A Dozen Ways To Measure Fluid

The demands of sophisticated automated processing systems, the need for ever‐tighter process control, and an increasingly stringent regulatory environment drive process engineers to seek more precise and reliable level measurement systems. Improved level measurement accuracy makes it possible to reduce chemical‐process variability, resulting in higher product quality, reduced cost, and less waste. Regulations, especially those governing electronic records, set stringent requirements for accuracy, reliability and electronic reporting. The newer level measurement technologies help meet these requirements.

The simplest and oldest industrial level measurement device is, of course, the sight glass. A manual approach to measurement, sight glasses have always had a number of limitations. The material used for its transparency can suffer catastrophic failure, with ensuing environmental insult, hazardous conditions for personnel, and/or fire and explosion. Seals are prone to leak, and buildup, if present, obscures the visible level. It can be stated without reservation that conventional sight glasses are the weakest link of any installation. They are therefore being rapidly replaced by more advanced technologies.

Other level‐detection devices include those based on specific gravity, the physical property most commonly used to sense the level surface. A simple float having a specific gravity between those of the process fluid and the headspace vapor will float at the surface, accurately following its rises and falls. Hydrostatic head measurements have also been widely used to infer level.