Catalytic-type/World leader in gas sensing innovation

Operating principle

Figaro offers a wide range of gas sensors for the detection of various gases, from explosive gases such as propane, toxic gases such as carbon monoxide, to air quality sensors for volatile organic compounds (VOCs) that are responsible for sick-house syndrome. Figaro offers a diverse portfolio of sensor technologies that can be matched to the unique requirements of each application.

  • MOS type
  • Catalytic type
  • Electrochemical type

Operating principle

Catalytic-type gas sensor consist of two elements: a detector element (D) which contains catalytic material and is sensitive to combustible gases, and a compensator element (C) which is inert. Combustible gases will burn only on the detector element, causing a rise in its temperature and, as a consequence, a rise in its resistance. Combustible gases will not burn on the compensator—its temperature and resistance remain unchanged in the presence of combustible gases. (Figure 1)
Normally a Wheatstone bridge circuit is formed with both elements as shown in Figure 2. A variable resistor (VR) is adjusted to maintain a state of balance of the bridge circuit in clean air free of combustible gases. When combustible gases are present, only the resistance of the detector element will rise, causing an imbalance in the bridge circuit, thus producing an output voltage signal (Vout). The output voltage signal is proportional to the concentration of combustible gases as shown in Figure 3. Gas concentration can be determind by measuring the output voltage.

■ Figure1 Basic measuring circuit

■ Figure2 Basic measuring circuit

■ Figure3 Relationship between gas concentration and output voltage

Warning and precautions for use of Catalytic-type gas sensors

  • Carefully read product information and other technical information provided by Figaro before using our products, and confirm specifications and operating conditions.
  • When designing an application circuit, please make sure that an accidental short circuit or open circuit of other electronic components would not cause excessive voltage, current or temperature exceeding rated values to be applied to the gas sensor.
  • When designing application products, please make sure that a gas sensor malfunction would not 1) cause adverse effects on other components, 2) directly or indirectly impair the safety of application products using gas sensors (e.g., malfunction, emitting smoke, fire, or any other unstable state of application products).
  • Consider adding safety measures for fail-safe where necessary, such as a protection circuit.

Cautions for safe use of Catalytic-type gas sensors

Applied voltage
Do not use the gas sensor if higher than the rated voltage is applied. If higher than the rated voltage is applied to the sensor, the lead wires and/or sensor elements may be damaged or sensor characteristics may be irreversibly impaired, even if no physical damage or breakage occurs.
Operating temperature
Do not use the sensor under higher temperature than the rated conditions. If the gas sensor is exposed to higher than rated temperature, an irreversible change of sensor characteristics would occur.
Environmental conditions
  • Avoid exposing the sensor where adhesives or hair grooming materials containing silicone or silicone rubber/putty may be present. If silicone vapors adsorb onto catalyst surface, the detector element will be coated, irreversibly inhibiting sensitivity.
  • Avoid highly corrosive environments. High density exposure to corrosive gases such as hydrogen sulfide, sulfur oxide, chlorine, hydrogen chloride, etc. for extended periods may cause corrosion or breakage of the lead wires or heater material. For information on specific gases and conditions for corrosive gases, please consult with Figaro.
  • Avoid contamination by alkaline metals. Sensor characteristics may be significantly changed if the sensor is contaminated by alkaline metals, especially salt water spray.
  • If the sensor is exposed to high concentrations of organic vapors (such as alcohol exceeding 10,000 ppm) for a long period, the filter may become saturated and sensitivity to organic vapors would temporarily increase. This change in characteristics is reversible.
  • Sensor performance may be affected if exposed to a high density of reactive gases for a long period of time, regardless of the powering condition. For information on specific gases and conditions, please consult with Figaro.
  • If water freezes on the sensor bead surface, the detector element may crack, which will irreversibly affect sensor characteristics.
  • This sensor cannot properly operate in a zero or low oxygen content atmosphere. It requires the presence of a certain minimum amount of oxygen in its operating environment in order for gases to combust on the detector element. The minimum amount of oxygen necessary will vary depending on the target gas.
  • If water condenses on the sensor element surface and remains for an extended period, sensor characteristics may temporarily drift. Light condensation under normal conditions of indoor usage would not pose a significant problem for sensor performance.
Handling
  • Sensor characteristics may be changed due to soaking or splashing the sensor with water.
  • Regardless of its powering condition, if the sensor is exposed in extreme conditions such as very high humidity, high temperatures, or high contamination levels for a long period of time, sensor performance may be impaired.
  • Avoid mechanical shock. Irreversible sensor drift or breakage of lead wires may occur if the sensor is subjected to a strong shock.
  • Under no circumstances should the sensor be disassembled, nor should the sensor can and/or cap be deformed. Such action would void the sensor warranty and would cause irreversible change in characteristics.
Storage conditions
  • To avoid shock, please keep the sensor in its original packing foam and sealed in a bag or container while in storage.
  • The sensor should be stored in a sealed bag containing clean air. DO NOT use silica gel.
Mounting process
  • Manual soldering is always recommended for mounting gas sensors.
  • Wave soldering may be used for this gas sensor if limited to the following conditions:
    (1) Suggested flux: Rosin flux with minimal chlorine
    (2) Transfer speed: 1-2 meters / min.
    (3) Pre-heating temperature: 100±20˚C
    (4) Solder bath temperature: 250±10˚C
    (5) Allowable soldering passes: 2 times maximum
  • The results of wave soldering cannot be guaranteed if conducted outside the above guidelines since some flux vapors may cause poisoning or drift in sensor performance similar to the effects of silicone vapors.
  • When a resin coating is applied on a printed circuit board for improved resistance to moisture and corrosive gases, the chemical solvent contained in coating material may affect sensor characteristics. Sample testing should be conducted to see if this process would adversely affect sensor characteristics.
  • Excessive vibration may result in irreversible sensor drift or cause the sensor element beads and/or the lead wires to resonate and eventually break. Usage of compressed air drivers or ultrasonic welders on assembly lines may cause such vibration to the sensor. Before using such equipment, preliminary tests should be conducted to verify that there will be no influence on sensor characteristics.
  • MOS type
  • Catalytic type
  • Electrochemical type