CALIBRATION OF 24 TON WEIGHING SCALE BY CORIOLIS MASS MASTER METER UNCERTAINTY CALCULATION IN LOAD AND IN LOAD DIFFERENCE
Aart Pruysen
Abstract:
At the MicroMotion Flowfacilities (= division of Emerson Process Management) in Veenendaal – the Netherlands, the Coriolis MicroMotion meters are gravimetric calibrated for the European users via the standing start and stop method. These flowfacilities have a “stand uncertainty” (Calibration and Measurement Capability; CMC) of 0.03%. The biggest scale of this facility, a 24000 kg scale for flowcapacities up till 660 t / h and based on gyroscopic balance, is calibrated since three years via a Coriolis MicroMotion mass master meter. The reason for this method is that placing 3 times 24 * 1 ton weights on the bottom of the tank is a very timely consuming task (2 days); the master meter method is much shorter in time and should hopefully lead to approximately the same uncertainty as performed with weights. The mass master meter method has a longer traceability chain than calibrating with weights so the question was if it is feasible to get again a CMC of 0.03 % at the final end. This paper explains the calibration method; latest results (February / March 2003) and, very surprisely, the reasons why the results are better than obtained from calibration with weights. This paper includes: - Theory of weighing of liquid - Introduction of three independent factors for the weighing scale: Weighing scale factor (WSF); Buoyancy Vapour Correction (BVC) and Immersed Pipe Correction (IPC) - Determination of the uncertainty of scale in load - Determination of the uncertainty of scale in load-difference, based on an agreed calculation method with NMi-VSL (Dutch National Standards institute) - Determination of the CMC of a gravimetric flowcalibration facility under operating conditions - How to minimise CMC for a special case - Results of this performed procedure This paper ends with two conclusions: - Reason why this master meter method gives a smaller uncertainty in relation to the calibration with weights - There is a need for a well defined, written calculation method when a scale is used from one load to another load (load difference). Is currently very dependent of the involved company (authority) for the assessment.
