Humidity and Temperature Sensors for Harsh Environments (AFTF-45 / KFTF-45)

July 14, 2026

Author: S. Becker

In many applications, humidity is not the actual problem—it is air quality. Chemical contamination, cleaning processes, or aggressive media often corrode conventional humidity sensors faster than expected.

The AFTF-45 (surface-mounted) and KFTF-45 (duct) humidity and temperature sensors have been developed specifically for such environments: wherever sensors must provide stable measurements over the long term, even though the air is anything but “clean.”

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In a nutshell

If humidity sensors fail regularly, the cause is often not the humidity itself, but the environment:

PTFE-protected sensor (Teflon membrane with very fine pores)
High resistance to chlorine, ammonia, cleaning chemicals
Ideal for swimming pools, food industry, animal husbandry
Durable alternative to standard sensors
Standard signals 0-10 V or 4-20 mA for BMS/DDC

The aim: reliable measured values even where other sensors fail prematurely.

Why classic humidity sensors fail in "hard" environments

In many projects, the same pattern repeats itself time and again: A standard sensor is initially installed—and it works just fine. There is no immediate indication that the environment is actually critical for the sensor system. Problems develop gradually and are often not noticed until the control system is no longer operating stably or the measured values are clearly no longer plausible. A typical scenario from real-world practice:

During the first few months, the sensor provides consistent, stable readings
Over time, slight deviations or delayed reactions may become apparent
After one to two years, significant drift or complete failures occur

The key point here is that this development doesn't happen all at once, but rather gradually in the background.

What Actually Happens Inside the Sensor

The cause lies in a combination of various stressors that are often underestimated in everyday life:

chemical compounds in the air, such as chlorine in swimming pools or ammonia in animal husbandry
Aerosols and cleaning residues that settle on surfaces
fine particles from processes or the ambient air

These substances act directly on the sensor element. Depending on the medium, the following occurs:

Chemical reactions alter the sensor layer
Particles deposit on the surface and affect diffusion
Over time, residue forms a kind of “film” on the sensor

The problem with that is: The sensor continues to measure—but under different conditions.

Effects on Measurement Behavior and Control

In terms of building automation, this means:

The sensor's characteristic curve is shifting slowly
Reaction time becomes slower
Measurement results can no longer be reliably reproduced

A particularly critical issue is that these changes are often not immediately recognized as errors. At first glance, the values appear plausible, but they consistently deviate from the actual conditions. In practice, this leads to:

incorrectly set control parameters
unnecessary energy consumption (e.g., excessive ventilation or dehumidification)
in the worst-case scenario, to quality issues in the process

The solution: sensors with PTFE protective diaphragm

When you look at the causes of failure described above, it becomes clear: The problem lies not in the actual measurement of humidity, but in the direct exposure of the sensor element to the environment. This is exactly where the AFTF-45 / KFTF-45 comes in—not through a “different measurement method,” but through significantly improved protection of the sensor technology.

At the center is a PTFE membrane (Teflon) located directly in front of the sensor element, which acts as a physical barrier. This membrane is designed to remain permeable to water vapor while simultaneously blocking the majority of harmful substances. Compared to conventional sintered filters, which tend to have larger pores, the PTFE layer is significantly finer and prevents particles, aerosols, or corrosive substances from reaching the sensor at all.

The main difference between the AFTF-45 and the KFTF-45 lies in the sensor protection.

Use of a PTFE membrane (Teflon) directly on the sensor
very fine pore size (significantly smaller than traditional sintered filters)
provides reliable protection against particles and chemical exposure

In practice, this means:

The key difference isn't apparent right away on Tag first Tag, but becomes evident over time:

Contaminants never come into direct contact with the sensor element in the first place
Deposits on the sensor surface are significantly reduced
Chemical reactions that cause aging proceed much more slowly

The sensor therefore operates continuously under more stable conditions—even if the environment remains “critical.”

Impact on Measurement Stability and Service Life

From the application's perspective, this has several noticeable effects:

more consistent measurements over longer periods of time
less drift, even under continuous load
fewer unplanned sensor failures

This is particularly evident in areas such as swimming pools and food processing. While unprotected sensors must be replaced regularly, the measurement quality of protected sensors is maintained for much longer.

Another factor that is often underestimated in everyday use: Maintenance and calibration intervals also change. When less buildup occurs and the sensor element is subjected to less chemical stress, the calibration remains more stable—and interventions are needed less often.

