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Another way to determine the level of liquid or granular solids in a tank is to weigh the tank when it's empty and when it's full and develop a scale. e.g., if a tank weighs 2000 lbs. when it's empty and it weighs 3200 lbs. when it's full, the material in the tank can be calculated to weigh 1200 lb when the tank is full. If the tank has a uniform shape, the material in the tank should weigh 600 lb when the tank is half full, and 300 lbs. when the tank is a quarter full. A ratio can be developed by dividing the total weight of the material by 100%. This means that 1% equals 12 lbs. If the tank is spherical or conical or other non-uniform shape, a table will need to be developed to determine the weight at each height of the tank because the values will not be linear.
The weighing system in this type of application takes advantage of the load cell technology to measure the weight. ill. 1 (below) shows an example of a weighing system used to determine the level of material in a hopper. Notice that this type of sensor depends on the weight of the material to determine the level in the tank. For this reason it's important that the material does not become wet because its weight would not be the same as when dry and the instrument would think the tank was fuller than it actually is. ill. 1 shows examples of these types of weighing systems for a variety of tanks.
Above: ill. 1: (a) Example of weighing system
that measures the amount of product dispensed from the tank into a truck.
(b) The load cells for this system
are mounted in the feet of the tank to measure the total weight of the
tank. (c) An example of two tanks that share load
cells to determine the amount of product in each tank. (d) Example
of load cells in the feet of a spherical
tank to measure the amount of product in the tank. (e) Example of load
cells in cylindrical tanks to measure the amount of product in the
tanks.
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