ISO 6145-7:2018

Overview

ISO 6145-7:2018 specifies a dynamic method for the continuous preparation of calibration gas mixtures using thermal mass‑flow controllers (TMCs). The standard applies to mixtures of non‑reacting species and describes principles, set‑up, operation and calculation of composition and uncertainties. When optimally applied, the method can achieve a relative expanded measurement uncertainty (k = 2) of ≤ 2%. Using pre‑mixed parent gases can extend achievable dilution to mole fractions below 10−6. Note: mass‑flow measurement by TMCs is not absolute and requires independent calibration.

Key topics and requirements

• Principle and device types: covers TMCs with a constant‑current heater and TMCs under constant‑temperature control, including heater, temperature sensors, Wheatstone bridge and control valve functionality.
• Set‑up: one TMC per gas component (dual‑channel units available); use pressure regulators, contamination filters and a mixing vessel. Recommended regulator range: 60 kPa to 600 kPa.
• Operating guidance: operate each TMC close to its full‑scale flow to minimize relative uncertainty - small fractional operation increases uncertainty relative to the actual flow.
• Calculations and uncertainty: methods for converting flows to volume fractions and for estimating sources and magnitudes of uncertainty (volume fraction, measurement uncertainty, uncertainty propagation).
• Range of validity and precautions: applicable only for species that do not react with materials in the flow path; select materials (e.g., corrosion‑resistant diaphragms) suitable for the gases used.
• Annexes: informative guidance on pre‑mixed gases for high dilutions, practical hints, and detailed uncertainty calculations.

Applications and users

ISO 6145-7 is practical for:

• Calibration laboratories preparing traceable calibration gas mixtures for gas analyzers and detectors.
• Metrology institutes and QA/QC teams validating analyzer response and generating multi‑component standards.
• Environmental monitoring, emissions testing and process control groups needing continuous, large‑volume blends.
• Manufacturers of gas‑blending systems and TMC suppliers implementing or verifying dynamic blending performance.

Benefits include continuous production of large quantities, straightforward preparation of multi‑component mixtures, and adaptability when combined with computerized control and gas‑blending systems.

Related standards

• ISO 6145 series (dynamic volumetric methods)
• ISO 6145‑1 (methods of calibration)
• ISO 6143, ISO 12963 (comparison methods for composition), ISO 7504 (vocabulary), ISO 19229 (purity analysis)

Keywords: ISO 6145‑7:2018, calibration gas mixtures, thermal mass‑flow controllers, TMC, gas blending, dynamic methods, measurement uncertainty, gas analysis.