Gravimetric level measurement in vessels

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 when it comes to the particular 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 short 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 form a meaningful value and therefore 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 may appear 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 beyond your vessel. Thus, they do not come into contact with the product. Neither 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 with regards to relative linearity error. This makes dosing possible when, for example, food is filled into bags ? and of course the packaging must contain the amount specified.
Another advantage is that the merchandise will not influence the measurement. The level could be determined gravimetrically for several contents ? solids, liquids, viscous substances, foaming substances ? even toxic or aggressive contents can be considered. 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 ensure 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 low height and a small installation volume. However, it usually is problematic if the vessels are already permanently installed and can only be positioned on modules with a great deal of effort. Then it really is probably advisable to bolt strain transducers to the supports. Any risk of strain transducers determine the compression of the supports due to the weight of the contents. Research 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. Used, however, it could always happen that, for example, struts are absolutely necessary. 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 when compared to product, the more accurate the gravimetric measurement. Finally, free-standing outdoor vessels could be suffering from natural factors. For instance, 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 merely have a minimal price and high accuracy, but the weight can be determined reliably, no matter where it is positioned on the platform. For optimal force introduction, bending or shear beams with the corresponding weighing modules are often installed under tanks, silos or vessels that stand on supports. To determine 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 maximum capacity of which reaches up to 300 t. With four supports, vessels of up to 1,200 t could be weighed with the highest accuracy. Alternatively, vessels can be suspended from S-type force transducers or fitted with strain transducers.
Note
Level could be measured in many different ways. Whether gravimetric as described, or with a submersible pressure sensor, float switch or pressure sensor ? WIKA gets the right sensor for each and every application. Would you like to buy the products? In our WIKA online-shop you will find some of our standard designs. If you have any questions, your contact will gladly assist you to.
Also read our posts
Bending beam and shear beam in weighing technology
Practical application of strain transducers

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