🌿 Semi-quantitative assessment of transparency of waters — height of the water column in a 600 mm × 25 mm glass tube at which a print sample or test mark is still clearly visible (transparent and slightly cloudy waters, laboratory), depth of disappearance of a matt white Secchi disc (20 cm in inland waters, 30 cm in marine waters) lowered on a tape or rope in surface waters, bathing waters and waste water, and visibility of a 25 cm disc assessed by divers at a given depth; result in metres to 0.1 m (below 0.5 m — 0.05 m) stating the diameter and type of disc; PN-EN ISO 7027-2:2019-04

Environment PN-EN ISO 7027-2

Overview

WHAT THE STANDARD COVERS. We read the text of the standard in the iTeh sample of ISO 7027-2:2019 (first edition, English text) — PN-EN ISO 7027-2:2019-04 adopts EN ISO 7027-2:2019 and ISO 7027-2:2019 as identical (PKN search): foreword, introduction, Clauses 1–4 and Clause 5 up to 5.3.1.1 — the sample ends on page 5. We do not read the rest of the diver method (5.3), the report or Annexes A (devices) and B (interlaboratory field study results); we know the titles from the contents.

ACCORDING TO THE TEXT OF ISO 7027-2:2019. The standard was prepared by ISO/TC 147 “Water quality”, Subcommittee SC 2 “Physical, chemical and biochemical methods”. The first edition of ISO 7027-2 together with ISO 7027-1:2016 replaces ISO 7027:1999. Introduction: turbidity is caused by undissolved or colloidal matter and small organisms (bacteria, phyto- and zooplankton); it changes the light conditions in surface waters by absorption and scattering and influences their trophic status; semi-quantitative methods serve for indicative assessment of light conditions; the measurement is affected by dissolved coloured substances and particles (e.g. sediments), and with the disc — by reflections on the water surface depending on light and wind. Warning: working in or around water is inherently dangerous; tests must be carried out by suitably qualified and trained staff. Scope (1): a) visual range in the transparency testing tube — transparent and slightly cloudy water (4); b) visual range in the upper water layers with the transparency testing disc — especially surface water, bathing water, waste water, often used in marine monitoring (5.2); c) visibility by divers at a given depth (5.3); quantitative methods (turbidimeters, nephelometers) — ISO 7027-1. Reference (2): CIE S 017/E (International Lighting Vocabulary). Definitions (3): transparency (permeability to light — here as visibility in water), turbidity (reduction of transparency of a liquid by undissolved or colloidal matter and small organisms), attenuation coefficient (fraction of a beam absorbed or scattered per unit thickness, in m⁻¹). Laboratory (4): where measurement on site is not possible; colourless glass tube 600 ± 10 mm, internal diameter 25 ± 1 mm, graduated every 10 mm, with an outlet to lower the level; shield against lateral light; print sample (black print on white, characters 3.5 mm, line 0.35 mm) or test mark (e.g. black cross); constant light source — low-voltage tungsten lamp 3 W; clean bottles (if necessary washed with hydrochloric acid approx. 1 mol/l or a detergent solution), glass or plastic, filled completely without bubbles; measure as soon as possible, and if storage is needed — in a cool, dark place (10 ± 5) °C for no longer than 24 h, bring to room temperature before measurement, avoid contact with air; tubes clean, not clouded, optically identical; the sample is mixed by hand without bubbles, poured into the tube and the level lowered until the print is clearly visible from above; read the height, mean of replicates; result to 10 mm with the manufacturer of the apparatus. Disc (5.2): the depth at which a matt white disc is no longer visible is a measure of the transparency of surface waters; the reading is not an exact measure — sun glare, current and differences in eyesight of the staff influence it; the method was developed by A. Secchi (1865) and modified by G. C. Whipple (1899) — “Secchi depth”; in strongly tidal waters and reservoirs with turbidity currents the results for phytoplankton are not very informative, and humic substances can considerably reduce transparency; sinking disc (e.g. 1.7 kg), hanging horizontally on a tape or rope, clean, without scratches; recommended diameters: inland waters 20 cm (e.g. six holes or black and white sectors), marine 30 cm (without holes or sectors); other discs — results not comparable; within a programme and sites always the same type, preferably the same staff; tape with centimetre scale or rope with marks every 10 cm (usually at least 10 m), regularly checked against a length standard; optionally a pole and weight in running waters, view scopes suppressing reflections. Procedure (5.2.2): look from a small height, on the shaded side of the boat or dock, without direct sunlight, best between 10 am and 2 pm; allow the eyes time to adapt; lower the disc slowly, determine the point at which it is just visible (if necessary the mean of disappearance and reappearance), looking perpendicular to the surface; slow movement does not stir up sediment; read on the tape; mean of replicates; water depth preferably at least 50 % greater than the Secchi depth (water background, not bottom reflection); a view scope allows working on the sunny side, but the user must demonstrate whether it improves the result; polarizing glasses absorb much light; influencing factors: material between surface and disc, state of the surface, sky reflection, reflection of the water body, surface and diameter of the disc, sun altitude and cloud cover, wind and waves, height of the observer, adaptation of the eye, shadow of the boat. Result (5.2.3): in metres, mean of measurements, stating diameter and type of disc; rounded to 0.1 m, below 0.5 m — to 0.05 m; examples: 4.6 m (Ø 20 cm, six holes), 0.45 m (Ø 30 cm, plain white). Attenuation coefficient (5.2.4, marine environment): where it cannot be measured, it is estimated as x / Secchi depth (m); x depends on the sea area and increases with decreasing salinity; recommended values 1.7, 1.84 or 2.3. Divers (5.3.1): white 25 cm disc, clean; another disc — diameter in the result designation (e.g. visibility at 5 m with a 30 cm disc).

