What are the commonly used gas sensors？

Common gas sensors can be categorized based on technical principles such as thermal conductivity, electrochemical, catalytic combustion, PID, infrared, and semiconductor. Let’s dive into the detection principles of each type:

      1. Thermal Conductivity Gas Sensor 
Principle:  Detects gas concentration by measuring differences in thermal conductivity.  
  - Composed of a heating element and a measuring element.  
  - The heating element raises the gas temperature, while the measuring element compares thermal conductivity between heated and unheated states to determine concentration.  

     2. Electrochemical Gas Sensor 

Principle:  Relies on redox reactions between gas molecules and electrodes.  
  - Contains paired electrodes (e.g., working and counter electrodes).  
  - Gas interaction generates a current or voltage proportional to concentration.  
  - Example: Used for detecting CO, H₂S, and O₂.  

3. Infrared (IR) Gas Sensor

Principle:  Exploits gas-specific infrared absorption properties.  
  - Includes an infrared light source and a detector.  
  - Measures absorption intensity at characteristic wavelengths (e.g., CO₂ absorbs 4.26 μm IR light).  
  - Converts light attenuation into concentration values.  

     4. Semiconductor Gas Sensor  

- Principle:  Detects gas via changes in semiconductor resistance.  
  - Uses materials like tin oxide (SnO₂).  
  - Gas adsorption alters surface resistance, correlating with concentration.  
  - Common for detecting methane, alcohol, or VOCs.  

5. Catalytic Combustion Gas Sensor

 Principle:   Based on the Wheatstone bridge circuit.  
  - A detection element (catalyst-coated) and a reference element form one bridge arm.  
  - Flammable gas triggers flameless combustion on the catalyst, increasing temperature and resistance.  
  - Voltage imbalance in the bridge reflects gas concentration.  
  - Ideal for methane, propane, and other combustibles.  

      6. Photoionization (PID) Gas Sensor
- Principle: Uses ultraviolet (UV) light to ionize gas molecules.  
  - High-energy UV photons break molecules into ions (positive ions and electrons).  
  - Ion current is amplified and converted to concentration (e.g., ppm-level detection).  
  - Highly sensitive to VOCs and low-concentration toxins.  

Each technology has distinct strengths and limitations:  
- Thermal Conductivity: Simple, durable, but low sensitivity.  
- Electrochemical: High accuracy, but limited lifespan.  
- Infrared: Non-invasive, selective, but higher cost.  
- Semiconductor: Fast response, but susceptible to humidity.  
- Catalytic Combustion: Reliable for combustibles, but requires oxygen.  
- PID: Ultra-sensitive to VOCs, but ineffective for inert gases.