In the pharmaceutical, building materials and food industries, on vessels and silos, on platforms and dosing systems, right up to kitchen and bathroom scales ? weight is measured everywhere. In industry, it becomes particularly interesting in terms of the level in vessels. There are various 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 ? Promising ? 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 part of weighing technology.
Gravimetric level measurement is particularly 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 are located outside the vessel. Thus, they don’t touch the product. Neither is it essential to open or modify the vessels. Contamination can thus be eliminated. Cleaning is also much easier. Furthermore, very precise measurement is possible ? 0.02 % of the rated load is not uncommon regarding relative linearity error. This makes dosing possible when, for example, food is filled into bags ? and of course the packaging must support the amount specified.
Another advantage is that the product will not influence the measurement. The level can be determined gravimetrically for several contents ? solids, liquids, viscous substances, foaming substances ? even toxic or aggressive contents can be considered. Furthermore, the measurement isn’t 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 into the force transducers, they also make installation super easy. The module and force transducer have a low height and a small installation volume. However, it really is problematic if the vessels are already permanently installed and will only be positioned on modules with a great deal of effort. Then it really 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 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 can’t be prevented. Ideally, the vessel should be mechanically decoupled. In practice, however, it could always happen that, for example, struts are essential. Then it depends on what great their supporting effect is. Similar problems can occur if the measurement is influenced by way of a high bearing weight. The low the weight of the vessel when compared to product, the more accurate the gravimetric measurement. Finally, free-standing outdoor vessels could be affected by natural factors. For example, if wind pushes or snow leads to 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, however the weight is also determined reliably, regardless of 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 these force transducers, divide the total weight of the vessel by the amount of supports. If the rated load exceeds 10 t, it is possible to switch to compression force transducers, the utmost capacity of which reaches up to 300 t. With four supports, vessels of up to 1,200 t can be weighed with the highest accuracy. Alternatively, vessels can be suspended from S-type force transducers or fitted with strain transducers.
Note
Level can be measured in many various 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? Inside our WIKA online-shop you will discover a few of our standard designs. For those who have any questions, your contact will gladly help you.
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