Refrigerant Leak Detection: Thermal Conductivity vs. NDIR Sensors

A refrigerant leak might seem like a small problem – until it isn’t. 

Even a slow, undetected leak can drive up energy costs, damage equipment, and release greenhouse gases into the atmosphere. In industries where efficiency and compliance matter, catching these leaks early is helpful and necessary.

To do that, businesses rely on refrigerant leak detection, and choosing the right sensor makes all the difference. 

Two common technologies stand out: thermal conductivity sensors and NDIR sensors. 

Compliance with standards like UL60335-2-40, particularly Annex LL, continues to shape how refrigerant leak detection is approached. These standards define how quickly and accurately a sensor must detect a leak, especially when working with low-GWP or mildly flammable refrigerants.

Each operates differently, with advantages depending on the application. Thermal conductivity sensors measure how gases transfer heat, while NDIR sensors detect refrigerants by how they absorb infrared light.

Understanding the strengths of each sensor helps in choosing the right one for HVAC, refrigeration, and industrial settings. 

Let’s break down how they work, where they perform best, and what to consider when selecting a detection method:

Understanding Refrigerant Detection Requirements

Refrigerants are used in HVAC, refrigeration, and industrial cooling systems to transfer heat and maintain temperature control. When leaks occur, they affect:

• System performance
• Increase energy costs
• Release greenhouse gases into the atmosphere.

To reduce environmental impact, refrigerant leak detection is now regulated by agencies like the EPA, ASHRAE, and international environmental bodies. 

The Montreal Protocol and Kigali Amendment led to the phase-down of high-GWP refrigerants such as R-410A and R-134a, pushing the industry toward lower-GWP alternatives like R-32, R-290 (propane), and CO₂ (R-744).

Different refrigerants require different sensing approaches. Some, like CO₂, need sensors that detect high concentrations, while others, such as hydrocarbons (R-290, R-600a), require sensors designed for flammable environments. Selecting the right detection method depends on the refrigerant type, application, and operating conditions.

In North America, UL60335-2-40 – specifically Annex LL – sets performance expectations for refrigerant leak detection systems. Updates in the fourth edition focus on tighter thresholds for accuracy, drift, and response time, especially for detecting low-GWP and mildly flammable A2L refrigerants.

How Do Thermal Conductivity Sensors Work?

Thermal conductivity sensors measure how gases transfer heat. Each gas has a unique thermal conductivity, meaning it absorbs and moves heat differently. By detecting these changes, the sensor can determine the presence and concentration of a gas.

A thermal conductivity sensor typically consists of:

• A heating element that generates a stable heat source
• A sensing chamber where the gas interacts with the heat
• Temperature-sensitive components that detect heat transfer variations
• Electronics that convert the data into a readable measurement

When refrigerant enters the sensing chamber, it alters the rate of heat transfer compared to the reference gas. The sensor measures this difference and calculates the refrigerant concentration.

Advantages of Thermal Conductivity Sensors

• Responds quickly to gas concentration changes
• Can detect a wide range of refrigerants, including CO₂
• Works well in mixed-gas environments
• Compact and energy-efficient design
  
  

Limitations of Thermal Conductivity Sensors

• Less selective, meaning they may require additional processing to distinguish between gases
• Affected by temperature and pressure variations
• Accuracy depends on calibration and the specific gas mixture

Thermal conductivity sensors are commonly used for CO₂ detection and gas monitoring in HVAC and industrial applications.

For refrigerants classified as A2L, these sensors must meet specific performance benchmarks outlined in UL60335-2-40, including response time and detection limits defined in Annex LL.

How Do NDIR Sensors Work?

NDIR (Non-Dispersive Infrared) sensors detect gases by measuring how they absorb infrared (IR) light. Each gas has a unique absorption pattern, allowing the sensor to identify specific refrigerants with high accuracy.

An NDIR sensor consists of:

• An infrared light source that emits a controlled beam
• A gas sampling chamber where refrigerant interacts with the IR light
• An optical filter that isolates the specific wavelength absorbed by the target gas
• A detector that measures how much IR light is absorbed

When refrigerant enters the chamber, it absorbs some of the infrared light at a specific wavelength. The detector measures the difference between the emitted and received light, calculating the gas concentration.

Advantages of NDIR Sensors

• High specificity – detects only the target gas
• Stable over time with minimal drift
• Largely naffected by temperature and humidity changes
• Works well for CO₂ and many hydrofluorocarbon (HFC) refrigerants
  
  

Limitations of NDIR Sensors

• Slightly slower response time in some applications compared to thermal conductivity sensors
• More expensive due to optical components
• Can be affected by dust or contaminants in the gas sample

NDIR technology is widely used in NDIR CO₂ sensors, NDIR sensors for refrigeration, and other refrigerant leak detection applications where accuracy and long-term stability are needed.

Because of their precision and low drift, NDIR sensors are often selected for systems seeking to meet UL60335-2-40 Annex LL requirements, particularly where consistent refrigerant identification and fast response are required for safety and compliance.

Comparing Thermal Conductivity and NDIR Sensors

4 Application Scenarios: Which Sensor is Best for Your Needs?

Selecting the right sensor depends on the refrigerant type, operating environment, and detection requirements. Some applications benefit from the fast response of thermal conductivity sensors, while others work better with the precision of NDIR sensors.

1. HVAC & Refrigeration Systems

• Best choice: NDIR sensors
• Used in commercial and residential HVAC systems to detect refrigerant leaks.
• High selectivity improves accuracy when detecting CO₂, HFCs, and low-GWP refrigerants.

2. Industrial Leak Detection

• Best choice: Thermal conductivity sensors
• Works well in environments with multiple gases, such as manufacturing plants and cold storage facilities.
• Detects refrigerant concentration changes quickly, helping reduce energy loss.

3. Automotive Air Conditioning & Transport Refrigeration

• Best choice: NDIR sensors
• Used in vehicle cooling systems to detect CO₂ (R-744) and HFO refrigerants.
• Provides stable readings, even in environments with temperature fluctuations and vibrations.

4. Energy-Efficient Smart Buildings

• Best choice: NDIR sensors
• Integrated into IoT-based monitoring systems for continuous leak detection.
• Helps buildings meet green building standards and lower energy consumption.

Both thermal conductivity and NDIR sensors support refrigerant leak detection, with the best choice depending on the specific needs of each application.

Choosing the Right Sensor for Refrigerant Leak Detection

Both thermal conductivity sensors and NDIR sensors are used for refrigerant leak detection, but they work in different ways. 

Thermal conductivity sensors respond quickly and perform well in mixed-gas environments, making them a good fit for industrial settings. 

NDIR sensors provide precise, long-term detection for specific refrigerants, making them ideal for HVAC, refrigeration, and automotive applications.

When compliance with standards like UL60335-2-40 Annex LL is required – especially when working with A2L refrigerants – sensor selection may be shaped by response time, stability, and detection accuracy. Understanding how each technology functions helps in making the right choice for the application.

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