ISO 9368-1:1990

Overview - ISO 9368-1:1990 (Static weighing systems)

ISO 9368-1:1990 specifies standardized procedures for verifying installations that measure liquid flow in closed conduits using the static weighing method. It defines test methods to assess systematic and random errors, flowrate stability and to calculate the overall measurement uncertainty for static weigh-tank systems. Normative annexes A–E provide worked examples and detailed guidance for error estimation, uncertainty assessment and stability analysis.

Key topics and technical requirements

• Scope and purpose: Procedures for checking flowrate measurement installations that use static weighing (weightank) for calibration and testing.
• Main installation items covered: diverter, weigh/tank, weighing device, receiving tank, timer, pumps and auxiliary instrumentation.
• Verification principle: Separate evaluation of errors for each item, then combine to obtain overall uncertainty - systematic errors corrected where possible and residual uncertainties quantified.
• Checks and methods:
  • Weighing device: checked with standard weights or standard volumetric tanks; successive substitution allowed if needed (see annex A).
  • Diverter: inspected for leak-tightness and correct operation; leakage and operation effects quantified (annex B).
  • Timer: must have sufficient discrimination so timer error in filling time is negligible (guidance includes example 0.01% discrimination; ISO references for high‑precision timing methods).
  • Density measurement: required when converting mass to volume; density accuracy and temperature control affect volumetric flow accuracy (refer ISO 4185 guidance).
  • Flowrate stability assessment: within and between integration intervals using low‑inertia turbine or other stable meters; procedures and worked examples in annexes C and D.
• Uncertainty calculation: Clause 7 and annexes describe combining systematic and random uncertainty components following ISO 5168 principles.

Practical applications and users

ISO 9368-1 is used by:

• Metrology and calibration laboratories performing flowmeter calibration.
• Hydraulic research facilities and test rigs for pump and turbine testing.
• Manufacturers and test houses that certify flow measurement systems.
• National metrology services and legal metrology authorities for certification and periodic inspection of static weighing installations.

Benefits include standardized, repeatable verification, documented uncertainty budgets and improved comparability of flow measurement results across laboratories.

Related standards

• ISO 4185 - Weighing method for liquid flow in closed conduits (fundamental method details).
• ISO 5168 - Estimation of uncertainty in flow‑rate measurement.
• OIML Recommendation 33 - Conventional values for weighing in air.

Keywords: ISO 9368-1, weighing method, liquid flow measurement, static weighing systems, flowrate stability, uncertainty estimation, diverter, weightank, flowmeter calibration.