Ancient Greek buoys tank level control

London -- The ancient Archimedes principle -- that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid – is now being used to resolve the problem of accurately measuring and monitoring the liquid contents of vessels and tanks.

Measuring and monitoring of liquid levels plays an important role in the majority of processing industries — a point highlighted by the recent official report identifying faulty level gauges as a key cause of the explosion at the Buncefield oil depot.

Currently, load cell weighing systems are the most widely used systems for level measuring and monitoring. However, these present significant problems when fitting, especially retrofitting, to tanks. The main alternative, level measuring systems are much easier to fit but can give inconsistent and low accuracy results.

Among the companies now employing the ‘Archimedes’ technology, Weightron has introduced the Displacement Weighing System (DWS), particularly for applications in the food and drink industry. The unit consists of a stainless steel cylinder suspended from a weight transducer mounted in a stainless steel housing. The unit fits to the top of the vessel with the transducer out of contact with the liquid.

A narrow window runs directly through the liquid to the base of the vessel so that the cylinder can hang freely. The force measured by the transducer is directly proportional to the length of the cylinder submersed in the liquid, according to Weightron.

In effect, the transducer measures the decrease or increase in weight of the cylinder through buoyancy as the liquid level rises or falls respectively, the company explains. By calibrating the system with the vessel empty and full the system provides an accurate reading of the weight of liquid in the vessel at any time, it said.

The results are not affected in any way by external force shunts and installations are achieving accuracies of better than +/-0.03% — compared to +/- 0.2% to 1% for standard level measuring systems, said a Weightron spokesman.

The system is suitable for a wide range of corrosive and non-corrosive liquids in both pressurised and un-pressurised vessels. The transducer can be integrated with a wide range of instrumentation including standard weight indicators and transmitters.

Interfacing with plant control systems and PLCs is very straightforward and a fully ATEX compliant version is available for use in designated hazardous areas, claims Weightron. The HMI, it adds, is designed for multiple vessel installations and the in-built comprehensive data logging capabilities provide essential traceability data..