The Development of a Fluid Consistency Transducer

Abstract

A robust fluid consistency transducer suitable for on-line industrial applications has been developed. Its particular application is the monitoring of primary sludge in sewage treatment plants. The transducer uses the effects of fluid loading on a mechanical resonator. This is in the form of a disk (sensor) mounted at the end of a rod about 2.5 m long. The disk is driven by longitudinal waves in the rod generated by making a section of it of magnetostrictive material.

Three classifications can be given to the liquid loading effects on the vibrating sensor:
(a) inertial or stiffness loading
(b) viscous dissipation of energy stored
(c) acoustic radiation of energy stored.

Of these, acoustic radiation has undesired interactions due to back reflections and gas bubbles. It must therefore be minimised. By using low frequencies, the radiation efficiency was made sufficiently small for these acoustic effects to be negligible.

In preliminary experiments, it was found that viscous dissipation was the more sensitive parameter to measure. Thus the change from sludge to water produced a large change in decrement but only a small frequency shift.

Various forms of sensors were investigated.
These included:
(a) a free edge circular plate vibrating in its fundamental flexural symmetrical mode,
(b) a solid centre circular plate vibrating with one nodal circle,
(c) a rigid circular plate in piston type motion,
(d) a sphere in translational motion,
(e) an 'oyster' shell consisting of two circular plates joined at the periphery with a gap in between, the two plates vibrating in phase in the fundamental mode,
(f) a torsionally resonant rod. This was found to interact sufficiently well without further attachment.

A wide range of sensitivities was achieved. The highest is applicable to light mobile liquids and the lowest to heavy thick fluids of which sludge is the extreme case.

Extensive user trials were carried out at Minworth where the separation of the primary sludge could be monitored. Designs were produced which were impervious to the high-speed scrubbing action of sludge. An important economic aspect of the equipment is that the line and electronics are common to the full range of fluids; only the end sensor needs to be changed.

Patent application is being made for the system

Date of Award	Nov 1975
Original language	English
Awarding Institution	Aston University