Humidity transmitters and probes are industrial devices for integrating humidity and temperature measurements into HVAC plants, industrial process systems, climatic chamber monitoring, GMP environments, pharmaceutical and chemical processes, and meteorological applications. Unlike portable thermo-hygrometers and stand-alone data loggers, transmitters operate as permanent installations, powered by direct voltage, with standard analog outputs (4-20 mA, 0-10 V) or digital outputs (Modbus RTU, Profibus, Profinet, EtherNet/IP, OPC UA, MQTT) for integration into PLC, SCADA and BMS (Building Management System) architectures.
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Types of transmitter
Transmitters differ in their installation geometry: wall-mounted (with a compact housing for mounting on a wall or bracket, with an integrated or stem probe); stem-type (with a probe up to 1000 mm long for insertion into air ducts, pipework and enclosed spaces); through-wall (with dedicated flanges for installation in large-section air ducts); for pressurized ducts (with gaskets and seals to withstand internal pressure); with interchangeable probes (for special applications at high temperature, high humidity, in vacuum or with liquids). The housings are typically of polycarbonate, ABS or aluminum, with a protection rating of IP65 or higher.
Sensors and accuracy
The sensors used are predominantly polymer capacitive types, with high-class accuracies (±1-2% RH and ±0.2-0.3°C) for professional industrial applications. The sensors are protected by filters (sintered PTFE, sintered stainless steel, semi-permeable membrane) that guard against contamination (dust, oils, droplets) while still allowing water vapor to pass through. The filters can be replaced periodically. For special applications (aggressive chemistry, sterilization) filters in PTFE, ceramic or titanium are available.
Outputs and protocols
The standard analog outputs are: 4-20 mA (the most robust over long distances, immune to EMC noise, widely used in the process industry); 0-10 V (for building applications, with short cables); 0-5 V (for building BMS). The digital outputs include: Modbus RTU (over RS-485, by far the most widespread, supported by practically every PLC); Profibus DP (for the process industry); Profinet (industrial Ethernet); EtherNet/IP (Allen-Bradley/Rockwell); MQTT and OPC UA (for modern IoT architectures). The most advanced transmitters support analog and digital outputs simultaneously for maximum flexibility.
HVAC and process applications
The main applications are: monitoring of air handling units (AHU) in commercial and industrial HVAC; process humidity control in paper, tobacco, pharmaceutical and food production; verification of laboratory climatic chambers (IQ/OQ/PQ qualification); monitoring of pharmaceutical warehouses and archives; control of operating theaters (30-60% RH required); monitoring of cold rooms and storage environments; meteorological applications (automatic weather stations); control of test environments (climatic ovens, salt spray chambers).
Standards and calibration
Industrial transmitters comply with the following standards: IEC 61326 for electromagnetic compatibility (EMC); EN 60204 for the electrical safety of machinery; the ATEX Directive 2014/34/EU for models intended for explosive atmospheres (with Ex ia or Ex d certification in zones 0, 1 and 2). Periodic calibration is carried out with humidity generators (saturated salt or flow divider) calibrated to ISO 17025, either in the laboratory or through an on-site service. The typical interval is annual, with an assessment of sensor stability over time.
Selection criteria
Selecting a transmitter takes into account: the humidity and temperature range; the accuracy required; the installation geometry (wall, duct, through-wall); the analog or digital output required by the existing PLC/SCADA system; the IP protection rating for the installation environment; ATEX safety certifications where applicable; the availability of replacement probes and accessories; the supplier’s periodic calibration service; technical support for commissioning.
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