Overview
ISO 23505:2025 - Petroleum and liquid petroleum products - Calibration of spherical tanks - External electro-optical distance‑ranging method defines a standardized external measurement method using an electro‑optical distance‑ranging (EODR) instrument to calibrate spherical tanks (radius > 1 m). The standard covers measurement procedures, equipment, data processing and the compilation of tank capacity tables for above‑ground, single‑shell, non‑insulated spheres (radial variation ≤ 1%). It can also be applied to spherical tanks carried above deck when a ship is docked. Spheroidal tanks are excluded.
Key topics and technical requirements
- Measurement principle: Determine sphere centre by intersection of vertical bisectors, measure slope distance and horizontal/vertical angles to tangential target points; compute radii from geometric relationships and average results to derive tank capacity.
- Minimum stations: At least 3 stations, equally spaced around the tank; stations should provide large, interwoven viewing spheres and meet station‑to‑wall distance constraints.
- Tangential target points: Minimum 8 evenly distributed tangential points per visible sphere (0°, 45°, … 315°). For radii > 8 m, up to 16 points may be used.
- Equipment requirements: EODR instrument meeting angular and distance accuracy per ISO 7507‑4:2010 (angle accuracy ~1 mgon / 3", distance accuracy without prism ≥ 2 mm), tripod and brace, dip‑tape, single‑corner prisms, ranging pole, paint/plate thickness meters.
- Safety & environmental: Laser class compliance per IEC 60825‑1 (Class 1 or 2) and electrical protection appropriate to hazardous area classification (see IEC 60079‑10‑1).
- Calculations & outputs: Procedures for outer/inner radius determination, average wall thickness, total internal height on vertical centreline, total and partial volumes at reference temperature and working pressure, and development of tank capacity tables.
- Quality considerations: Guidance on calibration uncertainties (Annex A), thermal effects on shells (Annex B), and calibration certificates (Annex C).
Applications and users
- Practical for: tank calibration service providers, metrology laboratories, petrochemical and petroleum terminal operators, surveyors, and classification societies.
- Use cases: custody transfer accuracy, inventory control, regulatory compliance, leak detection baseline, and pre‑insulation calibration.
- Advantages: non‑intrusive external method suitable for large tanks and docked ship tanks, providing reliable capacity tables for commercial and safety applications.
Related standards
Keywords: ISO 23505:2025, spherical tank calibration, EODR, electro‑optical distance‑ranging, tank capacity tables, petroleum tank calibration, external measurement method.