ISO 6145-8:2005

Overview

ISO 6145-8:2005 - "Gas analysis - Preparation of calibration gas mixtures using dynamic volumetric methods - Part 8: Diffusion method" defines a dynamic diffusion technique for producing very low‑concentration calibration gas mixtures. The standard covers generation of component mole fractions from 10‑9 to 10‑3, achieving a relative expanded uncertainty (k = 2) of ≤ ±2 % when procedures and controls in the standard are applied. It is intended for organic liquids (boiling points ≈ 40 °C–160 °C), hydrocarbons at ambient/indoor levels, and low‑concentration water vapor standards.

Key topics and technical requirements

• Principle: A liquid calibration component evaporates in a reservoir and diffuses through a diffusion tube into a controlled flow of a complementary gas. Composition is calculated from the diffusion mass flow and complementary gas flow.
• Concentration range: Mole fractions from 10‑9 to 10‑3.
• Uncertainty target: Relative expanded uncertainty U (k = 2) not greater than ±2 %.
• Apparatus & materials:
  • Diffusion cells and tubes preferably made of borosilicate glass to minimize sorption/desorption.
  • Use chemically inert tubing for gas delivery and transport.
• Diffusion tube dimensions (recommended ranges):
  • Length L > 0.03 m
  • L/diameter > 3
  • Diameter between 0.001 m and 0.02 m
• Flow & thermal control:
  • Complementary gas temperature in containment controlled (≈ ±0.15 K) to meet uncertainty requirements.
  • Keep the flow regime such that convective transport is avoided (recommend keeping Reynolds number below ~100).
  • Minimize path length between diffusion source and point of use to reduce adsorption losses.
• Mass‑flow determination:
  • Continuous weighing (suspension microbalance or load cell) or periodic weighing with pressure corrections for ambient variations.
  • Clean handling, equilibration (typically ≈ 24 h) and regular checks for drift (replace cell if diffusion mass flow drifts > 1 %/month).

Applications and users

• Calibration laboratories preparing low‑level gas calibration standards for analytical instruments.
• Environmental and indoor‑air monitoring labs needing trace hydrocarbon or humidity standards.
• Manufacturers of gas analyzers and calibration devices validating instrument response and accuracy.
• Research groups developing reference materials or evaluating sorption/transport phenomena in low‑concentration gas systems.

Related standards

ISO 6145 is a multi‑part series on dynamic volumetric methods. Relevant parts include:

• Part 1 (methods of calibration), Part 2 (volumetric pumps), Part 4 (syringe injection), Part 5 (capillary devices), Part 6 (critical orifices), Part 7 (thermal mass‑flow controllers), Part 9 (saturation), Part 10 (permeation), Part 11 (electrochemical generation).

Keywords: ISO 6145-8:2005, diffusion method, calibration gas mixtures, gas analysis, low‑concentration standards, diffusion cell, complementary gas, trace hydrocarbons, humidity calibration.