What is Dewpoint Temperature?

We know that we cannot paint outside if it is raining as moisture on the surface during the application process leads to adhesion problems with the coating. It does not, however, need to be raining for moisture to form on the surface. A bottle removed from a refrigerator quickly forms condensation as the temperature of the bottle, when brought into the warm, moist air, creates moisture on the external surface.

If air is sufficiently cooled, without changing its pressure or moisture content, the relative humidity will increase to 100% and the water vapour in the air will condense. The condensed water is dew and the temperature at which this occurs is the dewpoint temperature (Td).

Calculating the Dewpoint Temperature

By measuring the air temperature (Ta) and the relative humidity of the air (RH), the dewpoint temperature can be calculated.

Once the dewpoint temperature (Td) is determined, the surface temperature (Ts) should be measured.

We can then subtract the dewpoint temperature (Td) from the surface temperature (Ts) and delta T (TΔ ) can be calculated.

Delta T (TΔ) is the primary climate parameter within the coatings industry. It is generally accepted in the coating industry that the surface temperature should be at least 3°C (5°F) above the dewpoint temperature for a coating to be applied.