Frost/Dew Point Temperature and Relative Humidity Measurements: Primary and Secondary Calibration Methods

Process Insights GmbH, Calibration Center, Max-Eyth-Str. 30, 70736 Fellbach, Germany

^* Corresponding author: AFurtado@emea.process-insights.com

Published online: 7 April 2025

Abstract

Humidity measurements are crucial across various industries to ensure product quality, safety, and process efficiency. These measurements can be expressed as absolute quantities, such as frost/dew point, temperatures point, t_f/t_d, or as relative humidity, U, which relates to saturation at a given temperature. Chilled-mirror hygrometers are widely used in high-precision industrial and research applications due to their broad measurement range, stability, and accuracy. Ensuring metrological traceability requires rigorous calibration using traceable methods. The DAkkS-accredited Calibration Laboratory of Process Insights follows DKD-R 5-8 [1] guidelines and meets ISO 17025 [2] requirements, employing both primary and secondary calibration methods for t_f/t_d over the range [-90, 95] °C, and U over the range [1, 98] %rh. Primary calibration achieves the lowest uncertainties using in-house 1-pressure and 2-pressure humidity (U_tf/tf : 0.4 K - 45 mK and U_U: 0.10 - 0.39 %rh), while secondary calibration relies on stable CM hygrometers and commercial humidity generators (U_tf/tf : 0.54 K - 70 mK and U_U: 0.10 - 0.49 %rh). For relative humidity calibration, both t_f/t_d [1] and temperature [3] sensors require calibration. Humidity calculations follow established references, ensuring high accuracy [4, 7-8]. This work outlines the methodologies and challenges in maintaining traceability and accuracy in high accuracy humidity measurements at PI Cal.