--- title: Compressed Air Quality and Dew Point (ISO 8573-1) url: https://www.sarveshanalytics.com/learn/compressed-air-quality-and-dew-point.html description: Compressed air quality classes for water, oil and particles under ISO 8573-1, pressure dew point targets for each class, dryer types and where to measure dew point. updated: 2026-11-02 source: html --- Learn · Process primers # Compressed air quality, dew point and ISO 8573-1 Wet compressed air corrodes pipework, freezes outdoors and spoils products. Dew point is the number that says how dry it is. Beginner 3 min read Lesson 11 of 13[Process primers](https://www.sarveshanalytics.com/learn/track-process-primers.html) Helpful first: [Moisture measurement methods compared for dry gas](https://www.sarveshanalytics.com/learn/moisture-measurement-methods-compared.html) Quick answer ISO 8573-1 classifies compressed air purity by particles, water and oil. For water, classes 1 to 6 correspond to pressure dew points of minus 70, minus 40, minus 20, plus 3, plus 7 and plus 10 C or lower. Refrigerant dryers reach about plus 3 C; desiccant dryers reach minus 40 C or lower. A dew point analyser after the dryer catches dryer failure before moisture reaches the process. Updated 2 November 2026 · Figures from Sarvesh datasheets **After this lesson you can** - State the ISO 8573-1 water classes and their dew points - Compare refrigerant and desiccant dryers - Place a dew point analyser - Explain the pressure dew point ## ISO 8573-1: three contaminants The standard classifies compressed air by solid particles, water and total oil, with a class number for each. A specification such as 1.2.1 gives the class for particles, water and oil. Class 0 means a stricter value agreed with the user. ## Water classes | Class | Pressure dew point, at or below | |---|---| | 1 | minus 70 C | | 2 | minus 40 C | | 3 | minus 20 C | | 4 | plus 3 C | | 5 | plus 7 C | | 6 | plus 10 C | Always a **pressure dew point**: the dew point at line pressure. The same air at atmospheric pressure has a much lower dew point: [Dew point to ppm calculator for gases](https://www.sarveshanalytics.com/calculators/dew-point-to-ppm-calculator.html). ## Dryers - **Refrigerant dryers** cool air to about plus 3 C and drain the water; they suit general plant air (class 4). - **Desiccant dryers** pass air through an adsorbent that holds water, reaching minus 40 C (class 2) or minus 70 C (class 1); they suit instrument air, pharma, electronics and outdoor lines in cold climates. - **Membrane dryers** remove water by diffusion for small flows. ## Where to measure - **After the dryer,** to confirm the class and catch failure of a desiccant bed or a refrigerant dryer. - **At the point of use** for critical processes, since pipework can add moisture. - **On instrument air,** since a wet instrument line can freeze or corrode. ACCURRA-S M015 reads dew point from minus 80 to plus 20 C with a ceramic sensor, per the catalogue, at line pressure. It is supplied without a pump and has no auto-calibration, so it suits a pressurised line: [Compressed-air and instrument-air dew point (ISO 8573)](https://www.sarveshanalytics.com/applications/compressed-air-and-instrument-air-dew-point-iso-8573.html). ## Good habits Use stainless steel sample lines, short, with no dead legs: [Sample lines: materials, length, diameter and lag](https://www.sarveshanalytics.com/learn/sample-lines-materials-length-and-lag.html). State whether a dew point is at line pressure. Check the dew point against the class limit on a regular basis, and alarm when it rises: [Moisture measurement methods compared for dry gas](https://www.sarveshanalytics.com/learn/moisture-measurement-methods-compared.html). **In short** - ISO 8573-1 water classes 1 to 6 run from minus 70 C to plus 10 C pressure dew point. - Refrigerant dryers reach about plus 3 C; desiccant dryers minus 40 C or lower. - Measure after the dryer and at critical points of use. - Quote pressure dew point at line pressure. ## Check your understanding Answer in your head or on paper first, then open the answer. Recalling an answer teaches more than rereading. **What pressure dew point does water class 2 require?** Minus 40 C or lower. **Why use a dew point analyser after the dryer?** To catch dryer failure before moisture reaches the process. **Why must dew point be stated at a pressure?** At lower pressure the same air has a much lower dew point; the numbers differ by many degrees. ## Key terms - [Pressure dew point](https://www.sarveshanalytics.com/learn/glossary.html#t-pressure-dew-point) - [Dew point](https://www.sarveshanalytics.com/learn/glossary.html#t-dew-point) - [ppm](https://www.sarveshanalytics.com/learn/glossary.html#t-ppm) ## Frequently asked questions **What dew point is instrument air?** Often class 2, minus 40 C pressure dew point or lower, depending on the application and climate. **What is the difference between refrigerant and desiccant dryers?** Refrigerant dryers cool to about 3 C; desiccant dryers adsorb water to reach minus 40 C or lower. **Which analyser measures compressed air dew point?** ACCURRA-S M015, minus 80 to plus 20 C. **Does ISO 8573-1 cover CO and CO2?** Gaseous contaminants are handled in other parts of the standard; medical and breathing air have their own specifications. ## Related products [ ### ACCURRA-S Online analyser for trace, percent and purity O2, CO, CO2, CH4 and dew point; can measure two gases from one sample. View ACCURRA-S ](https://www.sarveshanalytics.com/safe-gas-analyser.html) ## Where this applies - [Compressed-air and instrument-air dew point (ISO 8573)](https://www.sarveshanalytics.com/applications/compressed-air-and-instrument-air-dew-point-iso-8573.html) - [Medical-air and breathing-air quality](https://www.sarveshanalytics.com/applications/medical-air-and-breathing-air-quality.html) - [ACCURRA-S M015 moisture analyser](https://www.sarveshanalytics.com/trace-moisture-analyser.html)