Catalytic Combustion Sensor and Methane Reaction Curve Analysis

In the field of combustible gas detection, catalytic combustion sensors are one of the longest-applied and most mature technologies. Especially in methane detection, their stable and predictable output characteristics make them a foundational choice in mining, petrochemical, and industrial safety systems.

Nexisense, with over 40 years of experience in gas sensing technology, provides a systematic interpretation of the reaction curve between catalytic combustion sensors and methane from both engineering and mechanistic perspectives, helping users better understand performance limits and application value.

Working Principle of Catalytic Combustion Sensors

The core of a catalytic combustion sensor consists of two resistive elements with similar structural and electrical characteristics:

• Detection element (active element): coated with catalyst

• Compensation element (inert element): does not participate in combustion reactions

When methane or other combustible gases undergo flameless oxidation on the surface of the active element, heat is released, increasing the element's temperature and changing its resistance. This change is converted into a voltage signal output via a Wheatstone bridge.

This mechanism means that the sensor output is essentially a function of the heat released during gas combustion.

Basic Shape of Methane Reaction Curve

Under ideal conditions, the catalytic combustion sensor’s response curve to methane exhibits highly regular characteristics.

Good Linearity

Within the 0% LEL to 100% LEL range, the sensor output signal and methane concentration usually show an approximately linear relationship.

This feature is one of the most important advantages of catalytic combustion technology, offering the following in engineering applications:

• Simple concentration calculation

• Low difficulty in signal processing circuit design

• Strong system stability

Mathematically, this can be approximated as:

V_out = K × C + V0

• V_out: sensor output voltage (mV)

• K: sensitivity coefficient (mV/%LEL)

• C: methane concentration (%LEL)

• V0: zero output voltage (in clean air)

Slope and Sensitivity Engineering Significance

The slope of the reaction curve directly corresponds to the sensor's sensitivity to methane. In actual products, typical methane sensitivity ranges from 55–65 mV/%LEL, meaning:

• Each 1% LEL increase in concentration

• Output voltage increases by approximately 60 mV

A higher slope makes signal changes more noticeable per unit concentration, enhancing system resolution. However, excessive sensitivity may increase susceptibility to noise and environmental disturbances, so a balance is necessary in design.

Linear Range Boundary Conditions

Good linearity mainly applies within the lower explosive limit. When methane concentration exceeds 100% LEL (~5% by volume):

• Oxygen gradually becomes insufficient

• Catalytic combustion reaction is limited

• Heat release no longer increases linearly with concentration

At this point, the output curve gradually deviates from linearity, flattens, or even decreases under extreme conditions. This is why catalytic combustion sensors are primarily used for LEL monitoring rather than high-concentration analysis.

Key Factors Affecting the Reaction Curve

In real applications, the reaction curve is influenced by multiple factors.

Ambient Temperature

Temperature changes affect:

• Catalytic reaction rate

• Element thermal equilibrium

• Resistance temperature coefficient

Typically, higher ambient temperature lowers the curve slope and sensitivity. Engineering-grade sensors often employ structural design or algorithms for temperature compensation.

Ambient Humidity

High humidity may form an adsorbed water film on the catalyst surface, slightly inhibiting gas diffusion and reaction, affecting signal stability. Although smaller than temperature effects, it cannot be ignored in high-humidity mines or underground spaces.

Catalyst Deactivation

Over time, catalysts may degrade due to:

• Poisoning by sulfur, silicon, or other substances

• High-temperature sintering

• Long-term high-concentration gas exposure

This manifests as a gradual decrease in reaction curve slope and lower output for the same concentration. Without regular calibration, this may cause delayed alarms or failure.

Engineering Countermeasures

To maintain usable reaction curves in complex environments, system-level controls are required:

• Temperature compensation circuits or algorithms to correct environmental effects

• Use catalysts with stronger anti-poisoning properties

• Establish regular calibration and performance evaluation mechanisms

• Monitor zero point and sensitivity drift at the system level

These measures do not change the physical nature of catalytic combustion but significantly extend effective service life.

Nexisense’s Practical Experience

In long-term applications, Nexisense continuously optimizes reaction curve stability:

• Maintains linearity through catalyst formulation and sintering process control

• Introduces compensation strategies in sensor structure to reduce environmental fluctuation impact

• Performs multi-point calibration with standard methane gas before shipment to ensure sensitivity consistency

These practices ensure reliable engineering performance under standard operating conditions.

FAQ

Can catalytic combustion sensors be used for high-concentration methane detection?
Not suitable. Designed for safety monitoring within the LEL range; reactions are limited at high concentrations.

Does the reaction curve change over time?
Yes, mainly as decreased sensitivity, requiring regular calibration.

Why are catalytic combustion sensors still widely used?
Because of their mature principle, good linearity, and simple system, they remain cost-effective in many industrial scenarios.

Conclusion

The reaction curve between catalytic combustion sensors and methane is the core foundation for quantitative combustible gas detection. Stable linearity, clearly defined sensitivity, and well-defined working range boundaries have made this technology a key support for industrial safety monitoring for decades. Despite physical limitations in lifespan and environmental adaptability, with proper use and maintenance, catalytic combustion sensors remain a mature, reliable, and well-validated technology path.