ISO 18213-6:2008

Overview

ISO 18213-6:2008 - part of the ISO 18213 series on nuclear fuel technology, tank calibration and volume determination - specifies a procedure for accurate in‑tank determination of liquid density in accountancy tanks equipped with dip tubes (bubbler probes). The method is applicable to tanks that use pneumatic bubbler systems with two or more probes of differing, fixed lengths. ISO 18213-6 describes a two‑step approach: (1) use a calibration liquid of known density to determine the vertical separation of probe tips, and (2) use that calibrated separation to infer the density of an unknown process liquid from in‑tank pressure measurements.

Key topics and technical requirements

• Scope & applicability: Tanks must have two or more fixed bubbler probes; liquids must be clear, homogeneous, and at thermal equilibrium.
• Physical principle: Hydrostatic relation P = g·H·ρ underpins the method; simultaneous tip pressure measurements from two probes yield probe separation S and, from that, liquid density ρ.
• Two‑step procedure:
  • Calibration step: fill tank with a calibration liquid (demineralized water preferred) whose density is well known; measure pressures to compute probe separation.
  • Density determination: measure pressures of the process liquid and compute density using the calibrated separation.
• Measurement considerations: Standardize readings to reference conditions (temperature, pressure); compensate for effects of bubble formation, gas mass in lines, flow resistance and air buoyancy (detailed adjustments addressed in ISO 18213‑4 and ISO 18213‑5 for slow/fast bubbling).
• Accuracy & uncertainty: With careful technique and water as calibration liquid, relative standard deviation on the order of 10‑4 (~0.2–0.3 kg/m3) is achievable. Clause 8 of the standard addresses uncertainty estimation including contributions from density, local gravity and changes in reference conditions.
• Supporting data: Equation for water density as a function of temperature (valid ~4 °C to 40 °C) is provided for reliable calibration.

Applications and users

• Practical uses: in‑tank density measurement for tank calibration, nuclear materials accountancy, volume determination, process control and verification of inventory for safeguards and regulatory reporting.
• Typical users: nuclear fuel plant engineers, metrologists, tank calibration teams, safeguards/accountancy auditors, calibration laboratories and instrumentation specialists working with pneumatic bubbler systems.

Related standards

• ISO 18213 series: Part 1 (procedural overview), Part 2 (data standardization), Part 3 (statistical methods), Part 4 (height determination - slow bubbling), Part 5 (height determination - fast bubbling).
• Equipment and operating procedures cross‑reference ISO 18213‑1 for required instrumentation and calibration practice.

Keywords: ISO 18213-6:2008, nuclear fuel technology, tank calibration, volume determination, liquid density, in‑tank measurement, bubbler probes, dip tubes, accountancy tanks, demineralized water, uncertainty estimation.