Method of determining a calibration or maintenance time interval

A method of determining a calibration time interval after which a specific measurement device of a given type for measuring a quantity to be measured on a measurement site of an industrial site shall be re-calibrated, comprising the steps of:

• operating said specific measurement device on said measurement site,
• determining a criticality (C) of the specific device,
• based on the criticality (C) setting a reliability target (RT) for the device, wherein the reliability target (RT) denominates the probability of the device to be compliant according to a predefined criterion at the end of the calibration time interval to be determined by this method;
• defining error ranges for measurement errors (E) of the device;
• selecting a reliability model for a reliability of the device as a function of a normalized time (tn) and a set of at least one parameter (cl, . . . , cm) from a variety of predefined reliability models;
• determining a reliability function for each error range by determining a separate set of parameters for the selected reliability model for each of the error ranges based on prescribed reliability expectation values (RV(tnpd)) for each of the error ranges, which a reliability function associated with this error range shall comply to at least one predefined normalized time (tnpd);
• wherein said sets of parameters (ci, . . . , cm) are determined such, that the resulting reliability functions (R(tn;
• ci, . . . , cm)) cover a normalized area ([0, I]2) defined by reliabilities (R(tn)) ranging from 0 to 1 and normalized calibration times (tn) ranging from 0 to 1,
• repeatedly calibrating said measurement device by:
• during calibration of said measurement device, determining the measurement error (E) of the specific measurement device and based on the measurement error (E) determining the error range corresponding to the measurement error (E) of the specific measurement device;
• determining a normalized calibration time as the normalized time, at which the reliability function determined for the error range corresponding to the measurement error (E) of the specific device and given by the selected reliability model and the set of parameters determined for this error range equals the reliability target (RT);
• and
• determining the next calibration time interval (T), after which the specific measurement device shall be re-calibrated, based on a product of this normalized calibration time and a predetermined reference time interval (Tr).