Few measurements are as common and widespread in terms of application, operation, and variety as is level. This measurement is defined as the determination of the position of an existing interface between two media. These media are usually fluids, but they may be solids or a combination of a solid and a fluid. The interface can exist between a liquid and a gas, a liquid and its vapor, two liquids, or a granular or fluidized solid and a gas.
Many techniques are available for the measurement of these interfaces, each with its own trade-offs of advantages and limitations. The best selection depends on the nature of the specific application, including the process to be measured, the degree of accuracy and dependability desired, and economic considerations and constraints. The design engineer must have a working knowledge of the various types of measuring devices available as a guide for the selection and implementation of a system best suited for a particular application. Following is a list of level-measuring methods in present use:
- Hydraulic head
- Variable displacer
- Sonic and ultrasonic
- Fiber optics
- Weight and load cells
- Rotating and vibrating paddle
- Hydrostatic tank gaging (HTG)
With this significant number of types and devices to choose for a specific application in order to select a viable method, it may be a daunting task, especially for a novice or beginner in the field of measurements. This article deals with the most common of level technologies, with a focus on head type and radar.
Donald Gillum has more than 40 years of experience as master instructor, department chair, and division director of instrumentation control systems and automation at Texas State Technical College in Waco, Tex. Prior to this, he spent 10 years at a petrochemical facility as an instrumentation and analyzer engineering technician. Gillum spent two terms on the ISA Executive Board, served as program evaluator and commissioner for ABET, and currently sits on the board of directors for ABET. He wrote the ISA book, Industrial Pressure, Level and Density Measurement, Second Edition. He obtained a B.S. from the University of Houston and is a registered professional engineer in control systems engineering.