STATUS (as of 03.10.2026). The PKN search engine (query “PN-EN ISO 7027-2”, 03.10.2026) lists PN-EN ISO 7027-2:2019-04 — English version, “Introduces: EN ISO 7027-2:2019 [IDT], ISO 7027-2:2019 [IDT]”, without a withdrawal note.

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database, data up to 17.09.2026, read 03.10.2026). The number 7027-2 appears in 2 records at 2 laboratories: AB 362 and AB 646. In the current PCA documents (read 03.10.2026; AB 646 issue no. 26 of 29.06.2026, AB 362 issue no. 28 of 16.07.2026) the number appears at the same 2 laboratories; in every document the number of page markers equals the number of PDF pages, without repetitions; a search of all PCA documents downloaded from BIP on 03.10.2026 found no laboratories outside the database copy. The form “PN-EN ISO 7027-2:2019-04” with clause 5.2 at both (“pkt 5.2” and “punkt 5.2”). None of these items is suspended. The “Laboratories” tab (query “PN-EN ISO 7027-2”) shows both laboratories — checked by a tab query on the labcoda.pl production server on 03.10.2026.

WHAT THE LABORATORIES TEST (items in PCA). AB 362: water — transparency by the standard transparency disc method (0.5–10.0) m. AB 646: surface water — transparency (visibility of the Secchi disc), direct measurement (0.1–10) m.

WHERE A SEEMINGLY CORRECT RESULT IS EASY TO GET. Disc: the result depends on the diameter and type of the disc — other discs give non-comparable results, so diameter and type are reported with the result, and a monitoring programme always uses the same one (5.2.1.1, 5.2.3). Lighting: direct sun, reflections, wind and waves, polarizing glasses and lack of eye adaptation change the reading (5.2.2) — the standard requires the shaded side and recommends 10 am to 2 pm. Bottom: in shallow water the disc is seen against the bottom — the water depth should be at least 50 % greater (5.2.2). Tape: a rope without periodic length checks against a standard carries its error into every result (5.2.1.2). Rounding: below 0.5 m the result is rounded to 0.05 m, above — to 0.1 m (5.2.3); the lower end of 0.1 m (AB 646) lies in the 0.05 m zone. Attenuation coefficient: the estimate from Secchi depth depends on the chosen x (1.7–2.3) and the sea area (5.2.4). Interpretation: in waters with much mineral suspended matter or humic substances transparency says little about phytoplankton (5.2, Note 3). Method: it is a semi-quantitative method — it does not replace turbidity measurement with a turbidimeter according to ISO 7027-1 (1).

WHAT WE DO NOT GIVE. We do not quote the full procedure of the diver method, the devices of Annex A or the field study results of Annex B — they lie outside the sample. We do not summarize ISO 7027-1 or CIE S 017 in this entry. We do not give transparency requirements for bathing waters or water quality classes.

Method principle

Transparency is assessed visually as the distance over which a standard object can be seen through a layer of water. In the laboratory the water level in a graduated glass tube is lowered until a print sample under the bottom becomes clearly visible from above; in the field a matt white disc is lowered on a tape and the depth at which it disappears is read (Secchi depth); divers assess the visibility of a disc at a given depth. The result is given in metres with a description of the device, because it depends on the disc diameter and the observation conditions.

Applications

• Transparent and slightly cloudy waters — transparency testing tube in the laboratory (1, 4)
• Surface waters, bathing waters, waste water and marine monitoring — Secchi disc (1, 5.2)
• Assessment of light conditions and trophic status; estimation of the attenuation coefficient in the sea (introduction, 5.2.4)
• Water and surface water — transparency with a disc — 2 accreditation scope records of 2 laboratories in PCA

Key parameters

Standard

Standard number

PN-EN ISO 7027-2:2019-04

Title (PL)

Jakość wody — Oznaczanie mętności — Część 2: Metody półilościowe do oceny przezroczystości wód

Title (EN)

Water quality — Determination of turbidity — Part 2: Semi-quantitative methods for the assessment of transparency of waters

Step-by-step procedure

1. Choice of method

   Tube (laboratory), disc (field) or divers (1). PN-EN ISO 7027-2:2019-04 — no withdrawal note (PKN, 03.10.2026).

2. Observation conditions

   Shaded side, no sun, eye adaptation, perpendicular line of sight (5.2.2).

3. Disc measurement

   Slow lowering, point of disappearance and reappearance, reading on the tape (5.2.2).

4. Tube measurement

   Lowering the level until the print is clearly visible, reading the height (4.2.3).

5. Result

   Mean; disc — m to 0.1 (0.05) m, diameter and type; tube — 10 mm and manufacturer (4.2.4, 5.2.3).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• Working in or around water is inherently dangerous; tests are carried out by qualified and trained staff (warning)

Frequently asked questions