What is the unit of water content in compressed air

Analysis of compressed air moisture content unit

The moisture content of compressed air is a core indicator to measure the degree of air dryness, and its unit of measurement needs to be comprehensively understood based on the testing principle and application scenarios. Here is a professional explanation of the commonly used unit system and technical connotation:

1. Mass concentration unit

1. g/m³ (g/cubic meter)
   • Definition: Under standard conditions (0℃, 101.325kPa), the mass of water vapor contained in each cubic meter of air
   • Application: It is often used to estimate the amount of condensate water in engineering calculations. For example, 1g/m³ water content corresponds to 1000m³ air producing 1 liter of condensate water per hour
2. mg/m³ (mg/cubic meter)
   • Conversion relationship: 1g/m³ = 1000mg/m³
   • Accuracy expression: Precision detection instruments (such as phosphorus pentoxide sensor) can display to the order of 0.1mg/m³

2. Volume concentration unit

1. ppmV (parts per million by volume)
   • Definition: The volume of water vapor accounts for parts per million of the total air volume
   • Conversion relationship: 1ppmV ≈ 1.2mg/m³ (under standard conditions)
   • Typical value: The moisture content of atmospheric air is about 1000- 10,000 ppmV, and the compressed air needs to be controlled below 100ppmV
2. %RH (relative humidity)
   • Definition: Ratio of actual water vapor partial pressure to saturated water vapor partial pressure at the same temperature
   • Industrial standard: The dryness of compressed air is usually required to be ≤20%RH (corresponding pressure dew point ≤-20℃)

3. Temperature parameter units

1. ℃ (degrees Celsius)
   • pressure dew point: At a certain pressure, the temperature at which the air is cooled to saturation is the core indicator to measure the dryness of compressed air
   • atmospheric dew point: The dew point temperature after air is expanded to atmospheric pressure, used to assess the risk of condensation in the air tank
   • Typical requirements: Instrument gas needs to reach a pressure dew point of-40 ℃, and corresponding water content ≤ 0.07g/m ³

4. Example of unit conversion
Take the pressure dew point of-20 ℃ as an example:

1. Looking up the table, the corresponding saturated water vapor partial pressure is 1033 Pa
2. Calculated volumetric concentration: 1033/101325 ≈ 1.02% → 10200 ppmV
3. Converted mass concentration: 10200 ppmV × 1.2mg/m ³/ppmV ≈ 12.24g/m ³
4. Equivalent expression: water content 12.24g/m ³ or 12240 mg/m ³

5. Selection of testing methods and units

1. on-line monitoring：
   • Capacitive humidity sensor: Direct display of %RH or pressure dew point
   • Resistance-capacitance dew point meter: Accuracy up to ±0.5℃, suitable for dew point range from-80℃ to +20℃
2. laboratory testing：
   • Mirror condensation method: Determine the dew point temperature through optical detection, accuracy ±0.1℃
   • Weighing method: Calculate water content by weighing difference before and after drying, and use it as an arbitration test method

Enterprises should select testing units based on specific application scenarios:

• Ordinary pneumatic equipment: adopting %RH or pressure dew point (℃)
• Precision manufacturing: Use mg/m³ or ppmV
• Special process: Pressure dew point and mass concentration need to be marked simultaneously

By establishing a water content unit conversion table, data from different testing instruments can be compared with each other to ensure that the quality of compressed air continues to meet the requirements of ISO 8573-1.