[Core Summary]
The temperature and humidity alarms in clean rooms and controlled environments are often not due to insufficient capacity of air conditioning units, but rather the cumulative result of deviations in measurement point selection and installation. The S+S Regeltechnik (S+S, Germany) RFTM series room-type multi-parameter transmitters and AFTF series wall-mounted/outdoor temperature and humidity transmitters have a humidity measurement range of 0 to 100% RH, with a typical humidity deviation of ±2.0%. Their outputs cover three types: 4...20 mA, 0–10 V, and Modbus RTU. During the selection phase, it is necessary to simultaneously consider six factors: humidity measurement range, allowable operating humidity, accuracy reference conditions, temperature measurement range, output protocol, protection level, and long-term stability, to avoid compensating for hardware errors through control logic after commissioning.
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Three root causes of frequent alarms
The over-limit alarms for clean room humidity are mainly concentrated in three areas: mismatch between the transmitter range and actual operating conditions; neglect of reference conditions for sample accuracy indicators; and inconsistency between the output signal and the building control system protocol. All three issues arise during the selection phase, and once operational, can only be addressed through offset compensation by the host computer, which is more costly than verifying the samples in the early stages.
Humidity range and allowable operating humidity
The RFTM series Modbus version and the AFTF series humidity range are both 0-100% RH. The real constraint on installation location is the "allowable operating humidity": the AFTF series is labeled as < 95% RH (no condensation), while the RFTM-PS-Modbus is labeled as 0-95% RH (no condensation). Clean areas are usually controlled at 45-60% RH, with sufficient margin; the purified water room and cleaning room adjacent to the clean room are close to saturation, and humidity-sensitive components are constantly on the verge of condensation, requiring the use of a probe-type structure with a sintered filter.
Reference conditions for accuracy indicators
The deviation values provided in the sample are all accompanied by conditions. The standard humidity deviation for AFF / AFTF is ± 2.0% within the range of 20...80% RH and at +25°C, and ± 3.0% outside this range. AFF-20 / AFTF-20 and AFF-25 / AFTF-25 are tightened to ± 1.8% within the range of 10...90% RH, and ± 2.0% outside this range. The temperature side standard model has a typical deviation of ± 0.4 K, and the -20 / -25 models have a typical deviation of ± 0.2 K, also with +25°C as the reference. If traceability of the measurement point uncertainty is required, conservative calculations should be made based on the deviation values outside the range.
Temperature range gear alignment with scale
The AFTF series supports four DIP switch settings: –35...+35 °C, –35...+75 °C, 0...+50 °C, and 0...+80 °C, with the factory default being 0...+50 °C. The RFTF room-type fixed model operates within the same temperature range of 0...+50 °C, as does the RFTM-PS-Modbus model. The setting directly determines the 4...20 mA scaling relationship. If the DCS side configuration does not match the on-site DIP switch setting, it will result in systematic deviations throughout the entire process.
Two routes for output protocols
For the analog route, choose Type I 4...20 mA (operating resistance calculated as Ra = (Ub – 14 V) / 0.02 A, two-wire or three-wire system) or Type U 0–10 V (load resistance > 5 kΩ). For the digital route, choose AFTF-Modbus-T3 or RFTM series Modbus version, with RS485 interface electrically isolated, Modbus RTU address range 0...247, and baud rate and address set by DIP switches in the power-off state. AFTF-Modbus-T3 internally calculates six data points: relative humidity, absolute humidity (g/m³), mixture ratio (g/kg), dew point (°C), enthalpy (kJ/kg), and temperature. One bus can replace multiple single-parameter transmitters.
Protection level and installation form
The RFTM series and RFTF room-type enclosures are made of ABS, with a protection level of IP 30, suitable for surface mounting on clean room walls or installation in a Ø55 mm concealed box. The AFTF series enclosures are made of 30% glass fiber reinforced polyamide, with a protection level of IP 65, suitable for return air ducts, technical interlayers, and outdoor applications. For walls that require regular wiping and disinfection, the IP 30 enclosure should be avoided in direct spray areas.
Long-term stability and recalibration
Both series exhibit long-term stability of ±1% per year, and are factory calibrated. On-site environmental accuracy fine-tuning can be performed by professionals. AFTF-Modbus-T3 offers a zero offset adjustment range of ±10% RH and ±5°C. The annual recalibration cycle should be determined based on this drift value and internal control limits.
Three typical configuration combinations
Clean zone room monitoring: RFTM series Modbus version, equipped with LA bus terminator, arranged with one measurement point every 30 m² as recommended in the sample. Supply and return air ducts: AFTF series Type I 4...20 mA, equipped with Ø16 mm V2A protective tube and replaceable sintered filter. Dew point or enthalpy value involved in interlocking: directly select AFTF-Modbus-T3, eliminating the need for secondary conversion by the host computer.
[Parameter Table]
**Parameter Source Description**: The values in the table above are taken from the official product samples of S+S Regeltechnik: AFTF (Rev. 2025-V29 GB), AFTF-Modbus-T3 (Rev. 2022-V32 GB), RFTM-PS-Modbus (Rev. 2022-V31 GB), and RFTF (Rev. 2023-V23 GB). Items marked with [Please verify] are not explicitly provided in the sample and should be confirmed with the manufacturer or agent before being included in the technical agreement.
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