The principle of servo gauge measurement is based on Archimedes law, which states “any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object.”
Typical servo gauges consist of three compartments: drum compartment, drive compartment, and power supply compartment. The drum compartment contains a precise machined drum on which the measuring wire is wound. The displacer is suspended by this wire into the tank. The drive compartment contains the drive train (servo gauge motor and electronics) and the measurement electronics.
A magnet coupling is used to convey torque between the drive train and the drum and forms the isolation between the process and the electronics and environment. The displacer is moved by a stepper motor in the drive compartment.
As soon as the displacer reaches the surface of the liquid in a tank, buoyancy reduces the apparent weight of the displacer. This is measured by the servo gauge electronics using the force transducer. By also keeping track of the exact position of drum and number of revolutions, the length of the expanded wire can be measured. In combination with the known position of the servo gauge (gauge reference height, GRH), the exact product level can be calculated. This level is then corrected for several typical tank uncertainties, such as product temperature, tank shell effects, changes in GRH due to hydrostatic tank bulging, etc.
This article comes from tanotocentre edit released