In-line weighing system

Eur. Ing. John Turland

Mass is one of the four base quantities of the International System of Units. But the reason for the growing importance of accurate mass measurement to industry is the tendency for more and more processes to be specified in terms of mass.

Whether the product is a pharmaceutical, a tin of paint, or a foodstuff, inevitably, the quality is the result of accurate mass mea-surement during its manufacture. The twin motivations of consumers‘ quality demands and external regulatory inspection have created a constant need to improve measurement of this critical process variable. The result is that the precision with which mass is mea-sured is now expected to be higher than that of virtually any other process variable.

There are several well accepted methods to determine mass. Most of these rely on the measurement of ‚weight‘, i.e. the influence of local gravitational attraction on the item to be weighed. But some, such as Coriolis mass-flow meters measure mass directly. For most practical purposes mass and weight can be considered as interchangeable terms although small variations in the local value of the Earth’s gravitational attraction mean this is not exactly the case.

Weighing is at its most demanding when it is ‚in-process‘, for example, determining the mass of a product inside a process vesseleither for inventory or process control purposes. Techniques in this field can be broadly divided into

• Load cell weighing systems – which can be applied to virtually any type of storage tank, mixing vessel or reactor. With proper care, system accuracies of 0.05 to 0.1% can be achieved.

• Mass flow meters – these can produce good results but are not helpful for measuring directly the mass of a product in a vessel.

• Mass inferred from level measurement – there are several non-gravimetric level measurement techniques (e.g. radar, ultrasonics) and they work reliably as long as, e.g. the effects of product foaming and phase boundaries are considered. But as a means to measure mass, they suffer from the fundamental disadvantage that a separate density measurement is required to compute mass.

However, one mass measurement method offers the accuracy of the load cell but with an approach which overcomes all of the installation considerations of the traditional load cell system – it is the „in-line weighing system“.

In-line weighing exploits Archimedes‘ Principle – i.e. a body immersed in a fluid is buoyed up by a force equivalent to the weight of the fluid displaced by that body. If a „displacer“ of a length equivalent to the required measuring range is suspended in a vessel, then its measured weight will vary according to the mass of the fluid displaced and this, in turn, will provide a measure of the weight of the vessel contents. To turn this concept into a practical instrument requires a means of weighing the displacer which is accurate, unaffected by process pressures and temperatures, and does not degrade the sealing of the process vessel.

The heart of the in-line weighing system is a unique design of load cell. This is flange-mounted at the top of the vessel, with the displacer suspended from a short bending beam inside the cell. The measuring gauges are installed on the outside of the strain gauge carrier tube and this feature seals them fully from the process and allows applica-tions with high process pressure and temperature. Because it is the product and not the vessel which is being weighed, external forces from e.g. stiff pipework have absolutely no affect on the result. Similarly, the supporting structure has no influence and so the contents of a vessel installed on a concrete foundation pad can be weighed per-fectly accurately whereas conventional load cell weighing would demand major struc-tural work. In contrast with some non-gravimetric techniques, an In-line weighing System is not affected by product foaming or phase boundaries. The in-line weighing System is available in two variants. The ILWS provides a strain-gauge output suitable for connection to most types of weighing controller. The ILWT is complete with an integral weight indicator, incorporating linearisation for vessel profiles, and a 4…20 mA output. The integration of a HART interface allows the unit to be calibrated and set up locally or remotely, and all versions are certified intrinsically safe and suitable for all zones and gas groups.

Helios

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Further information cpp-207