Overview
ISO 8761:1989 - "Work-place air - Determination of mass concentration of nitrogen dioxide - Method using detector tubes for short-term sampling with direct indication" specifies a short-term, direct-reading method for measuring the mass concentration of nitrogen dioxide (NO2) in workplace air. The standard covers detector-tube methods for the nominal range of approximately 1 mg/m3 to 50 mg/m3 and is suitable for personal (breathing‑zone) and general area sampling. It describes reagent systems, sampling procedure, evaluation, calibration and reporting requirements.
Key topics and technical requirements
- Measurement principle: Chemical reaction of NO2 with reagents coated on a solid support inside a detector tube producing a coloured stain; concentration is read from stain length.
- Reagent systems: Common reagent chemistries referenced include N-(1‑naphthyl)-ethylenediamine dihydrochloride, N,N′-diphenyl‑benzidine and o‑tolidine.
- Operational range: Approximately 1–50 mg/m3. Conversion at 293.2 K and 101.3 kPa: 1 ppm = 1.91 mg/m3 (and 1 mg/m3 = 0.52 ppm).
- Precision and performance: Maximum permissible random error (2×SD) for detector-tube measurements is ±25% at ~6 mg/m3; greater relative errors occur at lower concentrations.
- Detector tube and pump requirements:
- Tubes must be uniformly packed, leak‑free, with filling layers perpendicular to tube axis.
- Tubes should meet the manufacturer’s stated stability and expiry; dispose of used tubes per safety guidance.
- Use the detector‑tube pump specified by the tube manufacturer; pump volume per stroke must not differ by more than ±5% from the nominal value.
- Interferences and correction factors:
- Positive interferents include chlorine, ozone, halogens and chlorine dioxide (can produce similar stains); sulfur dioxide can bleach stains at high concentrations (>100 mg/m3).
- Apply correction factors for temperature, pressure and relative humidity and account for known interferents when interpreting readings.
- Calibration and testing: Manufacturers should calibrate tubes with NO2 calibration gas mixtures (Annex A normative). Field checks include pump leak and flow tests.
Applications and users
ISO 8761:1989 is intended for:
- Occupational hygienists and workplace safety officers performing short-term NO2 spot checks.
- Environmental and industrial consultants conducting breathing‑zone surveys.
- Manufacturers of detector tubes and detector‑tube pumps (for product specification and calibration).
- Regulatory or compliance teams needing a standardized short‑term method for NO2 in work-place air.
Practical benefits: rapid on-site indication, simple equipment (detector tubes + pump), and suitability for personal sampling where immediate qualitative/quantitative indication is required.
Related standards
- ISO 6879:1983 - referenced for performance characteristics and related concepts for air quality measuring methods.