Uncertainty is the component of a reported value that characterizes the range of values within which the true value is asserted to lie. An uncertainty estimate should address error from all possible effects that is both systematic and random and, therefore, usually is the most appropriate means of expressing the accuracy of results. This is consistent with ISO guidelines. However, in many measurement situations the systematic error is not address and only random error is included in the uncertainty measurement. When only random error is included in the uncertainty estimate, it is a reflection of the precision of the measurement.
Accuracy is a measure of the magnitude of error between the result of a measurement and the true thickness of the item being measured. An accuracy statement predicts the ability of a coating thickness gage to measure the true thickness of a coating to be measured. Accuracy statements provide the performance capability across the full functional measurement range of the gage. Often the measurement range is split into two sections ranging from 0 to a fixed value and then everything greater than that fixed value (up to the gage's measurement limit). Accuracy statements frequently include a fixed portion that remains constant across the measurement range, plus a variable portion that is related to the measurement result for a particular thickness. Such accuracy statements are critical since those with no fixed value imply an exact measurement at zero. To prevent conversion errors, accuracy statements are stated in both their imperial and metric equivalents. The following figure identifies a sample accuracy statement for a DeFelsko gage. Sample Accuracy Statement for PosiTector 6000 FS Gage
Any measurement made with a measuring device is approximate. If you measure the same object two different times, the two measurements may not be exactly the same. The difference between two measurements is called a variation in the measurements. The total error is usually a combination of systematic error and random error. Many times results are quoted with two errors. The first error quoted is usually the random error, and the second is the systematic error. If only one error is quoted it is the combined error. It does not mean that you got the wrong answer. The error in measurement is a mathematical way to show the uncertainty in the measurement. It is the difference between the result of the mesurement and the true value of what you were measuring.