Absolute and fractional uncertainty

When you take a reading there is always some uncertainty due to the need for a human to read off whereabouts it comes against a scale division. We write this uncertainty as a +- number after the reading and this is called the ABSOLUTE UNCERTAINTY. In advanced theory, it is written using the small Greek letter delta (squiggly d) as shown below.

 Absolute uncertainties are important if an equation involves ADDING or SUBTRACTING. Suppose we wanted to calculate the temperature change from two readings. First reading = 293+-1K and second reading is 373+-1K. So temperature change is 373-293=80K. Because we did a subtraction calculation we have to ADD THE ABSOLUTE UNCERTAINTIES. So Temperature change = 80+-2K.
You add absolute uncertainties when you subtract two readings; you also add absolute uncertainties when you add two readings.
The FRACTIONAL UNCERTAINTY is shown below

All we would need to do now is x100 to get the percentage uncertainty. So %U = fractional uncertainty x 100.
The rule for fractional uncertainties is the same as for percentage uncertainties. In any calculation where you multiply OR divide readings, you always ADD %U or fractional uncertainties.