Theory Of Operation - Siemens SITRANS F Operating Instructions Manual

Description

3.5 Theory of operation

3.5 Theory of operation
The Coriolis principle of measurement
The flow measurement is based on the Coriolis law of motion. Particles moving in a rotating /
oscillating system will resist imposed oscillations in a manner consistent with their mass and
velocity (momentum). Oscillation produced by a Coriolis flowmeter where the process media
is accelerated around bends results in phase distortions of the measuring tubes.
The SITRANS F C sensors are energized by an electromagnetic driver circuit which
oscillates the pipes at their resonant frequency. Two pickups are placed symmetrically on
either side of the driver to provide position signals for digital processing.
When the media flows through the sensor, Coriolis force will act on the measuring tubes and
cause deflection which can be measured as a phase shift between Pickup 1 and Pickup 2.
The phase shift is proportional to the mass flowrate.
The frequency (or period) of the vibration is a direct function of the process media density.
The frequency and amplitude of the driver is regulated to ensure a stable output from the 2
pickups. The temperature of the sensor tubes is measured to provide accurate
compensation for changes in the material stiffness. As a result the process media
temperature is also accurately measured.
The flow proportional phase signal from the pickups, the temperature measurement and the
driver frequency enable calculation and reporting of mass, density, volume, and temperature.
Digital signal processing (DSP)
The analog to digital conversion takes place in an ultra low noise sigma delta converter with
high signal resolution. With fast digital signal processing massflow and density values are
calculated using a patented DFT technology (Discrete Fourier Transformation). The
combination of this patented DFT technology and the fast DSP enables short response time
(< 10 ms) to changes in the measured values.
The built-in noise filter is configurable and can be used for improving the performance of the
flowmeter, in case the installation and application conditions are not ideal. Typical process
noise such as gas bubbles (two-phase-flow) can be reduced through the filter functions.
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SITRANS FC430 with HART
Operating Instructions, 03/2012, A5E03361511-01