A servo gauge is used on a tank containing a liquid. The gauge utilizes a displacer suspended on a support wire or tape which is wound on a storage wheel. The storage wheel is balanced by a counterweight. Any change in liquid level results in an imbalance which causes a servo motor to energize and rotate the wheel to bring the counterweight and displacer back into balance. The change in level is determined by counting the steps of motor operation. The changes in level are determined using the relationship between length and steps of motor operation. The level gauge is operable to continually correct calculation to compensate for changes in wire length per turn of the spool.
Numerous types of process instruments are available for measuring level, such as level in a tank. One such instrument is a displacement servo gauge. A servo gauge uses a counterweight or spring to balance the weight of a displacer. The displacer is suspended on a cable from a drum. The drum is coupled to the spring or counterweight. The spring or counterweight is selected so that an equilibrium condition exists when the displacer is at the top surface of the liquid. A change in the liquid level causes a corresponding change in the counterforce to move the gauge out of balance. Upon sensing such an imbalanced condition, a servo motor rotates the drum to raise or lower the displacer, as is necessary, until the gauge is again in balance. Rotational movement of the motor, which represents movement of the drum and thus the cable and displacer, is sensed to calculate the change in level.
Problems have been found associated with prior displacement servo gauges. Particularly, errors in level measurement can occur due to variations in drum diameter, as well as weight of the cable. Specifically, if the cable is wound in multilayers on the drum, then the exact length of cable relating to an increment of rotational movement of the servo motor shaft continually changes. This is due to the difference in circumferential length of the outer layers versus the inner layers when wound on the drum. Further, in the equilibrium state, the applied forces are balanced when the sum of the weight of the cable and the weight of the displacer is equal to the buoyancy force exerted by the liquid and the counterforce of the gauge. The weight of the displacer is a constant. In conventional gauges the counterforce weight is also constant. However, the weight of the tape varies with its extended length, which in turn varies with the liquid level. Thus, with conventional gauges, any change in weight of the tape requires a variation in the buoyancy by changing the penetration of the displacer and the liquid. This change results in an error in the reading of the level of the fluid, reflecting this change of penetration.
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