Measuring air humidity and climatic conditions is a check that cuts across numerous industrial, environmental and process sectors. The presence of water vapour in the atmosphere affects the quality of production processes, the preservation of products, the efficiency of HVAC systems, the prevention of condensation and mould in buildings, and the quality of application of paints and coatings. The technologies available comprise portable thermohygrometers, standalone dataloggers, industrial transmitters, dew point hygrometers and cloud-based remote monitoring systems.
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Humidity sensors
The most widespread relative humidity sensors are polymer capacitive ones, in which a hygroscopic film between two electrodes changes its capacitance as it absorbs water vapour, with typical accuracies of ±2 % RH and response times of the order of seconds. Resistive sensors exploit changes in the conductivity of hygroscopic salts. Chilled-mirror dew point sensors use a mirror cooled by a Peltier cell together with an optical system to detect the formation of condensation, providing primary references of very high accuracy (±0.2 °C dew point). IR sensors and electrolytic dew point sensors cover specific industrial applications, such as measurement in dry air, in gases and in aggressive environments.
Categories of instrument
The section brings together portable handheld thermohygrometers for combined temperature and relative humidity measurements, temperature and humidity dataloggers for monitoring controlled environments and archives, industrial RH/T transmitters and probes for integration into HVAC and process plant, dedicated dew point meters for checking climatic conditions before and during painting, specialised DPM (Dew Point Meter) instruments for coating processes, and remote environmental monitoring systems with IoT cloud for data archiving, alerts and reporting.
Industrial applications
The applications include climate control in the pharmaceutical industry (GMP rooms, warehouses, production areas), monitoring of historical archives, libraries and museums (UNI 10829 for cultural heritage), verification of laboratory climatic chambers, HVAC control in offices, hospitals, data centres and cleanrooms, verification of industrial painting conditions (relative humidity, surface temperature, dew point in accordance with ISO 8502-4), preservation of food and pharmaceutical products along the cold chain, remote monitoring of distributed sites, and humidity control in greenhouses, agricultural warehouses and silos. The spread of IoT sensors with wireless transmission has made continuous monitoring of hundreds of points possible at reduced cost and complexity.
Reference standards
Measurement of humidity in air is governed by OIML R121 (metrological characteristics of hygrometers), WMO Guide no. 8 for meteorological applications, UNI EN 17025 for calibration laboratories, and ASTM E337 for the psychrometric method. For painting, the ambient conditions must be verified in accordance with ISO 8502-4 (dew point at least 3 °C below the surface temperature). In pharmaceuticals, EU GMP Annex 1, ICH Q1A for stability testing and WHO TRS 961 apply. Calibration of hygrometers is carried out with saturated-salt, bath-temperature or flow-divider humidity generators, calibrated to ISO 17025.
Selection criteria and integration
The selection depends on the humidity range (0-100 % RH), on the accuracy required (±1 % RH for critical applications, ±3 % for general use), on the need for datalogging, on the connectivity (USB, Modbus, Bluetooth, LoRaWAN, NB-IoT) and on the type of installation (portable, fixed, wall-mounted, immersion). The child pages develop each category with its technical parameters and typical fields of application.
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