Julien M. Rey and Markus W. Sigrist
The objective is to develop a cost-effective and rugged photoacoustic (PA) sensor that can be used in various industrial applications. Such a PA sensor should be affected neither by intensity fluctuations of the light source nor by microphone or electronics drifts.
Photoacoustic (PA) gas sensors are based on the conversion of absorbed light energy into acoustical waves which can be detected with various type of microphones. The amplitude of a PA signal is directly proportional to the absorbed power in the gas sample, i.e. it depends on the power of the exciting light beam, but also on the responsivity of the microphone, the nature and pressure of the gas present in the PA cell and on the geometry of the cell. The proportionality of the PA amplitude with the intensity of the excitation light source, requires some kind of intensity normalization scheme (power meters or reference cells) for quantitative measurements.
To fulfill the requirements given above, we recently designed a novel PA scheme named “Differential Mode Excitation Photoacoustic Spectroscopy (DME-PAS)” [1-3]. This new PA method takes advantage of the selective excitation of two different modes in a resonant PA cell. Two excitation beams are sent to the DME-PA cell. The ratio of the absorbed energy corresponding to the excitation beams affects the ratio of the resulting acoustic mode amplitude. This acoustic mode amplitude ratio (and not the absolute PA signal value) is then used to derive the gas concentration. Two forms of the DME-PAS scheme have successfully been developed. Figure 1 shows a schematic of the setup.
In a first embodiment, two excitation beams (A and B) are sent to the DME-PA cell. The intensity ratio (IA / IB) of these two light beams reaching the cell affects the ratio of the resulting acoustic mode amplitude. A sample cell is placed along one of the beams before it reaches the PA cell. This allows to change the light intensity ratio IA/IB when an absorbing gas is present in this sample cell. Since the ratio of the acoustic mode amplitudes depends on the intensity ratio IA/IB of the beams reaching the DME-PA cell, it is thus a measure of the absorption in the sample cell. Measuring the acoustic mode amplitude ratio thus provides a means to determine the gas concentration in the sample cell. Using a simple black-body emitter as light source, a limit of detection of 25 ppm m-1 for acetone vapour was obtained.
In addition to the DME-PA gas sensor presented above [1,3], we recently built a second device which consists of only one cell that acts as both sample and PA cell. As light source, it uses a current modulated near-infrared (NIR) light emitting diode (LED) . A picture of this device is presented in Fig. 2.
This device was designed to measure water vapor. Figure 3 shows an example of a water vapor measurement. The linear relationship between the PA amplitude ratio and the water vapor concentrations is clearly demonstrated. Both sequential (one modulation frequency at a time) and simultaneous (both modulation frequencies at the same time) excitation of two acoustic modes of a DME-PA system have been investigated. The main drawback of the sequential modulation scheme is the additional time required for changing the modulation frequency in order to obtain the amplitude ratio needed by the DME-PA technique. With a total acquisition longer than 7 s, the simultaneous modulation scheme provides an improved measurement uncertainty. The uncertainty of water content measured with this setup is ±150 ppmV (simultaneous excitation, 15 s total acquisition time). This sensor is robust since it contains no moving parts and is very stable since it is based on the DME-PA scheme, which compensates for intensity fluctuations of the light source and for any microphone or electronics drifts. At the selected wavelength range (around 1400 nm) no significant interference with the main air components occurs. The LED-based photoacoustic sensor could thus represent a valuable alternative to conventional capacitive or resistive humidity sensors.
This project is supported by the Huber-Kudlich foundation and ETH Zurich.
1. J.M. Rey and M.W. Sigrist: “Differential Mode Excitation Photoacoustic Spectroscopy: a new photoacoustic detection scheme”, Rev. Scient. Instrum. 78, 063104 (2007)
2. J. M. Rey and M.W. Sigrist, Applicants ETH Zurich: “Differential photoacoustic detection of gases”, European Patent Application Nr. 06026139.3, Date of application: December 18, 2006
3. J.M. Rey and M.W. Sigrist: “Simultaneous dual-frequency excitation of a resonant photoacoustic cell”, Infrared Phys. Technol. 51, 515-519 (2008)
4. J.M. Rey and M.W. Sigrist: “New differential mode excitation photoacoustic sensing scheme for near-infrared water vapour sensing”, Sens. Actuators B: Chem. 135, 161-165 (2008)
5. J. M. Rey, C. Romer, M. Gianella and M. W. Sigrist
"Near-infrared resonant photoacoustic gas measurement using simultaneous dual-frequency excitation", Appl. Phys. B (2010), doi: 10.1007/s00340-010-3994
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