Overview

Level measurement is defined as the measurement of the position of an interface between two media. These media are typically gas and liquid, but they also could be two liquids. The first method of level measurement, a few thousands of years ago, consisted of a graduated stick that was referenced to an arbitrary datum line. In more recent times, the glass gage was developed as an evolution of the U-tube principle (this is described further in Chapter 6 on pressure measurement). Eventually, level measurement was used on pressurized tanks by connecting the upper end of the tube to the vessel. With equal pressure in the tube and the vessel, the liquid level in the tube was at the same point as the level in the tank.

Over the years, level measurement technology has evolved, and highly accurate and reliable devices are now on the market. New principles of measurement are being introduced, and existing principles are continuously improved upon. Many parameters need to be considered when applying level-measuring devices, depending on the type of level measurement selected. Ignoring such parameters may result in a measurement with a high error or one with a short life span. Like any item of instrumentation and control, level-measuring devices should be installed where they can be easily accessed for inspection and maintenance.

Level measurement is a key parameter that is used for reading process values, for accounting needs, and for control. Of the typical flow, level, temperature, and pressure measurements, flow tends to...