Eureka - a new approach to vessel weighing

Edited by the Engineeringtalk editorial team Jun 26, 2006

A new technology is opening up exciting opportunities for the accurate weight content measuring in liquid vessels, based around Archimedes' principle.

Measuring and monitoring the liquid contents of vessels and tanks plays an important role in the majority of processing industries.

Traditionally this is accomplished using either well established level measuring techniques or load cell weighing systems.

Load cell weighing systems provide the best accuracy but their fitting can be awkward, especially in retrofit applications.

Lifting vessels or modifying support structures to accommodate the mounting assemblies can be impractical and expensive.

Results will be influenced by force shunts such as pipe work or mechanical restraints and remedying this without major system redesign is often difficult.

Fitting load cells to an established group of vessels is therefore relatively difficult and expensive.

Level measuring is the main alternative and technologies include ultrasonics, capacitance, radar, differential pressure, vibrating forks and magnetic float switches.

(The differentiation should be made between level measuring and level control: a high proportion of level systems are used to provide only high and low level alarms to prevent vessel overflow or running out of raw materials).

Most level measuring systems can be fitted with minimum disruption via the top of the vessel.

Although these methods give reliable results in most applications they have a number of important short comings, especially with the growing demand for high accuracy systems.

For methods such as ultrasonics, which rely on signals reflecting from the liquid surface, the so-called dead band (the distance between the measuring transducer and the highest level that can be detected) can be problematic, especially in small or shallow vessels.

Additional stand pipes above the vessel are required to try to overcome this.

A clear line of sight to the liquid surface with minimum obtrusions over the whole measuring range is also essential.

By definition the technologies are limited to measuring level and, where applicable, converting this to volume.

Results can be affected by factors such as temperature, vapour-liquid interfaces, bulk density changes as well as mixers, stirrers and heaters.

Typical level accuracies are in the order of +/-0.2 to 1% and this can deteriorate further when vessels are being filled or emptied.

Translating these results to accurate weight readings therefore gives inconsistent low accuracy results.

A new technology is opening up exciting opportunities for the accurate weight content measuring in liquid vessels, based around Archimedes' principle.

The displacement weighing system, with its fully stainless steel compact design, is ideally suited to applications in the food and drink industries.

In essence the system is very simple, consisting of a stainless steel cylinder suspended from a weight transducer mounted in a stainless steel housing.

The housing is simply fitted to the top of the vessel with the transducer out of contact with the liquid.

All that is needed is a narrow window 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.

In effect, the transducer measures the decrease or increase in weight of the cylinder (through buoyancy) as the liquid level rises or falls respectively.

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.

The results are not affected in any way by external force shunts and installations are achieving accuracies of better than +/-0.03%.

The system is suitable for a wide range of corrosive and noncorrosive liquids in both pressurised and unpressurised 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.

The powerful HMI is designed for multiple vessel installations and the in-built comprehensive data logging capabilities provide essential traceability data.