Overview
ISO 8573-3:1999 - "Compressed air - Part 3: Test methods for measurement of humidity" defines guidance for selecting and using suitable humidity measurement methods for compressed air. The standard focuses on measurement of water vapour (not liquid or other states), specifying sampling techniques, measurement procedures, evaluation, uncertainty considerations and required test reporting. It also provides guidance for converting humidity statements (e.g., relative humidity to dew point) into the standard format: pressure dew point.
Key topics and technical requirements
- Scope: Measurement of water vapour in compressed air; limitations of methods and guidance on method selection.
- Sampling techniques: Describes full-flow and partial-flow sampling (bypass and extraction) and reduced-pressure sampling. Stresses probe protection from free water and representative flow control.
- Measurement methods: Lists and compares commonly used technologies such as spectroscopic (laser diode), chilled-mirror (condensation) - manual and automatic, chemical reagent tubes, and electrical sensors (capacitance, conductivity, resistance), and psychrometers. The standard notes each method’s contamination tolerance, application context and operational limits.
- Evaluation and conversions: Provides procedures for recalculating measurements to reference conditions (commonly 20 °C and 7 bar(e) unless otherwise agreed), and converting between relative humidity, dew point and mixing ratios (references ISO 7183 tables).
- Uncertainty and quality: Requires consideration of measurement uncertainty (confidence limits, calibration records, and equipment capability). Advises on estimating probable uncertainty and reporting confidence (typically 95%).
- Reporting requirements: Test reports must include system description, sampling point, measurement system and calibration details, the declared pressure dew point (actual and recalculated to reference), reference pressure, stated uncertainty and date.
Applications and users
ISO 8573-3 is practical for:
- Compressed air system designers and operators ensuring air quality for pneumatic tools, instrumentation and production lines
- Maintenance and reliability engineers monitoring dew point to prevent condensation and corrosion
- Instrumentation manufacturers selecting sensors and validating measurement performance
- Test laboratories and quality managers issuing certified humidity reports in accordance with ISO 8573 series
- Regulated industries (pharmaceuticals, food, electronics) where strict compressed air humidity control is required
Related standards
- ISO 8573-1 - Compressed air: contaminants and purity classes (reference for specifying purity levels)
- ISO 7183 - Reference data for saturation pressures/densities (used for conversions)
- ISO 3857-1, ISO 5598 - Vocabulary and terminology related to compressors and fluid power
ISO 8573-3:1999 is essential when you need standardized, traceable compressed air humidity measurement, validated sampling methods, and clear reporting of pressure dew point and measurement uncertainty.
