Hi Can anyone enlighten me on how to combine these values into an uncertainty budget. I have a calibration certificate for a flow meter that states an uncertainty with K=2 of 0.2% of reading, resolution of 0.001 kg and a repeatability of 0.01%. I am having trouble getting my head around how to produced a combined uncertainty from these values. I require the combined uncertainty for this budget to be used in another budget which will have the flow rate included. ?
If I just take the values as given, I would do the following, knowing that 0.2% Rdg =
0.002000 2000 ppm = 0.002 x rdg. k=2 is the expanded uncertainty with approx. 95% confidence. We need to convert the values to their standard uncertainties before combining.
Let's say the reading is 5 (looks like kg.).
If k=2, then [(0.002 x 5 kg) / 2] = standard uncertainty of 0.01 kg; assumes normal distribution.
Resolution (assumes digital roundoff) = 0.001kg/2 = ?0.0005, containment limits; 0.0005/sqrt3 = ?0.00029 kg standard uncertainty; rectangular dist.
Repeatability (of rdg) = 0.0001 x rdg = 0.0001 x 5 kg = standard uncertainty of 0.0005 kg.
To combine, sqrt[(0.01^2)+(0.00029^2)+(0.0005^2)] = 0.01 kg, combined std unc (the standard deviation of the combined uncertainties). Multiply by 2 and you have 0.02 kg expanded uncertainty with approx. 95% confidence for the reading (2 std deviations).

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