Measurement Of Uncertainty In Iso 17025 Implementation
Before entering in to ISO/IEC 17025 uncertainty concept
, sources and Measures, View the meaning.The error in measurement may dominate result of calibration due to all variables affect the calibration process in calibration laboratory. Uncertainty is calculated to give correct results of instruments calibration for further processing and to build confidence in the measurement taken.
Concept
* 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.
* 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.
* 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.
* 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 measurand. 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) (1.0)
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.
* An estimate of the measurement Y (output estimate) denoted by y, is obtained from Eq.(2.0)
Y =f(x1,x2,....,xn). (2.0)
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.
* The iso 17025 standard uncertainty of measurement associated with output estimate y, denoted by u(y), is the standard deviation of the unknown (true) values of the measurand Y corresponding to the output estimate y. It is to be determined from the model Eq (1.0) using estimate x, of the input quantities Xi and their associated standard uncertainties u(xi).
The ISO/IEC 17025 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.
* 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.
* 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.
Measurement Uncertainty as per iso 17025 standard
It's sometimes claimed that it's inappropriate to report the uncertainty associated with measurements, as clients don't require or understand it. In a customer centred world, it's considered inappropriate to dictate to them. Also, there is sensitivity about revealing to clients for the first time that the data provided for many years is in fact somewhat less certain than previously implied. For many, it would be easier to let sleeping dogs lie, but that is not a satisfactory medium term, let alone long-term strategy. The fact is, it is now widely accepted that having some knowledge about the uncertainty of a measurement is an essential part of making and using measurements.
What's the answer?
There are three issues that need to be addressed.
* First, measurement person themselves need to be persuaded that uncertainty is part of measurement and that effective decision making needs to take measurement uncertainty into account. Second, measurement laboratories need to develop the skills needed to evaluate their uncertainties and to feel comfortable with the fact that measurement, like the rest of life is not a certain process.
* Thirdly, like any other professionals, measurement person need to be able to explain to their clients the value of their work, including its limitations.
Just as uncertainty of measurement defines the region of uncertainty, it also defines the region of confidence. For, example if X Lab report the result of a chemical measurement as 10 2 mg/kg, Y Lab also reporting that that the result is not less than 8 mg/kg. or greater than 12 mg/kg.
Let's start by recognizing some facts of Measurements made to inform important decisions, such as does an imported food comply with regulations, or does a manufactured product meet the client's specification?
* Particularly where the results are close to the decision criteria, reliable decisions can only be made when we have information about the reliability (or uncertainty) of the results. Anything else is a delusion. There is growing recognition of the importance of this issue and as a result laboratories accredited to ISO/IEC 17025 now need to evaluate their uncertainties. Clearly, the more important the decision, the more important it is to have a reliable estimate of the associated measurement uncertainty (MU).
Finally the best measurement capability of Laboratory can gives confidence in the measurement.
by: jamesmiller
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