Sensitivity Improvement of Micromachined Convective Accelerometers

The sensitivity of a micromachined convective accelerometer filled with different gases at various conditions is theoretically and experimentally investigated. It is shown that the sensitivity of the accelerometer is proportional to the Rayleigh number for the contained gas. The Rayleigh numbers for nine gases are calculated, and the corresponding sensitivity improvements are predicted. To test our predictions, bulk silicon micromachining technology is used to fabricate and package several thermal accelerometers. A precision rotary stage is set up enclosing a packaged device into a gas chamber and rotating its sensitive axis against gravity. Data acquisition is done by a high-precision digital multimeter over full cycle rotations for seven candidate gases: nitrogen (N ₂ ), argon (Ar), carbon dioxide (CO ₂ ), sulfur hexafluoride (SF ₆ ), hexafluoroethane (C ₂ F ₆ ), octafluoropropane ( C ₃ F ₈ ), and octafluorocyclobutane ( C ₄ F ₈ ). The measured sensitivities are in good agreement with the theoretical predictions. An upper limit to the sensitivity improvement by boosting the Rayleigh number for the contained gas is measured. Sensitivity decline beyond a critical Rayleigh number is reported for the first time since the advent of the convective accelerometers without proof mass.