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). When the air is warmer, the air is filled with more energy and less likely to condense into a liquid, so the air can hold more of this moisture which we are able to feel on hot, muggy days.There are two basic methods of measuring Relative Humidity and Dew Point Temperatures in the field. This is because warmer air has the ability to hold more moisture than cold air. You will often see higher dew points when the air temperature is higher. Below is a rough guide on how the air will feel with certain dew point temperatures. The higher the dew point, the more humid the air feels and vice versa. Meteorologists and other scientists will use dew point as a way of expressing how sticky or muggy it feels outside. When you have a temperature of 80☏ and dew point around 60☏ the relative humidity will only be around 50%, but the air will feel much more uncomfortable. A rule of thumb to follow is when the dew point is higher, the more moisture there is in the air and the muggier it feels.įor example, you can have a temperature and dew point both at 30☏, and the air will have a 100% relative humidity, but it won’t feel humid outside. It is essentially a percentage of the amount of moisture the air can potentially hold at one time.ĭew point is a better way to measure how the air actually feels. This is different from relative humidity, which is defined as the ratio expressed as a percentage of the amount of atmospheric moisture in the air, relative to how much moisture the air can hold at a specific temperature. This often creates fog or dew drops on plants during the early morning hours of the day when this phenomenon occurs. The dew point, or dew point temperature is the temperature at which the air must cool in order to reach saturation. In other words, as soon as the air temperature drops to the dew point temperature, the air has cooled down enough to reach that saturated point to condense into tiny water droplets.