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 in terms of the particular level in vessels. There are many possibilities for weight measurement or weighing. The filling level could 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 comprises 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 form a meaningful value and thus the level depends upon 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 particularly suitable when uneven distribution can occur within the vessels.
Gravimetric level measurement: Advantages
This type of weighing has several advantages. Possibly the greatest advantage is that the force transducers used for gravimetric level measurement can be found outside the vessel. Thus, they do not come into contact with the product. Nor is it necessary to open or modify the vessels. Contamination can thus be eliminated. Cleaning can be much easier. Furthermore, very precise measurement is possible ? 0.02 % of the rated load isn’t uncommon regarding relative linearity error. This makes dosing possible when, for example, food is filled into bags ? not to mention the packaging must contain the amount specified.
Another advantage is that the product does not influence the measurement. The particular level can be determined gravimetrically for all contents ? solids, liquids, viscous substances, foaming substances ? even Vindictive or aggressive contents can be viewed as. Furthermore, the measurement is not falsified even if voids or uneven contents distribution occur inside the silo or tank, as the entire contents are weighed.
Suitable mounting modules not only make sure that the weight force is properly led in to the force transducers, they also make installation very easy. The module and force transducer have a minimal height and a little installation volume. However, it really is problematic if the vessels are already 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. Any risk of strain transducers determine the compression of the supports caused by the weight of the contents. They thus make it possible to weigh immovable vessels gravimetrically.
Gravimetric level measurement: Disadvantages
The disadvantages of gravimetric level measurement vary in significance according 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 example, struts are essential. Then it depends on what great their supporting effect is. Similar problems may appear if the measurement is influenced by a high bearing weight. The lower the weight of the vessel compared to the product, the more accurate the gravimetric measurement. Finally, free-standing outdoor vessels could be affected by natural factors. For instance, if wind pushes or snow results in a significant upsurge in weight.
Typical force transducers for the weighing of vessels
Single point load cells are classically used for scales. These not only have a low price and high accuracy, however the weight is also 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 look for the rated load of the force transducers, divide the total weight of the vessel by the number of supports. If the rated load exceeds 10 t, you’ll be able to switch to compression force transducers, the utmost capacity which reaches around 300 t. With four supports, vessels of up to 1,200 t can be weighed with the best accuracy. Alternatively, vessels could be suspended from S-type force transducers or fitted with strain transducers.
Note
Level could be measured in many various ways. Whether gravimetric as described, or with a submersible pressure sensor, float switch or pressure sensor ? WIKA gets the right sensor for every application. Do you want to buy the products? Inside our WIKA online-shop you will find a few of our standard designs. If you have any questions, your contact will gladly assist you to.
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Bending beam and shear beam in weighing technology
Practical application of strain transducers