Photo Acoustic Spectroscopy

direct NO measuring

PAS - Photoacoustic Spectroscopy

The PhotoAcoustic Spectroscopy (PAS) is a spectroscopic method which utilizes the photoacoustic effect. This method is field proven in several applications like emissions testing of vehicles, in environmental technology (detection of air pollutants), in the medical technology and in the biology.
It is highly accurate, stable and a direct measurement (independent of background gases).

It is based on the fact that infrared light energy is absorbed by gas molecules. In the photoacoustic spectroscopy the modulated light is converted into acoustic waves. The soundwave is then converted into an electric signal with the use of a microphone.

Measuring principle of the PAS (NO2) sensor:

photo acoustic spectroscopy principle

Performance of the PAS (NO2) sensor:

Allan-Variance

Linearity

allanplot
linearity

The sample gas is irradiated with modulated light of a predefined wavelength. IR-laser-diodes are often used as light source, because for many applications the particular wavelength of the examined material is in the infrared range. The light is electronically or mechanically modulated, for example by using a chopper.

Gas molecules absorb a part of the light, when the light frequency corresponds with an absorption band of the gas in the cell. The higher the concentration of the gas, the more light is absorbed.


The absorbed light causes heat and therefore a pressure rise. Due to the modulated light the pressure will alternately increase and decrease.
This generates an acoustic signal which can be detected by a microphone and then converted into an electric signal.

  1. Benefits at a glance

    Major advantage of the photoacoustic effect is the fact that sensitivity is basically not dependent on the optical path length.
    This allows high sensitivity from short absorption path length and highly linear concentration response over a wide dynamic measurement range from very low samplevolumes.

    No interfering signal and extremely accurate results

    The absorption is measured directly and not in relation to the background.
    Therefore the PAS is one of the most sensitive methods for the detection of gases and often used in trace-gas analysis.

    Low sample volume and small cell size

    The small size of the PAS cells enables the measurement of very small gas volumes.
    In contrast to conventional methods, the sample volume can be reduced drastically.

    Huge price advantage

    Another advantage is that the PAS is generally cheaper than other gas analysis methods. The reason for this are the microphones which are less
    expensive than (infrared-)detectors.

    Low amount of drift

    The response of the microphone is extremely stable. Therefore the drift is very low and calibration is seldom needed.

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