To find Relative Humidity (RH), you typically need to determine the actual amount of water vapor in the air and compare it to the maximum amount of water vapor the air can hold at that temperature. The general formula is:
Relative Humidity (%) = (Actual Water Vapor Content / Saturation Water Vapor Content) x 100
Here's a breakdown of common methods:
1. Using a Psychrometer
A psychrometer (or hygrometer) measures relative humidity directly. It typically consists of two thermometers:
- Dry-bulb thermometer: Measures the ambient air temperature.
- Wet-bulb thermometer: Has a wet wick around the bulb. As water evaporates from the wick, it cools the bulb. The difference between the dry-bulb and wet-bulb temperatures indicates the relative humidity. A larger difference means lower relative humidity because more evaporation is occurring.
You then use a psychrometric chart or a calculation to determine the relative humidity based on the temperature difference.
2. Using a Hygrometer (Electronic)
Electronic hygrometers measure relative humidity using sensors that respond to changes in moisture. These are typically digital and provide a direct reading of the RH. Common types of sensors include:
- Capacitive sensors: These sensors use a capacitor whose dielectric material absorbs moisture from the air. The change in capacitance is then used to calculate the relative humidity.
- Resistive sensors: These sensors use a hygroscopic material that changes its electrical resistance based on the relative humidity.
3. Calculation from Specific Humidity and Saturation Humidity
As the provided reference suggests, you can also calculate RH using specific humidity and saturation humidity.
- Specific Humidity: The actual mass of water vapor per mass of air (typically expressed as grams of water vapor per kilogram of dry air).
- Saturation Humidity: The maximum mass of water vapor that air can hold at a given temperature and pressure (also expressed as grams of water vapor per kilogram of dry air). This value increases with temperature.
The formula is:
Relative Humidity (%) = (Specific Humidity / Saturation Humidity) x 100
To use this method, you'll need to know or calculate both specific humidity and saturation humidity. Saturation humidity can be found using tables or equations if you know the air temperature.
4. Using Dew Point Temperature
Dew point is the temperature to which air must be cooled to become saturated with water vapor. At the dew point, relative humidity is 100%.
- Knowing Dew Point: If you know the actual air temperature and the dew point, you can use a psychrometric chart or equations to determine the relative humidity. The closer the dew point is to the air temperature, the higher the relative humidity.
In summary, measuring relative humidity involves determining how much moisture is present in the air compared to how much moisture the air could potentially hold at a given temperature. Several methods exist, each using different instruments and calculations.
