoUnfortunately most commercial engineering software is probably too general to know to check whether the sub floats or not automatically. That would require reading the engineer's mind. That check would need to be at least turned on by someone, if not coded into the software itself. And my experience is that few even consider turning these checks on. If you're writing the code yourself, then it should be easy to have all sorts of automated checks. Another HNer recommended using assertions to check that the sub floats when you want it to and sinks when you want it to: https://news.ycombinator.com/item?id=22396621 https://news.ycombinator.com/item?id=22396621
- Luc 7y agoHadn't encountered kips before: https://en.wikipedia.org/wiki/Kip_(unit) https://en.wikipedia.org/wiki/Kip_(unit)
- ggcdn 7y agoVery common in structural engineering, and I imagine that’s about it. Concrete is heavy.
- mcherm 7y ago> Just yesterday I made a trivial error crunching some numbers for when a construction site could turn off their dewatering. And how did you catch this error?
- ggcdn 7y agoIn this case, the result seemed odd, so I checked it by stepping away from the spreadsheets and FEM software, and simplifying the problem down to a quick hand calc. But if I hadn’t done this exact exercise on dozens of buildings, I might not know what ‘odd’ looks like. Luckily, in this field, you can usually come up with a quick check that gets you within +/- 20% of the more sophisticated answer. These kind of sanity checks should be baked into the design process, but unfortunately not everyone seems to do them.
- sitkack 7y ago> These kind of sanity checks should be baked into the design process, but unfortunately not everyone seems to do them. Could you please writeup an essay on this and publish it in an industry journal? This should be required, it should be a social and cultural norm across all engineering professions. The story of saving the Citicorp Center to uncontrolled oscillations from an incoming storm is fascinating. http://www.slate.com/blogs/the_eye/2014/04/17/the_citicorp_tower_design_flaw_that_could_have_wiped_out_the_skyscraper.html http://www.slate.com/blogs/the_eye/2014/04/17/the_citicorp_t... https://www.youtube.com/watch?v=um-7IlAdAtg https://www.youtube.com/watch?v=um-7IlAdAtg
- sunstone 7y agoImportant calculations like these ought be done in at least two independent ways. For example, a detailed version accounting for every nook and cranny and a reality check version that takes things in the large just to make sure both results pretty close. If they're not, then have someone else try again. Having said that, there's the famous (pretty simple) SAT question that all test takers got wrong, even the test setters. Oh, except there was that one guy who wrote "the right answer is not here." (It was a circle rolling around a bigger circle.)
- golergka 7y agoTwo days ago I discovered that Node-based server that I wrote converts user money balance from a number to the string somewhere completely silently, and after that money added to it is treated like string concatenation instead of integer addition. Made me shiver when I realized what was happening, good thing we're not live yet. (It's not real money, I'm a game developer, and would obviously never use JS's floats for real currency. But it still would be a product-killing bug).
- scandinavegan 7y agoThere was a big story in Sweden in 2008-2009 when a bridge collapsed due to a faulty drawing, killing one person and badly injuring two others. Two were construction workers on the bridge (killed and injured respectively) and a person in a car passing under the bridge (injured). The engineer had specified a 7 mm thickness of a part of a metal beam that was supposed to be 25 mm. She argued that the manufacturer should have realized that the drawing was a preliminary draft because it was undated, it was missing other information, and was "clearly" underdimensioned, but it was still manufactured and put in the bridge without anyone noticing until the collapse. The reason the story was extra covered in engineering magazines at the time was because the prosecutor argued that the responsible design engineer should go to prison, and three others (the owner of the design firm, a manager at the construction firm, and a coordinator who led the project) should get suspended sentences. No one from the manufacturing firm was prosecuted. The result of the trial was that the designer got a suspended sentence and a fine, and the design company was fined 1.5 million SEK. All others were freed. Most people agreed that the lack of process was to blame (the design should have been checked by someone else at the design firm, they should have clear markings on preliminary drafts, things shouldn't be manufactured without a formal handshake, and so on), but it's still an interesting question where the line should be on personal responsibility. Some forms of negligence in the workplace should be criminal, especially if it results in people dying, but it's pretty harsh if an individual engineer is personally responsible for putting a 7 instead of 25 (possibly due to copy-paste of work in progress) in a drawing or in a piece of code. It's hard to draw a clear line of the responsibility between the individual engineer and the company that they work for, which should have processes to catch these things. Links (in Swedish): https://sv.wikipedia.org/wiki/Kistaraset https://sv.wikipedia.org/wiki/Kistaraset https://www.nyteknik.se/bygg/ingenjoren-darfor-rasade-bron-i-kista-6408239 https://www.nyteknik.se/bygg/ingenjoren-darfor-rasade-bron-i... (has photos, including the drawing that says 7 mm instead of 25 mm)
- mongol 7y agoI come to think of a other highly publicised case in Sweden, where two employees were to clean a lime kiln and got hot steam and hot limestone dropped on them, one died and one severly burned. This case I think showed more obvious neglect from responsible managers, but no prison sentence was given here either. https://www.nordkalk.com/news/news/archive/2016/04/nordkalk-prosecuted-for-accident-in-lulea-2011/ https://www.nordkalk.com/news/news/archive/2016/04/nordkalk-...
- ryanmercer 7y agoOne time I had a shipment that was a few million JPY and just out of habit I put the currency in as USD, fortunately, the system said "hey dummy, that duty sure seems high" at which point I noticed the mistake and was like ahhhhhhh that could have been really bad. I was paranoid for like a week after that. I can't imagine being part of an error like this or that one Mars mission, oof.