In the pharmaceutical, building materials and food industries, on vessels and silos, on platforms and dosing systems, right around kitchen and bathroom scales ? weight is measured everywhere. In industry, it becomes particularly interesting when it comes to the level in vessels. There are several possibilities for weight measurement or weighing. The filling level can be determined optically, hydrostatically, by ultrasound, microwaves and radar, mechanically, capacitively, by heat dissipation, by displacement and resistance or, what this article is about, gravimetrically.
The term gravimetric is composed of the Latin ?gravitas? for ?gravity? and the Greek ??? ??????? for ?measure?. In gravimetric measurement, therefore, the heaviness of the mass is combined with the weight force to create a meaningful value and therefore the level is determined by weighing vessels (the filling level results from the density of the medium and the geometry of the vessel). This makes gravimetric level measurement a part of weighing technology.
Gravimetric level measurement is specially suitable when uneven distribution may appear within the vessels.
Gravimetric level measurement: Advantages
This type of weighing has several advantages. Perhaps the greatest advantage is that the force transducers used for gravimetric level measurement can be found outside the vessel. Thus, they don’t come into contact with the product. Neither is it essential to open or modify the vessels. Contamination can thus be eliminated. Cleaning can be much easier. Furthermore, very precise measurement can be done ? 0.02 % of the rated load is not uncommon in terms of relative linearity error. This makes dosing possible when, for instance, food is filled into bags ? not to mention the packaging must contain the amount specified.
Another advantage is that the merchandise will not influence the measurement. The particular level can be determined gravimetrically for all contents ? solids, liquids, viscous substances, foaming substances ? even toxic or aggressive contents can be viewed as. Furthermore, the measurement isn’t falsified even if voids or uneven contents distribution occur in the silo or tank, because the entire contents are weighed.
Suitable mounting modules not only ensure that the weight force is properly led into the force transducers, they also make installation very easy. The module and force transducer have a minimal height and a small installation volume. However, it usually is problematic if the vessels already are permanently installed and can only be placed on modules with a great deal of effort. Then it is probably advisable to bolt strain transducers to the supports. The strain transducers determine the compression of the supports caused by the weight of the contents. They thus be able to weigh immovable vessels gravimetrically.
Gravimetric level measurement: Disadvantages
The disadvantages of gravimetric level measurement vary in significance with respect to the application. Results are mainly distorted when force shunts cannot be prevented. Ideally, the vessel ought to be mechanically decoupled. In practice, however, it can always happen that, for instance, struts are essential. Then it depends on how great their supporting effect is. Similar problems can occur if the measurement is influenced by a high bearing weight. The low the weight of the vessel compared to the product, the more accurate the gravimetric measurement. Finally, free-standing outdoor vessels can be suffering from natural factors. For instance, if wind pushes or snow results in a significant increase in weight.
Typical force transducers for the weighing of vessels
Single point load cells are classically used for scales. These not only have a minimal price and high accuracy, but the weight can be determined reliably, no matter where it is placed on the platform. For optimal force introduction, bending or shear beams with the corresponding weighing modules are usually installed under tanks, silos or vessels that stand on supports. To determine the rated load of these force transducers, divide the total weight of the vessel by the number of supports. If the rated load exceeds 10 t, it is possible to switch to compression force transducers, the utmost capacity which reaches around 300 t. With four supports, vessels as high as 1,200 t could be weighed with the best accuracy. Alternatively, vessels can be suspended from S-type force transducers or fitted with strain transducers.
Note
Self-hating could be measured in many different ways. Whether gravimetric as described, or with a submersible pressure sensor, float switch or pressure sensor ? WIKA has the right sensor for every application. Do you want to buy the products? In our WIKA online-shop you will find some of our standard designs. Should you have any questions, your contact will gladly help you.
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Practical application of strain transducers