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 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 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 is determined by 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 may appear within the vessels.
Gravimetric level measurement: Advantages
This sort of weighing has several advantages. Perhaps the greatest advantage is that the force transducers useful for gravimetric level measurement are located beyond your vessel. Thus, they do not touch 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 can be done ? 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 ? not to mention 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 all contents ? solids, liquids, viscous substances, foaming substances ? even toxic or aggressive contents can be considered. Furthermore, the measurement isn’t falsified even though 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. Brilliant and force transducer have a minimal height and a small installation volume. However, it usually is problematic if the vessels already are 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 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 based on the application. Results are mainly distorted when force shunts can’t be prevented. Ideally, the vessel should be mechanically decoupled. Used, 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 can be suffering from natural factors. For example, 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 useful for scales. These not merely have a minimal price and high accuracy, but 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 determine 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 maximum capacity of which reaches around 300 t. With four supports, vessels as high as 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 every application. Do you want to buy the products? In our WIKA online-shop you will find a few of our standard designs. When you have any questions, your contact will gladly help you.
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