Having diagnostic information or logs from the calibration of gas ultrasonic meters is the first step in providing an audit trail of diagnostic parameters that can be monitored to assure meter performance. Confirming these diagnostic parameters are identical when the meter run is commissioned provides high confidence that the calibration from the laboratory has transferred to the field and additional uncertainty due to installation effects need not be applied to the overall system uncertainty calculation. Monitoring these diagnostic parameters, particularly Profile Factor and the AGA 10 Speed of Sound comparison to measured speed of sound for consistency gives high confidence that the meter is performing properly and the initial calibration is still valid. There are different levels of monitoring, from a fully automated CBM system with a 24/7 audit trail, to flow computer or SCADA polling of various diagnostic parameters which can be trended, to manual collection of maintenance logs and archived daily/hourly log files. Gathering and analysing the data can provide information of long term performance such as Profile Factor, often influenced by dirt build up, to sudden changes brought on by flow conditioner blockage or the presence of liquid hydrocarbons. Having operational procedures that include gathering and analysing ultrasonic meter diagnostics can significantly impact a user's ability to maintain accuracy in gas flow measurement. Ultimately, mandatory recalibration intervals can be extended or completely done away with once the diagnostic features and analysis that can indicate when recalibration may be required are accepted by the various metrology or weights & measures authorities.
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