Effect of Knudsen number on maximum response of a Thermal Conductivity Detector

Micro-Thermal Conductivity Detectors (μTCDs) have been designed, fabricated and tested in a gas chromatography system to show a correlation between detector design and the mean free path of the carrier gas. Our tests show that for a fixed detector design, the maximum possible response is limited by the Knudsen number (the ratio of the mean free path of the gas and chamber critical dimensions). Therefore, the miniaturization of μTCDs is limited by the relationship between detector design and the mean free path of the carrier gas. Several devices were fabricated to show that as the critical dimensions of the detector approach the mean free path of the carrier gas, the thermal resistance and sensitivity of the system are strongly affected. This work is the first to show how the carrier gas mean free path needs to be considered when designing μTCDs.