Relative humidity measures the amount of water vapor in the air compared to the maximum amount it can hold at that specific temperature. This value is expressed as a percentage, indicating how close the air is to saturation. Determining this measurement accurately uses psychrometry, a traditional method employing two specialized thermometers to capture the effect of evaporative cooling.
Understanding the Necessary Equipment
The standard apparatus is the psychrometer, which consists of two side-by-side thermometers mounted on a frame. One thermometer, known as the dry bulb, measures the ambient air temperature. This reading provides the true air temperature and is the baseline for the environment being tested.
The other thermometer is the wet bulb, which has its temperature-sensing bulb covered by a clean cotton or muslin wick. Before taking a measurement, this wick must be saturated with distilled or deionized water. The purpose of using pure water is to prevent salts or contaminants from interfering with the natural evaporation process on the wick.
As air moves over the wet wick, water evaporates, which draws heat away from the bulb and causes the wet-bulb temperature reading to drop. The amount of cooling that occurs is directly related to how much moisture is already in the air. The difference between the dry bulb temperature and the wet bulb temperature is the key variable that will be used to calculate the relative humidity.
Step-by-Step Measurement Procedure
The procedure begins with the preparation of the wet-bulb thermometer by thoroughly soaking the muslin wick with distilled water. The instrument must then be subjected to rapid air movement to promote continuous and complete evaporation from the wet bulb.
For a sling psychrometer, the device is whirled or swung rapidly in the air, held at arm’s length to avoid contamination from body heat or moisture. This movement needs to be maintained for a specific duration, typically between 45 to 90 seconds, to ensure the wet bulb temperature stabilizes. The correct duration is reached when the wet-bulb reading stops dropping.
Once the required period of airflow is complete, the readings must be taken immediately. It is necessary to read the wet-bulb temperature first, as its temperature will begin to rise as soon as evaporation slows. The dry-bulb temperature is then recorded, providing the two necessary data points for the final calculation.
Translating Readings into Relative Humidity
The recorded temperatures are used to determine the “wet-bulb depression,” which is simply the difference found by subtracting the wet-bulb temperature from the dry-bulb temperature. A large depression value indicates that the air is relatively dry, allowing for a high rate of evaporation and resulting in significant cooling. Conversely, a small depression means the air is near saturation, causing little evaporation and a smaller temperature difference.
To convert these two temperatures into a relative humidity percentage, a specialized resource known as a psychrometric chart or table is required. This chart graphically represents the complex relationships between the air’s properties. The dry-bulb temperature is located on the chart’s horizontal axis.
The wet-bulb depression value is then used to find the corresponding relative humidity curve. By cross-referencing the vertical line for the dry-bulb temperature with the appropriate curve, the state point of the air is established. This intersection point indicates the relative humidity percentage for the measured air.