What is the Dew Point Test?

When working with humidity and temperature climate chambers, it is important to have a solid understanding of thermodynamic states and in particular of dew condensation.

In the following article, we will discover what the dew point is and why it is important for our climate chamber tests.

Dew Point Definition

Thermodynamics identifies a series of thresholds that determine the transition of a substance from one state to another. These thresholds are called "points."

The best-known of these is undoubtedly the boiling point , which marks the beginning of the gasification process of a liquid. As we all learn in school, the boiling point of water at sea level is reached at a temperature of 100°C.

The dew point can be considered the inverse because it identifies the threshold below which a two-phase mixture (such as water and air) becomes saturated with vapor, resulting in a change of state. Condensation occurs when the air temperature drops, reducing the amount of energy available to maintain the water in the vapor form, which returns to its liquid state. 

It is what we also call “dew” in common language.

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How is the Dew Point measured?

The dew point is related to three parameters: temperature, humidity and atmospheric pressure.

Assuming that atmospheric pressure is constant (like the example of water at sea level), it is the other two parameters that are decisive.

For example, at 85° and 85% relative humidity, the dew point falls to 80.9°; at 23° the dew point falls to 12° if the relative humidity is 50% but to -10° if the relative humidity is only 10%.

One of the main standards for measuring the dew point of an environment is the ASTM D4230-02.

What is the Dew Point Used For?

The dew point is a crucial threshold for measuring the relative humidity of an environment. In fact, it is relative humidity that causes a gas to become saturated and condenses into dew. As the previous examples show, a simple equation can calculate the relative humidity of an environment by knowing its temperature and observing the dew point.

Meteorologists use the dew point to interpret the climate of an area, but it is also important for a whole series of laboratory tests , not least climate tests that use humidity and temperature chambers.

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The FDM Humidity and Temperature Climatic Chamber

In climatic chambers, temperature plays a fundamental role together with the other essential parameter: relative humidity.

When carrying out climatic tests relating to the corrosion of metals, for example, humidity is the first factor to take into consideration.

For this reason, it is extremely important to ensure that the sample we are testing is always above the dew point to prevent condensation from forming, which would alter the test values.

FDM climatic chambers guarantee the operator full control of all the parameters examined, thanks to our two highly technological controllers: stillFACE and revoFACE.

For further information, please do not hesitate to contact us.

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Reference Standards

• ASTM D1533 — Standard Test Method for Water in Insulating Liquids
• ISO 18434-1 — Condition monitoring and diagnosis — measurement techniques
• IEC 60068-2-30 — Damp heat cyclic (includes dew point cyclic evaluation)
• ASTM E337 — Standard Test Method for Measuring Humidity with a Psychrometer
• ISO 8573-1 — Compressed air — Contaminants and purity classes (include dew point classes)
• VDI/VDE 3514 — Temperature measurement in humidity and gases

Authoritative Sources

• NIST Humidity Calibrations — Metrological References for Humidity and Dew Point.
• ASTM D1533 — Water content measurement reference.
• ISO 8573-1 — Compressed air purity classes.

Frequently Asked Questions About Dew Point Testing

What is the dew point and how is it measured?

The dew point (Tdp) is the temperature at which air reaches saturation (100% RH) and vapor begins to condense. It is measured using three methods: chilled mirror hygrometers (accuracy ±0.1°C, metrological reference), capacitive HMP sensors (accuracy ±1°C, industrial use), and psychrometers (wet/dry bulb, low cost). For compressed air and process gases, ISO 8573-1 certified dew point transmitters are used

Which dew point classes does ISO 8573-1 define?

ISO 8573-1:2010 classifies compressed air into 6 humidity classes: Class 1 dew point ≤ -70°C (critical pharmaceutical), Class 2 ≤ -40°C (electronics), Class 3 ≤ -20°C (clean industrial), Class 4 ≤ +3°C (standard industrial), Class 5 ≤ ​​+7°C, Class 6 ≤ +10°C. Classes 1-3 require adsorption dryers; 4-6 require refrigeration dryers.

What is the difference between dew point and frost point?

The dew point is the condensation temperature above 0°C (droplet formation). The frost point is the freezing point below 0°C (frost forms directly by sublimation). In metrological measurement below 0°C, it is differentiated to avoid errors: at -20°C, the frost point, the absolute humidity is different from the theoretical dew point at -20°C. For precision calibrations, the NIST-traceable Modified Magnus Formula is used for the conversion.

Where is the dew point test used?

Industrial compressed air (ISO 8573-1): humidity class verification for pharmaceutical, electronic, and automotive processes. Insulating oils (ASTM D1533): water in electrical transformers, critical above 30-40 ppm. Natural gas: hydrocarbon dew point for pipelines. Environmental chambers: humidity control validation in extreme ranges. Electronic component testing: condensation prediction and MSL (Moisture Sensitivity Level) for reflow soldering.