This invention relates to single vibrating tube transducers which are typically used to measure mass, density, or mass flow.
Transducers of this kind are very well known. One example is described in our patent specification GB No. 2 062 865. Such transducers exploit the well known fact that fluid flowing in a vibrating tube which is clamped at its two ends will produce a phase difference between the transverse displacements of the upstream and downstream ends of the tube. The mass flow rate of a fluid in such a tube is a linear function of the ratio A of the phase difference to the resonant frequency of the tube, where
A=.phi./F
.phi. being the phase difference and F being the resonant frequency of the tube. The mass flow rate M is then give by EQU M=a A+b
where a and b are constants and are independent of the fluid type and flow rate.
Transducers of this type are subject to variations in accuracy due to external vibrations and shocks when they are used in hazardous environments. Problems have also been encountered due to the vibrating tube setting up an acoustic resonance between the tube and the instrument casing. Another problem is the fact that antinodes of vibration may be set up at points on the tube distant from the means for sensing the vibrations and will thus cause inaccuracies in phase difference measurement.
Known measures (GB No. 2 062 865) for overcoming some of these problems include slight ellipticity of the tube to define the vibrational mode; this imposes manufacturing problems. The nodes are typically clamped by nodal masses and in GB No. 2 062 865 these masses are extended towards each other around the vibrating tube and connected by a flexible bellows to form a sealed enclosure which can be evacuated to present contamination of the components by dust etc. which may cause inaccuracies. Maintenance of an evacuated enclosure imposes problems in use. Moreover the extended nodal masses have a centre of gravity displaced from the nodal point of attachment to the tube and shocks can be communicated to the tube too easily.
The object of the present invention is to provide more satisfactory solutions to the problems dealt with above.
The present invention is defined in the appended claims to which reference should now be made.