MEASUREMENT OF SOUND PRESSURE VIA TEMPERATURE-FLUCTUATIONS BY A COLD-WIRE MICROPROBE AT HIGH-TEMPERATURES

A miniature microphone, suitable for measuring periodical pressure variations in gases at frequencies between 0.1 and 6000 Hz and temperatures up to 1200 K, has been developed in order to characterize the frequency response of gas sensors. The temperature variations in the gas due to the heat of compression are measured by a platinum cold-wire probe, from which the pressure variations are calculated. An optimized probe design makes it possible to calculate the transfer function of the microphone by a consideration of thermal boundary layers of the wire holders and surrounding walls. Both a simple approximative formula for the present setup and a more generalized finite-element calculation are presented. Thus, absolute measurements of sinusoidal pressure variations in phase and amplitude with max. 0.2 Pa resolution are possible. Comparison with a fast chemical gas sensor and internal heating measurements demonstrate the proper operation of this device. Possible disturbing effects and operation limits are discussed with.respect to modified design or operating conditions in other applications for frequency response studies in physical chemistry.