When using a photoacoustic infrared instrument, a gas sample is introduced into the monitor's measurement chamber and the sample is exposed to a specific wavelength of infrared light. If the sample contains the gas of interest, that sample will absorb an amount of infrared light proportional to the gas concentration that is present in the sample.
Photoacoustic infrared analysis, however, extends beyond simply measuring the amount of infrared light that is absorbed; this technology actually detects what occurs after the gas is absorbed.
Always in motion, gas molecules move around the inside of the measurement chamber, generating pressure. When gases absorb infrared light, the temperature of the molecules rises and they begin to rapidly move. As a result, measurement chamber pressure increases, creating an audible pulse that can be detected. A highly sensitive microphone is located inside the photoacoustic infrared monitor to detect even the smallest of pressure pulses, enabling detection of even the lowest gas levels.
The monitor's optical filter allows only that particular light wavelength of the gas in question; a pressure pulse confirms the presence of that gas. The premise is simple; if no pressure pulse occurs, then no gas is present. The magnitude of the pressure pulse indicates the gas concentration present. The stronger the pressure pulse, the more gas that exists.
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