Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
D4230-20 – Standard Test Method Technical Guide
📐 Scope and Applicability
ASTM D4230-20 describes a standardized test method for determining the thermodynamic dew- or frost-point temperature of ambient air by condensing water vapor on a cooled surface. This temperature is referred to as the condensation temperature for brevity. The method is applicable across a range of condensation temperatures from 60°C to -70°C and is intended for continuous humidity measurement in the natural atmosphere on a stationary platform. Values are expressed in SI units.
⚙️ Test Method and Instrumentation
The test method utilizes a cooled-surface condensation hygrometer, commonly known as a dew-point hygrometer, which measures humidity by condensing water vapor on a polished mirror surface. The mirror is typically made of a nonhygroscopic material to prevent absorption. Operational procedures include careful site selection and continuous monitoring. Referenced ASTM standards include D1356 for terminology and D3631 for surface atmospheric pressure measurement. The table below summarizes key technical specifications.
| 🔍 Parameter | 📏 Specification |
|---|---|
| Measurement Range | 60°C to -70°C |
| Instrument Type | Cooled-surface condensation hygrometer |
| Platform | Stationary |
| Unit System | SI units |
For accurate measurements, ensure the mirror is clean and free from dust or contaminants that could affect condensation.
📊 Key Terminology and Definitions
This standard defines specific terms essential for understanding the test method. Key terms include nonhygroscopic material, which neither absorbs nor retains water vapor, and mirror, a polished surface for condensate deposition. The following symbols are used in calculations:
| 🎯 Symbol | 📐 Definition |
|---|---|
| e | Vapor pressure of water vapor in moist air |
| ei | Saturation pressure of water vapor over a plane ice surface |
| ew | Saturation pressure of water vapor over a plane water surface |
| P | Ambient air pressure |
| r | Mixing ratio |
| T | Ambient air temperature |
| Td | Thermodynamic dew- or frost-point temperature |
| RHi | Relative humidity with respect to ice |
| RHw | Relative humidity with respect to water |
Users should consult Section 8 for safety precautions, particularly when dealing with high or low temperature extremes.
❓ Frequently Asked Questions
🔍 What is the purpose of ASTM D4230-20?
It provides a standard test method for measuring humidity in ambient air using a cooled-surface condensation hygrometer to determine the dew- or frost-point temperature.
💡 What is the temperature range of the method?
The method is applicable for condensation temperatures from 60°C down to -70°C.
⚡ Is the method suitable for all environments?
It is specifically designed for continuous measurement in the natural atmosphere on stationary platforms, not for mobile applications.
📌 What are the key symbols and their meanings?
Key symbols include e for vapor pressure, ei for saturation pressure over ice, ew over water, P for ambient pressure, and Td for dew- or frost-point temperature.