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1.
Figure 6.

Figure 6. From: Detection of Molecular Oxygen at Low Concentrations Using Quartz Enhanced Photoacoustic Spectroscopy.

Calibration of the detector with tapered amplifier.

Andreas Pohlkötter, et al. Sensors (Basel). 2010;10(9):8466-8477.
2.
Figure 1.

Figure 1. From: Detection of Molecular Oxygen at Low Concentrations Using Quartz Enhanced Photoacoustic Spectroscopy.

Illustration of a QEPAS spectrophone. The light is focused through the acoustic resonator tubes and the prongs of the tuning fork.

Andreas Pohlkötter, et al. Sensors (Basel). 2010;10(9):8466-8477.
3.
Figure 5.

Figure 5. From: Detection of Molecular Oxygen at Low Concentrations Using Quartz Enhanced Photoacoustic Spectroscopy.

Calibration of the sensor without tapered amplifier. From the numerical fit, a coefficient of 4.44·105 %/V is determined. The noise level of 1.48 μV is shown as a broken line.

Andreas Pohlkötter, et al. Sensors (Basel). 2010;10(9):8466-8477.
4.
Figure 2.

Figure 2. From: Detection of Molecular Oxygen at Low Concentrations Using Quartz Enhanced Photoacoustic Spectroscopy.

Optical setup with tapered amplifier, electronics and data acquisition. The upper part shows a photograph of the implemented system while the lower part is a schematic drawing of the system.

Andreas Pohlkötter, et al. Sensors (Basel). 2010;10(9):8466-8477.
5.
Figure 8.

Figure 8. From: Detection of Molecular Oxygen at Low Concentrations Using Quartz Enhanced Photoacoustic Spectroscopy.

2f QEPAS signals of 21% oxygen in dry argon (circles) and ambient air (triangles). The contained water vapor leads to a signal enhancement due to higher V-T transfer rates.

Andreas Pohlkötter, et al. Sensors (Basel). 2010;10(9):8466-8477.
6.
Figure 4.

Figure 4. From: Detection of Molecular Oxygen at Low Concentrations Using Quartz Enhanced Photoacoustic Spectroscopy.

QEPAS measurements of ambient oxygen in air at p = 300 mbar. The graph shows the normalized 2f wavelength modulation signals measured without (circles) and with the tapered amplifier (triangles).

Andreas Pohlkötter, et al. Sensors (Basel). 2010;10(9):8466-8477.
7.
Figure 3.

Figure 3. From: Detection of Molecular Oxygen at Low Concentrations Using Quartz Enhanced Photoacoustic Spectroscopy.

Dependence of the QEPAS signal on the optical power, measured at C = 21% oxygen concentration in air. From the linear fit (line) a slope of 25.4 mV/W is derived.

Andreas Pohlkötter, et al. Sensors (Basel). 2010;10(9):8466-8477.
8.
Figure 7.

Figure 7. From: Detection of Molecular Oxygen at Low Concentrations Using Quartz Enhanced Photoacoustic Spectroscopy.

Long-term QEPAS measurement of oxygen in air with the amplified laser system. The signal is normalized to an optical power of P = 1 W. The signal variation is found to be 0.57% (RMS). The inset shows the same measurement with another scale.

Andreas Pohlkötter, et al. Sensors (Basel). 2010;10(9):8466-8477.

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