Not a “panacea,” but a targeted interpretation

The PTFE diaphragm does not replace proper planning of the measurement point, but rather serves as a useful complement to it. In extreme cases—such as direct contact with liquids or heavy spray—the installation conditions must still be taken into account. But: For the vast majority of applications involving chemical or particulate contamination, this design makes the crucial difference between “works in the short term” and “operates reliably for years.”

Typical applications - focus on aggressive air and cleaning

Swimming Pools & Wellness Areas High humidity, constant chlorine exposure, and warm, humid air containing aerosols. For more information on stable measurement values despite chlorine exposure, see here.
Food Industry & Hygiene Areas Harsh cleaning agents, spray mist and residues, and varying operating conditions. Here You will gain in-depth expertise on chemical exposure resulting from cleaning in food-processing areas.
Animal Husbandry & Agriculture (e.g., Poultry Farming) High ammonia levels, organic particles in the air, and consistently high humidity. We have everything you need to know about high-humidity sensors for detecting particle and ammonia contamination here compiled for you.
Industry & Manufacturing Welding processes, dusty or particle-laden air, chemical emissions. Discover more background information and technical details about the long-term stability of high-humidity sensors under challenging conditions here.

AFTF-45 or KFTF-45—which model is right for you?

For swimming pools and wellness areas, both surface-mounted sensors and duct sensors may be suitable, depending on the facility. The key factor is where the measured value is needed for control or monitoring.

AFTF-45 as a surface-mounted sensor

Wall mounting in swimming pools, wellness areas, or utility rooms
Direct measurement of indoor air
Suitable for areas with humid and contaminated ambient air

The AFTF-45 is suitable for situations where humidity and temperature need to be measured directly in the room—for example, in indoor swimming pools, shower areas, locker rooms, or adjacent utility areas.

KFTF-45 as a duct sensor

Installation in ventilation and air conditioning ducts
Measurements in Supply or Exhaust Air Ducts
Suitable for HVAC systems, recirculating air systems, and central dehumidification

The KFTF-45 is useful when humidity and temperature need to be measured in the air duct, for example, to control exhaust air, supply air, dehumidification, or heat recovery.

In larger swimming pool facilities, a combination of the two may be useful: The room sensor monitors conditions in the lounge area, while the duct sensor monitors which air is being exhausted, treated, or recirculated.

Differences: AFTF-35 vs. AFTF-45

The AFTF-35 and KFTF-35 are the right choice when high humidity and the risk of condensation are primary concerns.

AFTF-45 and KFTF-45 are useful when additional chemical or particulate contamination is present—for example, from chlorine, aerosols, cleaning residues, or contaminated ambient air.

As a simple guide to help you decide:

Condensation and moisture dynamics are the main problem: Series 35
Chlorine, aerosols, or contaminated air are the main problem: Series 45

Click here for our blog post for high-moisture applications (AFTF-35 / KFTF-35)

Important points for selection and use

Even with protected sensors such as the AFTF-45 or KFTF-45, the installation environment remains a critical factor for long-term measurement stability. In practice, it has been shown time and again that no matter how technically robust a sensor may be, if environmental conditions are not taken into account, this directly affects its service life and the quality of the measured values. Especially in areas with cleaning cycles, chemical exposure, or high particle density, it’s worth carefully selecting the installation location and considering typical influencing factors from the very beginning.

Key practical points to consider are:

Position not directly in the spray mist from cleaning processes
Regular visual inspection if heavily soiled
Select the appropriate design (room vs. duct)
Ensure integration into the building management system (BMS) via standard signals

A robust sensor is no substitute for good planning—but it does provide crucial support. By taking these points into account, you can ensure that the sensor technology can fully realize its potential even in real-world operations.

Choose the right humidity sensors for your pool and spa now

In swimming pools and wellness areas, high humidity, warm indoor air, chlorine exposure, and aerosols place special demands on sensor technology. The AFTF-45 and KFTF-45 are designed for harsh environmental conditions and ensure stable humidity and temperature measurements in contaminated ambient air.

For you, this means:

More reliable measurements in demanding swimming pool environments
a better foundation for ventilation, dehumidification, and climate control
less maintenance and replacement work
Easy integration into building management systems, DDC, PLCs, or HVAC systems
Suitable designs for in-room and duct measurements

Depending on the installation situation, choose the appropriate model: AFTF-45 for direct indoor air measurement or KFTF-45 for installation in ventilation and HVAC ducts.

Discover Humidity Sensors for Swimming Pools & Spas