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 with regards to 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 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 create 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 can occur 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 beyond your vessel. Thus, Solid don’t touch the product. Nor is it essential to open or modify the vessels. Contamination can thus be eliminated. Cleaning can be easier. Furthermore, very precise measurement can be done ? 0.02 % of the rated load is not uncommon when it comes 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 does not influence the measurement. The particular 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 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 super easy. The module and force transducer have a low height and a little installation volume. However, it usually is problematic if the vessels are already permanently installed and can only be placed on modules with significant amounts 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 depending on the application. Results are mainly distorted when force shunts cannot be prevented. Ideally, the vessel ought to be mechanically decoupled. Used, however, it could always happen that, for example, struts are absolutely necessary. Then it depends on how 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 can 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 useful for scales. These not only have a low price and high accuracy, however the weight is also 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 look for the rated load of the 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 which reaches up to 300 t. With four supports, vessels of up to 1,200 t can be weighed with the best accuracy. Alternatively, vessels can be suspended from S-type force transducers or fitted with strain transducers.
Note
Level can 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 each application. Would you like to buy the products? Inside our WIKA online-shop you will find some of our standard designs. When you have any questions, your contact will gladly help you.
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