Acoustic detector

Photoacoustic Spectroscopy.7 When modulated IR radiation is absorbed by a sample, the substance heats and cools in response to modulated IR energy impinging on it. This thermal hysteresis is converted into pressure waves that can be communicated to surrounding gases and detected by acoustic detectors (essentially a sensitive microphone in the enclosed sample chamber). In such measurements, the acoustic detector replaces the IR detector of the spectrometer. 

As a special feature the PAS detector can be combined with a magneto-acoustic detector (MA) for the measurement of oxygen (02). Oxygen does not absorb IR light but it is paramagnetic. This means that if a switched magnetic field is applied... 

GHz, i.e. they should appear in the submillimeter wave region. Although their intensity should be very small (-y 10 cm ), the submillimeter spectrometer built by Krapnov et al. using the acoustic detector might have been able to detect these transitions. In a search for the weak transitions, the fact could be used that the frequency separation of the a(/+ 1, A 3) - - (/, k) and s(/+ 1, A 3) <- (J, k) transitions can be determined with microwave accuracy from the known inversion frequencies in the ground vibrational state of ammonia (see Fig. 8). 

In the laboratories of BASF (Badische Anilin- and Soda-Fabrik) at Ludwigshafen, the importance of infrared spectroscopy for industrial purposes was realized as early as the 1930 s. The first IR instrument with a modulated beam was built by Lehrer in 1937 and modified to a double beam instrument with optical compensation in 1942. Luft described the first non-dispersive infrared analyzer in 1943. He used the gas to be analyzed as absorber in a photo-acoustic detector cell. Thus, the instrument was sensitive only to this gas. He also provided a survey of early industrial applications of infrared spectroscopy (Luft, 1947). 

Modulated infrared energy is absorbed by the sample. The material heats and cools in response to modulated infrared light. This response is converted into a pressure wave, which communicates with a surrounding gas. The pressure change is detected by an acoustic detector in the enclosed sample chamber. The necessary amount of sample material is less than 10 mg. The measuring time is about a few minutes, and the quality of the spectra is similar to that of the DRIFTS and ATR methods. The costs for the equipment are higher. A comparison of photoacoustic spectroscopy with all various FT-IR methods for the use in combinatorial chemistry was made by Yan et al. [12,13]. 

The EPRI [1985) survey reveals that a wide range of techniques have been employed in commercially available biofouling monitors including annular flow devices, externally heat tubes, unheated surfaces, condenser simulators, electrically heated wires, biofilm sampling devices and acoustic detectors. 

Various modem accessories (ATR crystals, acoustic detectors, infrared microscopes, polarization modulation technique) as well as hyphenation techniques have substantially expanded the field of application of infrared spectroscopy. Applications of IR spectroscopy to surface investigations (characterization of the surface, physi-sorption and chemisorption studies, catalytic properties) are reviewed in [7-9]. Applications of hyphenated techniques, in particular combinations with chromatography, are given in [8]. 

Fast leak detection by hydrogen meter, acoustic detector, cover gas pressure gauge, rupture sensor and manometer seal level meter... 

Acoustic detectors and cover gas pressure gauges are provided for medium leakage, less than Ikg/s... 

Park, H.S. Thursby, G. and Culshaw, B. High-frequency acoustic detector based on fiber Fahry-Perot interferometer. 2nd Europ. Workshop on Optical Fibre Sensors, Santander, Spain 2004. SPIE-Vol. 5502 (2004), pp. 213-216... 

An estimated 2.8 million capacitors are in use in the USA. About 2000 of them rupture every year, causing spillage into the environment. As in the case of transformers, there are no tools to predict capacitor failure. Soil samples have to be tested for contamination, and the solid PCB-containing material must be shredded prior to incineration. The utility industry considers it to be cost-ineffective to recover the part/parts of the contaminated capacitors. Portable screening devices such as x-ray fluorescence meters for transformer oil, acoustical detectors to detect the ultrasonic sound from a faltering capacitor, and an infrared scanner to measure the temperature-rise of the faltering capacitor have been studied in the field (Miller, 1982). 

Since the discovery of the photoacoustic (PA) effect by Bell in 1880, who used the Sun as radiation source, a foot-operated chopper for modulation and an earphone as acoustic detector, the PA effect has found numerous applications as a sensitive and rather simple technique for determining optical, thermal and mechanical properties of all kinds of samples. This article focuses on methods and instrumentation employed in spectroscopic applications. Since photothermal (PT) spectroscopy is discussed elsewhere in the encyclopedia, PT schemes are only briefly mentioned here, whereas emphasis is put on instrumentation used in photoacoustic spectroscopy. 

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