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The control and measurement of humidity is important for many industrial applications and to ensure the appropriate storage of materials and products. Humidity measurement techniques are diverse and each presents different challenges for use and calibration for a range of pressures and gases. Over the past few years, the development of humidity sensors and apparatus has matured to a level where traceable calibration is beneficial to all industries in which humidity and moisture measurement and control are important. This paper deals with a European project in which the overall objective is to develop or extend the measurement and research capabilities of the participating emerging NMI/DIs’ countries in the field of humidity measurements, where access to these types of facilities is currently limited.

The accurate knowledge of the saturation vapor pressure of liquid water and ice has a great importance in a host of applications where humidity measurements are involved since it enables the correct conversion among different physical quantities that quantify the water vapor amount of a humid gas mixture. More specifically, accurate saturation vapor pressure measurements of supercooled water are strongly required in meteorology and atmospheric sciences, especially in cloud microphysics studies. Considering the importance of this kind of measurements and the limited availability of experimental data in the literature, further measurements in the metastable region were carried out at the Istituto Nazionale di Ricerca Metrologica (INRIM). The sample cell used in the experiment is a U-shaped capillary tube made of borosilicate glass, whose temperature is kept constant by immersion in a thermostatic bath with a high temperature stability (about 1 mK). The cell is connected to a capacitive diaphragm pressure gauge measuring the equilibrium water vapor pressure, that varies between about 116 Pa at T = 252.25 K and 615 Pa at T = 273.25 K. In the present work, the measuring method and the experimental setup are described and the measurements results reported. The results obtained in the whole temperature range are compared with those available in the literature and with the commonly adopted saturation vapor pressure formulations. A complete uncertainty budget is also given, resulting in an expanded relative uncertainty ranging from 0.06% at 273.25 K to 0.20% at 252.25 K.

Humidity is among the most important measured parameters related to HVAC applications, the storage of food products, industrial and medical gases, textile, paper and many other products requiring humidity measurement and control within certain limits. Humidity measurement techniques are diverse and each presents different challenges for use and calibration for a range of pressures and gases. Over the past few years, the development of humidity sensors and apparatus has matured to a level where traceable calibration is beneficial to all industries in which humidity and moisture measurement and control are important.

The performance of high-temperature, polymer-based RH sensors was investigated in order to evaluate their long-term stability. The work is aimed at understanding how severe temperature cycles may alter the characteristics of relative humidity (RH) sensors, how they impact the measurements, and how they contribute to the measurement uncertainty. The tests involved 10 high-temperature RH sensor probes available from five European manufacturers. They were initially calibrated against suitable humidity standards in the range from 10%rh to 85%rh with air temperatures from 20 °C to 80°C, and subsequently, they were exposed to air temperature at about 145 °C for several cycles, each cycle lasting (50–100) h. After each high-temperature exposure, a calibration check at 80 °C was carried out. The test lasted until the sensor exposure time exceeded 300 h. The paper presents the characteristics of such probes, the investigation results, and the comparison of the specified versus the experimental performances.