11 Jul 12 16:10
Uncertainty means that you don't know.
If an analog instrument has a scale of 0-10,000 (you pick the units) and major divisions of 1,000 units, with minor divisions of 500 units then if you see the needle between 5000 and 5500 you have to guess at a value more precise than either 5000 or 5500. That guessing is the uncertainty. Typically acceptable values are half of the smallest increment (i.e., on this instrument any reading that is not divisible by 250 is implying less uncertainty than really exists and valid readings would be 5000, 5250, or 5500).
For electronic instruments we have lost these rules and I often see a 0-10000 unit instrument with an uncertainty of +/-2% of full scale (+/- 200 units) report a value of 5634.67821 units. In reality you know that the reading is between 5400 and 5800, but to imply that you know to 5 decimal places would be laughable if it weren't so sad.
For Square Edged Orifice measurement you have uncertainty in the pressure, differential pressure, and temperature instruments. You have uncertainty in your gas analysis. You have uncertainty in the ID of the meter tube and the bore of the orifice. There is even uncertainty in the constants used in the flow calculation. Combining all of these uncertainty values is described in API 14.3 Part 4 and is quite complex arithmetic. At the end of it there is a graph that shows total uncertainty to be a function of β ratio, nearly linear, and very horizontal in a range between 0.36 and 0.72. Outside this range the uncertainty increases slowly for a little while (which is where the wider range I mentioned above comes from) then turns sharply upward.
David Simpson, PE
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.