A guide to selecting level measurement systems, from Alan Hunt
THE demands of increasingly sophisticated automated processing systems, the need for tighter process control and an increasingly stringent regulatory environment are all driving the search for accurate and reliable level measurement systems. These demands are being met by advances in level measurement technologies offering improved accuracy and reliability.
Ongoing developments in level measurement technology are helping to transform the possibilities for getting an accurate and up-to-the-minute overview of process conditions. Gone are the days when getting a level reading meant personally visiting each and every sight glass around a site. Instead, new devices offer improved level measurement accuracy and present opportunities for reducing chemical process variability, improving product quality, and cutting both costs and waste, as well as satisfying stringent regulatory requirements, for accuracy, reliability and electronic reporting.
One of the best-known industrial level measuring devices is the sight glass. Widely used for many years, these devices have long offered the means for checking levels across many industrial sites. However, as a manual measurement method, these devices have traditionally had characteristics that have limited their application.
The material used for transparency, for example, can suffer catastrophic failure, with ensuing environmental implications as well as hazardous conditions for personnel and/or fire and explosion. Seals are also prone to leaks, and buildup, if present, can make it difficult to read the visible level. Consequently, these, plus other drawbacks, have seen the steady replacement of conventional sight glasses with more advanced technologies.
Other commonly-used level detection devices include those working on the specific gravity principle, where a simple float having a specific gravity between that of the process fluid and headspace vapour floats at the surface, accurately following its rise and fall. Hydrostatic head measurements have also been widely used to infer level.
The benefits offered by new level measurement technologies are seeing traditional mechanical and pressure-based measurement tools being replaced with systems that measure the distance to the fluid surface by a timing measurement. Commonly termed as “time of flight”, this technique measures the distance between the liquid level and a reference point at a sensor or transmitter near the top of the vessel. A pulse wave generated at the reference point travels through either the vapour space or a conductor, reflects off the liquid surface, and returns to a pickup at the reference point. The total travel time is then measured by an electronic timing circuit. Dividing the travel time by twice the wave’s speed enables the distance to the surface of the liquid to be accurately derived.
Magnetostrictive, ultrasonic, laser and guided-wave radar transmitters are among the most versatile technologies available. Each one measures level by using the change of a specific physical parameter at the process fluid surface, such as density, dielectric constant and sonic or light reflection. These technologies make use of the latest electronic techniques and incorporate embedded microprocessor-based digital computers for control, analysis, and communication functions.