Variable displacement sensors.
When a body is immersed or partly immersed in a liquid, it loses weight equal to the liquid weight displaced. Variable displacement level devices utilize this principle by measuring the weight of the immersed displacer.
Archimedes’ Principle
Archimedes’ Principle states that a body immersed in a liquid will be buoyed up by a force equal to the weight of the liquid it displaces. This upward pressure acting on the area of the displacer creates the force called buoyancy.
Principles of Variable Displacement
The float displaces its own weight in the liquid in which it floats. It will sink into the liquid until a volume of liquid is displaced that is equal in weight to that of the float. When the specific gravity of the liquid and the cross-sectional area of the float remain constant, the float rises and falls with the level. So, the float will assume a constant relative position with the level and its position is a direct indication of level. The amount of liquid displaced by variable displacers depends on how deeply the device is submerged in the liquid. With variable displacement devices, the amount of displacement varies with the level of the liquid.
The span of the displacer is the distance that the displacer will respond to the forces of buoyancy. Buoyant force depends on the amount of liquid displaced and the density of the liquid. It is important to note the relationship of specific gravity to the change in weight of the displacer as the level changes. Displacers used in liquids with lower specific gravity will not change weight as dramatically as those used in liquids with higher specific gravity. This is why displacer level measuring systems are not used in applications where they could be immersed in liquids of varying specific gravities.
Liquid-Liquid Interface Measurement
An advantage of variable displacers is that they are capable of detecting liquid-liquid interfaces as well as liquid-gas interfaces. When a displacer is used to determine the level of an interface between two liquids, it is always completely submerged.
Variable Displacement Level Measuring Devices
A displacer must be connected to a measuring mechanism which, when sensing the changes in buoyant force, converts this force into an indication of level. A displacer body can be suspended directly in a tank, or installed in a float chamber on the outside of the vessel. Torque tube displacer level instrument is suspended from an arm that is attached to a torque tube or torque rod. A knife-edge bearing supports the movable end of the torque tube. This type of bearing provides an almost frictionless pivot point. The torque tube must be sufficient strength to support the full weight of the displacer in the absence of buoyancy, or when the level is at minimum. It is a solid or hollow tube that transfers displacer motion to an electronic instrument or a pneumatic instrument that will produce a signal proportional to the changes in the weight of the displacer. Spring balance displacers are devices similar to torque tube displacers. In these devices, the torsional spring of the torque tube is replaced by a conventional range spring. The motion of the displacer is transferred to the indicator by means of magnetic coupling.
Applications
Variable displacement level devices are most often used for local level indication or control. Because displacers are immersed in process fluids, their material of construction must be compatible with the process. Displacers are also extremely sensitive to changes in the density of process liquids. Provisions must be made to measure and compensate for such changes in density when variable displacers are used.