Hygrometer

Hygrometers are essential for environmental monitoring in calibration laboratories, manufacturing areas, and storage facilities where humidity affects product quality or measurement accuracy. Common types include capacitive (polymer film sensor whose capacitance changes with moisture), resistive (measures conductivity change), chilled mirror dew point (detects condensation on a cooled surface), and psychrometers (wet-bulb/dry-bulb temperature comparison). Each type has different accuracy, range, and response time characteristics.

Calibration of hygrometers involves exposing the sensor to environments with known humidity levels and comparing the readings. Reference standards include chilled mirror hygrometers (for dew point), saturated salt solutions (which produce known RH levels in sealed chambers), and humidity generators (precision systems that produce air at controlled humidity). Calibration is typically performed at multiple humidity levels (e.g., 11%, 33%, 50%, 75%, 95% RH using appropriate salt solutions) and may include temperature dependence testing.

For calibration management, humidity measurement is notoriously challenging due to the difficulty of generating and maintaining stable humidity references, the slow response time of many sensors, and the sensitivity of humidity to temperature. Capacitive RH sensors can drift over time, especially in contaminated or extreme environments, and typically need calibration every 12 months. Many calibration laboratories monitor and record ambient humidity as part of their environmental control requirements (ISO 17025), making accurate hygrometry essential for all measurement disciplines, not just humidity-specific measurements.