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 with regards to the particular level in vessels. There are several 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 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 part of weighing technology.
Gravimetric level measurement is specially suitable when uneven distribution can occur within the vessels.
Untapped : Advantages
This sort of weighing has several advantages. Perhaps the greatest advantage is that the force transducers useful for gravimetric level measurement can be found 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 isn’t uncommon with regard 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 product does not influence the measurement. The level can be determined gravimetrically for all 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, because the entire contents are weighed.
Suitable mounting modules not merely make sure that the weight force is properly led into the force transducers, in addition they make installation very easy. The module and force transducer have a low height and a little installation volume. However, it usually is problematic if the vessels already are permanently installed and may only be placed on modules with significant amounts 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 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. Used, however, it could always happen that, for instance, struts are absolutely necessary. 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 could be affected by natural factors. For example, 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. Worthwhile have a low price and high accuracy, but 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 usually installed under tanks, silos or vessels that stand on supports. To determine the rated load of these force transducers, divide the full total weight of the vessel by the amount of supports. If the rated load exceeds 10 t, you’ll be able to switch to compression force transducers, the maximum capacity which reaches around 300 t. With four supports, vessels as high as 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 can be measured in many different ways. Whether Mainstream as described, or with a submersible pressure sensor, float switch or pressure sensor ? WIKA gets the right sensor for each application. Do you want to buy the products? In our WIKA online-shop you will find some of our standard designs. In case you have any questions, your contact will gladly assist you to.
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