The ACI Thermistor Outside Air Series is a single-point weatherproof outdoor temperature sensor for building automation and outdoor temperature monitoring. The sensing element is double encapsulated in thermally conductive epoxy to eliminate the effects of moisture, and terminates in two 14 in etched Teflon lead wires that are color coded by sensor curve. The unit is designed to mount under an eave or on the shaded side of a building with the sensing tube pointed downward so water and ice cannot settle inside it.
Specifications sourced from the ACI Thermistor Outside Air Data Sheet and ACI Outdoor Series Installation & Operation Instructions (Version 5.0). Verified against ACI source documentation on 26 August 2026.
Best For
- Outside Air Temperature Sensing
- Cold Storage Facilities
- High Dew Point / Condensing Environments
Key Features
Double-Encapsulated Element
One sensing point built with ACI's double encapsulation process and thermally conductive epoxy, which ACI specifies to eliminate the effects of moisture on the sensor.
Euro Enclosure
PC/ASA Plastic with UV Protectant, rated Weatherproof (no NEMA/IP rating published), with an enclosure operating range of -40 to 88 C (-40 to 190 F).
Color-Coded Teflon Leads
Two 14 in (35.6 cm) 22 AWG etched Teflon leads in Brown/Blue identify the curve at the panel and resist outdoor exposure.
Shaded-Mount Design
Intended for eave or north-wall mounting with the sensing tube pointed downward so water and ice cannot settle inside the tube.
Direct Controller Connection
Passive element wires straight into an analog input with no transmitter or power supply. The controller input must be set to the matching sensor curve.
Five-Year Coverage
Backed by the ACI Five (5) Year Limited Warranty. A NIST Certificate (3 points) is available on the same part number by adding the -NIST suffix.
Technical Specifications
| Sensor Type | Sensor Curve | Thermistor | Non-Linear, NTC (Negative Temperature Coefficient) |
|---|---|
| Number of Sensing Points | One |
| Number of Wires | Two (non-polarity sensitive) |
| Sensor Output | Lead Wire Colors | 20 kOhm nominal at 25 C (77 F) | Brown/Blue |
| Sensor Accuracy | +/-0.2 C (+/-0.36 F), 0 to 70 C (32 to 158 F) |
| Power Dissipation Constant | 3 mW/C |
| Sensor Stability | Sensor dependent; contact ACI for the curve in question |
| Response Time (63% Step Change) | 25 seconds nominal (63% step change) |
| Operating Temperature Range | -40 to 70 C (-40 to 158 F) |
| Storage Temperature Range | -40 to 70 C (-40 to 158 F) |
| Operating Humidity Range | 0 to 100% RH |
| Lead Length | Conductor Size | 14 in (35.6 cm) | 22 AWG (0.65 mm) |
| Lead Wire Insulation | Wire Rating | Etched Teflon (PTFE) colored leads | MIL-W-16878/4 (Type E) |
| Conductor Material | Silver plated copper |
| Enclosure Material | PC/ASA Plastic with UV Protectant |
| Enclosure Operating Temperature Range | -40 to 88 C (-40 to 190 F) |
| Enclosure Flammability Rating | UL94-V0 |
| Enclosure NEMA / IP Rating | Weatherproof (no NEMA/IP rating published) |
| Product Dimensions | See dimensional drawing on the back page of the data sheet |
| Product Weight | 0.46 lbs (0.21 kg) |
| Agency Approvals | CE, RoHS2, WEEE |
Documentation
ACI Thermistor Outside Air Data Sheet- ACI Outdoor Series Installation & Operation Instructions (Version 5.0)
Related Products
- ACI A/20K-O-BB : Weatherproof Outside Air Temperature Sensor, 20K Thermistor, Aluminum NEMA 3R Enclosure
- ACI A/20K-O-4X : Weatherproof Outside Air Temperature Sensor, 20K Thermistor, Plastic NEMA 4X Enclosure
Frequently Asked Questions
What is the sensor output of the A/20K-O-EH?
This unit reads 20 kOhm nominal at 25 C (77 F) with Brown/Blue lead wires. It is a passive element, so the controller supplies the excitation and resolves the temperature from the resistance curve.
Does the A/20K-O-EH need a transmitter?
No, provided the controller input is configured for the matching curve. If the run is long, the input is current-only, or the reading must be scaled at the sensor, use the ACI TT or TTM Outside Air transmitter series instead.
Where should the A/20K-O-EH be mounted?
Mount it in shade on the north side of the structure (south side in the southern hemisphere), 1 to 2 ft (0.3 to 0.6 m) below the eave and at least 4 ft (1.22 m) above grade. The sensing tube must point downward so water, ice and debris cannot settle inside it. Avoid chimney walls, vents, doors, dampers and locations above windows.
What is the Euro enclosure rated for?
The Plastic Euro Enclosure is PC/ASA Plastic with UV Protectant, rated Weatherproof (no NEMA/IP rating published), with an enclosure operating range of -40 to 88 C (-40 to 190 F).
What is the warranty on the A/20K-O-EH?
ACI covers this series under its Five (5) Year Limited Warranty. The full text is published in the front of the ACI Sensors and Transmitters catalog and on workaci.com.
ACI Five (5) Year Limited Warranty. This series is covered by ACI's Five (5) Year Limited Warranty. ACI publishes the full warranty terms in the front of the ACI Sensors & Transmitters catalog and on workaci.com; refer to that document for the scope of coverage.
- Sensor Type:
- Outside Air
- Output Signal:
- Temperature Only
- CO2 Element:
- None
- Humidity Element:
- None
- VOC Element:
- None
- Temperature Element:
- 20K
- Display:
- None
- Supported Protocols:
- None
- Enclosure:
- Plastic Euro
- NIST Certificate:
- No
