Micro Thermal Conductivity Sensor for Gas

General description

    The thermal conductivity gas sensor is manufactured on silicon with thin film deposition and silicon micro-machining techniques. This device determines gas concentrations of binary or ternary gas mixtures.

    The sensor structure consists of an integrated heater located on a thin electrical and thermal insulating membrane. Two thin film resistors are used for heating and measuring the temperature Tm of the membrane. Two reference resistors are integrated on the silicon beside the membrane area for the compensation of the ambient temperature changes. The four resistors are passivated to protect them from the effects of the gas and are electrically connected to a TO8 or TO5 base by gold wire bonding.

Applications

-Industry

•Determining gas concentration by measuring the thermal conductivity in binary mixture (Ar, H₂ in N₂) or quasi-binary mixture (H₂, He or CO₂ in air)

•Combustion control

•Monitoring of CH₄ concentration in Natural Gas for gas engine control

•Detection of refrigerant gases such as Freons (R-11; R-12, R-21, R-22), CFC or CF₃CH₂F Fluoroethane (R-134 or R-404) for leakage monitoring of cooling systems

•Measurement of Hydrogen (0 to 5% or higher), Helium (0 – 5000 ppm) or Xenon in air

•Security

•Industrial process control, horticulture, food storage, fermentation process control

•Micro vacuum device as miniature Pirani gauge

•Micro absolute humidity sensor in air based on thermal conductivity

•Beverage industry (CO₂ concentration)

-Domestic use

•CO₂ safety monitoring (0-4%) for Security, Fire Alarms or Indoor Air Quality

•Detection of cooking point in microwave owens

Main advantages

•Silicon micromachined sensor with very small dimensions (3.5 x 3.5 mm²) and for miniature gas volume (20 mm³)

•Short time constant (membrane time constant <5 ms)

•Low power, highly sensitive with a minimal heating power (< 6 mW)

•Independent of flow

•Integrated sensing resistance on membrane as well as temperature compensation resistors

•Reliable and reproductible sensitivity performances

•Small dispersion of the sensor specifications due to batch fabrication processes

•Linear output signal versus target gas concentration

•Very stable long term operation

•High mechanical resistance

•No field maintenance