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There are two basic methods of measuring Relative Humidity and Dew Point Temperatures in the field. These are addressed in ASTM E 337, Standard Method for Measuring Humidity with a Psychrometer (The measurement of Wet and Dry Bulb Temperatures). One is with a sling psychrometer and the second is with the newer electronic meters. The difference between these two is described in this article.
Many coating failures have been attributed to applying coatings when climatic conditions were not within specifications. 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.
Typically, most project requirements are a Relative Humidity below 85% and a minimum 5 oF between the surface temperature and the dew point. When Relative Humidity is around 50% and the Dew Point spread is 10oF to 15oF, accuracy in these tests are not critical. 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.
It is generally assumed that the most accurate method of determining Relative Humidity and Dew Point are the Sling Psychrometer. 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. 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. The sling psychrometer measures two parameters, Dry Bulb (ambient temperature) and Wet Bulb.
The dry bulb temperature (DBT) or ambient temperature is the temperature of the air. This is the temperature that you would get in the shade and not the temperature in direct sun.
The wet-bulb temperature (WBT) measures the temperature that results from evaporation, and it is directly related to relative humidity. When moisture evaporates, it cools the environment, reducing the temperature slightly. The WBT will vary with Relative Humidity (RH). When the relative humidity is high, evaporation is low and there is less of a cooling effect. When relative humidity is low (air is dry) evaporation increases and the cooling effect is greater. The difference between the wet bulb and dry bulb temperature, therefore, gives a measure of atmospheric humidity.
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 amount of moisture the air can hold is dependent on the atmospheric pressure. When the air is 100% saturated, evaporation will stop, and the Dry Bulb Temperature will be equal to the Wet Bulb Temperature. When DBT – WBT = 0 then RH = 100% It is strongly suggested that electronic meters be used instead of sling psychrometers for the best accuracy.
Dew Point (DPT) is the temperature to which air must be cooled to become saturated with water vapor. When cooled further, the airborne water vapor will condense to form liquid water (dew). When air cools to its dew point through contact with a surface that is colder than the air, water will condense on the surface.
If you are going to use a sling psychrometer, it is recommended that the following procedures be followed to minimize any errors. Prior to using the sling psychrometer, read the dry bulb temperature.
Reading will be more accurate if taking them in shaded conditions. Twirl the sling for approximately 30 seconds and red the wet bulb. Continue to twirl for 15 seconds and read the wet bulb. Continue until the wet bulb readings do not change. Use the dry bulb temperature you measured before twirling, then use the psychometric table or one of the online MTestsite to calculate RH and DewPoint.