The Role of Dew Point in Sustainable Environmental Management

THE ROLE OF DEW POINT IN SUSTAINABLE ENVIRONMENTAL MANAGEMENT

Overview »» The Dew Point determines the preservation environment that can be achieved because General Recommendation it determines what combination of Temperature and Relative Humidity can be achieved for Preservation: »» Temperature (T), Relative Humidity (RH), and Dew Point are interrelated Make it as cool as you can »» At a constant Dew Point – as the T rises, the RH falls, and as the T falls, the RH rises while maintaining a moderate »» Dew Point is typically the limiting factor of a mechanical system’s ability to provide a RH – understanding Dew good environment for long-term preservation of collections Point is the key.

Understanding Dew Point The Dew Point is the temperature atDew which Point air containing a specific amount of water becomes saturated. Dew Point can be used as an indicator of the total moisture content in an environmentDew Point Think of the diagrams below as a series of “buckets” – the bucket size increases or decreases with temperature (A), while the amountConcept of water inA: the bucket remains the same regardless of the temperature change (B). Concept A: Air Temperature Air Temperature 55o 60o 65o 70o 75o 80o 55o 60o 65o 70o 75o 80o (A) Capacity to hold water The capacity of air to hold water Capacity to hold water increases as air is warmed, and decreases as air is cooled

Concept B: Concept B: Air Temperature (B) Air Temperature 55o 60o 65o 70o 75o 80o 55o 60o 65o 70o 75o 80o The actual amount of water in the air does not change with changes in air temperature Actual water present Actual water present

Use IPI’s Dew Point Calculator to learn more about how dew point, temperature, and RH interrelate at: www.dpcalc.org

Relative Humidity is the percentage of the bucket that is full based on the moisture content of the air (actual water present) and the capacity of the airConcept to hold of water Relative (based Humidity on temperature) (C).

Air Temperature

Relative Humidity represents how saturated the air is with Absolute Humidity Actual water present water vapor (.0092 lbs/lb)

Relative Humidity

Air Temperature Move up 45o 50o 55o 60o 65o 70o 75o 80o to humidify

Capacity to The environment is at its Dew hold water Point when the air is completely Absolute Humidity Actual water (.0092 lbs/lb) saturated – i.e., 100% RH. The present

dew point can be reduced by 100% 84% 70% 59% 50% 42% cooling the air below the current Capacity to dew point (dehumidifying hold water to remove moisture), which reduces the capacity of the air Absolute Humidity (.0076 lbs/lb) Actual water to hold moisture (shrinking the present bucket). Excess moisture in the system will condense out. The 100% 84% 69% 58% 50% 42% 35%

lower the dew point, the lower Capacity to the RH can be maintained at hold water cool temperatures.

Absolute Humidity Actual water (.0062 lbs/lb) Move down present to

100% 84% 68% 58% 48% 41% 34% 29% dehumidify

Special Thanks to Peter Herzog of Herzog/Wheeler & Associates, St. Paul MN, for helping IPI develop these tools and concepts