Overview
ISO 7027-2:2019 - "Water quality - Determination of turbidity - Part 2" specifies semi-quantitative methods to assess water transparency (visibility) in laboratory and field settings. It complements ISO 7027-1 (quantitative methods using optical turbidimeters or nephelometers) by describing accessible, indicative techniques used for transparent to slightly cloudy waters and surface monitoring.
Key semi-quantitative methods covered:
- Transparency testing tube (laboratory; Clause 4)
- Transparency testing disc / Secchi disc (in situ surface and marine monitoring; Clause 5.2)
- Diver visibility measurement (visibility at depth; Clause 5.3)
The standard includes apparatus specifications, sampling and handling guidance, test procedures, expression of results and reporting requirements.
Key topics and technical requirements
- Apparatus specifications
- Transparency testing tube: colourless glass, 600 mm ± 10 mm length, internal diameter 25 mm ± 1 mm, graduations 10 mm (Clause 4.2.1).
- Print sample and constant light source (low-voltage tungsten lamp ~3 W) for tube method.
- Transparency testing disc: matt white, weighted to sink; recommended diameters - 20 cm for inland waters, 30 cm for marine waters (Clause 5.2.1).
- Measuring tape/rope with cm marks and optional view-scopes or poles for currents.
- Sampling and sample handling
- Clean bottles filled bubble-free; if stored, keep at 10 ± 5 °C and measure within 24 hours. Allow samples to return to room temperature before testing (Clause 4.2.2).
- Procedures
- Tube: fill tube, lower liquid level until test mark is clearly visible from above; read depth to nearest 10 mm (Clause 4.2.3–4).
- Disc (Secchi): lower disc slowly; record mean of disappearance and reappearance depths; avoid direct sunlight and reflections; best between 10:00–14:00 (Clause 5.2.2).
- Diver visibility: procedure and expression of results detailed for in-water assessments (Clause 5.3).
- Influencing factors
- Results affected by dissolved colour (humic substances), suspended solids, surface glare, wind, currents and individual eyesight - limitations noted in the Introduction and relevant clauses.
Applications and users
ISO 7027-2 is used for:
- Rapid or field-based assessments of water transparency in lakes, rivers, coastal and bathing waters
- Routine environmental monitoring, marine surveys, and preliminary surveys prior to detailed turbidity analysis
- Citizen science and resource-limited monitoring programs where semi-quantitative indication is sufficient
Primary users:
- Environmental laboratories, water quality monitoring agencies, marine researchers, dive teams, consultants and NGOs involved in aquatic monitoring.
Related standards
- ISO 7027-1:2016 - Quantitative determination of turbidity using optical turbidimeters/nephelometers (see note in ISO 7027-2).
- Prepared by ISO/TC 147 (Water quality) - provides harmonized terminology and measurement practices across the ISO 7027 series.
For accurate reporting and comparability, follow device specifications and document apparatus, measurement conditions and any deviations when using ISO 7027-2 methods.
Open preview (PDF)Water quality — Determination of turbidity — Part 2: Semi-quantitative methods for the assessment of transparency of waters