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Overall uncertainty of a sum of 2 equal quantities

Question asked by Mantu on Aug 27, 2010
Latest reply on Aug 29, 2010 by Dr_joel
I checked the uncertainty calculation formulae for sum and product of quantities. I want to know the overall uncertainty of measurement in case of sum of 2 equal quantities. Here I assume the uncertainty in one path of quantity x is say u(x) and I have two paths where x quantity flows to give a total of 2x quantity flow.
If I take the total quantity = 2x then the overall uncertainty would be the same as u(x) as 2 is a constant with no uncertainty of its own. However if I take the total quantity as sum i.e. x+x
then the standard uncertainty should be sqare root of [u(x) squared + u(x) squared] i.e. root 2 times x.
Which of the above is correct?  

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