Photo acoustic Spectrography
Photoacoustic gas measurement (e.g., Bruel-Kjaer gas monitor)is based on the same principles as conventional IR-based gas analyzers: the ability of CO2 and N20 and anaesthetic agents to absorb IR light. However, they differ in measurement techniques. While Infra-red spectrography uses optical methods, PAS uses an acoustic technique. When an IR energy is applied to a gas, the gas will expand and lead to an increase in pressure. If the applied energy is delivered in pulses the gas expansion would be also pulsatile, resulting in pressure fluctuations. If the pulsation frequency lies within the audible range, an acoustic signal is produced and is detected by a microphone. Potential advantages of PAS over IR spectrometry are higher accuracy, better reliability, less need of preventive maintenance, and less frequent need for calibration. Further, as PAS directly measures the amount of IR light absorbed, no reference cell is needed and zero drift is nonexistent in PAS. The zero is reached when there is no gas present in the chamber. If no gas is present there can be no acoustic signal.6 Despite being a superior method of measurement of CO2, photoacoustic spectrography did not gain as much popularity as IR spectrography.
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