Determination of optimal calibration intervals by balancing financial exposure against measurement costs

Abstract

Maintenance strategies associated with fiscal measurement systems have traditionally been based on time-based approach when calibration activities are scheduled at fixed intervals without much consideration to the previous performance of measuring instruments. The current economic challenges encourage abandoning the time-based maintenance in favour of other strategies, such as risk-based approach to maintenance. This approach is used to determine an appropriate calibration frequency by balancing financial exposure against measurement costs.There is not a universally applicable single best practice for determination of calibration intervals, this is reflected in a few officially published documents which provide only a general guidance. Addressing the need for better understanding of the mechanism required for determination of the optimal calibration intervals the detailed calculation algorithm suitable for practical use is developed based on the general guidance on risk-based approach to maintenance.The calculation algorithm is based on statistical analysis of calibration data of an individual measuring instrument which defines the progressive change of a measurement error as a nonstationary random process. The measurement error together with the selected loss function defines the financial exposure which forms the ‘total cost’ function by summation with the measurement costs, determined as the cost of ownership of the measurement instrument. A minimum of the total costs function determines the optimal calibration interval of the measuring instrument.The case study of the ultrasonic flow meters, operating in the custody-transfer gas flow measurement system, are used to illustrate the calculation algorithm. It has been shown that the determination of calibration intervals is a complex mathematical and statistical process requiring accurate and sufficient data including calibration results and registered costs. Application of the suggested calculation algorithm can be beneficial in assessment and minimisation of financial risks associated with currently implemented maintenance strategies as well as in review of calibration intervals to balance financial exposure and measurement costs.