Method of optimizing calibration time intervals between consecutive calibrations of a measurement device for measuring a quantity to be measured operated on a measurement site of an industrial site, comprising the steps of: - defining a given number of discrete measurement error levels (E i) for measurement errors of the device determined during its calibration, - defining a given number of discrete criticality levels (C j), for criticalities of the device determined based on a probability of a defect of the device and a severity of consequences of the defect, - assigning a risk category (R k) classifying a risk involved in operating the device on the measurement site to each pair of one of the measurement error levels (E i) and one of the criticality levels (C j), - determining the measurement error (E D) of the device by performing a calibration of the device and determining the corresponding measurement error level (EL) of the device, - determining the criticality (C D) of the device pertinent to the device on the measurement site of the industrial site and the corresponding criticality level (CL) of the device, - determining the risk category (RC) of the device based on the measurement error level (EL) and the criticality level (CL) of the device, and - in case the risk category (RC) of the device is lower or equal to a maximal allowable risk category (R max) performing a re-calibration of the device after a calibration time interval given by a product of a given reference interval (T R) and a predetermined risk factor (r k) assigned to the risk category (RC) of the device, -- wherein the risk factors (r k) are positive numbers larger than zero and smaller than a predetermined maximal value (r max) larger than one, and -- wherein risk factors (r k) assigned to lower risk categories (R k) are larger than risk factors (r k) assigned to higher risk categories (R k). - defining a given number of discrete measurement error levels (E i) for measurement errors of the device determined during its calibration, - defining a given number of discrete criticality levels (C j), for criticalities of the device determined based on a probability of a defect of the device and a severity of consequences of the defect, - assigning a risk category (R k) classifying a risk involved in operating the device on the measurement site to each pair of one of the measurement error levels (E i) and one of the criticality levels (C j), - determining the measurement error (E D) of the device by performing a calibration of the device and determining the corresponding measurement error level (EL) of the device, - determining the criticality (C D) of the device pertinent to the device on the measurement site of the industrial site and the corresponding criticality level (CL) of the device, - determining the risk category (RC) of the device based on the measurement error level (EL) and the criticality level (CL) of the device, and - in case the risk category (RC) of the device is lower or equal to a maximal allowable risk category (R max) performing a re-calibration of the device after a calibration time interval given by a product of a given reference interval (T R) and a predetermined risk factor (r k) assigned to the risk category (RC) of the device, -- wherein the risk factors (r k) are positive numbers larger than zero and smaller than a predetermined maximal value (r max) larger than one, and -- wherein risk factors (r k) assigned to lower risk categories (R k) are larger than risk factors (r k) assigned to higher risk categories (R k). -- wherein the risk factors (r k) are positive numbers larger than zero and smaller than a predetermined maximal value (r max) larger than one, and -- wherein risk factors (r k) assigned to lower risk categories (R k) are larger than risk factors (r k) assigned to higher risk categories (R k).