RH-Combo-Duct-Thermistor

Supported Applications

The ACI Relative Humidity Duct with Thermistor Temperature Sensor series is designed for commercial heating and cooling building management systems. These transmitters monitor relative humidity in ductwork, utilizing a capacitive sensing element with a factory-fitted hydrophobic filter to resist moisture ingress. The multilayer construction of the sensor provides resistance against dust, dirt, oils, and common environmental chemicals found in HVAC environments.

Operating Conditions & Performance

The series operates within an ambient temperature range of -40 to 149 °F (-40 to 65 °C) for storage and -40 to 140 °F (-40 to 60 °C) during operation. Relative humidity measurement covers a range of 0-100%, with an operating RH limit of 0 to 95% non-condensing on conformally coated PCBs.

Accuracy specifications vary by selected model and range:

• RH Accuracy @ 77°F (25°C): +/- 1% over a 20% RH Range between 20 to 90%; or +/- 2%, 3%, or 5% from 10 to 95%.
• RH Stability: Less than 2% drift / 5 years.
• RH Response Time (T63): 20 Seconds Typical.
• Thermistor Accuracy @ 0-70 °C (32 - 158 °F): +/- 0.2 °C (+/- 0.36 °F) for most series; A/10K-E1 Series is +/- 0.3 °C (+/- 0.54 °F); A/1.8K Series is +/- 0.5 °C @ 25 °C (77 °F).

Power requirements depend on the output signal and load resistance:

• 4-20 mA Output: 15 - 40 VDC / 18 - 28 VAC for a 250 Ω Load; 18 - 40 VDC / 18 - 28 VAC for a 500 Ω Load.
• 0-5 VDC Output: 12 - 40 VDC / 18 - 28 VAC.
• 0-10 VDC Output: 18 - 40 VDC / 18 - 28 VAC.

Configuration Options

The series offers configurable options for enclosure type, sensor element, output signal, and calibration certification. Units feature on-board DIP switches to select the desired output signal (4-20 mA default; 0-5 VDC or 0-10 VDC field selectable) without replacing the sensing element.

Enclosure Options:

• Euro (-EH): ABS Plastic with UV Protectant, UL94-V0. Features a gasketed cover and 304 Series Stainless Steel sensing tube/filter (or optional sintered filter).
• NEMA 4X (-4X): Polyethylene Plastic, 100% V2, NEMA 4X (IP 66). Features Schedule 40 PVC (White) sensing tube with slotted PVC without filter.
• NEMA 3R Plenum Rated (-BB): Aluminum enclosure. Features Schedule 40 PVC (White) sensing tube with slotted PVC without filter.

Sensor Elements:

• RH Sensor Type: Capacitive with Hydrophobic Filter.
• Temperature Sensors: Non-Linear NTC Thermistors available in 1.8K, 3K, AN (Type III), AN-BC, CP (Type II), CSI, 10KS, 10K-E1, 20K, 50K, and 100KS nominal resistance options.

Outputs & Connections:

• Output Signal: 4-20 mA (2-wire default), or field-selectable 3-wire 0-5 VDC / 0-10 VDC.
• Load Resistance: 700 Ω maximum for 4-20 mA; 4K Ω minimum for voltage outputs.
• Electrical Connection: Screw Terminal Blocks (Polarity Sensitive), supporting wire sizes from 16 AWG to 26 AWG.

Certifications & Calibration:

• Approvals: UKCA, CE, RoHS2, WEEE.
• NIST Certification: Available as an option (3 Points default: 20%, 50% & 80%; or 1% NIST Test Points with 5 points within a selected range).

Key Product Differences

The series distinguishes itself through specific enclosure materials and sensing tube configurations tailored to environmental needs. The Euro (-EH) style utilizes ABS plastic and stainless steel components, while the NEMA 4X (-4X) variant employs polyethylene with PVC tubing for different mounting requirements. The Plenum Rated (-BB) option provides an aluminum housing suitable for plenum spaces.

Accuracy tiers are defined by model selection: RH1 models offer +/- 1% accuracy over a specified 20% range, whereas RH2 and RH3 models provide +/- 2% or +/- 3% accuracy across the broader 10 to 95% span. Temperature sensor variations include specific resistance values (e.g., A/1.8K vs. A/10KS) which dictate nominal output at 77°F (25°C) and influence power dissipation constants.

Installation notes specify that the duct probe should be placed three to four duct segments down from bends or obstructions, mounted on top or sides of ductwork to avoid moisture damage. The transmitter requires a stabilization time of 30 minutes before accuracy verification.