Do you know the answer?

Last month I published a question posed by a subscriber/reader inquiring about the practicality of using inductive sensors for process level measurements.

A few subscribers responded and generally agreed it would be difficult if not impossible to apply inductive sensors to process level measurements.

10/16/2003



Last month I published a question posed by a subscriber/reader inquiring about the practicality of using inductive sensors for process level measurements.

A few subscribers responded and generally agreed it would be difficult if not impossible to apply inductive sensors to process level measurements.

Rodolfo M. Foresi of French based Total, a company involved in oil and gas exploration, and production, refining, and base chemicals and polymers summed it up when he wrote,

"Regarding the question on using inductive sensors for level measurement. My experience is confined to process industries, specifically oil and gas, however I see no technical reasons why inductive sensors couldn’t be applied in process industry level measurement.

"That said, I see three major differences between level sensors used in the process industry and inductive sensors used in the steel industry:

First is rangeability; process sensors usually measure levels starting from a few centimeters (inches) and extend to several meters (yards) whereas inductive sensors work almost exclusively in the millimeter range.

"Second is accuracy and repeatability. When a sensor remains relatively close to the point of measurement, such as in the steel industry, the +/- accuracy is a few microns. However, as a sensor gets further and further from the target, accuracy and repeatability become more difficult to achieve. Inductive sensors located a long-way from the target might only be capable of accuracy and repeatability of a few centimeters. This may be sufficient for some process industry applications, but it’s not sufficient for custody transfer applications.

"Third is target suitability. Steel is ferromagnetic, thus it is a suitable target for inductive sensor measurement. Few, if any liquids are ferromagnetic, thus creating a suitable target environment would be challenging.

"Many principles have been used for level measurement, indicating the problem is not easy. Instrument engineers would like for all liquids to be clean, not coating, not viscous, with very low freezing point, etc. Unfortunately liquids such as crude oil aren’t that 'friendly,' thus level measurement related problems arise.

"I do not believe that inductive sensors will become a viable, widely used liquid-level measurement technology in the near, or not so near, future. This leaves the instrument engineer to continue to hold their breath each time a level sensor is specified, anxiously waiting to see if the technology and installation produces the required results.”

—Dave Harrold, senior editor, Control Engineering, dharrold@reedbusiness.com





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