Coriolis mass flow metering has been established as the most accurate widely-used industrial flow measurement technology since its introduction in the mid 1980s. Coriolis meters operate (Fig. 1) by oscillating a flow-tube (typically 1-300 mm in diameter), at the natural frequency of a selected mode of vibration, the so-called drive mode. Two sensors monitor the flow-tube vibration as the process fluid passes through. The frequency of oscillation (in the range 50Hz - 1kHz depending on flow-tube geometry) is determined by the overall mass of the vibrating system, and hence for a given flow-tube, this varies with the density of the process fluid. Accurate determination of the frequency of vibration thus enables the process fluid density to be calculated. The geometry of the flow-tube is arranged so that Coriolis forces act to give a phase difference between the two sensor signals, roughly proportional to the mass flow of the process fluid (which in the largest meters may approach 1 tonne/s). While the flow-tube is essentially a mechanical device with a few electrical transducers (sensors and drivers), the transmitter is an electronic and computational device which drives and monitors the flow-tube, and which generates the measurement data. A long-term research programme at the University of Oxford has been developing all-digital transmitter technology [3, 4, 5] with various improvements including a very fast response time [6] and an ability to operate in two-phase flow [1-5]. The transmitter architecture in Fig. 1 includes audio quality analog-to-digital converters (ADCs) and digital-to-analog convertors (DACs), with 24-bit samples delivered at 48kHz. Field Programmable Gate Arrays (FPGAs) are chips consisting of configurable logic blocks, capable of carrying out complex digital algorithms in real time and in parallel. FPGA tasks include interfacing to the ADCs and DACs, generating the drive waveform and pre-filtering of the measurement data. This architecture is used in Invensys Foxboro's commercial product, the CFT-50 Coriolis transmitter, which was used in the trials.
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