Sensor Devices with High Metrological Reliability

Abstract

At present, a great number of embedded sensor devices provide monitoring of operating conditions and state of equipment, including nuclear reactors of power plants. The metrological reliability of measuring instruments built in equipment determines the validity of measurement information. The quality of production, operating costs, and the probability of accidents depend on the validity of measurement information coming to control systems. The validity is particularly important in such fields as nuclear power engineering, cosmonautics, aviation, etc. For some products in definite periods of their operation, even a short-term loss of confidence in measurement accuracy is unacceptable. The key problems of the measurement information validity are related to the sensor metrological reliability, since their components age and their parameters drift with time. Sudden failures can also happen. All this can result in control errors. The sensor devices used to monitor the condition of technological equipment and the parameters of a technological process, are, as a rule, subject to a variety of influencing quantities. Possible consequences of these influences are, for example, depositions, magnetization, and so on. In some cases, the effect of the influence quantity can be weakened by a careful design of the sensor. For example, the rate of fouling of a sensor surface can be reduced by polishing the surface. However, it is not always possible to develop a sensor device immune to influencing factors over a long period of operation. Sometimes, economic reasons may play a role as well. At present, the traceability of measurements is provided by periodic calibrations or verifications (hereinafter both of these procedures will be referred to as calibrations). Accordingly, within the period of operation the probability of a metrological failure depends on the length of the calibration interval (CI). The state of a secondary transducer can be verified by supplying electrical signals of reference values to its inputs. As demonstrated in (Fridman, 1991), between 40% and 100% of all measuring instrument failures are due to metrological failures. Improvements in production quality result in decrease of the number of failures, the share of metrological failures being increased because with the technology improvement the share of sudden failures decreases. It is not expedient to apply fundamental assumptions of the classical reliability theory (e.g., mutual independence of failure rates and stability of a failure rate) to measuring instruments. Usage of methods based on these assumptions leads to crude errors in the CI determination. To decrease the risk of getting unreliable information, usually the CI is no more than 2-3 years. However, the cost of a sensor device calibration is typically 35–300 euro, and the