[Core Summary]
The temperature and humidity transmitter consists of a humidity-sensitive element, a temperature-sensitive element, and a signal conditioning circuit. It converts the relative humidity and temperature of the measured air into standard signals or bus data that can be directly read by the control system. Common points of confusion surrounding this term include: the difference between relative humidity and absolute humidity, the physical meaning of dew point temperature, the types of humidity-sensitive elements, the functional boundaries of temperature compensation, and the basis for selecting between Modbus RTU and analog output. This entry uses the sample parameters of the S+S Regeltechnik (S+S, Germany) RFTM series and AFTF series as a reference for an item-by-item explanation.
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Definition and functional boundaries
Temperature and humidity transmitters differ from pure sensors: sensors output raw physical quantities that have not been calibrated, while transmitters complete calibration, linearization, and signal conversion internally, directly outputting signals that can be connected to PLCs, DDCs, or monitoring platforms. S+S Regeltechnik's RFTM series are room-type multi-parameter transmitters, while the AFTF series are wall-mounted/outdoor temperature and humidity transmitters. Both utilize digital humidity sensors and integrate temperature sensors, with calibration completed at the factory, and environmental accuracy fine-tuning can be performed by professionals on-site.
Core Term Comparison
Relative Humidity (RH, % RH)
The ratio of water vapor partial pressure to the saturated water vapor pressure at the same temperature. It varies with temperature, and the relative humidity of air with the same water content decreases as the temperature rises. The relative humidity measurement range for both the RFTM series and the AFTF series is 0 to 100% RH.
Absolute Humidity (g/m³)
The mass of water vapor contained in a unit volume of air does not vary with temperature. The AFTF-Modbus-T3 internally calculates absolute humidity from relative humidity and temperature, with a measurement range of 0...80 g/m³.
Dew Point (°C)
The temperature at which air cools to saturation with water vapor and begins to condense under isobaric conditions. The dew point is used to assess the risk of surface condensation and serves as a direct criterion in the design of cold surfaces in clean rooms, cold aisles in computer rooms, and pipeline insulation. The dew point output range of AFTF-Modbus-T3 is –20…+80 °C.
Humidity-sensitive components
An element that converts changes in humidity into variations in electrical quantities. The aforementioned series from S+S Regeltechnik utilizes a digital humidity sensor, characterized by minimal hysteresis and excellent long-term stability, with the sample providing a long-term stability of ±1% per year.
Temperature compensation
The output of humidity-sensitive components drifts with temperature, necessitating correction using simultaneously measured temperature values. The accuracy specifications provided by the sample are all tied to a reference temperature: the typical deviation of ±2.0% for AFF/AFTF standard humidity is valid within the range of 20...80% RH at +25°C, and ±3.0% outside this range. Citing accuracy values without reference conditions is a common misinterpretation during the selection phase.
Modbus
Based on the RS485 physical layer master-slave serial communication protocol, the RTU mode employs binary frames. The Modbus model from S+S Regeltechnik utilizes an electrically isolated RS485 interface, with a slave address range of 0...247. The baud rate and address are set through DIP switches in the power-off state. A single bus can be connected to multiple transmitters, replacing the need for point-to-point analog cable laying.
Selection criteria for output format
Analog output is suitable for retrofit projects with few points and surplus existing AI channels: U-type output 0–10 V, load resistance > 5 kΩ; I-type output 4…20 mA, operating resistance calculated as Ra = (Ub – 14 V) / 0.02 A. Digital output is suitable for scenarios with many points and requiring derived quantities such as dew point or enthalpy: AFTF-Modbus-T3 standalone provides six data points: temperature, relative humidity, dew point, absolute humidity, mixing ratio, and enthalpy, eliminating the need for conversion by the host computer.
Terminology Comparison Table
[Parameter Table]
**Parameter Source Description**: The range, deviation, power supply, and protection level listed in the terminology comparison table and parameter table are taken from the official samples of S+S Regeltechnik: RFTM-PS-Modbus (Rev. 2022-V31 GB), AFTF (Rev. 2025-V29 GB), AFTF-Modbus-T3 (Rev. 2022-V32 GB), and RFTF (Rev. 2023-V23 GB). The calibration cycle and zero offset range of RFTM are not specified in the sample, marked as [Please verify].
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