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Goodbye, Fundamental Kilogram & Ampere

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Roger.Knight
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Re: Constants
Roger.Knight   9/15/2014 4:41:38 AM
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I'm so glad someone finally mentioned tolerances.  I remember my tutor telling us at the end of the first year of my physics degree "we only tried to teach you one thing this year: the importance of error bars in measurement". They stressed that a measurement was totally meaningless unless it had an associated tolerance or error bar, and any calculation or transformation on the measurement must also be done on the tolerance figure, so at the end you know the accuracy bounds of your result - so no guessing how many decimal places to quote.

mhrackin
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Re: Constants
mhrackin   9/12/2014 11:13:26 AM
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And that is why the true derivation of the "IRS" acronym is "Infinite Resource of Stupidity."  That's one reason I support the Fair Tax.

zeeglen
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Re: Constants
zeeglen   9/12/2014 10:31:22 AM
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Quoted from USA IRS Form 1040-ES

"When estimating your 2014 net earnings from self-employment, be sure to use only 92.35% (.9235) of your total net profit from self-employment."

That's right, make a rough estimate of what you expect to earn by the end of this year, then calculate with four (yup, four) significant figures. 

zeeglen
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Re: Metric/English converstions
zeeglen   9/12/2014 10:12:09 AM
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Many cheap digital calipers display to .001 inch and 0.1 mm, depending on selection of measurement units.  Since 0.1 mm is approximately .004 inch, it is better to do the measurement using the inch display then convert to metric with a calculator - four times better resolution.

Wnderer
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Re: Metric/English converstions
Wnderer   9/12/2014 9:38:32 AM
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PCB design is another place where rounding can get you into trouble. For example when you're creating a footprint in mils from a drawing in mm. If you convert the average spacing to mils and round to the nearest mil and then place the pins using this average spacing, the pins at the end will be off. You need to convert each individual pin location from mm to mils and then round.

bk11
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Re: Constants
bk11   9/12/2014 9:28:39 AM
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I've come across schematics where the designer used, for example, a 4.99k resistor in series with a 10 ohm resistor, because the design called for 5k.  They were both 1% tolerance.

Bill_Jaffa
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Re: Metric/English converstions
Bill_Jaffa   9/12/2014 8:14:13 AM
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1.00 inch = 25.4 mm exactly: that's the legal definition of the inch. So resolution and precision are defined by that relationship.

MeasurementBlues
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Metric/English converstions
MeasurementBlues   9/12/2014 8:07:29 AM
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Following this line of conversation on digits.

WHenI was responsible for deisnging a portable printer for the Psion Organiser, the mechanical edngineer who designed the case drew the diagrams in inches but the cases were being made in taiwan and I had to convert to millimeters. The question was , how many digitis did one need? So we had drawings with three decimal places in inches and three decimal places in millimeters. I'm sure we didn't need such precision.

 

MWagner_MA
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Re: Constants
MWagner_MA   9/12/2014 7:26:15 AM
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I agree I see this much too often on engineering drawings from mechanical engineers (sorry guys :-)) running 3D cad tools.  They are so engrossed in running the tool I see silly things like a dimension of 3.001 with a 20mil tolerance.

Bert22306
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Re: Constants
Bert22306   9/11/2014 6:54:01 PM
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There's never an excuse for false precision, but there are plenty of examples of needing a lot more than just three significant digits. The immediate examples that come to mind are any navigation system, even ancient ones where you had to manually flip through almanacs.

The more we rely on automation, the more the need for accuracy and precision becomes acute.

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