Gunter Honig

Mass flow meter with vibration sensor

The mass flow rate of a fluid, for example a gas or air, for example passing through a duct or the like, is determined by placing a vibrating element in the path or stream of air flow, and measuring the damping thereof due to the air flow; the damping can easily be measured, in accordance with the invention, by connecting the vibrating element in a circuit having a servo system including a controlled variable amplifier and a comparator, such as a proportional-integral (PI) controller (14) comparing the signal on the vibrating element with a reference establishing a vibrating amplitude and, in turn, controlling the amplification of the variable amplification amplifier (8) to maintain a constant amplitude of vibration, the control signal controlling the variable amplification amplifier being a measure of the damping, and hence of mass flow rate. To eliminate damping due to extraneous conditions, such as clamping of the mechanical vibrating elements, connection of electrodes and the like, an alternating current (10a) can be fed to the vibrating element, the alternating current having a frequency which is high with respect to the vibration frequency, and then being filtered out by a band-pass filter (17) tuned to the alternating current frequency, the level of the signal across the vibrating element being subtracted from the overall damping signal to thereby eliminate errors due to inherent damping of the mechanical system independent of fluid flow.

Vibrator damping flow meter

An electromagnetically excited vibrator with arms carrying baffle members at their ends that are immersed in the flow to be measured is mounted at one or more vibration nodes to reduce loss of vibratory energy to the casing of the flow meter. A flat magnetically conducting cross shape can be mounted at a node in the middle, provided each cross beam of the cross is vibrated in flexure, and the two cross beams vibrate in phase opposition (counter stroke). Such a vibrator is excited by a similarly crossed pair of E-shaped cores with exciter and secondary windings on the middle legs of the cores, or by a coaxial re-entrant core with permanent magnet wafers set in the rim at respective locations separated by an air gap from the arms of the cross. In each case the vibrator can be firmly affixed to the central leg or legs of the core. A two-ended rod system can be excited by magnetostriction, operating differentially on a split middle portion of the core, while the latter is supported at two nodes located between the split middle portion and the respective ends that carry baffle members. Evaluation circuits in all cases measure the vibration damping caused by fluid flow.