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why is the US not Metric
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Cerebus:

--- Quote from: bsfeechannel on November 02, 2019, 04:17:40 pm ---
--- Quote from: Cerebus on November 02, 2019, 04:12:55 pm ---
--- Quote from: bsfeechannel on November 02, 2019, 03:33:34 pm ---4' 11" and 1/16 is exactly 1.5 m. 3/8" is approximately 10 cm. Piece of cake: 1.7 m - wide minimum blind.

--- End quote ---

Want to try that again?  :)

You're trying to work out: 4' 11 1/16" - (2 x 3/8").  3/8" is approximately 1 cm, not 10 cm and the talk of clearance ought to make it clear that the blind is meant to be narrower than the window (recess) not (considerably) wider.

The exact answer is 4' 10 5/16". It's remarkably easy to do in your head as long as one knows 1" = 16/16" and 3/8" = 6/16"; just re-cast it as:
 4' 11 1/16" - (2 x 3/8")
=> 4' 10 17/16" - (2 x 6/16")
=> 4' 10 17/16" - 12/16".

Junior school 'fractions' practice basically.

.

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You see? You prove my point. Imperial is error-prone. Ditch that rubbish.

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No, imperial can't shoulder the blame here for any error proneness. You made errors in your imperial to metric conversion, understanding the question and not noticing that you had a clear "blind longer than window recess" result. Nothing to do with imperial, all your errors where made in the conversion/problem domain. I think anyone who deliberately chooses imperial if they have a choice of using metric is crazy but you clearly demonstrate that understanding and manipulating the problem is much more important than they units you use to express it in.

I'm lucky, I was educated in an era when the UK was actively using both imperial and metric units so I have no difficulty in thinking in either, although I'll pick metric if I have a choice. It also means that I'm primed to immediately notice that your "3/8" is approximately 10 cm" was off - I instinctively know how big 3/8" and 10cm are. I think it's fairly safe to assume that you're 'metric to the core' and so just don't have the instinctive grasp of imperial units that would allow you to immediately spot you'd made a gross error. Thus I think one ought to be very, very careful if forced to work in an unfamiliar unit system. I suspect an 'imperial to the core' American engineer might be prone to just the same errors as you if working the other way around.

What I'll never do is mix the two systems of units in the same job. Your "4' 11" and 1/16 is exactly 1.5 m" is not strictly correct, 4' 11 1/6" is actually 1.500 187 5 m exactly. For fitting blinds 187.5 microns 'off' is neither here nor there but tolerancing in metric and imperial simultaneously is exactly the kind of thing that can bite you in the unmentionables if you try and combine metric and imperial units. If you're working on everyday things one might choose 1/32" as an imperial tolerance and 1 mm as a metric tolerance as 'natural' units if you're working exclusively in one or the other, but if one is using a mixed unit system what does one pick 1/32" = 793.75 microns, or 1mm = 37.37 thou ~ 5/128" (exact = 5/127")? This way lies madness...

The other thing that any metric weenies need to watch out for if forced to use imperial - nominal imperial sizes of threaded connectors are often very much in name only -  a 1/4" bsp (british standard pipe) thread has nothing that measures in at 1/4". Metric threaded connectors are much saner. If I need imperial threaded connectors for something I check three times to make sure I've got the right variant out of the confusing variety of them. My pet hate is things that combine the two systems and then use similar sized but incompatible threads; PC cases typically use both M3 and 4/20 UNC threads at the same time - M3 for the baseboard standoffs, 4/20 UNC for most other things. An M3 screw will start to fit a 4/20 UNC tapped hole and then jam, a 4/20 UNC screw will fit an M3 tapped hole so that it won't pull straight out, but also so that it won't tighten.
CatalinaWOW:
The comments on recipes are intriguing.  In the US virtually all units are volume (cups, fluid ounces, teaspoons and tablespoons). The exceptions are usually for things marketed in a standard size, the 1lb can for example. My experience with European cookbooks, confirmed by the examples given above show a mixture of volume and mass units.  Do cooks resort to scales or balances for these?
Simon:
converting to metric and calculating is not the most accurate way. Not an issue here but it illustrates how scientific calculations can become a nightmare. If the problem is resolved in fractions and the result converted it will be more precise.

I found when doing calculations for my coursework that an error would quikly introduce itself if not enough digits were used and I found myself carrying out the whole calculation in one go on a calculator but breaking the operations down to wirte them out on paper so that the tutor could see the way i had done it. Obviously I never wrote out every last digit and if you calculated in steps like i wrote using the numbers I wrote the result would be different from the one I gave that was carried out in "full resolution" on the calculator with all the digits available.

For example it's not 1500mm exactly but 1500.18mm (and the rest), not worth worrying about for a window blind though.

You can of course just work in inches with decimal point.
xrunner:
Well I'm in the US and I prefer metric myself. That's all I use on my model railroad projects and since I got a 3D printer it's all metric anyways (length and temp). I do have a box of bolts and nuts which unfortunately I've had all mixed up (imperial + metric), I really should sort all that out.

But if I deal with friends it has to go back to inches and Fahrenheit. They could use meters if they had to but when it comes to temps - nope, has to be Fahrenheit. I gave a soldering iron tip thermometer to a friend as a gift and he really cringed that it only displayed Celsius, but has since come around. Most wouldn't have a clue what an average indoor room temp would be in Celsius.  :P
SL4P:
In the US, they have to retain Imperial measures, so they can measure their soft drink bottles and ammunition!

How the 7.62 and 9mm got through I can’t imagine!  Maybe they snuck in after WW2
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