Characteristics of a microbridge type MEMS sensor for the thermal conductivity measurement of gases by a steady state method

Thermal conductivity λ of gases was successfully measured by a simple steady state method using a microbridge type MEMS sensor fabricated on a Si substrate. The sensor consisted of a hot wire with two adjacent thermocouples on the surface of a SiO₂ microbridge. The temperature increase of the microbridge was measured by supplying step-like electrical power Q to the hot wire. Due to the small heat capacity of the microbridge, the temperature increased to a saturated value ΔT within several tens of millisecond. In moving gas, the difference of the ΔT between upstream and downstream thermocouples gives the flow velocity. In static gas, this difference intrinsically becomes zero, and the heat flow Q_G from the hot wire to the surrounding gas is calculated using the Q to yield the same ΔT for different gases with known λ. Once Q_G is obtained, the λ of any unknown gases can be estimated by measuring the Q to yield the same ΔT.