Processing facilities and other facilities routinely include tanks for storing liquid, solid, or other materials. For example, storage tanks are routinely used in tank farm facilities and other storage facilities to store oil or other materials. Processing facilities also often include tanks for implementing industrial processes. Storage tanks could include above-ground structures and below-ground structures.
Often times, it is necessary or desirable to measure the amount of material stored in a tank. This may be useful, for example, during loading of material into the tank or unloading of material from the tank or during liquid stock accounting. As a particular example, “legal metrology” often requires highly accurate measurements from level gauging instruments installed on the roof of a tank, such as during custody transfers or when levying taxes or duties. In bulk storage tanks, an error of one millimeter in a level reading can correspond to several cubic meters of volumetric error. This can result in losses of thousands of dollars for one or more parties. Moreover, this can have negative effects on stock reconciliation, which involves attempting to track where materials are located and how materials are lost.
Among other approaches, servo gauges have been used to measure the level of material in a tank. A servo gauge is an electro-mechanical type of automatic tank gauge that typically raises and lowers a displacer, which sinks in material within a tank. The servo gauge can determine the level of material in the tank based on changes in the displacer's apparent weight, which changes depending on (among other things) whether the displacer is hanging in the air or submerged in the material.
The displacer of a servo gauge is typically suspended by a thin yet very strong wire, which is often spooled on a grooved measuring drum. The servo gauge rotates the drum to raise and lower the displacer. By continuously measuring the apparent weight of the displacer, the servo gauge can sense whether the displacer is above, partially submerged, or fully submerged. The servo gauge may then attempt to keep the displacer at a fixed position relative to the surface of the material in the tank. By doing so, the servo gauge can calculate the material level in the tank based on the amount of wire spooled off the drum. It may also be possible for a servo gauge to measure the density of different layers of material (such as swater and oil) in a tank.