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Dew Point or Relative Humidity: Who Knows? Who Cares?

WEDNESDAY, APRIL 24, 2019

By Don Schnell, Polygon US


On coating projects, there can be confusion over the usefulness of these two different ways of measuring humidity in the air. Many specifications mention both units of measure, but they seem to conflict.

Let’s clarify with some definitions.

Dew point temperature is the temperature at which condensation will occur in a particular air sample. It is important to recognize that when the air temperature is lowered to the dew point temperature for that air sample, the water will find somewhere to condense. This is when dew forms on the top of a tank, for example.

Photo: Shutterstock

Consider what is happening when you see condensation on your favorite cold drink. The air adjacent to the surface of the can has cooled to the dew point temperature and, therefore, must release its moisture. The surface of the can is a convenient landing pad for that moisture. The condensation will not form when the dew point temperature is below the temperature of the surface of the can. (This is why refrigeration-type dehumidifiers work.)

Relative humidity, on the other hand, is relative to temperature and is a measure of how much moisture is in the air as a percentage of how much water the air can hold at its current temperature. Air that is at 50 percent relative humidity contains half of the water that it can hold at that temperature. When you cool the air, you lessen its ability to hold water. If the air is cooled to 100 percent relative humidity, the water begins to transition from a gas to a liquid, otherwise known as condensation. If there is no convenient surface on which the liquid can condense, it rains. At 100 percent relative humidity, the dew point temperature equals the dry bulb temperature.

So how does this information apply to a coating project? When your surface temperature approaches the dew point temperature, corrosion rates increase and condensation can form. Science tells us that when the relative humidity exceeds 50 percent, metals corrode at a much faster rate. Psychrometrics show us that at any temperature, when the dew point temperature is around 20 degrees below the surface temperature, the relative humidity at the surface is around 50 percent.

When dry abrasive blasting, to preserve the blasted surface, it is necessary to maintain the dew point temperature at a level 15-20 degrees below the surface temperature. This is the same as maintaining the relative humidity below 50 percent at the surface. It’s important to remember the relative humidity only matters right at the surface, which is almost impossible to measure accurately. This is why we most often compare the surface temperature to the dew point temperature.

If you are applying coating or curing coating, it is only important to control condensation. In theory, this would dictate a difference of less than 1 degree between the surface temperature and the dew point temperature of a relative humidity of not more than 99 percent at the surface. In practice, we know that the instruments are not that accurate, nor are the instrument users. This is why many coating specs require that the surface temperature be not less than 5 degrees above the dew point temperature.

*Claims or positions expressed by sponsoring authors do not necessarily reflect the views of TPC, PaintSquare or its editors.

 

ABOUT THE AUTHOR
Don Schnell, Polygon US

Don Schnell is a regional sales director for Polygon US, a temporary climate control provider. He directs and coaches salespeople in seven company offices on the East Coast of the U.S. Services include humidity and temperature control for industrial coatings projects, commercial construction, water damage restoration and many other commercial and industrial applications. With more than 25 years in the commercial building and industrial arenas, he has helped pioneer much of the dehumidification technology used today in construction drying applications. Don has several published articles on various climate control subjects and has presented technical papers at many national events. He entered the industrial coating business after attending three years at the North Dakota State University School of Architecture. Hobbies include golf, bass fishing and woodworking.

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