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Roger.Knight
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Re: Constants
Roger.Knight   9/18/2014 6:56:36 AM
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Hi MeasurementBlues,

that is exactly the information that an error bar is designed to convey, and as your metrologist has pointed out - the uncertainlty in your initial measurement has to be derived from somewhere. Whether that is a highly formal procees or an engineering estimate is irrelevant in this discussion, the important thing is the mindset that the uncertainty gets transformed along with the value.  Once you have that mindset it can become obvious, say in the case above of converting the units for PCB pad placement, that if you do the conversion of the distance from each pad back to , say, pin 1, then you have the same uncertainlty in the position of each pad.  But if you convert the distance between pin 1 & pin 2, then pin 2 to pin 3.. then the uncertainties grow with each successive calculation

MeasurementBlues
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Re: Constants
MeasurementBlues   9/17/2014 10:52:36 PM
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And the actual value is between what and what?

MeasurementBlues
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Re: Constants
MeasurementBlues   9/17/2014 10:51:09 PM
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@Roger, you need to carry that a step futher by adding a level of confidence.

Calibration and metrology people will tell you that your definition repeated blow is also meaningless. That is, with what level of confidence can you say that the actual value of a measurement is within the upperand lower tolerance limits? Are you 90% sure? 95% sure? 99.99% sure?

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.

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
User Rank
CEO
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
User Rank
Blogger
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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CEO
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
User Rank
Manager
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
User Rank
Blogger
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.

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