Definition
The percentage ratio of the amount of water vapor in the air to the maximum water vapor capacity it can hold at the same temperature. 0% indicates completely dry air, 100% saturated air. As temperature changes, relative humidity also changes; therefore, dew point measurement is generally more reliable for process control.
Detailed Explanation
Relative humidity is defined as the ratio of the partial pressure of water vapor in the air (Pv) to the saturated vapor pressure at the same temperature (Psat). Saturated vapor pressure is strongly dependent on temperature and is calculated using the Antoine equation or ASHRAE psychrometric correlations. As a result, the relative humidity of an air mass containing the same amount of water vapor changes significantly as temperature varies.
As a rule of thumb, a 1°C temperature rise at room conditions corresponds to roughly a 3% drop in RH. This behavior makes RH a meaningful parameter for human comfort, but insufficient on its own for process control. Even when the absolute moisture content remains constant, the RH reading fluctuates continuously with temperature changes.
Calculation
RH = (Pv / Psat) × 100
Pv: partial pressure of water vapor in the air (Pa) Psat: saturated vapor pressure at the same temperature (Pa) — from steam tables or the Antoine equation RH: relative humidity (%)
Practical Example
On a winter morning in Istanbul, outdoor air may be 5°C at 85% RH. When this air is drawn into an office building and heated to 22°C, the absolute moisture content remains the same but relative humidity drops to approximately 26%.
In this dry environment, occupants experience mucosal irritation, eye discomfort, electrostatic discharges, and cracking in wooden floors. To reach the comfort range (40–60% RH), a steam or ultrasonic humidifier must be integrated into the system. This example shows that heating alone cannot control humidity, and humidification is an independent engineering problem.
Engineering Note
RH measurement in industrial processes has two fundamental limitations:
1. Temperature dependence — two environments with the same absolute moisture content can read very different RH due to a temperature difference. For this reason, dew point control is preferred in pharmaceutical packaging or electronic assembly processes. 2. Loss of accuracy at low temperatures — capacitive RH sensors lose accuracy down to ±5% below -10°C. Dew point measurement is more reliable in cold storage and freezer environments.
In GMP, ISO 9001, and pharmaceutical applications, RH sensors should be calibrated every 6 months; in general industrial applications, annually. Salt solution reference environments (LiCl, MgCl₂, NaCl) or chilled mirror dew point meters are used as calibration references.
