| 摘要 |
<p>The invention concerns a sensor for the measurement of carbon dioxide concentration within an air flow being input to a measuring cell (1) connected to at least one electroacoustic element (2, 3) for transmitting and receiving airborne acoustic waves within the measuring cell (1). The output signal of the sensor corresponds to the propagation velocity of these waves and an indirect measure of carbon dioxide concentration, with an automatic offset correction functionality for compensation of undesired offset variations of the signal, caused by e g temperature or humidity variations. Preferably, an accumulating function for minimising the influence of said offset variations is included, which for humidity variations may consist of a material with high porosity and surface density of polar molecular groups, e g cellulose. In between an inner and outer sheath (4, 5) surrounding the measuring cell (1). Temperature control of the inner sheath (4) is effected by a temperature sensor (11), a resistive heating element (12) in good thermal contact with the inner sheath (4), and a control circuit (13). The electroacoustic elements (2, 3), together with an amplifier (14) constitutes a free running oscillator, the frequency of which is a monotonous function of the sound velocity within the measuring cell (1). The offset correction functionality is based on measurement or calculation of the average sensor signal, extreme values, or other statistical parameters calculated during a certain period of time, being shorter than the time lapses of the undesired offset variations.</p> |
