Level Gage

an instrument used in industry to measure or monitor the level of a liquid or bulk material in a reservoir, tank, or piece of production equipment. Depending on where they are installed, level gages may provide continuous measurement or discrete monitoring of one or several levels. Level sensors serve as level gages in automatic control systems and regulation systems for production processes.

Liquid-level gages are classified according to the principle of operation into mechanical, hydrostatic, electric, sonic, and nuclear types. The simplest design is the gage glass, which uses communicating vessels; it permits direct observation of the liquid level in a closed vessel. Mechanical gages may use float mechanisms, in which the sensing element floats on the surface of the liquid. Other types have buoyancy displacer elements, in which the buoyant force acting on the displacer is measured. The displacement is communicated by means of a mechanical linkage or remote control system (electric or pneumatic) to the measuring system of the gage. In level measurements with hydrostatic gages, the pressure of the liquid column in a tank is balanced against the pressure of another column that fills the measuring instrument or against the reaction of a spring mechanism in the instrument.

Electric level gages may use capacitance- or resistance-measuring devices. In capacitance-measuring gages, the capacitance of the sensing element varies in proportion to the level variation of the liquid. Resistance-measuring gages measure the resistance between electrodes positioned in the medium being measured; the wall of the tank or piece of equipment may serve as one of the electrodes. Acoustic gages use the reflection of ultrasonic vibrations from the interface between the measured liquid and a gas. In nuclear gages the facility containing the liquid to be measured is irradiated with gamma rays from radioactive elements; the intensity of the radiation subsequently measured varies with the volume of the liquid in the facility. All fluid-level gages may be designed for use with both open tanks and pressurized equipment.

The simplest level gages for bulk solids are designed with sensing elements in the form of plates that are in contact with the surface of the substance. Any level change is transmitted remotely to a second measuring instrument. Capacitance-measuring and nuclear gages are also used for this purpose.