The load cell strikes back

The technological buzz words of the late 20th Century were Quality and Reliability. The 21st Century will still have this ethos, even though it is likely to be tempered by increasing cynicism about whether much of the struggle for quality is designed to keep management consultants and the forestry commission in business.

The precision of measurement is an obvious area where improvements can lead to better control, higher quality, lower product variation and lower costs. One would think, therefore, that the relatively low cost of the best measurement equipment would not receive more than a passing thought.

However, the 1980s-90s were also characterised by an obsession with new technologies and the short term price of everything. This provided an environment in which the load cell method of vessel content measurement took second place to `sexy' coriolis mass flow and radar level determination whose typical instrumentation accuracy of 0.25 per cent often leads a loop measurement capability of no better than 1 per cent. When average performance is not acceptable, alternative measurement solutions such as the tried and tested load cell need to be considered.

Load cells are based on Newton's second law which states that force is equal to mass times acceleration. The acceleration provided by gravity means that a force transducer or load cell can measure one of the fundamental physical properties of matter - mass.

The shear force in an isotropic and homogenous structure is uniform across the structure, and strain gauges bonded to the neutral axis of such a structure will have their resistance altered linearly in proportion to the force. As the structure's weight changes, the load cell system senses the shear force, detects a proportional resistance change and generates an output signal.

Tanks on the battleground

Tank level is a poor indicator of vessel contents, since the level will change with expansion, contraction or other dimensional irregularities, and changes in density are ignored. Level measurements are inherently inaccurate and unsuitable for high performance process control or inventory management applications.

Temperature, pressure and flow measurements have been the traditional focus of process measurements. Historically, mass flow data was obtained using inferential measurements for a first order calculated approximation of mass flow. The coriolis mass flow meter's marketing success provided legitimacy to a more accurate (although intrusive) in-line paradigm and quickly became the measurement of choice where performance expectations were high.

Unfortunately, the promises of the coriolis mass flow meters have yet to be validated consistently below the 0.25 per cent threshold, and their rangeability, maintenance and long term stability are all limited compared with load cells.

A broad range of transducers and companion instrumentation is available for load cells. Transducer selection criteria include consideration of total weight in addition to vessel dynamics, prevailing wind and/or seismic forces.

In conclusion, load cells offer the opportunity to make simple, accurate, cost effective and reliable measurements of the mass contents of all process vessels. The technology is well proven and feedback from the experience of engineering many hundreds of applications has resulted in almost all application problems being identified and overcome. The year 2000 should find load cells back as the first choice for vessel content measurement. PE

Peter Zecchin is process weighing manager at Nobel Systems. The author would like to acknowledge the help of E Laderoute of Nobel's sister company, BLH Electronics, in preparing the article