What is Dew point? What is the relation between dew point and relative humidity? Importance of dew point for paint application.

What is the dew point?

 Dew point is the temperature point at which the air can hold no more water (water vapor) and is one of the factors in the heat index. It is always lower or the same as the air temperature.

This temperature depends on the pressure and water content of the air. When the air is cooled below the dew point, its moisture capacity is reduced and airborne water vapor will condense to form liquid water known as dew. When this occurs through the air's contact with a colder surface, dew will form on that surface.

Why does the dew point matter?

It’s important to know the dew point when painting because water must be able to evaporate more quickly than the solvents in the paint for paint to properly cure. When there is too much moisture and humidity for water to evaporate quickly enough, condensation may form during the application and drying process, stopping the paint from hardening.   

Typically, the optimum temperature for applying paint is 6°C above dew point or, for painting metal surfaces, 3°C above dew point. 

How to check dew point?

The easiest and most efficient way to measure dew point is with a dew point meter. These digital devices are capable of estimating the probability of condensation on a surface by determining the dew point.

standards of measuring dew point?

ISO 8502-4 & ASTM D3276-07 

How to calculate dew point?

Let's see the example:

Dew point vs. humidity

The dew point is the temperature to which the air must be cooled in order for it to become saturated. It provides a measure of the actual amount of water vapor in the air – so the higher the dew point, the more moisture in the air.

Relative humidity increases as the air temperature drops to the dew point or the dew point rises to the air temperature (since the humidity is relative to the air temperature). Once the air temperature and dew point meet, the air becomes saturated and the relative humidity reaches 100%. The dew point can never be higher than the air temperature.

Similarly, as the air temperature rises and/or the dew point drops, the relative humidity decreases because the air is getting farther away from saturation.

Bellow chart is relation between dew point and relative humidity:

Importance of dew point for paint application:

For painters and companies who rely on coatings in their manufacturing process, the dew point can present a real headache.
Dew point can affect the way paint cures and dries. When painting below dew point, moisture can condense on the surface while it is still drying. This leads to a surface defects such as a milky, patchy appearance once cured, blistering, delamination, loss of adhesion, or it could even prevent the paint from curing at all.

To avoid dew point related issues with paint, there is several things that can be done. For most coatings it is recommended to paint at least 3 degrees above dew point.

It is important to ensure the surface being painted and the surrounding air is a temperature above dew point, when in an enclosed space use dehumidifiers and heaters to control the environment and maintain a constant temperature and humidity level.

To avoid issues related to dew point and paint, it is important to ensure that the surface being painted and the air surrounding it are at a temperature above the dew point. This can be achieved by controlling the environment in which the paint is applied, such as by using dehumidifiers or heaters to maintain a consistent temperature and humidity level