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Math Help - Combined Uncertainties

  1. #1
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    Combined Uncertainties

    I am looking for some help with a very basic problem. There are three devices used for measuring volume. Device A and Device B measure volumes separately that are then combined and measured by Device C. Device A and B measure with a relative uncertainty of +- 5% with a 90% confidence interval. Device C measures with a relative uncertainty of +- 4% with a 90% confidence interval. When comparing the results from A + B with C what is the acceptable deviation? How far off can A+B be from C while still within tolerance. Thanks for any help in advance.
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  2. #2
    Grand Panjandrum
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    Quote Originally Posted by Kostya View Post
    I am looking for some help with a very basic problem. There are three devices used for measuring volume. Device A and Device B measure volumes separately that are then combined and measured by Device C. Device A and B measure with a relative uncertainty of +- 5% with a 90% confidence interval. Device C measures with a relative uncertainty of +- 4% with a 90% confidence interval. When comparing the results from A + B with C what is the acceptable deviation? How far off can A+B be from C while still within tolerance. Thanks for any help in advance.
    The error from the average of A and B is \pm 5/\sqrt{2}\ \%, the error of the difference of this average from C is \pm \sqrt{(5/\sqrt{2})^2+4^2}\ \% .

    But you really need to consult your notes to know why.

    CB
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