How one compact transmitter keeps air quality, energy efficiency, and occupant comfort in perfect balance.
Walk into a well-controlled building on a hot summer afternoon and you immediately feel the difference – cool, dry air without the stuffiness. Behind that comfort is a continuous, invisible loop of measurement and adjustment. At the heart of it? A humidity and temperature transmitter.
The Rotronic HygroFlex1 is designed precisely for this job. Part of the latest generation of HVAC transmitters from Rotronic – a brand trusted by process engineers and building managers worldwide – the HygroFlex1 combines field-proven sensor technology with an accessible price point, making precision climate monitoring practical at scale.
This article breaks down what the HygroFlex1 is, how it works, why humidity and temperature monitoring matter in HVAC systems, and what makes this transmitter a reliable choice for your next installation.
What Is the Rotronic HygroFlex1?
The HygroFlex1 is a wall, duct, or space-mount transmitter that simultaneously measures relative humidity (RH) and temperature, and transmits those values as electrical signals to a building management system (BMS) or controller. It is not a standalone display device – it is the sensing and signal-transmission layer of a larger climate control loop.
At its core is the Rotronic Hygromer® IN-1 capacitive humidity sensor – the same sensor used across Rotronic’s professional instrument range. Paired with a fast-response thermistor for temperature, the HygroFlex1 delivers two calibrated analog outputs that a connected controller can act on in real time.
What is a Hygromer® sensor?
The Hygromer® IN-1 is a capacitive thin-film polymer sensor. As the moisture content of surrounding air changes, the capacitance of the sensor element changes in a precise, repeatable way – allowing the transmitter to calculate relative humidity accurately, even after years of continuous use.
KEY SPECIFICATIONS AT A GLANCE
| Parameter | Specification |
|---|---|
| Humidity sensor | Rotronic Hygromer® IN-1 |
| RH accuracy (wall & duct) | <±2 %RH (10–90 %RH at 23 °C ±5 K) |
| Temperature accuracy | ±0.3 °C (wall/duct) | ±1 °C (space mount) |
| Operating RH range | 0–100 %RH (non-condensing) |
| Operating temperature | −20 to 60 °C (wall/duct) | −20 to 50 °C (space mount) |
| Humidity response (t63) | 30 s (35–80 %RH step, without filter) |
| Temperature response | 4 s |
| Long-term stability | <1.5 %RH per year |
| Analog outputs | 4–20 mA current / 0–1/5/10 VDC voltage (freely scalable) |
| Supply voltage | 12–28 VAC / 15–40 VDC |
| Housing / IP rating | ABS / IP 65 (wall & duct) | IP 30 (space mount) |
| CE / EMC | 2004/108/EC compliant |
Understanding the Field: HVAC and Indoor Climate Control
HVAC stands for Heating, Ventilation, and Air Conditioning – the integrated systems that regulate temperature, humidity, and air quality inside buildings. From office towers and hospitals to warehouses and data centres, HVAC systems are the invisible infrastructure that keeps people comfortable, products safe, and equipment operational.
Relative humidity – the percentage of water vapour in the air relative to its maximum capacity at a given temperature – is as critical as temperature itself. Too high, and moisture condenses on surfaces, promoting mould growth and corrosion. Too low, and static electricity builds up, wooden structures warp, and occupants experience discomfort.
Maintaining the right indoor humidity level requires continuous, accurate measurement – which is exactly what a transmitter like the HygroFlex1 provides.
Why ±2 %RH accuracy matters:
A controller acting on inaccurate humidity data will overcorrect – either running humidifiers when unnecessary (wasting energy) or under-dehumidifying (risking condensation and air quality issues). At ±2 %RH, the HygroFlex1 gives controllers data they can trust.
Where the HygroFlex1 Is Used
The HygroFlex1’s flexible mounting options and robust operating range make it suitable across a wide variety of HVAC scenarios:
- Commercial Buildings: The wall-mount version (Type W) monitors zone-level humidity and temperature, transmitting live readings to a connected building management system for automated climate control.
- Air Handling Units (AHUs): Duct-mount versions (Types D and F) installed inside AHU ducts monitor supply and return air conditions, enabling precise humidity control in the air stream.
- Museums & Archives: Tight humidity control protects artefacts, artworks, and documents. The HygroFlex1’s long-term stability of <1.5 %RH/year ensures reliable readings over extended periods without frequent recalibration.
Available Variants: Choosing the Right Version
The HygroFlex1 is available in four physical form factors, each designed for a specific installation scenario:
| Version | Type Code | Best For | IP Rating | Display |
|---|---|---|---|---|
| Wall Mount | Type W | Room / zone monitoring | IP 65 | Optional |
| Duct — 100 mm | Type D | Smaller ducts, AHUs | IP 65 | No |
| Duct — 300 mm | Type F | Large ducts, main trunks | IP 65 | No |
| Space Mount | Type S | Flush-mount, controlled spaces | IP 30 | Optional |
All versions support 4–20 mA current output. Select duct versions (HF135) additionally offer 0–10 VDC voltage output. Both outputs are freely scalable using the Rotronic SW21 software.
What Makes It a Smart Choice for Your Project?
- Proven sensor technology. The Hygromer® IN-1 sensor has been Rotronic’s field-proven element for decades. Specifying HygroFlex1 means industrial-grade sensing in a cost-optimised housing – not an unknown capacitive element from an unverified supplier.
- Field-adjustable calibration. The transmitter can be adjusted at 35 %RH and 80 %RH – the two standard reference points used in field calibration kits. Readings can be verified and corrected on-site without returning the unit to a laboratory.
- Wide power supply range. Accepts 12–28 VAC or 15–40 VDC, making it compatible with almost any BMS panel power rail without additional voltage conditioning.
- Software configurability. Output scaling, calibration offset, and humidity adjustment are all configurable via Rotronic SW21 software – useful when integrating into existing BMS loops with fixed input ranges.
- CE / EMC compliant. Certified to 2004/108/EC for electromagnetic compatibility.
- Low long-term drift. At <1.5 %RH per year, the HygroFlex1 maintains its calibration accuracy over extended periods of continuous use.
Conclusion
Humidity and temperature are two of the most consequential variables in any HVAC system. The Rotronic HygroFlex1 brings the measurement accuracy and long-term stability needed to monitor and control these variables reliably, at a price point that makes large-scale deployment practical.
Whether commissioning a new building, upgrading an ageing sensor network, or tightening control in a critical environment, the HygroFlex1 is a transmitter worth specifying with confidence.
