The amount of moisture the air can hold is dependent on the atmospheric pressure. Relative Humidity (RH) is the measure of how much moisture is in the air divided by the amount of moisture the air can hold times 100. The difference between the wet bulb and dry bulb temperature, therefore, gives a measure of atmospheric humidity. When relative humidity is low (air is dry) evaporation increases and the cooling effect is greater. When the relative humidity is high, evaporation is low and there is less of a cooling effect. The WBT will vary with Relative Humidity (RH). When moisture evaporates, it cools the environment, reducing the temperature slightly. The wet-bulb temperature (WBT) measures the temperature that results from evaporation, and it is directly related to relative humidity. This is the temperature that you would get in the shade and not the temperature in direct sun. The dry bulb temperature (DBT) or ambient temperature is the temperature of the air. The sling psychrometer measures two parameters, Dry Bulb (ambient temperature) and Wet Bulb.

Even with laboratory grade sling psychrometers, the expected error is in the 5% to 7% range (ASTM E337-84) and it would be expected to see even greater errors with the psychrometers typically used on coating projects. Sling psychrometers used by meteorologists are “laboratory grade” with mecury indicators and have much greater accuracy than sling psychrometers typically used by inspectors and contractors. It is generally assumed that the most accurate method of determining Relative Humidity and Dew Point are the Sling Psychrometer. However, when the Humidity is close to 85% (or whatever the requirement is) and the dew point/surface temperature spread is about 5oF, it is important that readings be accurate. When Relative Humidity is around 50% and the Dew Point spread is 10oF to 15oF, accuracy in these tests are not critical. Typically, most project requirements are a Relative Humidity below 85% and a minimum 5 oF between the surface temperature and the dew point. When trying to determine Relative Humidity and Dew Point temperatures, an understanding of the wet bulb, dry bulb, relative humidity, and the dew point is useful in getting accurate values. Many coating failures have been attributed to applying coatings when climatic conditions were not within specifications. The difference between these two is described in this article. One is with a sling psychrometer and the second is with the newer electronic meters. These are addressed in ASTM E 337, Standard Method for Measuring Humidity with a Psychrometer (The measurement of Wet and Dry Bulb Temperatures). There are two basic methods of measuring Relative Humidity and Dew Point Temperatures in the field.

Dew point temperature