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Economical, Nonintrusive Measurement of Process-Material Volumes in Process Vessels

Stage: Development

A process vessel for a high-solids slurry may employ a complete-sweep agitator, which fully sweeps a vessel’s entire volume, to ensure the total mixing of chemicals. These agitators, however, present challenges for the implementation of commercially available process-level-measurement instrumentation to measure process-material volume. Existing mechanical level floats, radar or sonar measurements, differential pressure measurements, load cells, and nuclear level gauges all suffer from implementation and/or interpretation difficulties. Researchers at NREL have developed a device and method to determine the process-material volume within a vessel. NREL’s nonintrusive, economical, and easy approach measures the gas-void volume within an enclosed process vessel that may contain varying amounts of liquid or solid process material. The volume of process material in a vessel is the difference between the empty-vessel volume and the measured gas-void volume.

While several different commercially available techniques exist to measure level or volume in a vessel, these techniques are often incompatible with full-sweep-agitator vessel construction. Elevation instruments include physical floats, radar, sonar, capacitance, and differential pressure (liquid head), but these suffer interference from the full-sweep agitation. Process-mass measurement techniques may use load cells to measure the gross weight of the vessel with process fluid, but for small vessels, such as pilot-scale equipment, the signal-to-noise ratio tends to be low. External piping/tubing connections that can be bumped or moved also make using load cells problematic. While nuclear level gauges can measure the radiation absorption by a process fluid, nuclear level gauges are expensive, require special use permitting, and provide possibly difficult-to-interpret information for non-uniform process materials.

NREL’s device to measure vessel-void volume uses only a few simple and relatively inexpensive components: a gas-accumulator vessel of known volume, selector valves, and pressure and temperature sensors. After the accumulator’s pressurization with a gas, its pressure is equalized with the pressure in the unvented process vessel. Measurement of the initial and final pressures and temperatures within the process vessel and accumulator yields the void volume through the ideal gas law. The volume of process material in a process vessel is the difference between the empty-vessel volume and the measured gas-void volume.

To learn more about Economical, Nonintrusive Measurement of Process-Material Volumes in Process Vessels, please contact Eric Payne at:

ROI 20-39.

Applications and Industries

  • Laboratory or industrial chemical processing
  • Process-vessel manufacturers
  • Solution-phase chemical reactions
  • Mixtures and colloids
  • Biochemistry and biosynthesis


NREL’s device and method

  • Use inexpensive pressure and temperature instrumentation,
  • Are compatible with full-sweep agitators,
  • May be preferable for small laboratory or pilot-scale process vessels,
  • Use a differential measurement scheme for accurate and fast volume measurements, and
  • Measure the empty volume—not the fluid level—in a process vessel for direct information about the filled volume.