Measurement and Concept of ISO 17025

Before entering in to concept, sources and Measures, View the meaning. The error in measurement may dominate result of calibration due to all variables affect the calibration process. Uncertainty is calculated to give correct results of calibration for further processing and to build confidence in the measurement taken.

Concept of ISO 17025:-
1. Quality of measurements has assumed great significance in view of the fact that measurements  (in a board sense) provide the very basis of all control action. Incidentally, the word measurement should be understood to mean both a process and the output of that process.
2. It is widely recognized that the true value of a measured (or a duly specified quantity to be measured) is indeterminate, except when known is terms of theory. What we obtain from the concerned measurement process is at best an estimate of or approximation to the true value. Even when appropriate correction for known or suspected components of error have been applied, there still remains an uncertainty, that is a doubt about how well the results of measurement represents the true value of the quantity being measured.
3. A statement of results of measurements (as a process) is complete only if it contains both the value attributed to the measured and the uncertainty in measurement associated with that value. Without such an indication, measured results cannot be compared, either among themselves or with reference values given in a specification or standard.
4. The uncertainty of measurement is a parameter, associated with the results of a measurement that characterizes the dispersion of the true values, which could reasonably be attributed to the measured. The parameter may be, for example, standard deviation (or a given multiple of it), or the half - width of an interval having a stated level of confidence.
Y= f(X1,X2,....,Xn)
The model function f represents the procedure of the measurements and the method of evaluation. It describes how values of the output quantity Y are obtained from values of the input quantities Xi.
5. An estimate of the measurement Y (output estimate) denoted by y, is obtained from Eq.
Y =f(x1,x2,....,xn).
It is understood that the input values are best estimates that have been corrected for all effects significant for the model. If not, necessary corrections have been introduced as separate input quantities.
6. The standard uncertainty of measurement associated with output estimate y, denoted by u(y), is the standard deviation of the unknown (true) values of the measured Y corresponding to the output estimate y. It is to be determined from the model Eq using estimate x, of the input quantities Xi and their associated standard uncertainties u(xi).
The standard uncertainty associated with estimate has the same dimension as the estimate. In some cases the relative standard uncertainty of measurement may be appropriate which is the standard uncertainty associated with an estimate divided by the modulus of that estimate and is therefore dimensionless. This concept cannot be used if the estimate equals zero.
7. The standard uncertainty of the result of a measurement, when that result is obtained from the values of a number of other quantities, is termed combined standard uncertainty.
8. An expanded uncertainty is obtained by multiplying the combined standard uncertainty by a coverage factor. This, in essence, yields an interval that is likely to cover the true values of the measurand with a high level of confidence.